Landracing Forum
Tech Information => Aerodynamics => Topic started by: MLSE on June 27, 2011, 12:53:49 AM
-
I am looking for some help from someone who KNOWS what the effect of discharging an extreme volume of hot exhaust flow (1000 h.p. worth of exhaust flow) underneath the chassis of a production class sedan over 200 mph? I am not an engineer but it would seem to me that with the air already being packed under the car (due to the lack of a front air dam) that adding the exhaust volume has to have some sort of significant effect, either good or bad. Does this add to overall lift (and drag) on the chassis or would the exhaust flow displace cooler denser air beneath the car and reduce drag?
Also since "exhaust" in the rule book sates only that it "can be modified. must point away from fuel lines etc, away from the salt and past or away from the driver" (and could be discharged through the hood without changing the body contour) what would be the effect on aero over 200 mph - of the same high volume, high temperature flow passing over the sedan or coupe production body? Many big time turbo drag race cars are now just shooting the turbo discharge straight up through the hood.
Sure hope somone out there knows something (or everything) about this subject .
Thanks in adcance for any help you can give.
-
I have been pondering this alot too. Running the exhaust out of a two 4in. collectors through the car will be tough in the Datsun we are building (slowly). Shooting it straight out of the lower fender will look cool but will it hurt the aero? Could always turn them up through the hood like a tractor pull truck! Anyone that can comment on this with some knowledge will help MLSE and I out tremendously.
-
The reason I pointed mine straight back on the RATICAL was because of thrust and aero! If you have rake on the car the underside is going to be negative pressure.
-
Some people have been known to go through a lot of trouble to get the exhaust out the back of the car....... 8-)
-
I am in the planning and building stage of a bellytank, so i have been studying a lot of different things with aerodynamics and reseraching a few ideas i have, this was one of the thoughts i had also.
a lot of late nights at the computer reading and all i can come up with that may be closely related to this matter are some research papers from (aeronautical research committee report and memoranda no. 1683) and a LOT of very usefull tech and information if you google (the meredith effect) i still havnt read or understood all of the information that i gatherd from the search.
maybe someone here can check it all out and see if it would be anything helpfull with using the exhaust flow to give a boost to performance.
Most of the above information has to do with using the heated air from fighter aircraft engine cooling radiators to boost the airplanes speed.
the supermarine spitfire was the first to make use of this concept and the designers of the P-51 mustang said the meredith effect was a major factor for the P-51's high speed.
at 25,000ft altitude the hot air from the engine cooling radiators running through special ducting had 1/3 as much power as the proppeller had at full power.
you would think that the exhaust having a lot more heat than cooling radiators would contribute to a LSR cars speed.
i hope this is helpfull, i am new here, just mainly lurking and studying for my own build up.
-
A little off the subject, but still of intrerest (at least to me):
When Dan Hostetter built his ever evolving '32 Coupe he ran 8 individual pipes out the back of the car. He remembered a show car roadster from the late '50's or early '60's with that exhaust system (Green and white body, I can still remember the car on the cover of Hot Rod magazine). The only reason he did it was that he thought it looked COOL, and had always wanted to do in. His little Desoto Hemi was injected so it didn't really benefit from individual stacks.
He wanted more power so he went back to a conventional header setup. When he ran the car it was less stable that before. The 4 horizontal tubes on each side acted as sort of a bellypan and had cleaned up the air under the car.
So much for unintended concequences!
Tom
-
Exhaust gases are two things, hot and high velocity, which makes them, once they have exited the pipe, a high energy, low pressure gas stream. They can provide thrust and also if exited in the right area they can assist air flow over or under the car. I would never have the exhaust just point out of the car at 90 degrees or down, maybe up if you wanted down force but to the rear is best especially if you can use it to assist your aero package. I think Blue's (Eric) comment about, testing, testing, testing etc. fits here.
If you wonder how much force the exhaust can develop, you should see the vids of the fuel funny cars that lost the headers on one side, it nearly blows them over backwards!!
Rex
-
Rex;
It may be a B-29 that I'm thinking of, but somewhere I read about using the engine exhaust gasses for additional thrust. I think there was an "augmentor tube" involved that sucked in air to add to the thrust.
Regards, Neil Tucson, AZ
-
Tom --
The green and white roadster had an Olds (Cad?). The one you're thinking of was Scotty's Muffler Service from dear ol' Berdoo. Eight pipes outta an Ardun. They were machined from Ford torque tubes, I believe.
Would be good ballast, too, for an LSR car.
Best regards,
Stan
-
Exhaust gases are two things, hot and high velocity, which makes them, once they have exited the pipe, a high energy, low pressure gas stream. They can provide thrust and also if exited in the right area they can assist air flow over or under the car. I would never have the exhaust just point out of the car at 90 degrees or down, maybe up if you wanted down force but to the rear is best especially if you can use it to assist your aero package. I think Blue's (Eric) comment about, testing, testing, testing etc. fits here.
If you wonder how much force the exhaust can develop, you should see the vids of the fuel funny cars that lost the headers on one side, it nearly blows them over backwards!!
Rex
Rex, where would you exit the exhaust on my car? With a bellypan we'll have to remove to change classes, out the back would be a pain. Out the side is the easiest, but....
-
We ran our out the back fro a whole bunch of reasons
The Reverend, who drew the car wanted it to have clean lines, and therefor clean aero
there were also thoughts about the benefits of hot exhaust filling the void behind the car
I wanted a bit of length in the exhaust so it would run well
It took a bit of effort to get it all to fit and to deal with the heat, and it can make changing the engine hard work
However, it seems to work well and the engine runs nicer with the whole exhaust than with just headers
G
(http://i227.photobucket.com/albums/dd35/Jarman-Stewart/th_P3100953.jpg) (http://s227.photobucket.com/albums/dd35/Jarman-Stewart/?action=view¤t=P3100953.jpg)
-
I purposely routed my exhaust to come out the rear deck lid. My car has a stark drop off behind the rear window area and generates low pressure. The low pressure makes lift, and while I dont know if it really helps piping the exhaust into the low pressure area makes me sleep better at night.
~JH
-
Don't ya think that al this topic is becoming a bit dramatic?
Wouldn't ya wanna select the best tuned length for the system and then exit it in such a manner that a pressure zone isn't balking it's release, and then do it as simple as possible.
Ya might find routing all the pipe to an optimum exit location might tend to turn all yer plumbing into a horsepwer robbing mess.
Sumtimes ya gotta compromise!
-
a lot more likely to dramaticaly increase drag from trying to pick up 5-10 hp with equal length headers
-
Don't ya think that al this topic is becoming a bit dramatic?
Wouldn't ya wanna select the best tuned length for the system and then exit it in such a manner that a pressure zone isn't balking it's release, and then do it as simple as possible.
Ya might find routing all the pipe to an optimum exit location might tend to turn all yer plumbing into a horsepwer robbing mess.
Sumtimes ya gotta compromise!
To followers of the Church of Horsepower that would read like gospel, If you were a disciple of the prophet Aerodynamics you may see that as heresy.....Thankfully this is an ecumenical message board where we all act like we get along.....The better your aero, the less power you need,the better your aero the faster you go for the same power......... powerful engines are expensive to build, the better your aero the cheaper your fun.......
That horsepower that is "robbed" may well be un-necessary in a car with a half decent shape. That increase in power you cite by "exiting in such a manner that a pressure zone isn't baulking it's release " may well be inconsequential.
For one, it's low pressure that is causing drag even more than the wall of air you think you are pushing...... so dumping exhaust into a low pressure zone is going to help........
To a surprising number of people this discussion is interesting...and important, to some people it's what it's all about..
Sort of like the Hokey Pokey.
-
Drag improves as the temperature of the air increases . If there was a way to spread the hot exhaust down the side of the car without adding turbulence there should be a gain .
-
I have been pondering this alot too. Running the exhaust out of a two 4in. collectors through the car will be tough in the Datsun we are building (slowly). Shooting it straight out of the lower fender will look cool but will it hurt the aero? Could always turn them up through the hood like a tractor pull truck! Anyone that can comment on this with some knowledge will help MLSE and I out tremendously.
You can choose to imagine the air as flowing over the car or the car passing through the still air. Either way the closer the air follows the shape and the less "wake" you leave the better. (This part is easy to understand, it takes energy to stir something...if you leave a trail of eddys you are losing energy, the energy it takes to stir it up...)
Think about what exhaust travelling at 90 degrees to the direction of travel will do to the air that you are trying to leave , shall we say, as you found it.
-
Power equals thrust times velocity, so one extra pound of thrust at 400 mph is 1 lb * 400*1.5 ft/s = 600 lb*ft/s, a little over one horsepower for each pound of additional thrust. Bear in mind this is net, delivered horsepower - equivalent to at least five thermal horsepower of fuel consumption if it had been generated by putting in a bigger engine.
This is why engineers of the Thirties and Forties were so eager to derive extra thrust from engine exhaust - with or without augmentors - and from coolant heat. It also motivated efforts to use exhaust flow to pump cooling air.
-
If the exhaust flow is being used to raise the temperature of the air hitting the rear tyres, it could have a significnat effect. Drag and lift from the tyres is directly proportional to air density. Raising the temperature 10 deg C will cut both drag and lift 3%. This would be a big boost as the wheels are a major lift inducer and create significant drag.
High hot gas volume in the leading edge of any part will reduce the drag as well as the aerodynamic force: lift or downforce.
-
the effects of dumping the hot exhaust under the car would depend on several things, the shape and design of the underbody being the largest factor.
depending on the shape and design of the underbody and the exit location of the hot exhaust it could do anything from adding more down force and increasing traction, to lowering the overall drag of the vehicle and thus giving better aerodynamics and increasing speed.
-
aerodynamic force is directly related to air density flowing past the body.
half the air density and you half the drag.
half the air density and you half the lift.
aerodynamic force varies with velocity, double the velocity and you quadruple the drag.
even small well thought out plans for the exhaust can give BIG benefits in several ways.
it is well worth doing some research and doing more with the exhaust than dumping it without any second thought.
if it can drive turbochargers and make thousands of HP it can be used for many other things also.
-
without straying to much from the original post.
another thing to do some research on is the german heinkel HE100 fighter airplane first flown in 1938.
the HE100 used exhaust ejectors to boost air speed and had a unique evaporative or steam cooling system for the engine along the wings leading edge.
if rules permit engine cooling water could be circulated through double body panels at strategic locations and used to radiate heat to the surounding air flow without any additional drag and should reduce the density of the surounding air and make some gains in performance by reducing the air density along the skin.
-
Thanks Superford, seems you've had your nose down and tail up, good research.
That Heinkel was a complicated piece of kit, good looking shape......
Me thinks the allies were lucky this thing was pushed aside....check out the speeds and range under "Prototypes" here...
http://en.wikipedia.org/wiki/Heinkel_He_100
btw, show us some pics of your tank, we're all interested here.
-
Dr;
I suspect that the He100 was too complicated for its own good. A case of a designer outsmarting himself-- TOO many pumps! :?
Regards, Neil Tucson, AZ
-
Here is a different approach to exhaust flow that I have been thinking about, especially for a streamliner (if I ever get to build one).
Why not put the exhaust, single pipe, out the front center of the nose of the belly tank or streamliner?
Here is my thinking; torpedoes and Johnny Weissmuller. First, Weissmuller won 5 Olympic gold medals in the 1920's. When interviewed during his movie years as Tarzan on how he swam so fast, he said: the fastest way to swim in water is to swim out of the water. He explained how he arched his back to bring most of his torso out of the water, except for his arms and legs.
I remembered Weissmuller when I heard about the U.S. Navy having 300 mph torpedoes. I don't mean that the 300 mph torpedoes actually move "out of the water," but that some way the water is moved out of the way, I guess? Therefore, IF...IF...the Navy must create some form of cavitation of the water at the torpedo's nose and along the sides, then I could believe there are 300 mph torpedoes.
Alright, apply this theory to a streamliner to echo what John Burk said in Reply #15: "... spread the hot exhaust down the side of the car ... for a gain." Those New Jersey guys always have good ideas.
If someone tries my theory and gets a record, I'll be the first to buy him or her a steak dinner. :cheers:
Now, that's my food for thought on exhaust flow.
-
i have been in racing for several years, i am currently in iraq working as a civillan for the US government for the last 5 years.
thanks Dr Goggles, your tank is one i have ben lookng at for sometime and the way the exhaust exits the rear. the meredith effect could be implemented on the tank exhaust at the rear by using an exhaust augmenter where the exhaust exits the tank, smooth the body panels on the sides of the engine where the air louvers are and replace them with a NACA duct on each side. the large suction action of the exhaust augmenter would be like a vacume cleaner running and pulling cool air in the NACA ducts around the hot engine compartment and out the back, contributing more heat and expanding gasses to the air stream behind the car, filling the void behind the vehicle and increasing aerodynamics.
an earlier post beat me to the punch, it got to late for me last night so today i was going to elaborate about the american and the russian 300MPH torpedos, they use compressed gasses released along the front and sides of the torpedo to lower the drag and increase the speed, the gas bubbles create a boundry layer of gas around the torpdo body, pushing the water out of the way, water being many times more viscous than air and subsequently creating more drag.
i think the best way to release the exhaust gasses would be to imagine 2 long, flat header collectors 1/8 to 1/4 inch tall but 36inch in width surounding the body about its circumferance, releasing the hot exhaust gases into the air strem smoothly around the entire body diameter.
the ideal placement would be as close to the nose as possiable so the hot gasses could cover as much of the body over the longest distance, if you have a bullet shaped nose, which a LOT of the tanks and streemliners fall under this, you can imagine a bell shape being placed over the nose and it having a small air gap between it and the body for the exhaust to escape and the outlet device would have minimal effect on the vehicles aero. the bell shape with a circuler exhaust inside would distribute the exhaust evenly, if you only had a straight pipe sticking out the front, the gas distribution would be all over the place, the aero impeded, plus the on coming air would be working against the exhaust flow, trying to push back down the pipe.
-
imagine the exhaust augmenter working in reverse capped over the nose of the vehicle.
the exhaust augmenter works a lot like the bernoulli principal in design and function.
the vehicle moving at high speeds would actually help to pull the exhaust out of the system and improve exhaust flow.
-
....that distant crunching and grinding noise is the cogs in the Reverend's head starting to turn...
I think I see an exhaust augmenter in the Spirit of Sunshine's very near future......
Have a biscuit Superford , that is the single best peiece of info I have seen posted here for some time......
-
the russians & americans have used the idea of ejecting compresed gasses to lower hydrodynamic drag and there by increasing the speed of torpedos and entire submarines, they use the nuclear reactors to recover air from the sea water, store it in tanks and during a battle if they need a sudden burst of speed to evade an enemy torpedo they release the air along the hull of the submarine and reducing the hydrodynamic drag enormously. plus all of the air bubbles in the water help to throw off the tracking system of the torpedo.
-
non streamlined shapes have to worry more about pressure drag and streamlined shapes have to worry more about skin friction drag. so how to get the most benefit from the exhaust would depend on the shape that you are pushing through the air.
the worse your vehicles aerodynamics the greater the effect of dumping all of the exhaust and what ever hot air you can extract, into the wake behind the vehicle to help fill the void behind the vehilce and to decrease the pressure drag.
for the more streemlined vehicles the largest benefit would be gained from letting the exhaust flow over the body to decrease the skin friction drag.
never mind the name of the picture, it is was a side note for some research i was doing.
-
Is the "Propster" alive :? and well :-o or is that a pusher Canard plane ???
-
I am pretty sure that photo was of the rear end of a Vari-EZ. I built one a long time ago.
Don
-
Drag increases with the density of the air. Drag increases with area.
Surface area is not important when one is dealing with pressure drag, but it is important when dealing with viscous drag. Drag is influenced by other factors including shape and texture.
As drag increases, acceleration decreases. Eventually one can imagine a state when the
drag and weight forces are equal. It will not cease to move, but rather it will cease to accelerate. We
have reached terminal velocity. There can be no speed greater than or less than this one.
a streamlined shape has much less drag than a non-streamlined shape. Whatever drag exists for a streamlined shape is composed primarily of skin-friction drag with the pressure drag being very small. The increase in skin-friction drag occurs because the streamlined body has more area exposed to the airflow and thus has a greater area over which the boundary layer may act. A streamlined shape also experiences almost no boundary-layer separation.
as i stated before, where and how to introduce the hot exhaust depends to a great extent on the shape and aerodynamics of your vehicle and also the benefit you wish to receive from the use of the exhaust.
you can gain more downforce and traction or reduce the drag and gain more speed.
-
The key parameters affecting straight-line acceleration are engine torque, gear ratios, aerodynamic
drag and the traction limit of the rear wheels.
Acceleration can be either traction limited or power limited. The approach used to define the acceleration is first to consider the effect of the engine and gear ratios then to superimpose the effect of the traction limit and aerodynamic drag.
By using only the heat from the vehicle cooling systems and engine exhaust we can manipulate 3 of the 4 key parameters in acceleration to a lesser or greater extent, discarded heat can not do anything for gear ratios, but it can power turbochargers to increase engine torque dramaticly and increase acceleration.
Aerodynamic drag, lift and downforce, can be alterd and influenced with discarded heat to decrease overall drag and increase speed and acceleration. depending on the body shape and if the wheels are open or closed, the redirected heat can do anything from reducing tire drag and lift, to reducing body friction drag and reduce pressure drag.
lastly the discarded heat can be used to create downforce and thereby create more traction with the tires and increase acceleration, if the vehicle is traction limited.
you could increase your downforce on the salt and at the same time reduce drag and run a turbocharger to boost engine performance. using an exhaust augmenter you can increase engine coolling and at the same time reduce drag.
engine exhaust is a very powerful thing, 1/3 of the fuel burnt in an internal combustion engine is actually used to power the crankshaft, 1/3 is waste heat that is radiated from the engine and pickd up by the cooling system and the last 1/3 is sent out the exhaust as a waste product.
66% of the power from the burnt fuel is left for us to do with as we see fit, building the propper header, using coatings and wraps we can recover a lot of the exhaust gasses and using exhaust augmenters and redirecting the heat from the cooling system we can recover a lot of the radiated heat and also the heat from the cooling system.
-
I am pretty sure that photo was of the rear end of a Vari-EZ. I built one a long time ago.
Don
I agree, had a ride in one that was there at bendover for a fly in convention of EZ home built planes.
-
MLSE, sorry about my ramblings, but I thought I would add a little insight to what the exhaust and radiated heat could be capable of, without knowing more specific details about your vehicle and class rules, I can only assume that by you saying production class, you will be very restricted as to what you can do to your car shape and underpinnings.
So for starters you will have to worry a LOT more about pressure drag than you will skin friction drag. We can minimize that as well as a few other problems with the proper management of the radiated heat from around the engine compartment and the cooling radiators, as in water, transmission and oil coolers.
If you are not running engine oil and transmission coolers, it may be worthwhile to think about adding them so we can add the recovered heat into our system, if not the power you lost from the internal friction of the transmission and the friction and radiated heat from the engine being transferred to the oil, will be lost, so no matter how negligible, it is a source of power we can use to our advantage.
If we do not recover this source of energy and put it to use, your competitor will.
I’m sure records are set and broken by fractions of seconds and tenths of miles per hour, look at all the small details and in the end they will add up to be a contributing factor to the overall package.
I have lost races by .002 seconds before.
If rules permit we can build a type of duct system around the engine to recover the radiated heat and direct it to the underbody, increasing engine cooling and improve aero forces.
I will assume also that you will be very limited to what you can do to the actual vehicle underbody. We can’t install a full underbody tray, but can we use air flow directors and vortex generators. Are air dams, splitters, diffusers and spoilers permitted.
I wish I had a rule book to review, is there one on line I can look at.
Without knowing more about your particular class and the type of car you have I can only speculate.
Give me a LOT more details about what you have and what you wish to achieve.
-
Now what we need is a Submarineer, did any of you guys serve on a bubbler??
-
..how about if this "energy" is applied to a "lever" arm to multiply the mechanical
advantage...say to an impeller or "prop" .......to impart yet greater thrust by said "prop"...
-
"Drag improves as the temperature of the air increases" (Previous quote)
Is this why I always go faster in the afternoon qualifing than I do in the morning on the back up run..it's the excuse I use..................JD
-
this is an example of what heat on or around a surface can do for boundry layer and airflow.
using the same sphere all 3 tests, the only difference was the temperature of the sphere.
-
What were the temperatures involved?
Pete
-
Just curious, what were the real temperatures of the sphere in each of those tests? One might be able to use that effect by placing the water tanks as part of the outer body work of the vehicle, or using the skin a as flat surface radiator in the return flow to the tank, to allow it to get to maximum available temperature without overheating the rest of the water in the cooling system.
That is, as long as extreme temps aren't required to produce a significant effect.
-
Boundary layer heating becomes more efficient with lower Reynolds number.
The skin friction is reduced as a function of the ratio of the skin temperature to the ambient temperature. The result is an effective drag reduction.
The velocity profile can be modified by controlling the pressure gradient using surface heating.
Heating the surface under a turbulent boundary layer reduces skin friction.
-
the american F117 nighthawk low visability airplane used slits along the entire back width of the tail as an exhaust outlet, very wide and narrow.
i have seen motorcycle rear fenders turned into the exhaust pipe, a mirror image welded to the fender and the exhaust directed between the 2 parts and blowing out the trailing edge.
have double panels on the car and dump the exhaust into them.
the spectre car as an example, imagine if you will the entire rear half of the vehicle being double walled and the exhaust flowing thru it and out the very trailing edge, the super heated body will lower the skin friction drag and the exhaust dumping into the ideal location at the rear will lower the pressure drag at the same time.
most of the streemliners dump the exhaust at a less than desireable location.
-
There would be very little restriction to the exhaust as using the body for the exhaust would be a lot of area.
Even if you lost some power the gain in aerodynamics at high speed would more than offset this and still have a faster speed.
Remember each time you double your speed drag quadruples.
-
Jimmy six.
I do not know the speeds you run at, but I am quiet sure that the air density difference is what allowed you to reach a faster top speed.
Even though the cool morning air is denser and your engine is making more power, the warmer evening air is less dense and more beneficial to lower drag.
double a given speed, drag quadruples.
at elevated speeds, a small gain in aerodynamics will be as good as a big gain in engine HP.
I have seen thousands of man hours and several thousand dollars thrown at engines and chassis in the name of speed, when it would have been much easier and cheaper to build in aerodynamics to the overall package in the beginning.
Think outside the box, I used to like to read smoky yuniks articles.
-
Not to come across as a "Private Eye" or anything, but the authoritative tone that you take with your posting would lead one to believe that you have vast experience in this area. Nothing that you have posted yet says "in my opinion", but is all stated as fact without reference to any source material.
That and you won't give a simple answer to a direct question posed by 2 members in response to a single example that you have given.
At least your posts aren't all cut and paste stuff from other sources.
I checked your other posts, and your profile, didn't find the answer there, so I'll ask here, mainly because I and a lot of others like me spend a lot of time online looking for good info on this type subject, and when we think we've found it we spend a lot of hard earned time, money, and effort on trying to incorporate it into our builds. It is only natural that we would like to know that we can trust the source of this information. A little provenance goes a long way.
What is your background that provides you such an authoritative voice on such an esoteric subject.
And.
What is your name?
Franklin, perhaps?
If not please forgive me, but I seem to be seeing a familiar pattern here. :|
-
L :-D L, my :-o my the propster lives!!! he has planted the seeds lol but a darn intresting read for sure
-
Hmmm, I have never had a response like this so far in my career.
I have devoted my entire human existence since the age of 10 to the internal combustion engine and anything to do with speed. I do not have children or a wife, I do not have time for them, and my only passion is speed and performance. When most people were out partying or spending time with friends, I was studying in schools and spending late nights in the shop modifying cylinder heads or building engines or chassis. I am currently working for the US government in a technical field in IRAQ the past 5 years.
as i type this i am only a few miles from syria and turkey, in the northern iraqi dessert.
I assure you that I am very well versed and educated in everything from aerodynamics to thermal dynamics and composites; I am a machinist, welder, fabricator and engine builder.
I have constructed many winning race cars from the ground up and consulted with the teams after they were on the track and winning.
I quoted a lap time to a team owner that a car would run to within .01 of a second, 3 months before I made delivery of it, the 3rd lap the first time ever on the track, it ran the time i had quoted.
I only wished to impart some of my understanding and knowledge that I have gained.
I have been busy answering E mails from people that have flooded me with questions since I started posting on this thread.
Things that come natural to me and I take for granted amazes most people when I start elaborating on them.
i elaborated at length and show you many examples, pictures and drawings. more so than i have seen elsewhere.
To answer your question the first ball is at 70, the second one is at 140 and the 3rd one is at 240.
These temperatures are easily obtainable on a vehicles body surface using several methods.
NASA only started doing research into this in the 1990s.
I am so sorry if I offended anyone, I assure you it will not happen again.
-
Hey keep the posts coming . Very interesting to say the least !!
-
the american F117 nighthawk low visability airplane used slits along the entire back width of the tail as an exhaust outlet, very wide and narrow.
However this configuration was not used for either propulsion or aerodynamic improvement but solely to hide the exhaust heat plume from the rear. It caused no end of headaches keeping the exhaust duct intact and the heat coatings at the rear attached.
Jim Dincau (34 years at the lazy L)
-
the supermarine spitfire was the first to make use of this concept and the designers of the P-51 mustang said the meredith effect was a major factor for the P-51's high speed.
at 25,000ft altitude the hot air from the engine cooling radiators running through special ducting had 1/3 as much power as the proppeller had at full power.
Well i googled it and after reading through what was available on line, the consensus (and there were differing opinions) seemed to be that at BEST, oil and water cooler drag could be eliminated but no thrust was generated.
-
I was merely giving that as an example as to the shape and style, the same as the motorcycle fender.
The interior of the F117 exhaust ducts were lined with the same heat absorbing tiles as used on the space shuttle heat shield, to absorb heat and to protect the airplane.
All of this was used to help shield the exhaust so it could not be spotted by infra red signature, it mixed fresh cool air with the exhaust just before expelling it and the way that it was designed it could not be spotted from below.
-
superford...thank you much for sharing your knowledge...could you share some
pictures of the many race cars you built from the ground up..?...it would be neat
to appreciate that....level of race winning engineering..
Joe :)
-
The interior of the F117 exhaust ducts were lined with the same heat absorbing tiles as used on the space shuttle heat shield, to absorb heat and to protect the airplane.
I think not
-
It just struck me. 57Tbird?
-
The oil cooler was a necessary evil for the airplane to function; overcoming the drag of the cooler was a big accomplishment and talked about for many years.
If it were not for the technological advances that allowed for the drag of the cooler to be overcome the airplane would have been much slower. As I stated earlier, at 25,000ft altitude the cooling system duct was putting out 1/3 as much thrust as the propeller was making at full power. The cooling system, making use of some ingenious ducting overcame its own drag, thus free power.
the cooling system for a 12 cylinder 1,700HP engine in 1944 was rather large, ovecomeing its own drag with only ducting was something that books have been written about and is still talkd about 70 years later.
-
I would think by now you would have given a name and other information that others have ask for. Can you do that and answer the other questions on what you have built and run. It's not much to ask for from the membership.
-
:cheers: :cheers: :cheers:
If my warped sense of humor offends you I am sorry Come out and identify yourself so that I may apologize!!
-
It just struck me. 57Tbird?
T-bird couldn't spell :-D
.01 sec :-o :roll:
JL222
-
Great brain - tickling stuff, Super Ford.
Sombody needs to send you a late rulebook - you knowing the various classes / restrictions would make your comments even more stimulating and specific!
Guaranteed this individual ain't Propster and CERTAINLY not 57 T Bird,
poster child for every wannabee on the planet.
-
No offense here at all, yet. In fact I'm finding your posts very interesting and supportive of a lot of my own thinking and experience.
I believe that you'll find you're not an isolated case on this board. A lot of us have very broad experience and education, and it's probably safe to say that the majority of us have dedicated very significant portions of our lives to mastering questions of speed and performance, in widely varied applications, and to the detriment of our personal lives and relationships. Most "normal" people don't seem to understand our obsessions, and we gravitate to those who do, ending up here.
I would still like to know in more detail what qualifies you to make the statements that you have with the certainty that you express in your writing. A lot of this stuff is based on research that was done with a certain set of parameters in mind, and may or may not be applicable to our varying applications.
Again, I find that what you have posted agrees with my own theories and experience, and supports a lot of the thinking that I do. I would like to know the source so that I can rationally rely on it as confirmation, rather than taking it at face value and investing a lot of resources only to find out that I was completely wrong and grasped confirmation that was also wrong without doing due diligence.
You apparently haven't been on this board long, so may not know that there has been more than one case of "experts" showing up and spouting their drivel without being involved in the actual sport or understanding the unique applications that we have to deal with.
Like I said before, forgive me if I'm wrong, and I hope I am.
Please identify yourself and give us something to hang our hats on in order to identify yourself and your experience.
Welcome to the board.
Thank you.
-
Superford 317,
It doesn't matter who you are or what your qualifications are. Your statements and theorys are really stimulating. We have all grown accustomed to dealing with a government that spouts convoluted statements and it is up to us to sort out what fits our own situation. Don't take offense at our curiosity as to your qualifications. Please keep posting . I passed this string on a couple of days ago, with the caviat: " I think this guy's got it". Thanks for your participation.
Doug :cheers: :cheers: :cheers:
-
i have hard data and research to back up everything i do and say and have studied for many years.
-
There are some things a fellow never forgets. The film is threaded through the projector. The lights are doused. The hum and clatter of the Super 8. It is time for another high skool drivers education movie.
Two black gentlemen my age are driving along the highway and happily chatting away the time. The car is a mid 1950's American bomb. The driver feels drowsy. Sirens scream and the next scene appears. The two fellow are alongside the road in body bags. The car is upside down and the camera zooms in on a hole in the muffler. A serious deep voice says "It was carbon monoxide from this leaking muffler that killed those two. The exhaust was sucked up through the windows and cracks in the floor. Proper exhaust system maintenance is critical. The spent gases must exit beyond or alongside the rear of the vehicle."
This was good advice then and now. It is dangerous technology to do anything different. Do not bathe the bottom of the car in hot exhaust.
-
a better description with pictures so it can be understood more easily.
-
SF you mentioned surface finish back a page or so, what do you think of the "sharkskin" now popular in swimming
and its effect on boundry layer ?
-
Intrested why can you make that guarantee? Courious minds need to know
-
SF317, this is Jon -- the owner of the site. I've been looking at the back-and-forth between the others on the Forum - and you, and I notice that you're steadfastly hoarding personal information. So be it since it isn't required for membership. But since just about everyone else DOES tell us a bit of that data -- I guess you'll need to be prepared to accept whatever hassles they throw to you about asking who you are, and substantiating the information you list - such as your experience and education, whether formal or learned at the racetrack.
There's one thing that I do want from you, though, and it's yet another of the sore points that have developed over the past bunch of years. That is the posting of material that may well be copyrighted, such as the drawings and sketches and articles and photos that you have or may soon post. I can't, as the boss and owner, allow non-attributed material to be on the site lest I get hit with suits for infringement of copyright and so on. Yes, I know that there can be allowances made for material that is posted only for information and not for profit -- but I'd like you to play on the safe side (for me) and tell us from what source any such material you list comes. It's a requirement for others - and so it shall be for you.
Thanks.
Jon a/k/a Seldom Seen Slim
-
Rex;
It may be a B-29 that I'm thinking of, but somewhere I read about using the engine exhaust gasses for additional thrust. I think there was an "augmentor tube" involved that sucked in air to add to the thrust.
Regards, Neil Tucson, AZ
B-29s used exhaust to drive "Power Recovery Turbines" that recaptued power from the exhaust and delivered it (through a fluid coupler and a reduction geartrain) to the crankshaft, 700 HP worth at takeoff IIRC (so 700 of the roughly 3500 IIRC).
The closest I can think of an augmenter for round engine aircraft comes after the war with the DC-6 family (DC-6A, DC-6B, VC-118, C-118A, I don't recall all the designations) and some Convairs when equipped with the R-2800 Pratt and Whitney.
It (R2800) was equipped with an "ejector stack" exhaust system, on the aircraft in question.
On the DC-6s it fed into a cavity of the accessory section cowling (open on one side) that would appear to help with extraction of cooling air after it had done it's job.
In training early in my career of working on them, an instructor claimed that, on the Douglas, it was worth 400 HP in Equivalent Shaft Horsepower at cruise (cruising between 18,000 and 25,000 feet, with 25,000 feet being the highest ceiling I remember them being certified to).
Since there are some here that are questioning sources, I am a certified A&P, untill recently I was employed at an airline (for over a decade) that primarily flies DC-6 type aircraft (the models I mentioned in parens earlier).
In the group of people I worked with were alot of folks that worked on R-3350 powered aircraft that were still using the PRTs, it's an idea that won't leave me alone.
I know a Turbonique Drag axle would be illegal, I suspect that our frienly class chair wouldn't go for a power recovery turbine hooked to the diff either. :(
It would be interesting to try though, even if it was just for time. ;)
Not nit pciking, I know manta22 mentioned he wasn't certain, so please don't take this as me "calling BS", 'cause I'm not.
He brought up a point I find interesting and I thought I would elaborate, with what I've learned over the years from both experience and what some call "book learnin". :)
-
Thanks for the input-- good info. If I remember right, that turbine engine power recovery system was called a "turbo compound" engine. I'm dredging up brain cells from when I was in junior high school on this stuff. :-P
Regards, Neil Tucson, AZ
-
If a hot surface area on the vehicle makes it have less drag, funneling exhaust over the surface will not have much of an effect. The action and speed of the relative wind will not allow the hot gas to remain on the suface long enough, not to mention that there just plain is not enough of the gas to cover the vehicle.
A better alternative is just to paint the vehicle flat black. In the Bonneville summers, the sunny side of the vehicle can get up to 150 degrees (just guessing from touching the skin of my black car out there.)
Don
-
A better alternative is just to paint the vehicle flat black. In the Bonneville summers, the sunny side of the vehicle can get up to 150 degrees (just guessing from touching the skin of my black car out there.)
A more important question is how hot is the black car skin at the 2 mile marker -- I would bet it is within a fraction of a degree of the outside air temp. Painting it black also increases the ability of the skin to radiate heat, as black paint has higher emissivity in the infrared that other colors.
I strongly doubt that painting a car black is worth any measurable speed advantage. The cooling power of a 200+mph airflow far exceeds the radiant energy received from the sun.
It might make the car easier to find out on the salt but I suspect it is worthless for aerodyanmics. Next time you make a pass feel how hot that sun side of the car is when it comes to a stop. Or better yet put a cheap digital thermometer probe on a body panel not heated by exhaust or other internal heat source and watch how fast it cools as the car gets up to speed.
Larry
-
I seem to recall a thread on compound superchargers somewhere on this forum
Very interesting stuff.
However, I'm not sure I would want to get in a black car out on a salt lake especially if the skin of said car was being heated by the exhaust. And of course there is what WW added
There are some things a fellow never forgets. The film is threaded through the projector. The lights are doused. The hum and clatter of the Super 8. It is time for another high skool drivers education movie.
Two black gentlemen my age are driving along the highway and happily chatting away the time. The car is a mid 1950's American bomb. The driver feels drowsy. Sirens scream and the next scene appears. The two fellow are alongside the road in body bags. The car is upside down and the camera zooms in on a hole in the muffler. A serious deep voice says "It was carbon monoxide from this leaking muffler that killed those two. The exhaust was sucked up through the windows and cracks in the floor. Proper exhaust system maintenance is critical. The spent gases must exit beyond or alongside the rear of the vehicle."
This was good advice then and now. It is dangerous technology to do anything different. Do not bathe the bottom of the car in hot exhaust.
I don't think that would get through Tech
G
-
Nexxussian...turbo-compounding is allowed but puts you in supercharged class but it does not pressurize the engine...but you could use a super charger and gear a turbo or two back to the crankshaft [or?] like the Wright R-3350
Rule book pg 48 section 4.ff
We might catch up to what the aero guys were doing 70 yrs ago :-o
JL222
-
Turbo-compounding is alive & well in the new Detroit Diesel DD16 engine in Freightliner trucks. The turbine is mounted behind the turbo & the drive is fed into the trans. I believe the gain is something like 48 hp. If you see a truck with a DD15 or DD16 badge on the cab, ask the driver if you can check it out. Truckers love to talk.
Sid.
-
Detroit claims 85HP in their training. I've been through almost all of it as a dealer tech.
Rumor has it that they are getting ready to phase it out and try to make up the power loss elsewhere, like in the injection timing, cam timing, compression areas. Supposedly they are doing this without any increase in fuel consumption.
While the physics of the idea is sound, they are finding that warranty claims are getting very expensive. For example if a turbo shells out (not uncommon on any heavy truck engine) it frequently takes out the APT (auxiliary power turbine) due to fragments of the impeller tearing up the APT rotor, as well as the ATD (after treatment device, think catalytic converter) due to oil contamination.
-
Hi all, new guy here from Melbourne Australia. Been lurking on this site for awhile now and this thread has finally prompted me to join the forum. Thought I'd make my first post both an introduction and a contribution to the thread. Can trace my interest in LSR back to the age of five and have been studying and researching the technical aspects and the history of the sport since then. That said, I make no claim about being an expert on the subject, or any other for that matter, I have no formal training or qualifications in anything ( I left school young ). Also, I have no previous vehicles that I can point to as proof of any ability. As such you can make your own judgement on the technical merit of what I write. My opinions expressed are just that, they may or may not be fact. Shoot me down in flames if you can. Sorry to make this post rather long, I'll generalise to simplify it as much as I can. I hope it will be worth your while.
My aim with this post is to attempt to clear up some grey areas in the understanding of exhaust flow and how it relates to vehicle dynamics. It is true the exhaust contains considerable energy that is wasted and I haven't yet seen a vehicle take full advantage of this energy. As most everyone would know the exhaust flow can create thrust that contributes to pushing the car forward, this is true however not completely understood.
First, a brief refresher course. Rocket propulsion and Jet propulsion both come under what I term as a ' reaction engine'. That is, they propel themselves forward by the expulsion of matter (called the propellant) in the opposite direction. Newton's third law of motion states that ' for every action there is an equal and opposite reaction'. This matter could be in the form of a solid or liquid but is most commonly a gas produced by a chemical reaction in the combustion chamber. Rocket propulsion differs from Jet propulsion in so far as with a rocket, all of their propellants are carried on board the vehicle and the expulsion rate and force generated is largely independent of outside conditions. With a rocket propelled vehicle, the vehicles velocity has no relationship to the engines exhaust velocity. Rockets can, and do travel faster than their own exhaust velocity. With Jet propulsion it's slightly different. Only one of the propellants used in the combustion process is carried on board the vehicle, the other, Oxygen, is provided from the surrounding air. Jet propulsion works by adding energy to the gas stream flowing through the engine. As such a jet propelled vehicle cannot travel faster than it's own exhaust velocity.
At this point it's important I try and explain the difference between gross thrust and net thrust. Imagine a jet propelled vehicle held stationary with the engine at full throttle. In this condition gross and net thrust are one and the same. Now if the vehicle is let go and begins to accelerate two things happen. The gross thrust or 'overall' thrust will begin to rise (usually through a ram effect making the compressor more efficient and moving up through a pressure regime) however the net thrust, that which is actually propelling the vehicle will begin to fall. That's because the momentum added to the gas stream, the ratio of inlet velocity to exhaust velocity is reducing. With a Rocket, gross and net thrust remain the same irrespective of the vehicle's velocity.
Now lets apply this to a car or bike at Bonneville. The primary source of propulsion is with a piston or even turbine engine providing torque to rotate drive wheels. Sufficient traction provides the thrust to propel the vehicle forward. Secondary would be thrust generated from any rear pointing exhaust outlet. Being air breathing, the exhaust momentum would be classed as Jet propulsion and would conform to the above statement. I believe most vehicles at Bonneville would outrun their exhaust velocity and the net thrust at top speed would be a negative value. That's not to say it doesn't exist, your way of visualizing it just has to change. Up to the point of going negative it was providing more than what it was costing, now, when negative it should be seen more as drag minimization rather than ' free thrust'.
I'll touch briefly on the Meredith effect for radiators. The theory is that the air flowing through gets heated and leaves the duct at a higher velocity. In practice, through losses, there is no momentum increase and the net thrust is negative. Once again, the design is really based on drag minimization. However, if you were to merely quote the gross thrust output it would be an impressive figure.
So what can you do about this? First up, the exhaust has to point rearward and the velocity as high as possible to delay the point of going negative. Most exhaust outlets are way too big and velocity suffers as a result. The major pressure drop is through the exhaust port and the rest of the system is essentially ducting. Ideally you want a short pipe to keep losses to a minimum, direct it rearwards and then (now you can take this to the bank) put a nozzle on it. I'll avoid talking about the interaction between exhaust and vehicle aerodynamics in this post as it's growing way too long. My only objective here was to be thought provoking.
Cheers,
Dave.
-
As a younger, thinner man I had the opportunity to work on both the Turbo Compound CW 3500 and the Convair 340 with exhaust augmentation. Or really cool headers on a P&W 2800. I have no idea how much thrust it was supposed to add. But it looked like it should be good. I don't know how the exhaust collectors on our DC-6 aircraft with 2800s would have added to thrust.
-
the thrust from the exhaust collector on the P&W2800 was small, but the thrust from the cowling surounding the air cooled engine was dramatic. recovering and using the radiated heat with ingenious ducting.
a lot of engineering time and effort went into that and in the long run probably saved countless american lives.
many times people think they are looking at the big things and overlook the small things, that in reality are very big.
i was doing some CFD and wind tunnel testing for some government research a few years ago and had to make a presentation on this very thing.
-
superford thats really cool..!!!...what year was that and which tunnel.?..
-
Power equals thrust times velocity, so one extra pound of thrust at 400 mph is 1 lb * 400*1.5 ft/s = 600 lb*ft/s, a little over one horsepower for each pound of additional thrust. Bear in mind this is net, delivered horsepower - equivalent to at least five thermal horsepower of fuel consumption if it had been generated by putting in a bigger engine.
Plagerism 101! Go here and look at post #13. It seems like somebody else said the above quote first. Or are you "Piolenc" and moved from the Philippines to Iraq?
http://cr4.globalspec.com/thread/54975/Spitfire-Efficiency-of-Cooling-and-Thrust
And just to add a little precision to this thread the constant is not 1.5ft/sec but 1.466 ft/sec.
Very interesting thread and I have greatly enjoyed all posts even a few assumptions which cause me to fire up the old neurons. No time now, but I’ll try to make some “point-outs” later.
-
. . . direct it rearwards and then (now you can take this to the bank) put a nozzle on it.
My bank likes to see numbers on the money. How about a concrete example, say a specific change to a specific exhaust system of a specific engine, with estimates of before and after shaft power and exhaust thrust. Your choice of engine, but something famiiar to racers, like a Hayabusa or small block Chevy, would be nice.
-
Right here is what I love about this board.....
A bunch of itchy minds that fall hungrily on anything that falls anywhere within their field of expertise or could be remotely linked to their pursuits in landspeed racing.The difference between this and most message boards is that people here are watching and looking for applicability, they aren't armchair experts or internet trolls.
Between them they've sniffed out plenty of stuffed shirt professors but also given their approval to others who have proved they can mix it theoretically or practically.
You can't get cut and paste past them, if you say you've been involved in something then via the six degrees of separation they will check your bona fides.
superford thats really cool..!!!...what year was that and which tunnel.?..
That there is a guy who has ridden over 270mph on a sit on bike , his credentials are public knowledge, he has a right to know that you speak from experience and have verifiable credentials. If not , he knows not to take on board anything that challenges his prior held beliefs about this topic , it's not personal.
-
Geeze, I didn't mean to completely derail the thread into a peeing match over credentials, LOL. Here's a question to maybe get it back on topic. :-)
It seems to be generally accepted, from my reading, that about the best that can be done with exhaust flow is to help fill in low pressure areas, or areas of separation. When I asked a similar question of A2 his response was that the effect is so small that most disregard it and test without exhaust flow. As I understand it, he based this response on prior experience with varied types of cars that actually had experimented with it in the past, primarily because the exhaust flow has such low volume when compared to the overall mass of air displaced by the vehicle.
In most conventional vehicle types I can see that this is probably very true. LSR vehicles, however, are not conventional, especially streamliners. They are by definition designed to displace as little air as possible, and most have very powerful engines pushing a higher than normal volume through them. It seems to me that these vehicles could see significant gains from properly placed exhaust.
The "low" volume of the exhaust gas possibly could also be enhanced. Has anyone experimented with water injection into the exhaust stream? This would flash boil due to the heat of the exhaust gasses and provide extra volume that could be put to use.
-
Hmmm, I have never had a response like this so far in my career.
I have devoted my entire human existence since the age of 10 to the internal combustion engine and anything to do with speed. I do not have children or a wife, I do not have time for them, and my only passion is speed and performance. When most people were out partying or spending time with friends, I was studying in schools and spending late nights in the shop modifying cylinder heads or building engines or chassis. I am currently working for the US government in a technical field in IRAQ the past 5 years.
as i type this i am only a few miles from syria and turkey, in the northern iraqi dessert.
I assure you that I am very well versed and educated in everything from aerodynamics to thermal dynamics and composites; I am a machinist, welder, fabricator and engine builder.
I have constructed many winning race cars from the ground up and consulted with the teams after they were on the track and winning.
I quoted a lap time to a team owner that a car would run to within .01 of a second, 3 months before I made delivery of it, the 3rd lap the first time ever on the track, it ran the time i had quoted.
I only wished to impart some of my understanding and knowledge that I have gained.
I have been busy answering E mails from people that have flooded me with questions since I started posting on this thread.
Things that come natural to me and I take for granted amazes most people when I start elaborating on them.
i elaborated at length and show you many examples, pictures and drawings. more so than i have seen elsewhere.
To answer your question the first ball is at 70, the second one is at 140 and the 3rd one is at 240.
These temperatures are easily obtainable on a vehicles body surface using several methods.
NASA only started doing research into this in the 1990s.
I am so sorry if I offended anyone, I assure you it will not happen again.
Is this proof that our government is conducting some "Black list" experiments with advanced, possibly alien, technology? A secret base deep in the Iraq dessert. A totally dedicated speed master. No news coverage. I demand a Senate hearing. Keep Big Government out of LSR.
-
Nexxussian...turbo-compounding is allowed but puts you in supercharged class but it does not pressurize the engine...but you could use a super charger and gear a turbo or two back to the crankshaft [or?] like the Wright R-3350
Rule book pg 48 section 4.ff
We might catch up to what the aero guys were doing 70 yrs ago :-o
JL222
Yeah, but that bumps the record I'm after from the high 170s up to the mid 230s (IIRC). :(
Oh well, mabe in years to come.
Neil, yes, "Turbo Compund" is what I've read it being called.
-
A vehicle moving through the air experiences drag, friction drag and pressure drag.
On a streamliner the boundary layer on the surface remains attached for almost the entire length, the drag is primarily surface friction inside the boundary layer.
Drag can be reduced significantly using boundary layer heating; decreased drag means a faster top speed and quicker acceleration.
On a streamliner drag comes primarily from skin friction drag, when heat is added to the boundary layer, skin friction is reduced.
As stated earlier, the closer to the front of the vehicle that the heating is started the more effective it becomes and the farther back it is started the less effective it is, if only the front portion of the vehicle is heated, the warm air is carried under the boundary layer and down the surface, the more of the vehicle that is heated the more effective the drag reduction becomes.
Heating the surface air causes the boundary layer to thicken and causes some of the surface roughness and imperfections to be hidden also.
As with most things though there is a diminishing rate of return, as more heat is added.
The two best ways to achieve the surface heating on a vehicle powered with a liquid cooled internal combustion engine is with the engine coolant, turning the body into a radiator or using the exhaust between two body panels and radiating the heat.
By using the exhaust between the body panels it can be dumped into the wake behind the vehicle in the most advantageous area.
You may have to get a little more creative with keeping items cool that you never had to worry about, that is for another thread.
-
...superford this is really groovy stuff....!!
..you mentioned "I have constructed many winning race cars from the ground up and consulted with the teams after they were on the track and winning"....
would you be so kind as to share abit on "one" of the many cars and its winning..ie a team name...where did it race/compete..possibly a pic as you have made it look easy sharing MANY other pics....
..also....you mentioned re: "i was doing some CFD and wind tunnel testing for some government research"...what year was that and which tunnel....?..
thank you in advance..
Joe
-
Drag Reduction with Boundary Layer Heating
The benefits of reducing the drag of either a new or existing aircraft configuration are obvious. An aircraft’s endurance is directly proportional to the lift to drag ratio. Decreased drag also translates into faster top speed, quicker acceleration, shorter take-off distances and lower direct operating costs in the form of fuel savings. In order to project military air power, or on the commercial side, receive better range and fuel economy, reducing drag during the cruise portion of a flight is the most critical. During cruise, the drag of the aircraft primarily comes from profile drag (skin friction), induced drag (drag due to lift), compressibility drag, separation drag and interference drag. Of these, skin friction (from the “wetted” elements of the aircraft) typically accounts for more than 50% of the total. By applying active surface heating in the turbulent regions of the aircraft’s boundary layer, the skin friction is reduced as a function of the ratio of the skin temperature to the ambient temperature. The result is an effective drag reduction method that can be retrofitted to existing aircraft.
RHRC conducted drag reduction tests using boundary layer heating through a NASA Small Business Innovative Research program using the NASA Dryden F-15B Flight Test Fixture and a T-39 Sabreliner. RHRC's research proved that drag reduction savings are possible at full flight Reynolds and Mach numbers. RHRC also measured the amount of power required to achieve these savings. In general, boundary layer heating becomes more efficient with lower Reynolds number.
Note that the turbulent region of the boundary layer is not typically at the front of the vehicle. On a streamlined shape, the boundary layer is very thin at the front of the vehicle, then gradually thickens as the air flow moves down the body, and transitioning from laminar flow to turbulent flow prior to separation.
http://www.centennialofflight.gov/essay/Theories_of_Flight/Skin_Friction/TH11G3.jpg
http://www.rollinghillsresearch.com/Aero_Research/Boundary_layer_heating.htm
Another method of managing boundary layer thickness and transition to turbulent flow is to use suction to pull the thickening boundary layer off the panel and help re-attach the flow as a laminar low friction flow.
http://web.mit.edu/hml/ncfmf/12BLC.pdf
NACA ducts also help strip off the boundary layer, so it is conceivable that proper placement of NACA ducts to provide essential cooling could also help manage flow separation near the rear of a stream liner.
Larry
-
i thought eventually i would get someone to comment on the suction if i kept pointing you in the propper direction. it took longer than i thought it would.
very early on i made the comment about (if turbochargers can be used to produce thousands of HP they can be used for many things)
supose a vehicle running in a NA class but with a turbocharger to pull suction on the body at strategic locations, only where required to improve body airflow at certain areas, this would be very easy to do,perforated panels where the aerodynamics are the worst, running various hoses to different parts of the body as required.
you will loose some power running the turbo, but as i like to say, double the speed and quadruple the drag, again a little loss at the turbo for a gain in aero.
if you will read and try to understand a lot of what i say and imagine applying it to different parts of the vehicle and its various subsystems, a lot can be done.
imagine the vehicle being composed of hundreds of different packages assembled to form the whole, most of the packages can be manipulated in your favor for big gains in the end.
-
...superford this is really groovy stuff....!!
..you mentioned "I have constructed many winning race cars from the ground up and consulted with the teams after they were on the track and winning"....
would you be so kind as to share abit on "one" of the many cars and its winning..ie a team name...where did it race/compete..possibly a pic as you have made it look easy sharing MANY other pics....
..also....you mentioned re: "i was doing some CFD and wind tunnel testing for some government research"...what year was that and which tunnel....?..
thank you in advance..
Joe
Hey Joe. It seems to be falling on "deaf ears", "blind eyes", or something like it :-)
-
i thought eventually i would get someone to comment on the suction if i kept pointing you in the proper direction. it took longer than i thought it would.
It had nothing to do with you "pointing in the right direction", it had a lot to do with you not mentioning a well known and far more practical method of boundary layer control and dwelling on surface heating which in my humble opinion is one of those theoretical options that in real life are way too complicated for this application.
On the salt, the biggest challenge most of the car builders have after developing sufficient power, is trying to keep the internal temperatures of their vehicles within manageable limits and figuring out how to cool things off with little or no internal drag.
Many of these cars are on the ragged edge of heat stroking the driver or melting components like hoses, wires and such due to high internal temperatures. Piping hot exhaust and hot coolant water around the inside of the body is a complication most of these cars really need to avoid. Not to mention the safety implications of piping lots of 1100 deg exhaust and or 230 degree water all over the car, and what happens if the car crashes and sprays that scalding water all over the driver or develops an exhaust leak in an unfortunate location.
Larry
-
I think you would have a hard time sliding that turbine powered sucker through as Naturally aspirated. Somehow I think it would fall into the Turbo compound group
-
Superford317, Im nobody and make no claims that I am. My opinion is that if you dont identify yourself very soon and verify your claims of vast knowledge you will be quickly losing your audience. In enjoy reading your ideas but Id feel a lot better if I knew you werent just some j@ckoff like we've had post on here in the past. I mean no disrespect but the crowd will be turning against you shortly if you fail to follow my advice
-
Get your pitchforks and fire brands at the ready.
DW
-
Thank you DW.
MeGawd!
Even the MODERATOR has joined the villagers heading for the castle!
-
Superford, you're starting to look more like a stock Taurus with a big hole in the muffler. Step up & show us your stuff!! :evil:
Sid.
-
See the fine print.
-
.....i certainly feel that anybody should be able to post here "without" disclosing
identities or "anything" else that others might demand...!!!
..i do like to be able to have open discourse...in the process of discerning how much stake
i may choose to put into information....and generally in conversation...say someone says
"chgghsbhshshs" and "snbfddfh" "i know this d/t my extensive experience gnn snhshhs"...
i would in normal conversation form an appreciation for the info and where it is coming from especially
when it could potentially be applied to contraptions that folks put their loved ones in....
when folks offer up of their own volition (sp?) their accomplishments and case reports of their experience i might like to discuss aspects of that as a topic of interest for further info and appreciation......certainly folks shouldnt be "obligated" beyond their own comforts to share in the conversation....
-
This is an interesting topic. There are safety issues with this, such as heat, coolant scalding, carbon monoxide poisoning, smoke or oil from the exhaust hindering the drivers vision, etc. It might be a good idea to look at other ways to go faster and to discuss those.
-
The largest, and most obvious issue with suction as a way to control boundary layer is that the holes traditionally used for that are likely to plug quite rapidly in the salt environment.
IIRC the navy tried it in the late 50s or early 60s, and they had the same issue, even with the maintenance schedule they had for experimental aircraft (read as S***LOADS) they couldn't maintain the system on one aircraft to where they could rely on it (done for low speed handling improvements, so having it not work, and not be able to tell, untill the aircraft stalls on one side, at too high a speed).
Or at least that's what I read about it.
There was a company at one poit in time that was selling titanium leading edges for select Cessna wings (I haven't heard of them since the "recession") that were laser drilled for the same effect (came complete with an STC, and a large vacuum pump).
I've only seen one of these systems in person, but the owner, and all others I've heard from say the bane of them is they get packed full of bugs, and are a PITA to clean out.
I've mentioned the same issue several times in a row as any of the cars on the salt hhave a coating of salt on any area with a negative pressure, even after only one pass,
I personally have no issue with cleaning the car after each pass, but it would be dificult to get small holes to clean in a timely maner, and large holes would be difficult to supply with enough flow to generate the necessary boundry layer reduction.
If you have a suggestion as to how to avoid that issue, or clean the systems out faster I would love to hear it.
-
I was paying attention to Superford until I saw that he used "loose" when he meant to spell "lose." One of my internet pet peeves.
Don
-
reverse that sucker and flush and blow :-D
-
I didnt demand anything, I merely advised him. What he does is up to him, I just pointed out that he is losing his credability is all.
-
The largest, and most obvious issue with suction as a way to control boundary layer is that the holes traditionally used for that are likely to plug quite rapidly in the salt environment.
...
I personally have no issue with cleaning the car after each pass, but it would be dificult to get small holes to clean in a timely maner, and large holes would be difficult to supply with enough flow to generate the necessary boundry layer reduction.
If you have a suggestion as to how to avoid that issue, or clean the systems out faster I would love to hear it.
Good point, perforated panels and narrow slots would likely not be desirable in either the salt flats or the dry lake environments.
As I mentioned in my post, the NACA duct by its very design strips off the boundary layer. As the air flows down the leading edge ramp and spills over the curved inlet edges toward the rear of the duct, the air closest to the surface (the boundary layer) is the first to go. The free stream flow then would be inclined to re-attach to the surface just behind the NACA duct, especially if you had some mechanical suction being applied to the duct with an internal fan to assist the flow.
One interesting thing to note is that the seam between panels could also be constructively used for such an air bleed system. Many modern cars place their outdoor thermometer sensor in the door jam of the front edge of the drivers side door. There is air flow into that seam area. If you applied suction into a body panel seam on a streamliner only a 1/16" inch wide or so just in front of the area you are trying to preserve attached flow, I suspect you could strip off a good portion of the thick boundary layer at that point (depending on the width of the gap, applied suction level and shaping of the panel immediately behind the air bleed gap).
One advantage of using a panel seam, is cleaning would be trivial, pull the panel off in the pits and clean the now exposes edges of the seam.
Those are just two ideas that come to mind. The only way to know if it works is to do some before and after tuft testing to see if the surface turbulence behind the air bleed slot is reduced. There may be a critical value of suction needed, too little might do you little to no good, and too much might also cause problems like deforming the panel.
Fun to play with as a mind puzzle but only testing will tell you if it is a workable idea. If you have to pull cooling air someplace anyway, it only makes sense to try to locate the inlets in places that would help you maintain attached flow.
Larry
-
hotrod, i wish to apologize to you, i have pulled 5 straight 19 hour days and when i made my last post it was without very much thought or consideration.
when i went to my room last night, i thought about it for a long while and it really stuck in my mind all day.
i fell asleep 5 times today reviewing some research, i really need some rest.
for obvious driver safety, the exhaust heated panels and the water flowing along the body can be used on the rear half of the vehicle and electric heaters can can be used at the front.
exhaust augmenters, cooling ducting and venturies can be used to pull a vacume on the trouble areas of the body to help the aero.
-
"...electric heaters can can be used at the front"
Calculate the electrical power required to heat a large surface in a high velocity airflow--beaucoup watts!
Regards, Neil Tucson, AZ
-
a large area would not have to be heated, a band around the body as close to the front as possible would give a measurable benefit as the radiated heat would be carried along the boundary layer.
on a streamlined body think of the boundary layer as a cushion of air sitting under a blanket.
-
I've resisted posting again, but the temptation for searching got the better of me.
http://mb-soft.com/public/lowdrag.html
"In 1966, Northrop and the U.S. Air Force ... built two of the largest X-planes ever flown: the X-21As. The experimental twin-jets started life as weather-reconaissance Douglas WB-66 jets, electronic warfare versions of which saw service over Vietnam. Under a distinctive humped back, each X-21 sported a swept laminar flow control wing lined with thousands of spanwise razor-thin slits that were in turn perforated with over 815,000 minuscule holes, each of which sucked away turbulent air into a vast internal network of nearly 68,000 ducts, all leading to a pair of high-pressure pumps under the wings. The B-66's main engines were moved from their under-wing pylons to aft shoulder mounts like those on a typical business jet.
The X-21s were meant to prove not only that active laminar flow was achievable but that such a system could be manufactured, maintained, and operated in an everyday environment. "The X-21As proved conclusively that...[laminar flow control] is both effective and viable," experimental-aircraft authority Jay Miller writes in his book The X-Planes. "However, they also demonstrated that LFC incurred certain maintenance penalties that were not easily overcome...[and] that production technology for manufacturing LFC surfaces and related components was...prohibitively expensive for all but experimental aircraft."
The X-21A program had demonstrated that active laminar flow could be achieved using a hand-built wing that required constant maintenance--much of it devoted to keeping the pinholes from clogging with dust, dirt, and bugs--and enough power on board to run the hungry pumps. Active laminar flow control seemed to be a laboratory oddity with no hope of practical application. Unfortunately, that may be nearly as true today as it was in 1966.
The size and shape of the pinholes--and tuning the exact amount of suction applied through them--are the keys to the success or failure of any active laminar flow control system. In the 1940s and '50s, the trick was drilling the holes small enough or finding a porous wing material strong enough. In the '60s and '70s, the holes got smaller and more precise, but the problem became one of keeping them from clogging with dust and dirt."
Air & Space/Smithsonian Magazine, JUN/JUL 1995.
-
what i have been proposing is only use the vacume on the most trouble areas, if its only 24 sq/in on each side of the body at the worst areas, it is a start to better aero package.
-
It's worth trying.
-
superford:
"...the radiated heat would be carried along the boundary layer..."
Not "radiated" heat-- I think you mean "conducted" heat.
BTW--Did you do that electrical power calculation?
Regards, Neil Tucson, AZ
-
I sort of doubt the air wizzing by would pick up much heat . The underhood air in a production car at cruising speed after passing through condenser and radiator is only about 2 degrees warmer than outside . The guy who designed the 1970 Firebird fairings and hood scoop told me that . The air conditoning department got worried about the intake air bypassing the condenser and didn't realize .
-
Sure, it's worth a try (boundary layer control), and if you could work it where a panel gap was where you are drawing the air, it would definetly expedite cleanout. :-D
Of course, if it was a production based car, it could get the nickname "mister stich" if you tried that.
Not that it would be leagal in a production class.
Hotrod, how far to each side would you expect the boundary layer would a NACA duct strip away the boundary layer?
Sparky, I thought of that, but my luck it would fail the system, catastrophically. :-o
I'm sure our freindly officials would take a dim view of a "cleaning" procedure that could result in spreading pieces of the body around in a hurry, but maybe I'm wrong, someone could test that, on their car; when I am far, far away. :-D
I could try driving water backwards through the system, but visions of the car looking like a sprinkler aren't the best, not to mention the soft spot that would leave in the salt.
The officials wouldn't like that either. :-(
-
Hotrod, how far to each side would you expect the boundary layer would a NACA duct strip away the boundary layer?
That is a very good question!
It would effectively remove the boundary layer buildup for its entire width, and I suspect an area about 1/2 its width on each side of the scoop, but that is only a scientific wild "ass" guess. In the area behind the scoop you would have a very thin boundary layer and the thicker boundary layer areas on both sides "should" be influenced by that faster laminar flow nearby. What I am visualizing is that the nearby boundary layer would be gradually accelerated and in effect blown away by the laminar flow behind the NACA scoop. Sort of like what would happen if you have a fast flowing stream of water injected into a slower moving body of water. The slow moving water is accelerated and the fast moving water slows down as the two exchange momentum.
That would be something to be tested by someone in the A2 tunnel, or with tuft testing.
But that would be my expectation for the results of the testing.
Larry
-
The F1 guys have been using blown diffusers to great effect this year - so much so that the FIA are trying to put severe controls on them and causing a fair old row. Clever engine mapping to cut cylinders (which makes the engines sound horrible) means they can still blow high pressure exhaust gasses through the diffuser to increase grip and overall aero effieciency even when the driver is off the throttle in lower speed corners. You'll read this refered to as cold blown vs hot blown. Renault Lotus go as far as exiting the exhausts forward to blow gasses into the front of the sidepods and then out to the diffuser to maximise the hot blown effect. These guys sepnd millions just to get a fraction of a second improvement per lap (not saying that's a good thing but it's the way it is) so you can safely say that it works for them. Worth more investigation I'd say. Here's a link to start - there are lot's more.
http://en.espnf1.com/f1/motorsport/story/21825.html
Robin
-
Is the "Propster" alive :? and well :-o or is that a pusher Canard plane ???
He`s alive, But as for "Well"................... :evil:
-
:cheers:
-
Sorry for the long post but it is somewhat of a complicated subject so I wanted to explain it as simply as I could.
Most of this post will pertain to coup class or full bodied vehicles, as streamliners have a super efficient body design and handle the airflow around them and the wake behind them quiet well with little pressure drag.
Several posts ago I mentioned vehicle surface shape and texture having an influence on drag and “maj” from Australia later asked what I thought about the “sharkskin” and its effect on boundary layers, its not an easy answer and like many things will depend on the shape, size and speed of the vehicle it is applied to.
Shark scales are called placoid and have tiny ridges on them that are parallel with the direction the shark swims in, and differ from one shark species to the next, the faster the particular shark species could swim determined the size of the riblets.
The sharkskin, riblets, chevrons, bumps or V’s, I prefer to call them riblets, whatever you wish to refer to them as, do there work in turbulent boundary layers. The size of the riblets will depend on the thickness of the boundary layer at the vehicles surface and the speed of the vehicle through the air.
In LSR racing, pressure drag is more detrimental than friction drag.
Air has mass and it is displaced and moves around the vehicle as the vehicle passes through it, the force produced and the amount of air displaced is determined by the vehicle shape, speed and air density. Air moving past the vehicle sticks to the surface slows down and forms a layer, the air layer adjacent to the vehicle surface remains attached to the body surface, the air above that layer slows a little less and so on until there are several slower than free stream air velocity layers and this is called the boundary layer. How thick the boundary layer is depends on the vehicle speed, shape and air density. As a vehicle accelerates it disturbs more air than it does at slower speeds. Air moving in the boundary layer causes friction drag on the vehicle. Movement between the air layers remove energy and convert it to heat, lost energy from the boundary layer will cause a transition to turbulent flow and can take place even over a smooth vehicle surface.
The boundary layer can be laminar or turbulent. Laminar layers have less vehicle skin friction than a turbulent layer and drag will be less. If the boundary layer is laminar, changes in the speed of the air in the layer is gradual as it moves away from the vehicles surface. If the boundary layer is turbulent the air speed is chaotic and vortices form inside the boundary layer and will separate away from the vehicles surface because it has lost energy and is acted up on by the free stream air pressure.
Turbulent air has a quick change in speed and pressure and forms vortices that react with each other, increasing friction drag at the vehicle surface.
As the boundary layer moves from the front to the back of the vehicle it looses energy and will thicken and become turbulent or an abrupt change in the body shape or vehicle surface texture can cause the flow to totally separate from the vehicle and form a turbulent area behind the vehicle called a wake. The wake will cause a lower air pressure immediately behind the vehicle than there is at the front of the vehicle and that is what creates the pressure drag, think of it as a large suction pulling on the back of the vehicle and slowing its acceleration rate. Vortices are formed in the wake and will continue far behind the vehicle until the energy is overcame by the air viscosity and turned into heat. That is why I stated in many of my earlier posts that it is best to dump all of the air and heat that you can into the wake behind the vehicle, to help reduce the low pressure area and lessen the pressure drag on the vehicle.
-
Now this is where all of the bad things start to happen, the free stream air flow treats the boundary layer as if it were the actual vehicle surface, so when the boundary layer grows in size as it becomes turbulent and wants to totally separate from the vehicle surface and become even larger, drag increases dramatically and at the same time pressure drag increases behind the vehicle.
Even though turbulent boundary layers cause more drag than laminar boundary layers because of higher skin friction, it moves the point at which the boundary layer would otherwise separate from the vehicle causing lower drag pressure, it is still much better than the boundary layer becoming totally separated from the vehicle surface. The boundary layer if controlled can cause a reduction in drag. The longer the boundary layer can be forced to stick to the vehicle surface and conform to the surface shape before separating, the drag will be lower, even if it is only one inch on each side and one inch at the top you will still have a lower drag than if it were allowed to totally separate.
By re energizing the slowing boundary layer and forcing it to be turbulent for a longer period it can delay the flow separation and even finesse it to become reattached to the vehicle surface, thereby lowering the vehicle drag.
To get the best drag reductions, the size of the riblets will have to be tailored to the boundary layer thickness and the speed of the vehicle and the pattern of the riblets on the vehicle surface will have to be a random pattern , if the riblets are in uniform rows the drag will actually increase dramatically.
What the riblets do are, turbulence is increased on the peak of the rib but laminar flow is maintained in the groove and even though the surface area is increased the shear stress is lowered.
The riblets will lower the thickness of the boundary layer and reduce the surface turbulence. The effectiveness of the riblets to reduce drag can be on the order of 5% to 14% depending on many variables from air density, vehicle speed, shape and to how clean the riblets are, to work properly the grooves need to be very clean. In a turbulent boundary layer riblets can reduce friction drag to a level below what a flat plate is.
In the early 1980’s the initial R&D for the riblets was done at NASA langley research center. When the 3M Company read the research papers they started development of a riblet adhesive film that could be applied over surfaces. The adhesive film was named “Scotchcal MarineDrag Reduction Tape” the first known use of the 3M drag reduction tape was in 1987 in the Americas Cup on the ship piloted by Dennis Connor on the ship “The Stars and Stripes” after winning 4 straight races the rules were changed and the riblet tape was prohibited.
3M does not sell the tape or market it, the price of manufacture was said to be to cost prohibitive so production ceased.
I have used vortex generators, turning vanes, boundary layer suction, surface riblets and .040 air jets blowing at 50psi, all to keep the boundary layer energized and attached to the vehicle surface to reduce drag.
-
"""I have used vortex generators, turning vanes, boundary layer suction, surface riblets and .040 air jets blowing at 50psi, all to keep the boundary layer energized and attached to the vehicle surface to reduce drag"""
how did the vehicle (s) perform before compared to before after..ie what kind of results..?..
curious..
-
lol :cheers: :cheers: is that #2 or #3 joe
-
...i told my wife.."ill love you no matter how big your butt might get"...
doesnt mean it worked real well.....
i guess i like to know abit about how the f' something turned out....
-
Streamliners and golf balls are very different. Pressure drag behind the ball causes more drag than friction drag. By upsetting the boundary layer, a turbulent flow is set up, that actually weakens the effect of flow separation. The wake shrinks, thus drag drops.
Streamliners have an efficient shape and getting laminar flow with minimum separation over the body is the goal. Golf balls are spheres and have an inefficient shape. Dimples can make a golf ball travel up to twice the distance.
It all depends on whether moving the point of flow separation will give a drag reduction, and whether making the boundary layer turbulent will move the location of the separation. A car with a sharp corner somewhere near the back will cause the flow to separate there, whether it is laminar or turbulent, so forcing it to be turbulent cannot reduce drag. The shape of a streamliner is much different. There could well be regions on the streamliner where the separation point could be moved further back through the use of dimples or some other kind of device, but the drag reduction would certainly not be as dramatic as it is for a sphere. Shear stress is higher in turbulent flow than in laminar flow. Unless you can benefit by moving the separation point back, the dimples cause a drag increase, rather than a decrease. Golf balls and streamliners operate at Reynolds’s numbers that are much different.
The dimples on a golf ball create turbulence. Without them the air would lose energy and actually separate from the golf ball and increase drag dramatically. The dimples Add energy to the air and it stays attached to the golf ball all the way around. This actually reduces the drag.
The dimples disturb the boundary layer so that airflow separation happens further down, the turbulent air has more energy and the wake is smaller.
Drag on a sphere is primarily created by pressure drag on its back half that is created because the boundary layer separating and forming a wake and creating a low pressure area behind the sphere. If we could somehow minimize that separation, the drag would be significantly reduced. The boundary layer is a thin layer of air that lies very close to the surface of a body in motion. It is within this layer that the pressure gradient develops that causes the airflow to separate from the surface.
The reason we do not see dimples on streamliners is that these only work well on non streamlined bodies. The primary form of drag on these kinds of shapes is caused by pressure drag. Streamliners are dominated by friction drag. These streamlined bodies, have a shape that creates a much more gradual pressure gradient. This promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.
-
YEP reincarnation is a fact! :cheers:
-
I'm confused(daily state), the OP claims not to know about our classifications yet, the signature claims a 1000 HP lakester build in progress.
DW
-
Dan (Grasshopper), Your confusion will go away when you walk on the salt without leaving a foot print. Tony
-
Soggy is good?
-
As i stated in some of my preavious posts, i have never been to a LSR race, never seen the cover of a rule book. All i know is what i see from this forum and the pictures.
With many years spent at the track, more time spent in a class room than i prefer, tunnel testing and CFD. one can make very well educated assumptions and observations.
As someone in an earlier post hinted at, you don’t have to read any of this or you can assume it is all a lie, do your own research and gather your own opinions.
As evidence, 4600 hits in the past month must mean someone is interested and think it is worth reading.
Aerodynamics is aerodynamics.
i am building a bellytank for enjoyment and the thrill of the speed, i dont know about the classifications.
later i may try to run in a specific class for a set speed goal.
Some day i would like to travel to the great white dyno but will probably never race outside the ECTA.
Since i began my work for the US government in iraq i have been to the USA for 18 days since 2006, i havnt turned a wrench on my tank since then.
-
The riblets should not be placed on the entire vehicle surface, that will only worsen the aero problems. Some thought and research will have to be done to determine where they will need to be placed.
The riblets are only miniature vortex generators, depending on the application they are not much more than scratches on the vehicle surface.
Some sort of testing will have to be done to determine the optimal size of the riblets, wind tunnel testing, CFD or tuft testing and using trial and error. There are many variables that will determine the size of the finished riblets, the final deciding factor is the height of the boundary layer.
The riblets should be between 75% and 85% of the height of the localized boundary layer thickness they are placed in. On a vehicle with numerous boundary layer separation problems and locations, there will be many different sizes of riblets.
Vehicle speed, shape, air density, surface texture and surface angle of attack, along with a list of other minor details, will determine the height of the boundary layer and the riblet will be designed to fit the particular boundary layer it will be placed in.
Removable body panels can be constructed with different riblet designs that are optimized for a specific track and speed range, with a specific air density range in mind.
The optimal distance the riblets layout should be placed from the transition point where the boundary layer begins to separate from the vehicle surface, should be between 4 and 6 times the height of the boundary layer.
A universal size riblet can be used but the efficiency will be greatly reduced, a generic riblet may have the effectiveness of 10% to 30% of a designer riblet. Depending on how lucky you get.
As long as the riblets are placed in a very random pattern and not in organized rows and placed reasonably close to the transition area some gain will be noticed.
On UAV’s that had equipment attached to them later that had not been in the original design from the factory, designer riblets made a 8% reduction in fuel consumption or a gain in top speed of 3% to 5% and thereby increased there loiter time.
On track performance gains from the use of riblets will vary with the down force package used and the particular track with top speeds attained, the number of turns and the length of the straights, as well as all of the other variables mentioned above. In a particular race where the aero package was pushed beyond what it was designed for, the use of pre planned body panels with designer riblets brought the fuel consumption and speeds back into previous parameters.
The more research and development a particular vehicle has done to its aero package the less need there will be for the riblets as the body shape and design have already been optimized.
When things are attached to a vehicle surface that were not in the original design criteria and start to affect the aerodynamics and upset the air flow, that is when localized use of the riblets can be used to help bring the airflow back into order.
-
Brings to mind somthing that PP told me one time---you can have the millions you spent trying to achieve perfection in the wind tunnels, nullified by the sun hitting one side of your car and the panel distorting :-o
-
Building a bellytank without a rule book :roll: but does research on aero :?
Name some cars or your BS.
JL222
-
Building a bellytank without a rule book :roll: but does research on aero :?
Name some cars or your BS.
JL222
It's pretty clear that "superford317" is an external payload integrator on UAV's. He deals in a lot of separation effects. I would welcome him coming to LSR and improving the knowledge base of aerodynamics. However, like John, I think he needs a little more time applying these ideas to ground vehicles before making blanket statements. LSR and UAV's move in the same air, yet suffer many different effects stemming from their different mission requirements.
As far as suction or blowing, this is called "active circulation control" in the industry. We've been working on it for 70 years, and the effects and maintenance issues are well quantified. A short summary of the current state of the art:
1. Suction is not worth the maintenance/reliability of the slots and/or holes.
2. Blowing is capable of massive increases in lift in takeoff/landing configurations and is not worth the power in cruise conditions.
3. Achieving efficient blowing effect (Cu) with low temperatures and power levels is the current frontier. Current studies are focused on pulsed jets to improve the circulation control effects vs. power required.
It is possible that LSR vehicles could use active circulation control to affect separation and make significant reductions in drag. It would require a lot of full scale testing with tufts and an understanding of jet effects.
-
"BLUE" you are very close in your statement :-D
Before my present line of work, i spent considerable time in 3 different racing leagues before i was approached for my present work.
in your post you are hitting on my present work that i am prepapring for my next line of posts for land racing.
i have read a lot of your posts BLUE and you seem to be very knowledgable also.
-
It pains me to say this, as he has been one of my biggest hecklers, but “sparky” several posts ago made a comment about a discussion we were having about pulling a vacuum on the vehicle surface, he said “reverse that sucker and flush and blow” he may have been more correct than he knew at the time.
At the beginning of the last series of posts I made on riblets, I said please forgive me for the rather long posts, the same for this series of posts; this is a complicated subject that will need some explanations along the way, so here it goes.
Most of my previous posts dealt more with turbulent boundary layers and full bodied vehicles, most of this series of posts will deal with laminar boundary layers and stream liners.
The fastest speed a human has ever traveled was 24,791mph by Apollo 10 on its return trip from the moon on MAY 26 1969. There was no air to generate drag or friction heat on the surface, Outer space is very close to being a vacuum.
The fastest “Publicly Acknowledged’ airplane ever built was the Lockheed SR-71 Blackbird with a speed of MACH 3.2+, the SR-71 surface temperatures, due to aerodynamic heating, were around 800F at the nose, 1,300F on the engine cowlings and the cockpit canopy was over 550F. The Blackbirds JP-7 fuel was circulated behind the titanium outer skin on the wings leading edges and circulated around the electronics to help keep them cool. Asbestos was used in the hottest areas for protection also.
A lot of research and millions of dollars have been spent on pulling suction on boundary layers, from the air force to NASA, small scale wind tunnel tests to full scale flying models implementing the process, but have been proven too complicated and the holes that the suction is pulled through on the vehicle surface have proven to be VERY hard to keep clean and operating properly. Blowing of air out of the vehicle surface through holes can be very beneficial also, as we shall see.
The method used and even the specific locations it should be employed will depend on the vehicle shape and the boundary layer, depending on if it is laminar or turbulent.
On a LSR streamliner with a laminar boundary layer, everything is nice, orderly and smooth and the car should be moving like a bat out of hades, but if you suddenly forced the boundary layer to detach it would be like popping a parachute and look like one also if you could see the airflow patterns.
The boundary layer was first identified in 1904 by a young German physicist named Ludwig Prandtl, who was recognized later as the father of modern fluid mechanics, it was only a theory in a research paper, and it was many years later before it was actually proved to exist
You can take the exact same engine and depending on the aerodynamics of the vehicle it is placed in the vehicle can run 100MPH in one body design or run over 200MPH in another body design from nothing but aerodynamic changes in the body shape and surface texture.
Drag depends on velocity, as vehicle speed increases more power is required to overcome the drag. As speed is increased aerodynamic drag goes up quickly. Double the speed and you quadruple the drag.
Small gains in reducing aerodynamic drag can be as good as large gains in horsepower.
What is the boundary layer, you can’t really see it, smell it or taste it when it is attached to the vehicle in laminar or turbulent form, but when you lift from the accelerator you can really feel it is there from the deceleration caused by the aerodynamic drag forces and the pressure build up at the front of the vehicle.
Air has mass and there for the vehicle has to push it out of the way as it accelerates through it. The force generated from the air resisting being moved and the volume of air that is moved will depend largely on vehicle shape, vehicle speed and the air density. The air moving past the vehicle sticks to the surface slows down and starts forming layers that will vary in speed from zero to near zero speed at the surface of the vehicle to free stream airspeed several layers above the vehicle surface.
Each layer subsequently above the other moves a little faster than the one under it as you move above the vehicle surface until the outer most layers are at free stream speed. The layers between the vehicles surface and the top most layer just before reaching free stream speed is called the boundary layer. The boundary layer, as long as it is still attached can be considered as not much more than a thin film on the vehicle surface.
Viscosity is what can be considered as how sticky or gooey the air is and will be a factor in determining how thick the boundary layer will be. Pressure does not affect viscosity, at pressures of 7,000psi, it would only affect the air viscosity by about 10%; Temperature has by far the greatest effect on air viscosity. The height of the boundary layer will become smaller as viscosity is reduced. As the height of the boundary layer shrinks it becomes more stable. The velocity profile of the boundary layer can be greatly influenced by controlling the vehicle surface temperature, which in turn controls the viscosity. Viscosity in air is due to the transfer of energy between the different moving air layers, the energy being friction and heat, the layers in the boundary layer move at different speeds and so the air’s viscosity comes into play as stress builds in the different moving layers. Air exerts pressure forces and viscous forces on objects.
Viscosity in air is determined by the rate at which the air speed changes over distance and the energy that is transferred between the different layers.
As the temperature of a liquid increases its viscosity decreases, the air surrounding you is exactly the opposite. The viscosity of air increases as temperature increases.
Why the difference, as a liquid is heated the bond between the molecules is less so it tends to separate and become thinner. The bond holding the molecules together (cohesive) is greater than the energy transfer between the molecules (momentum).
In air, the bond between the molecules is a lot less while the movement and energy transfer between the molecules is a lot greater. As more heat is applied to the air the more the molecules move and the greater the energy transfer. The two are dominated by entirely different forces that react differently with energy input, in our case heating or cooling.
-
There are four states of matter, solid, liquid, gas and plasma, as you step up in matter, it contains a LOT more energy than the one preceding it. Think of it as hooking your air compressor to a plasma cutter and see what comes out. You are going from a lower state to a higher state. The same as going from the liquid to the air.
The air moving along the vehicle surface will create shear stress on the surface, you can think of it as the stress forcing the air to rise up like a wave as the water approaches a beach, at the surface the air speed is at or near zero and as you move up in the height of the boundary layer the layer speeds increases. The lower layers are slower and the top layers are slightly faster, so the bottom layers are retarded and held back in there forward movement as the layers above move past the layers under them.
The height of the boundary layer can be influenced by how smooth the surface of the vehicle is, the air viscosity, air density, vehicle shape and the vehicle speed.
As the boundary layer moves along the vehicles surface it will tend to grow taller as it moves farther back, the boundary layer is normally the thinnest at the front of the vehicle.
Air has very low viscosity so it does not want to stick to shapes very well as they move through it, air has less than 2% the viscosity of water. To keep the air attached and in laminar form where it will create the least drag along a vehicle body, a lot of restrictions will be placed on the body as to shape and design.
Taking the above mentioned streamliner with its very efficient body design as an example, the pressure gradient in the boundary layer will be distributed from front to back along the vehicle surface and the pressure in the boundary layer will decrease as the air flow moves from the front to the back of the vehicle, causing the boundary layer to stay attached and form a laminar boundary layer. If there is a rise in the boundary layer pressure in the direction of the airflow, caused by surface texture or body irregularities, the boundary layer will start to become turbulent and will totally separate from the vehicle surface if the pressure gradient becomes high enough, as I stated in an earlier post “THIS IS WHEN ALL OF THE BAD THINGS START TO HAPPEN”.
What really caused the boundary layer separation to happen though? The laminar boundary layer will follow the low pressure gradient. As was mentioned earlier, the speed of the air in the boundary layer is at or near zero at the surface of the vehicle and rises slowly as you climb up through the different layers until you reach free stream air speed at the top most layers. The slow moving air in the bottom layers at the vehicle surface has very little energy due to its slow speed and as the upper layers are moving faster they have more energy. As the boundary layer encountered the vehicle surface irregularities and the pressure in the boundary layer began to rise it will cause the slow moving bottom layers at the vehicle surface to slow more and actually stop and reverse flow and start forming vortices, the boundary layer goes from being laminar to turbulent at this point, the bottom layers in the boundary layer because of their low energy will have a high sensitivity to pressure variations, as the bottom layers slow down they become thicker, it would be like a rock in a moving stream stopping the flow of water, it will actually force the water striking the face to move down forming vortices and eddies and it will start to erode the base of the rock as the water flow is reversed and forced back up stream. A lot of people have found out about vortices and eddies when swimming.
The shear stress in the boundary layer at the vehicle surface will decrease as the square root of the distance from the area where the separation took place.
The most effective way to promote laminar boundary layers and delay the transition to turbulence is with the initial vehicle body design that will promote stabile pressure along the length of the vehicle. A well designed streamliner has very graceful curves, no sharp bends or drop offs and gradual tapering body lines to encourage the air to stick to the vehicle surface, conform to its shape and promote laminar air flow and low drag. At the rear of the streamliner it should be gradually tapering so that the air is accelerated inwards to prevent a void at the rear and setting up turbulence. By minimizing the pressure gradients along the body, laminar air flow can be carried out over large distances on the vehicle surfaces, so that the friction levels are at laminar instead of turbulent levels, laminar flow generates much lower drag.
Corners and sharp bends cause a rapid deceleration of the airflow and can lead to boundary layer separation.
In certain conditions the boundary layer can become detached and actually reattach its self to the vehicle surface, when this happens it is called a laminar separation bubble.
Laminar to turbulent boundary layer transition can be controlled or delayed by influencing the speed of the boundary layer by altering the pressure gradient at the vehicle surface.
Boundary layer control can be split into two categories, active and passive control. The velocity of the boundary layer can be influenced by controlling the pressure gradient at the vehicles surface. The stability of the laminar boundary layer depends on the speed of the boundary layer near the vehicle surface.
Turbulent boundary layers have more energy than laminar boundary layers and can handle higher pressure changes on the vehicle surface without becoming detached, but turbulent boundary layers produce much more friction drag than laminar layers, attached turbulent layers are still many times better than detached layers which cause many times more drag due to the ensuing pressure drag and the wake developing behind the vehicle, than turbulent boundary layers which are attached to the vehicle surface.
Active control measures require direct energy input to do there job and passive control requires no direct energy input for the work they do.
Examples of active boundary layer control systems are boundary layer heating, cooling, suction, blowing and even using flow driven vortex generators.
Examples of passive boundary layer control systems are fixed vortex generators such as the riblets, dimples V’s and vortex generating vanes.
-
The boundary layer control systems that are not controlled smartly from an external source will all have to be designed and tuned for there specific locations and the job they will perform on the vehicle surface, according to the boundary layer type and thickness they will be placed in.
The active boundary layer control systems with external controls can be used in a lot wider range of uses and conditions and can even be turned off when not required.
When steady vortices are introduced into separating boundary layers by passive means or steady blowing of air is used, active, they can postpone or prevent the boundary layer from separating from the vehicle surface. Turbulent flow can follow the vehicle shape easier than laminar flow.
The blowing of air into the boundary layer at an area close to separation reduces high pressure areas by accelerating the air in the boundary and can lead to reattachment of the flow.
Passive vortex generators transfer energy from the faster moving outer layers to the vehicles surface to delay boundary layer separation from the vehicle.
When an active system is used such as blowing air through small holes in the vehicle surface upstream of the separation location, it increases the mixing between the slow moving air at the vehicles surface and the faster moving outer boundary layers. When the air jets are randomly pulsed into the separating boundary layer, it is much more effective at delaying boundary layer separation than the steady blowing of air. Huge delays in the separation of the boundary layer can be gained with low levels of pulsating air flow into the boundary layer forming oscillating vortices.
The blowing of air into the boundary layer has a mixing effect and suction on the boundary layer through the surface of the vehicle has a calming effect on the boundary layer. The blowing of air through the vehicle surface will cause the boundary layer to become taller and suction will decrease the height of the boundary layer. A thin boundary layer is more stable than a thick boundary layer. Blowing air increases the speed of the boundary layer and the suction will decrease the boundary layer speed. Stresses in the boundary layer are increased by blowing and decreased by suction. With either blowing or suction, the effects on the boundary layer are greater on the downstream pressure gradients than at the spot where it is introduced into the boundary layer. Pressure fluctuations at the vehicle surface are regulated by speed differences in the whole boundary layer. The differing pressure gradients at the vehicle surface are related to turbulence and vortices that occur upstream. With blowing, skin friction will increase downstream because of the turbulence introduced to the boundary layer. Surface pressure will increase locally, immediately before and after the area where the air is blown and the pressure above the area where the blowing takes place will be less. Suction reduces turbulence at the vehicle surface. At the localized area where the suction is initiated, there will be lower pressure before and after the area and a higher pressure above the initiation point. Suction will have the exact opposite effects of blowing on the surface boundary layer. Downstream pressure gradients will greatly increase with surface blowing and be reduced with surface suction.
The boundary layer can be controlled and made to bend around small surface irregularities by using active vehicle surface heating and cooling. Removing heat from the vehicle surface boundary layer will have a stabilizing effect and adding heat into the boundary layer will have a destabilizing effect. Boundary layer transition from laminar to turbulent can be delayed by cooling the vehicle surface and can be brought about faster by adding heat to the vehicle surface. Upstream conditions will affect blowing more than suction, differing boundary layer speeds and pressures will have an influence on blowing more than it will with the suction. An advantage to suction or blowing at the vehicle surface is it can be turned off and on at will. Surface suction is plagued with the problem of keeping the suction ports clear and clean, obviously blowing will not have this problem.
A lot of control can be had over the boundary layer by the use of surface heating, cooling, suction or blowing. These forces can move the location of where the transition from laminar to turbulent flow takes place, control the thickness of the boundary layer, hasten or delay and even reverse the onset of laminar to turbulent flow.
The more turbulent the flow in the boundary layer the more effective these control systems are. On fully turbulent attached boundary layers a reduction in drag of 50% to 80% can be had. A vehicle with laminar boundary layers using surface cooling and low level spot suction could see drag reductions of up to 15%.
Using vehicle surface cooling in a laminar boundary layer will have another stabilizing effect in that it will promote a downward viscosity gradient in the boundary layer, the air will be less viscous as it comes closer to the vehicle surface, thereby reducing friction between the different boundary layers. The opposite is true with surface heating as it causes the highest viscosity at the vehicle surface. Using surface cooling and suction a boundary layer can be bent around some pretty sharp turns.
Viscosity of the air is responsible for boundary layer separation; any control over the viscosity can have significant effects on boundary layer separation. Temperature has the greatest effect on air viscosity, lower the air temperature and decrease viscosity or increase the air temperature and increase viscosity.
If you are working on a streamliner or a full bodied vehicle with a turbulent boundary layer, surface cooling and suction will keep the boundary layer attached longer, adding a calming effect to the boundary layer, helping to reduce the amount of turbulence and encouraging it to stay attached over longer distances and reducing the height of the boundary layer also. The surface cooling will cause a downward viscosity gradient in the boundary layer, reducing skin friction. If there are areas where the boundary layer is about to become separated, due to a high pressure gradient from the vehicle surface, use blowing and surface heat just before the separation area to add more energy to the boundary layer.
Using full body blowing at low pressure is another alternative, blowing around 4psi at a vehicle speed of 190 MPH, caused a 30 percent reduction in surface friction of a turbulent boundary layer. The low pressure process can be used on either turbulent or laminar boundary layers. The low pressure blowing reduces surface roughness and surface speed of the boundary layer, with the benefit of lower surface friction. The low pressure air lifts the boundary layer above the vehicle surface allowing it to flow over the surface more freely and reduce the effect of surface roughness also. Think of it like an air hockey table. The boundary layer increasing in height as it goes from the front to the back of the vehicle will be reduced because of the lower surface friction, but the boundary layer height will initially be slightly greater due to the blowing effect.
By applying suction and blowing on a vehicle at the same time can be very helpful. You can apply blowing at the front and suction at the rear. To avoid over suction and creating more drag, the suction volume should be lees than the blowing volume. Blowing will reduce the vehicle surface friction drag and the suction will reduce the vehicle pressure drag. Low pressure surface blowing can be used where the surface pressures are high to reduce shear stress and the suction can be used in areas where the boundary layer is slowing and in danger of separation. By using blowing with suction it is more effective than only suction by its self. When using suction combined with the blowing, if the blowing rate is higher than the suction, a thrust can be generated that will actually add to the vehicle acceleration.
Depending on the volume and pressure, blowing can be used to revive a slowing boundary layer in danger of separation or it can be used to raise the boundary layer ever so slightly and reduce friction drag at the vehicle surface.
It does not take very much heating, cooling, blowing or vacuum to influence a laminar boundary layer because it has such low momentum and energy.
By using surface cooling and suction it would reduce the height of the boundary layer and promote a more stable boundary layer that is less sensitive to surface irregularities and surface pressure gradients. By using active boundary layer control methods the streamliner body design can be more flexible in its design and shape also. The rear body should be tapering to the inside so the boundary layer is accelerated to the inside, creating low pressure gradients, that in turn help to keep the boundary layer attached and hopefully laminar for a longer time and distance.
Body design and boundary layer type will dictate the form of active or passive designs that will be used on the boundary layer. A particular vehicle may need surface cooling and suction at the front of the vehicle and may have to switch to blowing and surface heat towards the middle of the vehicle and then switching back to surface cooling and suction near the rear.
Since the boundary layer is so thin and low energy, which ever active system is used it does not take much energy to influence the boundary layer. Applications at the troubled areas are actually preferred over full body applications. If the blowing air pressure is too high it can literally blow the boundary layer off the vehicle and too much suction will increase surface friction due to pulling the boundary layer down against the vehicle surface too much. Localized suction will have up to 60% of the effectiveness compared to full body suction, but the full body suction will hold the boundary layer on for longer periods but cause more friction drag.
Actually moderation with any of the active controls is preferred. Pulsing blowing air is preferable over continuous blowing; low pressure is better than high pressure blowing and more effective.
-
This is most interesting to read, Superford317! Bundle of horsepower seems to be possible to be carried in ice and batterys instead of inactive lead bars. I really hope you continue with a discussion on differences with a flat and a round belly and whats happening under the car at the close proximity to the ground. As usual most of my previous thoughts and belives are put on end and backward!
-
I am very happy that people are enjoying my posts and get a little more insight as to how and why things function as they do.
It took a considerable amount of my free time to make the lasts 2 posts as to riblets and the laminar boundary layer.
I started a while back on 2 more lengthy subjects I think will be eye opening and informative that everyone will enjoy, I still have a ways to go on them before they are ready. I have so many things I want to cover; it will take a while to get it done.
The one you were asking about, flat and round bottoms and there close proximity to the ground, I hope to do it a little later also.
It may be 3 or 4 weeks before I get to do very much, next week I have a meeting in Dubai UAE and then from there to the Philippines for a few weeks.
-
Superford,
In your discussions you imply that the boundary layer on a streamliner can be laminar over the entire length of the body. I find this hard to believe especially with the present day cars that are running in the 400 mph area. Just looking at the Reynolds numbers for this type of vehicle would suggest that the transition from laminar to turbulent boundary layer may occur only a few feet from the nose of the car and that the majority of the car's body is subject to turbulent,( hopefully attached) air flow.
You do spend a lot of time discussing different ways to influence the boundary layer, heating, cooling, sucking and blowing but from a practical stand point related to the majority of land racing cars this is technology that is costly and very hard for the average builder to apply. I find it all very interesting but most not very practical for my build, but please continue to post as it is very interesting.
Rex
-
In the time you took to write all of that, Poteet and Main made 43 runs on the long course :-D
-
317 hopefully I haven't caused you as much mental anguish as your detailed explanations will cause me before I get my pea sized brain to open up and down load this to understanding level. Then to try to formulate an action plan to allow evaluation and assessment for several areas of concern for the skin package for my new lakester---
My working with laminar flow is limited to glider wings in the 60s-70s ---we cross hatched the gel coat on them with 320 grit on the leading edges to past the spar cap-- but that was primarily to expedite evaporation of rain droplets
how do overlapped skin joints affect the non laminar boundary flow an separated flow?
Thanks for challenging our pea sized AERO brains :cheers:
-
Interesting! I kept my "pea brain" focused [with the aid of some Bud], through the posts. There were times when I said: Damn! show me a picture. A little visual would help. The posts by 317 are very stimulating for an old guy's aging grey matter. Thank you for your well spent time.
Doug :cheers: :cheers: :cheers:
-
I just made it back from a much deserved vacation, while I was looking over the south china sea, I started thinking and then started writing. i haven’t been idle, I made several articles I will post as time permits, I still need to refine and polish them up a little more. I enjoyed writing them and I think they will be very informative on many subjects to do with aerodynamics.
I hope everyone has kept “sparky” out of trouble since I have been away.
-
Please forgive the long post as this is a very complicated subject with many variables involved and I hope to explain it without a lot of big words, so that will require a lot of explanations along the way. I could easily make a book on the subject; I almost have here without trying.
I have been asked several times about vehicles with flat bottoms and rounded bottoms, the differences and what I thought about them.
This post covers flat bottom and rounded bottom lakesters in both free stream air and in close ground proximity with open wheels, full body vehicle underbody airflow, underbody tunnel design and downforce generation from the underbodies. I tried to make my thoughts as organized as I could, but it is still a bit jumbled.
There are 2 debates with LSR cars. Which are better, vehicles with flat undersides low to the ground and little airflow underneath or vehicles that are round or teardrop shape with more ground clearance and letting the airflow underneath. As with anything else related to LSR it will depend on many things, such as the speed of the vehicle, how much research and development will be done to the initial body design and the desired weight of the vehicle, the size of the vehicle and even the wheelbase of the vehicle. The teardrop shape with high ground clearance will have a little less drag, be less prone to getting air born in a spin, better directional stability and generate less down force. The rounded vehicle with a flat bottom ran close to the ground can generate a lot of down force if needed, be more prone to becoming air born in a spin and generate a little more drag.
The earliest known land vehicle with a streamlined body shape was developed by a Belgium race car driver and engineer Camille Jenatzy, he was nicknamed the “red devil” and set the land speed record 3 times. The car was raced in 1899 and was named the “never satisfied” and was the predecessor to all single seat race cars.
Aerodynamics is the study of the motion of air and how it interacts with objects. The things normally observed in aerodynamics are temperature, velocity, density and pressure. In the previous set of posts I discussed briefly the cohesive bond of the air molecules and how it is affected by temperature. Because air has viscosity it has resistance to being moved. The viscosity of air is less than 2%of the viscosity of water. Air resists the motion of a vehicle moving through it because of the cohesion between the air particles. The resistance is the force required to break the cohesion between the air particles and make room for the moving vehicle.
Air has volume, viscosity, weight and compressibility. Air consists of 77% nitrogen and 23% oxygen. Air is not considered compressed until its density has increased by 5%. Compressibility effects of the air are usually ignored at speeds less than 228MPH. Because air is compressible, its density will change with pressure, as pressure increases the weight of the air increases.
At sea level air is 800 times less dense than water. The viscosity of air is the single most important factor in the resistance of a moving vehicle through the air. Measurements of air are normally taken with dry air, with no humidity, because the presence of water vapor changes the density of the air. At sea level, air pressure is 14.7psi at 60’F, 1 pound of air occupies a volume of 13.1 cubic feet. As air temperature is reduced, the air pressures decreases and the air density increases.
The earth’s gravity exerts a force that keeps objects pulled to the ground. The viscosity of air helps to prevent movement through the air and the weight of the earth’s atmosphere exerts a pressure of 14.7psi at sea level and decreases with altitude. Air pressure and density decrease as altitude increases. At an altitude of about 19,000 feet air density and pressure is roughly reduced by 50%. You would think that with 50% less air that the aerodynamic drag would be much less and performance would be much greater, but at the same time there is only half as much air for the engine to use to produce power. That is why turbochargers are frequently used in airplanes to recover the lost engine performance. During WWII both sides used turbochargers, superchargers, nitrous oxide and water injection to try and gain an advantage over each other in air superiority.
By lowering the ride height, it lowers the center of gravity of the vehicle, thereby reducing weight transfer during acceleration on rear wheel drive vehicles. Rear traction under acceleration can be improved by increasing the vehicle ride height because rearward weight transfer will increase. By lowering the front of the vehicle and raising the rear, high speed stability and downforce will be increased because of the angle of the roof and the underbody.
The vehicles tire traction can be increases by adding more weight to the vehicle, thus increasing the downward pressure felt by the tire and increasing grip. The weight of the vehicle can be reduced, thereby increasing the rate of acceleration. The lost downward force on the tires can be replaced with aerodynamic down force, to create the same effective weight on the tires. At lower speeds the effective weight on the tires will be less, so the rate of acceleration can be increased. At higher speeds the down force will come on gradually and increase tire traction as the speed is increased.
Because vehicles operate close to the ground, the ground will influence the airflow underneath and around the vehicle. The ground will have a big effect on the vehicle performance, either good or bad depending on the vehicle design and the ground clearance.
Placing downforce generating devices or lift generating devices in close ground proximity will increase their effectiveness. Wings, vehicle underbodies or spoilers will have a greater effect and will actually aid each other in close ground proximity. The closer they are ran to the ground the greater there effect will be, unless the ground clearance is reduced so much that the boundary layer between the device and the ground touch or the pressure on the surface of the device becomes too great and causes the airflow to separate. The boundary layer from the vehicle contacting the ground boundary layer will cause the drag to increase dramatically. The 2 intermingling boundary layers will have a drag greater than the sum of the 2 separate boundary layers.
If rules do not permit a flat bottom, front air dam or the cleaning up of the underside roughness, it would be better to increase the vehicle ride height, in a situation like this, to reduce the lift generated and lower the drag. Rules permitting, the vehicle underside can have the floor rounded to the outside on each side near the door sills, so the high pressure air can be bled off to the outside. Large engine, transmission and differential coolers can be placed horizontally under the vehicle to clean up the
-
roughness. Large engine oil pans made very wide and long with an aerodynamic shape can be installed. The exhaust can have separate pipes, flattened and flared out as much as possible, ran side by side down the center of the vehicle underbody, to mask the underside roughness and lower the drag.
The highest aerodynamic down force with the least amount of drag is generated at the vehicles underbody. The flat underbody is much simpler to produce and will generate large amounts of down force. The underbody with venturies built into it will generate larger amounts of down force. The flat underside with a front air inlet, side skirts and rear diffuser will mock a venture and allow the ground to serve as the bottom side of the venture, will be the best option. These Additions added to the flat floor to mock a venturi and increase down force, will be at a substantial cost in production time and research.
Daniel Bernoulli was a Swiss physicist and mathematician during the mid 1700’s. His experiments showed that velocity and pressure are related. Bernoulli’s work was very critical in designing aircraft in the twentieth century. Bernoulli’s equation helped in the design of the carburetor and explained how it functions.
Collin Chapman, working for team Lotus on F1 cars, designed the Lotus model 78 F1 car; the first race it won was the Long Beach GP race in 1978, Mario Andretti was the driver. The Lotus model 78 was the first ground effects car in racing and had a venturi shaped underbody. Venturi shaped undersides were banned in F1 in 1983, in an effort to try and reduce vehicle speeds. The venturi shaped floors were replaced with flat floors. By shaping the underside of the vehicle like a venturi the air under the vehicle can be accelerated there by lowering the pressure under the vehicle and creating down force. Even a small decrease in pressure over such a large area as the vehicle underside, can create substantial down force.
Drag force depends on vehicle velocity. Too much drag and you will be pulling a lot of unwanted air along with you that will be a hindrance to acceleration and top speed. In any form of racing top speed and acceleration are very important. To maximize acceleration and top speed you should minimize drag. Turbulent drag and friction drag do not increase at the same rate with increasing speed. Turbulent drag will increase at a much faster rate with increasing speed than frictional drag. When dealing with aerodynamics underneath a vehicle, there will be interference due to the ground being in close proximity and will to some extent, have an influence on the aerodynamic flow around the vehicle, it can be good or bad depending on if the vehicle underside is smooth or rough, the shape of the vehicle underside, the desired ground clearance and even the wheel base of the vehicle.
There will be a boundary layer generated on the underside of the vehicle as well as the ground, its effects can cause everything from generate down force to make the vehicle uncontrollable and become air born. A vehicle with a flat underside will have a minimum lift and drag at around 5in to 6in of ground clearance and downforce will start to be generated from there, as the ground clearance is reduced, and drag will gradually start to increase also. Under vehicle air velocity will increase as ground clearance is decreased. Velocity varies with area, if you reduce the area by 50% you will double the air velocity. At higher ground clearances the air flowing through the underbody will be affected by viscosity less, as the ground clearance is reduced it will be affected more by the viscosity of the air, as the speed of the airflow is increased. The most downforce will be generated from a flat floor, with a ground clearance of between 1.5in and 2.75in. As ground clearance is reduced, maximum downforce will gradually shift to the rear of the vehicle. The vehicle underside at a minimum should be looked at to reduce aerodynamic drag forces and to try and reduce its effects, to increase speed and stability by decreasing underbody pressure build up.
The use of airdams, side skirts and reducing the vehicle ground clearance can reduce the airflow underneath the vehicle, thereby reducing aerodynamic drag. Interference from the ground can cause pressure to build up underneath the vehicle and generate lift and in severe cases, cause the vehicle to become air born.
I have never seen it in person, only in magazines and internet photos. Using speed demon as an example, it has side skirts to prevent air from entering under the body from the sides, has a smooth flat underside with a very gradual angle from front to rear to prevent pressure build up and the gradual upsweep at the rear of the vehicle will help to generate down force. A lot of time and effort went into preventing pressure build up under the vehicle, preventing lift and making the vehicle more stable at high speeds. This also encourages better airflow over the body by reducing the amount allowed under the body. At the very rear of the body, the floor making the very gradual up sweep will act as the diffuser and generate down force from the flat underbody. Due to its underbody design, speed demon generates down force from the underbody. If speed demon were to become unstable and spin, the underbody design will greatly contribute to pressure build up and be more likely to become air born. Rear down force on speed demon would be greatly increased if there were side panels on each side of the up swept floor at the rear of the vehicle extending down close to the ground. This would aid in the diffuser operation greatly, but at the same time cause the tendency to become air born in a spin to increase also.
The vehicle with the higher ground clearance and rounded aerodynamic shape will tend to be more neutral in down force and drag. The nose being higher in the air, will generate less down force because it forces less air up over the nose and across the upper body. This body design allows the air to go straight around the vehicle 360 degrees instead of having to bend around it, thereby reducing drag. The teardrop shape in open airflow has very good aerodynamic qualities, but when it is forced to run close to the ground, the airflow becomes constrained and behaves differently. The ground prevents the formation of symmetrical air flow and results in an increase in drag. By installing a flat bottom on the teardrop shape, its qualities of being close to the ground will be greatly enhanced and will counteract most of the negative characteristics. If there were pressure trying to build up under the vehicle, the rounded bottom would bleed the high pressure air off to the outside. The rounded bottom with air flowing equally down both sides, top and bottom, would be like a rudder and aid in directional stability. The rounded bottom would generate very little if any lift, there is no floor area for the positive pressure to act against , if there were high pressure air under the vehicle it would bleed back into the free stream air due to the sharply rounded bottom . A flat bottom vehicle will have a smaller frontal area, than a rounded bottom vehicle. You can take 2 very aerodynamic shapes and mate them together and they can suddenly become very un-aerodynamic, because of the interference between the airflow patterns and boundary layers.
The best compromise of time money and effort would be a teardrop body design with its rounded and tapered shape to cut through the air. The vehicle should also have either a flat bottom or venturi shaped underside, both will generate down force, providing increased traction.
I like the design of the “spirit of sunshine” tank, teardrop body design with the flat floor close to the ground. With minimal horsepower, it has proven it’s self very capable of high speeds and good stability. It has one of the better body shapes with the engine exhaust at the most advantageous location. The vehicle with the flat bottom close to the ground, will usually generate a vortice off of the back corners of the vehicle where the rounded body meets the flat floor, increasing the drag slightly. The rounded vehicle with high ground clearance will usually not generate the rear vortices and have less drag.
The rounded vehicle with the flat bottom in close ground proximity will have a wider range of choices that can be used because of the design. The vehicle can be lowered to the ground, side skirts added and a front air inlet installed, to generate downforce from the underbody. The vehicle ground clearance can be increased to between 5in and 6in to generate little to no downforce and reduce the drag from the underbody. At a cost to a little more drag but substantially more downforce, the air inlet can be allowed to let more airflow into the underbody region, add side skirts and a custom rear diffuser and increase the down force dramatically over only allowing air flow over the top of the vehicle body only.
To take full advantage of the benefits of either a flat floor or the installation of an airdam, side skirts will have to be installed, to block the air from the sides flowing into the newly created low pressure area underneath the vehicle. The closer the side skirts can be ran to the ground, the more downforce will be generated. With a side skirt gap of .800in, it could cause a loss of as much as 50% of the downforce from the underbody.
A plain flat floor with a few degrees angle and an up sweep of the floor at the rear of between 5 degrees and 13 degrees will generate down force. The more elaborate you make the design the more down force will be created. Adding side skirts or center vanes to the rear up sweep area, which is the diffuser, will produce more down force. Changing the floor form being flat to a venturi shape will greatly increase the down force if the ground clearance is kept reasonably low.
A well designed venturi contoured underbody, vehicle side skirts and a low vehicle ground clearance will generate thousands of pounds of down force. A F1 car running under the constraints of a rule book the size of a new york phone book, dictating the underbody shape, size and locations, as well as the ride height and no side skirts, all in an effort to reduce down force and to reduce speeds, still generates around 600 pounds of down force from the underbody with a flat floor.
Where the venturi tunnel meets the rear diffuser, the lower edges should be left as sharp and squared off as possible, because there will be vortices created on each side of the diffuser, aiding the airflow and helping it to remain attached to the diffuser. The roof of the venturi, where the corners meet the wall, should have a large radius to prevent airflow separation. If you use a larger angle on the front air entrance or the rear diffuser to shift the center of pressure forward or backwards, it will be more sensitive to the main floor angle and ground clearance.
-
Aerodynamic downforce generated from the underbody has the lowest drag to downforce ratio of any of the aerodynamic downforce generating devices. Underbody downforce can be divided into active and passive.
Passive, being downforce generated when the vehicle is moving through the air and forcing the air to flow under or over the body to generate downforce.
Active, can be having a secondary power source pulling the air from under the vehicle, that is sealed with side skirts all the way around to the ground, causing a suction force to be generated under the vehicle. The active downforce will be irrelevant of speed, it will have the same downforce at rest as it will at 150MPH. If you reduced the pressure to -1psi on an area of 5 feet by 10 feet, you will generate over 6,000 pounds of downforce.
Texas native Jim Hall and Hap sharp founded Chaparral cars. Chaparral came from combining their last 2 names. Chaparral was the first race team to use fiberglass as a structural element and the first team to use scientifically designed airdams and spoilers. Chaparral built the car that inspired modern ground effects cars, the Chaparral 2J, also known as the vacuum cleaner. The Chaparral 2J had two 17in fans powered by a 45hp snowmobile engine and used plastic skirts to seal the underside of the car to the ground and pulled the air from under the car to generate downforce. It was outlawed by the sanctioning body after its first season of use. 8 years after the Chaparral 2J fan car, In F1, Brabham built the BT46B using side skirts to seal the underbody to the ground and a fan that was said, to be used to cool the engine, that pulled the air from under the vehicle, generating tremendous downforce, but was banned soon after by F1 also. If you reduced the pressure by only .18psi over a 5,000 square inch area it would generate 900 pounds of downforce. Not that it would matter to us, but the Chaparral 2J generated .4 to .5 more G force on a skid pad with the fans on than with them off. The closer the skirts hug the ground the more downforce will be generated.
Flat floors or venturi contoured floors depend on mass airflow to generate downforce, the more airflow allowed in at the front air inlet and the more airflow ejected out the rear diffuser, the more downforce will be generated. Vehicles with airdams, side skirts and splitters rely on preventing air from going under a vehicle, reducing underbody drag and tire drag, thereby having a lower drag coefficient.
For vehicles that cannot be fitted with flat floors or venturi shaped floors there are several things that can be done to help them, rules permitting of course. The use of diffusers, airdams and splitters can be almost as effective as a flat floor, as far as down force and drag reduction are concerned, but will require more work and testing.
Oncoming air at the front of the vehicle goes through stagnation, slowing down and increasing in pressue.
An airdam fitted to the front of a vehicle, will wrap around the front of the vehicle and extend down close to the ground, reducing the size of the gap between the front of the vehicle and the ground. As the vehicle ground clearance is reduced with the airdam, downforce will increase and the drag will be reduced. At very small airdam to ground clearance, drag will begin to increase because of boundary layer interference and the pressure gradient becoming too high can actually stop the airflow under the airdam and cause the drag to increase and the downforce to decrease.
An airdam with very low ground clearance, will speed up the air flowing under it into the underbody, lowering the air pressure and creating some down force this way. The airdam reducing the airflow under the vehicle will reduce the drag generated by the tires, exhaust, underbody and frame. The benefits of the airdam are 2 fold, reduce the airflow going into the underbody thereby reducing underbody roughness and drag and to create a low pressure area thereby generating downforce.
The stagnation point at the front of the vehicle, where the air hits and builds up pressure, will be lowered closer to the ground when the airdam is fitted. With the fitting of the airdam and the lowering of the stagnation point, more air will be forced around the sides of the vehicle and more air will be forced over the top of the hood and at the same time less air will be allowed to pass into the vehicle underbody.
The air pressure on the hood will increase and at the same time the air pressure under the vehicle will be decreased, because of the airflow being changed. The difference in the pressure differential causes most of the added down force.
The negative pressure or suction on the vehicle underside will extend to about the middle of the vehicle, generating more down force on the underbody.
As down force is added at the front of the vehicle it will usually take away at the rear of the vehicle.
If a vehicle, that has already had a flat underside installed, has an airdam installed also, drag will usually be increased because the underside is already smooth, so there is nothing to create drag. The airdam its self creates drag, so total drag will usually be increased.
A splitter can be added to the airdam lower leading edge closest to the ground and will stick out horizontally towards the front of the vehicle. The splitter picks up down force from the high pressure stagnation point at the front of the vehicle that was just lowered closer to the ground with the addition of the airdam. The splitter should be between 3in and 6in in length and does not need to protrude further than the thickness of the stagnation area.
The low pressure area that previously existed under and behind the airdam, with the addition of the splitter, will generate even more down force than before, because the size of the low pressure area will be extended by adding the splitter, thus increasing the floor area for the low pressure to act on and the low pressure will be reduced even more also. The high pressure area above the splitter and the low pressure area under it will cause a large pressure differential that generates the down force.
A diffuser can be added to the back of the splitter and generate even more down force from the airdam and splitter. The diffuser is an extension of the splitter, extending horizontality under the airdam and the back end of it being turned up. The splitter, airdam and diffuser form a simple venturi, the splitter extension being the throat and the rear upsweep being the diffuser, the rear upsweep will be the expanding cross section area for the airflow. The size of the splitter passing under the airdam and the size of the diffuser can be any size you want to make it, but the larger the size the more down force will be generated. The addition of the diffuser to the back of the splitter will lower the air pressure even more in the low pressure area under and behind the airdam, thereby increasing the down force even more.
On a vehicle with a flat floor and rear diffuser, the vehicle center of pressure can be moved forward or backwards by moving the location of the entrance of the vehicle floor to the diffuser. The highest downforce will be generated at the transition from the vehicle floor to the diffuser entrance. The diffuser entrance can be moved by changing the angle of the diffuser ramp. The angle of the diffuser floor can be between 5deg and 13deg, with an angle of 9deg to 10deg being most effective, diffuser angels over 14deg will cause the pressure to become too great and cause the airflow to separate, reducing downforce.
There will be vortices formed at the sides of the diffuser, that will improve the airflow through the diffuser.
The main job of the rear diffuser is to slow the speed of the under vehicle airflow and let the pressure rise to that of the external free stream airflow, before exiting into the free stream air. Some down force will be generated by the diffuser because normally the pressure in the diffuser will be lower than external pressure. Downforce is created not only in the diffuser but also under the entire floor area. The diffuser drives the airflow for the complete vehicle underbody. Because of the angle of the diffuser floor, its internal volume will increase as the aiflow moves to the back, causing the air from the underbody to expand as it passes through the diffuser, pulling air through the underbody. If the angle of the diffuser is too great, the airflow will separate, because it will not be able to overcome the pressure in the diffuser, causing pressure to build up under the vehicle. The angled flat floor will generate downforce by its self; the addition of a diffuser will increase the velocity of the air under the vehicle. The job of the diffuser is to convert the airflow’s kinetic energy or dynamic pressure into pressure rise or static pressure. The expansion of the air from the underbody in the diffuser slows the air, increasing the pressure. As the air is slowed down it is forced to become denser as the pressure increases. The diffuser can be a simple upsweep in the flat floor at the rear of the vehicle, but adding side plates that drop close to the ground and forming a tunnel will increase its effectiveness greatly. The side plates will allow the diffuser to generate more downforce at a lower speed. There will be a counter rotating vortice generated at the inside of the trailing edge of the diffuser side plates, enhancing airflow through the diffuser. The attached vortices inside the side plates will trail downstream into the wake behind the vehicle and cause a vortex induced suction. At very low ground clearances, the effect of the counter rotating vortices will be reduced as ground clearance is reduced. The counter rotating vortices will help to keep the arflow attached to the diffuser surface longer than it would be expected to at higher angles. The diffuser angle should be between 9deg and 10deg to be most effective. The smooth flat floor of the vehicle leading to the diffuser will allow a larger area over which the low pressure air can act, creating more downforce. The diffuser itself does not create downforce, it is the area in front of the diffuser that the low pressure acts on that creates the downforce. Another benefit of the diffuser is that by it being located at the very rear of the vehicle, allows all of the mass airflow to help fill the void behind the vehicle, reducing the wake size and the resulting pressure drop and the induced pressure drag. The diffuser will reduce the turbulence in the wake, thereby decreasing the pressure drag.
At the lower angles for the diffuser, the downforce will be gradual as the ride height is reduced and at very low ground clearances the falloff of downforce will be more gradual also. At the higher angles for the diffuser, downforce will build more quickly and falloff quicker at reduced ground clearances.
It was becoming a lot more common, in F1 to dump the exhaust into the diffuser, so the hot expanding gasses can increase the effectiveness of the diffuser. As the vehicle is cornering and slowing down, the airflow will be reduced under the vehicle, lowering the downforce at the time when it is needed the most. At low vehicle speeds there will not be enough airflow under the vehicle to support the twin counter rotating vortices that help drive the underbody airflow through the diffuser and contribute to generating downforce. F1 race teams went to a lot of effort and research to dump the exhaust into the diffuser, the vehicle electronics were set up to keep the engine RPM’s up during cornering when the driver was off throttle and to dump more fuel into the exhaust so there would be more mass airflow generated to keep the vortice structures supported and generating more downforce in the underbody and diffuser at low speeds. Just in the last few weeks, F1 decided it will be banning the practice of dumping the exhaust into the diffuser for next year.
The downforce of the flat floor with a rear diffuser can be increased even more with the addition of a front air inlet and side skirts. The flat floor between the air inlet and the diffuser is where the low pressure acts and creates the largest majority of the downforce. The inlet and diffuser simply aid the airflow into and out of the underbody region. The leading edge of the inlet should have a radius to help with the airflow. Downforce will increase as the inlet angle is increased, because at the steeper inlet angles it lets the flat floor area be larger. The inlet angle should be between 5deg and 17deg. The shallower inlet angle will pull more of the surrounding air into the inlet, but at a sacrifice of floor area. With the addition of side skirts, front air inlet and a rear diffuser the flat floor can be made into an approximation of a venturi.
The larger the floor area can be maintained, the more downforce will be created. Efficiency in the inlet and diffuser can be sacrificed somewhat, for the sake of a larger floor area, for the low pressure to act upon.
A teardrop shape has the lowest drag in free air and as the ground clearance is reduced the drag will increase dramatically. The fineness ratio can be used to make an educated guess between teardrop shape bodies to determine which is more aerodynamic. The fineness ratio is the relation between the length to width. The ideal fineness ratio should be from 5 to 5.75. The teardrop shape should have the blunt nose forward and the thin end to the rear. The area where the greatest thickness should be is recommended to be about 0.25 to 0.30 percent of the length of the body from the front end. As people like to say “it’s not how you open the hole in the air, it’s how you close it” this being said, the body should gradually tapper from the thickest point to a point at the rear, this area being the most important from an aerodynamic standpoint. The airflow being accelerated to the inside at the rear will encourage the air to remain attached to the body as long as the pressure does not become too great and force the
-
airflow to become detached. A large portion of the blunt nose at the front can actually be cut off flat without greatly harming aerodynamics and drag, but irregularities or too steep of an angle on the rear of the teardrop shape can greatly increase drag, due to boundary layer separation.
A torpedo shape with a gradual bulge in the top and bottom at a ground clearance of 6in to 7in will have 50% more drag than free stream, if the ground clearance approaches near zero, the same shape will have the drag increase 500%. By making the bottom flat the drag can be reduced dramatically at low ground clearances.
In the early 1980”s professor A. Morelli of the University of Italy, proved a streamlined body in close ground proximity, can have the same drag coefficient as a streamlined body in free air. It was called the morelli body and based on frontal area had a drag coefficient of 0.05.
The debate has been going on for years and will likely still be going on long after I am dead and gone. Which is better, flat and low VS round and tall will depend on the budget of the vehicle build, the amount of time one wish’s to invest, desired speed, available horsepower, if downforce is desired from the aerodynamics, suspension and even the surface being ran on. Considering there are several fiberglass tanks radially available, there is a good design that saves a lot of time and effort with construction, a flat bottom can be added to one if desired also. The vehicle with the lowest drag, which manages the airflow most efficiently, is the rounded body with the taller ground clearance, so it would be better for vehicles that are power limited. The flat bottom vehicle close to the ground will generate a lot of downforce if desired, have slightly more drag and be more prone to become airborn in a spin situation.
If extra ballast weight is carried to aid traction, you can choose to generate downforce with the underbody and carry less weight in the vehicle. The vehicle being lighter will have a faster acceleration rate. The downforce will be gradual and increase with vehicle speed, producing a linear downforce and traction curve. The lighter vehicle will produce less wear and tear on parts due to its lighter weight. Racing on a very rough surface will generate fluctuations in the downforce and traction, as the body makes undulations over the bumps. Spring rate will have to be calculated to account for the added weight of the aerodynamic downforce at high speed.
It has been proved in wind tunnels and at the track, vehicles can be designed with close to the same drag weather they are running in close ground proximity or in free stream air. In the end it will depend on if you want to generate downforce with the shape, have more confidence in a spin of not becoming airborn or the ease of construction.
As an inverted wing close to the ground has its ground clearance reduced, its downforce generation properties are magnified, as the ground clearance decreases, until it is very close to the ground then drag increases and downforce falls off quickly. A regular lifting airfoil will have its properties increased as ground clearance decreases. The same for any aerodynamic device, diffusers, flat vehicle undersides for downforce, splitters and airdams, all work better in reduced ground clearance or ground effect as it has become known.
Whatever tendencies a vehicle has built into its underside will be increased as its ground clearance is reduced. Most full bodied modern vehicles will generate between 140lbs and 180lbs of lift due to its underside roughness and shape and design of the body. If nothing is changed, but the ground clearance is reduced, the amount of lift generated, will increase as ground clearance is reduced.
A vehicle with nothing but a flat smooth underside installed can be neutral, generate lift or generate downforce, depending on the angle of the underside and the effect will be magnified with reduced ground clearance.
Everything being equal though, as ground clearance is reduced, drag will increase, because of the viscous effects of the airflow under the vehicle will be increased as ground clearance is reduced. As ground clearances become smaller air velocity will increase, thus increasing its viscous drag. As the ground clearances fall below 5in to 6in, the effects of the ground will begin to be felt and be magnified as the ground clearance is reduced.
-
Superford, you sometimes give me a headache with all the reading but I have to admit that I appreciate your interpreting and writing all this stuff out instead of plagiarizing from others and using cut and paste. We've had those experiences on this website in the past so a lot of us are a little gun shy when we see these types of posts.
Pete
-
SF317-
Your threads actually make my hair hurt but dont stop. I have to print them and reread them a couple of times but it is really eye opening.
Joe
-
In using a single turbo system ive seen cars use a single exhaust straight through all way to the rear of the car coming off the left side header and in other single turbo setups ive seen an x-pipe that leads to duals at the back. Which one would be a better choice? Or does it really matter? I would think that the less bends and extra tubes that get tied in would lead to a smoother exhaust flow.
-
Meant to say less bends and "LESS TUBES"
-
When you ask a good question...........be prepared for the best answer! This read is great............and so far I have yet to break 100 mph!
-
I've been reading this thread and find it really gets the gray matter stirred up. I have a lot of experience as an aircraft mechanic and have been an aircraft enthusiast for most of my life (well actually an enthusiast for just about anything that has a motor or wheels for that matter) and have read about the X-21 program and understand how is just wasn't practical on that scale. However, I do see how it could be used on a smaller scale in LSR to clean up problem areas. For instance the area around an intake scoop. Suction could be used to bring the airflow into a laminar flow over and around a scoop. Same goes for any other problem area. For those running at very high speeds these same suction holes could be used to increase drag at the end of a run by turning off the suction or even blowing air out to create more drag by disrupting the laminar flow over a larger portion of the body. Of course there would be handling considerations to creating drag at high speed in this manner. Aerodynamic braking such as this may allow for higher potential speeds as less space would be need to get the vehicle stopped and allow for a longer run under power.
-
Chris,
One of the misconceptions that is in this thread is that streamliners operate with laminar flow. If you calculate the Reynolds number for a car moving at 350 mph and one foot back from the nose it is 19.4 x 10(to the 7 power) ( 194000000) which is way beyond the transition point value for laminar to turbulent flow. The stardard calculation for the Re is: Re= (air density)(velocity)(length)/viscosity. If you use calculate it at STP (Standard Temperature and Pressure) and use English dimensions it simplifies to: Re= 6300(Velocity ft/sec)(lenght ft). If you use this formula you can see that the Re quickly exceeds 1,000,000 within the first .06 inches of the body length.
Some things can affect the position of the transition zone, and one is the shape of the body, if the pressure gradient is negative it can assist in keeping the boundry layer thin and extend the length of the laminar flow region, which is the lowest drag regime. This is why using such shapes as the NACA 66000 shapes is good as these were designed specifically to have extended areas of laminar flow. I am not sure if suction would cause a turbulent flow regime to become laminar again. Probably a good question for "Blue".
Rex
-
Rex, I'm not an engineer so I'm just trying to apply logic to what I've been learning. As I understand it turbulence creates drag and anything that can be done to decrease turbulent flow will reduce drag. I would think every designer has to make compromises in the design of a vehicle that create drag and this may be one way around some of those compromises. I'm definitely interested in what Blue and SuperFord have to say. This brings me to another question, are the horsepower gains made from a ram air system worth the increased drag created by the scoop. I know that is not a simple question to answer and there are many variables to consider. I'm new to LSR but really enjoying the mental stimulation all this is bringing. Chris
-
Chris,
I contend that most of the air flow over any 300 mph+ car is turbulent the trick is to assure that it remains attached. High drag occurs when the air becomes unattached and then generate large trailing vortex formations that cause drag.
Regarding air inlets I feel that if the inlet is sized properly to the engine air requirements at the cars estimated maximum speed and then the rest of the scoop is designed to ensure attached flow then the scoop will have a positive affect on the cars performance. I think that a properly designed and placed NACA inlet is probably the best possible design regarding drag and inlet but some cars do not allow you to put them in the proper air flow and they are not optimum.
I have posted these pics already of a scoop that I built for Steve Nelson's 4V fuel lakester. This car ran 187 in 2010 and 197 at the 2011 August Speed Week. The engine was basically the same as 2010 but I feel that the scoop did contribute to a part of the performance increase. The inlet is sized for about 115% of the engine air flow requirement at 200 mph and the general plan shape of the scoop is a NACA 66018 air foil shape. The area ratio of the inlet area to the maximum cross section area is about 8:1 so the scoop should do a good job of converting the energy of the inlet air into inlet pressure. The scoop is sealed to the body and the body is sealed to the injector inlets so there is very little air leakage. The scoop has internal turning vanes to aim the inlet air at each of the injector inlets. The opening in the top is to allow us to prime the injectors to get the motor started.
Rex
-
Rex, I love the look of that scoop, I am working on ideas for mine. Any pictures of the whole car?
-
Tman,
This is a pic of the car several years ago, the only outward change is the addition of the scoop and I do not have a currrent pic of the car with the scoop mounted. It is a very classic "tank" exceptionally well built and great detail and finish. Steve has worked very hard to get the car where it is today and I am very proud to be associated with the car. It did set the 4V Fuel Lakester record this year, around 195 mph. The tank is from a Grumman Albatross.
Rex
-
I was talking to Steve , he said that he'd been into the salt for a long time but since his kids had grown up " we have a bit more time , and my wife and I both love it here"....I said , with reference to his wife loving the salt " and for that you are a lucky man"
He replied " indeed, I am very, very lucky man". :cheers:
It is a great car, a great mix of style and business.
-
Steve's Lakester is inspiring!!! we have visted several times :cheers:
-
OK, you have posted other shots of it in the past. I think I saw your modified at their pit but was chasing parts at the time and did not stop :-(
-
Tman,
This is a pic of the car several years ago, the only outward change is the addition of the scoop and I do not have a currrent pic of the car with the scoop mounted. It is a very classic "tank" exceptionally well built and great detail and finish. Steve has worked very hard to get the car where it is today and I am very proud to be associated with the car. It did set the 4V Fuel Lakester record this year, around 195 mph. The tank is from a Grumman Albatross.
Rex
We pitted next to this Lakester at W.F. it is a very nice car. SB
-
Wow! That's a very nice looking lakester! :cheers:
-
Rex, That is beautiful metal work! I have aspirations of being able to do work like that. I guess maybe I am using the laminar in the wrong context. By laminar I mean the airflow is attached, and in my understanding keeping it attached or reattaching it is the goal of aerodynamic as applied to LSR. My thought is on a scoop like that to have an inner and outer skin with suction holes near the front of the scoop body to get the airflow to reattach sooner. After watching some of the videos from A2 it seems this is an area that causes drag in most applications. Chris
-
OUTSTANDING, REX,
Looks like some inner workings for air control.
Re:Your semi self- critique - as a hafas scoop builder myself -making sure the air TURNS the corner made me look dumb a few years ago when I partnered with a cylinder head wiz - big wakeup call on the TURN part.
I learned you can't wish air to do your bidding without helpin' guide it.
-
A lot of research and tax payer money went into NACA submerged inlet development, in the beginning there were 5 separate scientific investigations undertaken over a period of about 10 years, by some of the best scientist and engineers in the country. The name of the NACA submerged air inlet comes from the organization that first developed it, National Advisory Committee for Aeronautics, the predecessor to NASA. One of the NACA scientific investigations was completed and came up with the NACA duct with the divergent parallel walls, wanting a higher pressure recovery they launched another scientific investigation and came up with the curved divergent ramp NACA duct, that we know today, that has higher pressure recoveries. The first investigations were done small scale, in later investigations performed on full scale models; pressure recoveries were even higher, because of the difference in the boundary layer thickness, from small scale to full scale. NACA ducts are efficient and have little or no drag. For the best results the NACA duct should be located where the boundary layer is the thinnest. That will normally be near the front of a surface it is placed in, as the boundary layer will become thicker, the further it travels over a surface. The engineers that designed the NACA duct, wanted the lowest drag with the highest pressure recovery they could possibly get.
Velocity ratio is the amount of air that can flow through a given size at free stream speeds as opposed to actual real world, operational airflow through a similar size. The NACA duct forms a vortex on each wall that will force more outside air to flow into the inlet than normally would, generating a small ram effect. The two vortices improve the pressure recovery and improve air flow through the duct. The vortices are formed and work best when the air flow through the duct is at a continuous high speed, at lower inlet air speeds the vortices will break down and the duct will lose part of its pressure recovery. If a NACA submerged inlet were used to supply air to the engine intake manifold, it would be more suited to a situation like LSR than most other forms of racing, because the throttle is normally wide open or close to it the entire run, keeping the air flowing through the diffuser, preventing the vortice structures from breaking down. In racing where the driver is continuously on and off the throttle starting and stopping the airflow, the vortices would never get a chance to form and start there ram effect. A NACA submerged duct with curved convergence walls will have a 20% to 30% higher pressure recovery than a NACA duct with straight parallel convergent walls and have velocity ratios 20% to 50% higher also, due to the twin vortices functioning more efficiently with the curved convergence walls than with the straight parallel convergent walls. If you look at the curved convergence wall NACA duct closely, you will notice that the walls follow the contour of a vortice, that is why they are more efficient, they promote the vortice structure and increase the efficiency of it. The air flowing over the top and down the walls of the NACA duct is what forms the vortices, so all the corners should be left sharp to promote the creation and structure of the vortices. The early engineers didn’t realize that the vortices were being formed inside the duct, they found out by accident, after the straight walls were reshaped to follow the airflow streamlines along the sides of the duct, thus giving us the curved convergence walls we all know today.
The NACA duct has an advantage over other types of inlets for certain inlet and airflow requirements. The placement of the NACA duct is more critical than for other types of inlets because of boundary layer thickness and local air velocity. As the speed of the vehicle increases boundary layer thickness will increase also, decreasing the effectiveness of the NACA inlet. A lot of thought should go into where to place the NACA inlet beforehand. A well designed NACA inlet placed at an ideal location had a pressure recovery of .97 at a speed of 245MPH and a pressure recovery of .95 at a speed of 400MPH. The small differences between the pressure recoveries were due to the boundary layer thickness increasing with the speed, reducing the efficiency of the twin counter rotating vortices. A NACA duct has good pressure recovery when used in a region of low air velocity and a thin boundary layer. NACA ducts are best for systems that need only a small amount of air flow diffusion. Pressure recovery will be better if placed in a thick boundary layer but the velocity ratio will decrease. NACA ducts have reduced internal ducting and fewer bends, with less weight and also have a large reduction in drag compared to conventional external scoops. The divergence of the walls of the duct gradually lets the air expand and reduce the chance of separation. NACA ducts do not have as good a pressure recovery or velocity ratio as normal external style scoops, which can have a velocity ratio of over 100 percent, creating a positive pressure in the scoop and diffuser system, having a ram effect, and thereby increasing power at high speeds. The NACA duct external drag can actually be a negative number in certain situations, because of the removal of the boundary layer behind the inlet.
The NACA duct should have a width to depth ratio of between 3 and 5. That is the ratio of duct entrance width to entrance height. The NACA duct should have a ramp angle of between 5deg and 7deg. If the duct entrance ramp angles become too steep, the airflow can separate from the surface of the ramp and reduce airflow through the duct dramatically. The ramp angle can be up to 10deg without a significant pressure loss, less wall divergence should be used for smaller ramp angles. Ramp angles of 15deg or more will result in a large loss to the pressure recovery characteristics, a steeper ramp will make a shorter duct but as ramp angle increases efficiency decreases. All corners and edges should be left sharp to aid in the generation and structure of the vortices. The curved divergent ramp walls improve the pressure recovery, because the twin counter rotating vortices function more efficiently than a straight parallel wall NACA duct. The counter rotating vortices generated on each side of the ramp, force the extra airflow down the center of the ramp between the 2 vortices and into the entrance. About 50% of the body of the twin vortices will extend out of the NACA duct, above the surface of the vehicle, through the boundary layer and pull high energy air from above the boundary layer and into the duct. The pressure recovery at the end of the ramp and at the duct entrance can be over 90 percent after diffusion, in ideal situations. Pressure recovery can be improved with higher ramp angles up to 10deg and the divergent walls result in reduced pressure loss. The pressure loss at the ramp inlet will be dependent on the pressure along the ramp and the thickness of the boundary layer the submerged inlet is placed in. If turbulent airflow is allowed to flow into the submerged ramp, it will increase drag and cause pressure recovery to be reduced due to interfering with the formation of the vortices and causing the airflow to become detached from the ramp.
The best pressure recoveries will be with curved divergence walls. Wall divergence is the ratio of the width of the ramp entrance to the width of the submerged entrance. The divergent walls reduce the pressure loss by the air entering the duct and reduce boundary layer air flow into the duct. Thicker boundary layers reduce the pressure recovery at the submerged entrance, because it will reduce the efficiency of the twin counter rotating vortices, if the boundary layer is sufficiently thick enough so that the vortices will not be able reach above the top of the boundary layer, it would reduce pressure recovery and efficiency by as much as 30%. Pressure recovery will be increased if deflectors are used, regardless of boundary layer thickness. The use of the deflectors will increase the drag somewhat, and the height of the deflectors should be in accordance to the height of the boundary layer they are placed in. With the use of deflectors placed on top of the walls and the use of curved divergence, pressure recovery will be increased. The difference in pressure recovery with duct angle of attack is small. When the duct is located in a curved surface, deflectors will be necessary.
The lip is the shaped protrusion above entrance at the end of the submerged ramp. The shape of the lip must give a high speed at low inlet velocities. The lip should have an airfoil shape. When increasing duct divergence it will increase the angle of attack of the lip and by increasing ramp angle decreases the angle of attack of the lip. Adding curvature to the inner surface of the lip improves flow characteristics, adding curvature to the outer surface only will increase flow loss, by adding curvature to the inside and the outside of the lip surface will benefit flow the most. The lip should have a radius to prevent flow separation. Increasing camber and nose radius will have the best effect on flow. By submerging the lip below the surface in which the entrance is placed in, will be more efficient, but if the lip protrudes slightly above the surface the effect is not harmful. Pressure distribution over the lip will be changed drastically if the ramp angle is changed.
The entrance is the opening at the end of the ramp and is below the body surface, it is the area between the ramp floor and the lip. A square entrance has greater pressure loss than a rectangle entrance; hence the pressure recovery of a square entrance is less than that of a rectangle opening. Because the rectangle entrance is more efficient, that is why we use the width to depth ratio of between 3 and 5. The rectangle air entrance is more efficient because being flat and wide lets the twin vortices function more freely and gives more room between them, in the ramp, to ram the airflow between them and into the entrance at the base of the ramp. The rectangle entrance being more shallow and wider, will allow the body of the vortices to extend above the surface of the vehicle further and hopefully through the boundary layer. The square entrance will not let the bodies of the vortices stick out above the vehicle surface as far and thus loose part of their ram affect due to the ramp being a deeper depth. If the NACA duct is placed in a situation where the airflow is ramming into the entrance due to circumstances outside, efficiency can be increased with Corner baffles, small triangles added to the top corners above the entrance, where the top of the wall meets the entrance.
The NACA submerged inlet and the rear diffuser under the rear of a lot of race cars, function in almost the same way and have a lot in common. The twin counter rotating vortices inside the diffuser ramp and having the peak negative pressure at the entrance to the ramp from the vehicle surface, the peak pressures will follow the entrance to the ramp as the ramp angles are changed and it moves the ramp entrance locations.
The NACA duct has had a bad rap for a long time by people that were improperly designing it or improperly using it. Designed properly and employed in the correct area and situations, the NACA submerged duct can have pressure recovers and velocity ratios approaching those of a conventional style external scoop with a lot less drag. The reason we do not see NACA submerged inlets in a lot of racing classes is because of their poor performance in on-off accelerator situations, so that would cover pretty much all race classes that involve cornering and braking.
Methods can be employed to further increase the airflow through the NACA duct and through the pipe and ductwork leading from the inlet. That is for another set of post at a later date, as time constraints permit.
-
I think we've got a winner -- for the longest-ever single post :-D. I'd like some input from the rest of the forum folks - - on whether I should sooner or later add this monologue to the landracing.com archive of basic background information. Is it valid stuff? I'm not well-versed enough in the field to make the judgement, so will rely upon your (all of you) opinions. Let me know - and thanks.
-
With what my limited knowledge of NACA ducts is ---I think that this is "MOST likely" All spot on stuff
-
Thanks, Sparky, and thanks to the others that told me that this stuff appears to be straight scoop (pardon the pun). I'll get it into the archives for one and all to peruse whenever.
-
Superford, do you intend to give any hints as to sizing for engine requirements for example? Wayno
-
ah these intrepid LSR racers :evil: and ( OUR) quest to cheat Mother Nature out of her DUE :cry:
-
I hope SuperFord317 took a breath in the middle. A good dissertation though. All these comments and posts are pieces of the puzzle!
NACA ducts are typically miss-applied more often than not. Any hole will let air in it's just how efficiently will it work?
Here is another dissertation with some history that will give you another good overview:
http://www.flyingmag.com/scoop-naca-scoop
As for sizing here is another: [I have one but it's not for public use, sorry! :-(]
http://www.melmoth2.com/texts/NACA%20inlet%20sizing.htm
Just connecting a round stub to a NACA duct has some issues.
-
These are by far the best definitions on NACA submerged ducts that I have seen. Superford has a knack of of drawing a good picture with words. I gathered as much information as I could find before I built the one on my lakester and used an on line drawing to scale it. I didn't really have a clue how it really worked even after I gave it the back yard tuft and weedblower test. I could see that it did work though.I just dump it in to a large air air box without the engine running. On the low blower setting every was really smooth except the tufts in the ramp had a different slightly angled pattern. It showed no turbulence anywhere. On the hi blower speed the tufts in the ramp tightened up and angled a little more toward the center at the air box end and after a few seconds the tufts at the back of the duct (on the outside of the body skin) really tightened up. I took this to mean that without the engine running the air box had built pressure and was dumping air back outside. Somehow it did that without any sign of turbulence. I didn't know why the tufts angled in the throat but thanks to Superford's reference to the vortexes involved in the ducts I think I can "see the air".
"The proof of the puddin" will be during a run when I can incorporate the extra MAP sensor I've installed to data record pressure in the air box.
Thanks to Superford for all his work, to Slim for all the info he's made available here on this forum and to all the entries the members have posted.
hitz
-
As can be seen, I hate short yes or no answers :-D.
If I answer something it will be with a full explanation, with the how’s and whys presented in a way that can be easily understood.
Currently i am working on 5 different posts for landracing at the same time and I hope to post more of them when I get a little more time, hopefully in a few weeks I will be in Cambodia and will have a lot more time to devote to all of these future posts I have started.
-
Holiday in cambodia? :-D
I bet I am one of a handfull that gets that line?
-
Woody,
Your post are always so thought provoking!!! and you always leave us a little short on what the parameters are and also how we should interpret them. I assume that the air speed is around 300 mph (440 ft/sec) and from your illustrations the inlet area looks some what larger or very close to equal to the area of the round ducting tube that it is connected to. Looking at the internal velocities it is very interesting that the center line velocity in the duct inlet appears to be zero and that there is an area in the internal duct in which the air has stagnated to zero velocity. Your public awaits your explanation.
A note to all that are following this thread, you need to read both of the attachments that Woody has provided, the first goes over some of the ground that superford posted and also provides some incite as to the proper application of NACA ducts. Because of the nature of land racing vehicles many of the parameters that would govern the use of a NACA duct on an airplane can be avoided as we are really mostly only dealing with air speeds and inlet conditions at or near the vehicles maximum speed. Obviously cars that are very powerful and need to optimize acceleration will have additional needs for an adequately designed engine air inlet.
Rex
-
Woody,
Your post are always so thought provoking!!! and you always leave us a little short on what the parameters are and also how we should interpret them. I assume that the air speed is around 300 mph (440 ft/sec) and from your illustrations the inlet area looks some what larger or very close to equal to the area of the round ducting tube that it is connected to. Looking at the internal velocities it is very interesting that the center line velocity in the duct inlet appears to be zero and that there is an area in the internal duct in which the air has stagnated to zero velocity. Your public awaits your explanation.
Rex
Rex, the purpose of the posts is to provoke everyone, but thoughtfully! :-D
We all have our thoughts, experiences and opinions but since much of what we do has interactions it is difficult to find the right combination. Even when we do we can't be sure it's the optimum. What CFD has shown me is that just about everything regarding fluid flow is case specific. That's why we need testing, experiments and nowadays simulation! Like you, when I look at these I wonder why is it doing that? I try to find experimental papers with enough info to model it and see if the model is good enough to use.
The NACA duct shots were a specific mass flow at the round duct (hairdryer inlet) with no external flow just to see what the velocity profiles would look like in the ducts [baseline]. Then we look at it with the exterior flow to see what has changed and what the pressure recovery might be. Beyond that my former Japanese coworkers had a saying when questioned, "Not need know!" (Proprietary!) :cry:
Now I have to go finish the tire stuff on Buddy's build diary! :cheers:
-
Woody:
Yeah, ditto what Rex said. I would like to understand what I am seeing. Are the inlet and outlet areas similar? And according to Bernoulli, the zero velocity (blue) areas are the highest pressures, right? The velocities appear to range from 0 mph to about 255mph. The flow appears to be stagnant (highest pressure) in the trough and in front of the inlet but with nominal speeds of about 255mph mid-stream. Then there are other speeds of zero (in the diverging cone area) and 300 mph on the opposite side of that high pressure area. Geeze, a lot is going on in there! I know Woody, you said CFD is “case specific” and now I finally see the magnitude of what you are saying. You said it was done with a specific mass flow so that would equate to some constant velocity right? The pics we see are at a single speed?
What is interesting about the CFD picture is how it points out how incredibly difficult instrumentation would be. Just look at the widely deviating pressure distribution inside. If you put in only one pressure sensor, and made assumptions from that data point, you would be clueless as to what is really going on! Even a couple of sensors in the wrong spots would be giving you very bad information. That might lead to some really bad decisions on jetting or head-scratching when reading the plugs and looking at scoop pressure in the data log.
-
I talked to an engine builder several years ago who had built an intake manifold for a medical supply salesman, for the use of 3 dozen tiny medical cameras for a year. Instaled them in intake manifolds of several different combinations they built. But them on the dyno. My question, What did you learn?
I will never forget his reply--"We learned there was more "stuff" going on in there, moving different directions than one would ever imagine and some of them were fuel dropletts larger than a quarter."
That kind of research was probabaly on the front end of "wet flow" and "charge quality" buzz words that we read about today.
-
Woody:
Yeah, ditto what Rex said. I would like to understand what I am seeing. Are the inlet and outlet areas similar? And according to Bernoulli, the zero velocity (blue) areas are the highest pressures, right? The velocities appear to range from 0 mph to about 255mph. The flow appears to be stagnant (highest pressure) in the trough and in front of the inlet but with nominal speeds of about 255mph mid-stream. Then there are other speeds of zero (in the diverging cone area) and 300 mph on the opposite side of that high pressure area. Geeze, a lot is going on in there! I know Woody, you said CFD is “case specific” and now I finally see the magnitude of what you are saying. You said it was done with a specific mass flow so that would equate to some constant velocity right? The pics we see are at a single speed?
What is interesting about the CFD picture is how it points out how incredibly difficult instrumentation would be. Just look at the widely deviating pressure distribution inside. If you put in only one pressure sensor, and made assumptions from that data point, you would be clueless as to what is really going on! Even a couple of sensors in the wrong spots would be giving you very bad information. That might lead to some really bad decisions on jetting or head-scratching when reading the plugs and looking at scoop pressure in the data log.
I hear the mental gears grinding! :cheers:
CFD visuals, like the little cameras, show that there is a lot more going on than meets the eye. [Be careful what you ask for!] But that helps you plan your experiments and where to place the sensors and what to expect them to tell you. Also to see if you have correlation in the model. Computer models are all "perfect" the real world is not! Sorry, there are no exact answers!
In this case the round duct area is equal to the NACA duct inlet area: 1" X 4" = 4 in².
Attached are some pressure plots of the same run. These were looking for the lowest pressure so you will not see the pressure field associated with the low velocity at the top of the transition duct [~2psi range]. You have to look at several resolutions to "find" what you are looking for. Also some flow trajectories colored with velocity. You will note that since we are pulling the air in that airflow at the NACA duct start is reversed! If you tested this duct on a flow bench I would expect to see this. If we start pushing the air in with a 300 mph airstream then I expect this and the downstream flow to change. Or if you did this on a flow bench and blew across the NACA with a leaf blower then we would try to simulate that condition to see if it agreed with the model.
The fun part is just trying to finger all this Subaru out! :-D
-
Thanks guys, I dug out some NACA Duct info last night after reading a TON of posts on here about them. Stainless, woody and others. I will be using one for my intake.
-
Thanx Woody, for the follow-up note and clarification. Your CFD pics have rammed the point home on how chaotic the pressure distribution really is inside of the scoop.
In Post #167 Superford says “The NACA duct forms a vortex on each wall that will force more outside air to flow into the inlet than normally would, generating a small ram effect. The two vortices improve the pressure recovery and improve air flow through the duct.”
I know what vortices looks like and their powerful effect. Somehow I don’t see how that effect on a NACA wall is accomplished as clearly as Superford is saying. I’m not doubting him at this point, but since he doesn’t post references I would like to look into this some more. Is there any chance you could explore these wall vortices in CFD and give us a pic? :wink:
-
Saltfever, there is a NACA duct forum here: http://www.landracing.com/forum/index.php/topic,4062.0.html
There are some diagrams of the vortex that is formed. I will add that to my list of CFD Subaru to do!! :-D
More eye candy - think of a vortex generator as an external NACA duct! The vortices roll outward instead of inward! :-o
-
Thanx, Woody. :-) Some interesting information in the other thread on how sensitive the location is for a NACA duct.
-
Intentional vortex generation is used in many aerodynamic environments.
Here is the exact same process used to develop increased lift at the wing root and help maintain attached flow at high angles of attack.
The fuselage chine on the F16 behaves very much like a one sided NACA scoop. As the high pressure air on the lower side of the chine spills over to the low pressure side as a "sheet" it rolls up into a high energy vortex that trails aft along the top of the wing root. This significantly increases lift at high angles of attack which would cause flow separation on the wing root if the chine did not exist.
(http://img709.imageshack.us/img709/8031/bsw2260f18.jpg)
Larry
-
Here is a head on quartering view of the vortex condensation rolling up over the top of the wing.
(http://img408.imageshack.us/img408/2682/bsw2278f16headon.jpg)
Larry
-
Forgive me for being simple but how would any of this apply to exhaust flow on a vehicle? If one was to run the exhaust out of a NACA duct would this help air flow and not interupt the boundry layer as much as if you were to just have the collector bologna cut with the fender or ran straight out the rear?
Just thinking and keeping my mind busy here at work.
Zach
-
This discussion has gradually moved to more general topics. The discussion of using exhaust flow energy to help manage air flow is pretty much confined to the first few pages of the topic. It has now broadened out to discuss related aerodynamic issues. The NACA duct and its use is highly dependent on the local flow over the cars body and how the boundary layer behaves.
Larry
-
As I set typing this, I am watching dust devils crossing back and forth over the Iraqi dessert and watch in amazement at the power they have and the size of some of them.
Vortices are generated by almost all aerodynamic devices, no matter in lift or down force. Vortices are generated by sharp edges; drop offs and where 2 sharp angles meets each other. Vortices can create more lift, generate more down force or simply generate drag and hinder vehicle performance. Vortices can delay boundary layer separation, allow a wing to have a larger operating envelope, allow a thick turbulent boundary layer to become thinner and even cause a detaching boundary layer to become reattached to the vehicle surface.
As with most anything in life, you don’t get anything for free, the same is true for Mother Nature. If you try to increase the down force from a wing you increase the drag, if you increase the speed of an object through the earth’s atmosphere an increase in drag will result and increase the aerodynamic forces on the vehicle surfaces.
Evolution in nature over billions of years has created a lot of surprises we are just starting to unravel and understand. Everything from the placoid scales on shark skin, I made a rather lengthy post about earlier, that decrease drag on sharks while they swim, to vortices that are formed on the wings of bats, bumblebees and many other creatures in nature that allow them to fly.
If air had no viscosity there would be no drag, no lift or no down force. The viscosity of the air is what makes all of this possible.
Bill Lear invented vortex generators; he used them on his jets. Vortex generators can increase takeoff weight, lower takeoff and landing speeds.
Vortices help to suppress boundary layer turbulence and will reduce the thickness of the boundary layer. The core of the vortex is not always circular and the core radius is always changing.
Some of the factors that will influence vortex development on vehicles are the pressure gradient from the outer air stream, the boundary layer on the vehicle surface, the amount of turbulence in the free stream airflow and the turbulence level in the core of the vortex. The vortex generator size has a large effect on the strength of the vortice.
As the speed of the free stream airflow increases, the vortice will be stretched, decreasing the radius.
A flat bottom vehicle in close ground proximity will generate vortices at the trailing edge of the vehicle where the upper body meets the underbody and if a rear diffuser is ran, vortices will be generated from each corner of the diffuser. If vortices are generated on the vehicle flat underbody, they will increase airfow and increase the amount of down force generated from the underbody, as the vehicle ground clearance is reduced, the vortices will increase the suction on the underbody. As the ground clearance is reduced the suction on the vortex generator will increase, creating stronger vortices but vortice decay will be greater, due to the increase in vortice speed increasing friction and energy loss, turning the lost energy into het.
We owe our entire human existence to vortices, if it were not for vortices, bumblebees would not be able to fly and there for not pollinate our crops.
Scientists for many years could not understand how a shark could swim so fast or how many of nature’s creatures could even fly, according to scientific research bats and bumblebees should not even be capable of flight. Putting these creatures in smoke chambers with high speed cameras revealed what was going on and what allowed them to fly. Vortices were the answer, that allowed them flight by increasing airflow over there wings.
Weather you realize it or not many people around the earth are affected by vortices in one way or another. Tornadoes and hurricanes are some of the most powerful forces on earth and are examples of vortices and the power they possess, good or bad.
The mushroom cloud formed from a nuclear explosion is a vortice also. High flying jet aircraft leaving visible contrails is due to vortices.
Vortices in NACA submerged air inlets, on wings in down force or in rear diffusers, all aid airflow.
In our case, vortices are the rotation of gasses around a common center.
The earths spinning motion is what gives hurricanes and tornadoes there rotation and the vortices, both in nature and manmade may become visible due to moisture condensation or when dust is caught up in the vortices. Tornadoes develop from the clouds down to the ground and the dust devils develop from the ground up.
As we understand more and more about vortices it has spawned new nomenclature and even different branches of science. A new form of drag has recently sprung up and talked about more in racing, vortex drag, as more and more devices are being used in racing to generate down force, more vortices are being formed and thus an increase in drag.
Vortices are formed off of each wingtip, in lift or down force. Twin vortices are formed in underbody rear diffusers, on each side wall of a NACA duct and even off of the front and the rear of a tire on open body vehicles.
Some of the vortices are good, helping to make boundary layers stick to surfaces longer and some are bad, only parasites that do nothing but create drag.
Vortices can be maintained on vehicle surfaces for a considerable time if surface heating, suction or blowing are used to aid them.
Some of the vortices can trail behind vehicles for quite a ways, lasting until the viscosity of the air takes enough energy away from the vortices to cause them to break down, turning the lost energy into heat.
Jumbo jets form strong wingtip vortices that trail behind the airplane and last for a considerable time, before breaking down. Small airplanes landing behind jumbo jets have crashed form the turbulence the vortices produce; there are strict rules in place that prohibit small airplanes from landing behind jumbo jets for several minutes.
Counter rotating wingtip vortices are produced by wings in lift or wings in down force. A lifting wing has a low pressure area on top and a high pressure area on the bottom, the( HIGH ) pressure air will travel to and try to mix with the ( LOW ) pressure air and that is where the vortices are produced, as the air travels over the wing tip and falls over the edge and starts swirling. The same thing happens with a wing in down force, there is a high pressure area on top of the wing and a low pressure area below the wing and the ( HIGH ) pressure air will try to travel to the area of ( LOW ) pressure at the wing tip, forming the vortice where the two pressures meet.
As a lifting wing gets close to the ground the wingtip vortices begin to break down thus decreasing induced drag. As the wing drops below one wing span from the surface, ground effects start to take over and become stronger as it gets closer to the ground, the lift to drag ratio can be increased by 50% from ground effect on the wing.
There are several different types of vortex generators that can be used on vehicles or aerodynamic devices, to create vortices, some being capable of generating multiple vortices at one time. Some are known as half delta, rectangular or wing type, are but a few of them.
The qualities of vortices generated by vortex generators can be influenced by the angle, height, air flow and edge radius of the generator.
Vortex generators can be broken down into 2 classes. Vortex generators that do not reach above the vehicle boundary layer, known as sub boundary layer vortex generators and vortex generators that reach above the vehicle boundary layer into the free stream airflow. The drag generated by sub boundary layer vortex generators will be substantially lower than the drag from the taller vortex generators.
The strength of the vortex generated by a regular vortex generator will be much more than the vortices generated by a sub boundary layer vortex generator. The strength of the vortices will be reduced over time by the viscosity of the air taking energy away from the vortices and converting it into heat.
Sub boundary layer vortex generators placed on a vehicle surface will be affected by the boundary layer pressure gradients similarly. Over a distance equal to 40 sub boundary layer vortex generator heights the vortice strength was within 3% in a zero pressure gradient and a adverse pressure gradient.
If placed on a highly angled surface, sub boundary layer vortex generators would be better than regular vortex generators. On an angled surface sub boundary layer vortex generators will have a longer life without breaking down.
CAPS IN ( ) are transposition corections PER SF 317 request ---sparky 11/16/2011
-
Shall I archive this Superford post, too?
-
Except maybe for this part: :-(
Counter rotating wingtip vortices are produced by wings in lift or wings in down force. A lifting wing has a low pressure area on top and a high pressure area on the bottom, the low pressure air will travel to and try to mix with the high pressure air and that is where the vortices are produced, as the air travels over the wing tip and falls over the edge and starts swirling. The same thing happens with a wing in down force, there is a high pressure area on top of the wing and a low pressure area below the wing and the low pressure air will try to travel to the area of high pressure at the wing tip, forming the vortice where the two pressures meet.
Air can only flow from high pressure to low pressure - it's a one way street!
http://centennialofflight.gov/essay/Theories_of_Flight/Vortex/TH15G1.htm
-
If air had no viscosity there would be no drag, no lift or no down force. The viscosity of the air is what makes all of this possible.
This is complete nonsense. As is the notion of air flowing from low pressure to high pressure. It is clear that Superford has no understanding of fluid dynamics.
It would better serve the community if all of his postings were put in one place where people that want to read the random, repetitive, inaccurate, and contradictory regurgitations of information gleaned from other sources can do so, and where such information would not contaminate authoritative contributions.
Superford’s most useful contribution would be to cite his sources, so anybody who is interested could read the information in its correct context and thus be able to evaluate its authority and applicability.
-
Thanks both Woody and IB. I'm not versed well (at all) in aero stuff, so rely on folks that are (you two, in this case) to vet the information given to help determine what should be saved in an easy-to-find location for use as reference material. So far, on this subject, we've got one "Heck, no!" and one "Well, most of it". Any others care to help decide?
-
SSS---IMO---this is a simple brain fart transposition ---shall we now call him Rick Perry lol---he had the direction correct --just miss labled high goes to low ---- didn't mention the role of spill plates or end plates on wings to stop or curtail that flow ---YMMV---
-
Thank you sparky, you took the words out of my mouth, over 30 fully typed pages on Microsoft word and I get complaints about 2 words I get backwards in my haste. I try to proof read as much as I can, but I usually skip over the simple things and concentrate on the more complicated things. I hope slim will switch the 2 words around if not, oh well. Some of the posts are so long and I try to make them as understandable as possible so I repeat things as I see necessary to make certain areas more understandable to the less initiated or to new members that want to learn about such things.
Some of the things I am working on I do because there simply isn’t good sources on line, the NACA submerged inlet as example, I simply could not find good information on them, so I spent considerable time on the subject and I think a lot of people got a better understanding of them as a result and I have gotten several E mails thanking me for my work.
As the old saying goes you can’t please all of the people all of the time, but I do what I can.
If at any time there is a lot of clamor wanting me to stop I will be more than happy to do so.
As I complete 1 post I start 2 others and sometimes it gets a bit overwhelming trying to keep it all organized and straight.
Thank you and I am so sorry for the slip up I will do my best to make sure it doesn’t happen again.
IO thank you for your input.
We all make mistakes, to err is only human, such as SSS last post, thanking IB when we all knew he was refering to IO.
Sometimes I wish I could copy and paste or plagiarize someone else, it would keep me from making mistakes like this and save me a LOT of time, research and thought.
I enjoy doing this and helping to improve other peoples knowledge also and I am increasing my understanding as I delve into some of these subjects also.
-
Actually there is excellent information on line regarding the NACA submerged inlet ducts.
The best place to go is directly to the source (ie NACA), NACA was responsible for the research that designed the ducts that now carry their name in the 1940's and early 1950's.All their technical reports are freely available on line. NASA currently runs a reports server that has all these reports but a few years ago they "improved" the online report system and it is now very difficult to find these old reports unless you know the magic search keys to find them. If you put in generic search keys in their report server you get literally hundreds of hits on common terms.
It is better to go the UK mirror for their reports that still have the old original report server structure where you can pull up reports by their year of publication and follow the development of research in chronological order. These reports also cover some very good research on engine design and detonation research and airfoil shapes during the war that is still very valuable information. Most of these reports were developed under the pressure of wartime research or during the early cold war jet development up to the Korean war, so they frequently came out with an initial report and a final report that are nearly identical, so you may find more than one report that contains essentially the same information. Some of them were also called technical notes and have TN numbers.
The easiest way to pull up these reports is to go to the UK mirror at:
http://naca.central.cranfield.ac.uk/
There you can pull up all the reports for a given year and thumb through them (this is good for days of very interesting reading) or if you have a clue what you are looking for use the search function to pull up reports that contain your search terms.
In the case of the NACA ducts, at the time they were doing the research they were called "submerged inlets" and "low drag inlets" Those search keys will pull up over 20 different reports about half of which cover inlets operating at low supersonic speeds and above, and the other reports at low to high subsonic speeds.
Best to read them in chronological order (year or publication is in the file name) for NACA ducts you can start with these reports.
http://naca.central.cranfield.ac.uk/reports/1945/naca-acr-5i20.pdf
http://naca.central.cranfield.ac.uk/reports/1948/naca-rm-a8i29.pdf
http://naca.central.cranfield.ac.uk/reports/1948/naca-rm-a8b16.pdf
http://naca.central.cranfield.ac.uk/reports/1948/naca-rm-a7i30.pdf
http://naca.central.cranfield.ac.uk/reports/1949/naca-rm-a9f16.pdf
http://naca.central.cranfield.ac.uk/reports/1950/naca-rm-a50e02.pdf
http://naca.central.cranfield.ac.uk/reports/1951/naca-tn-2323.pdf
Do not go to this web site unless you are willing to lose the better part of a month reading fascinating reports!!!
Larry
-
The "fix" is in :-D
-
WOW thanks Larry!!
-
Thank you sparky, you took the words out of my mouth, over 30 fully typed pages on Microsoft word and I get complaints about 2 words I get backwards in my haste. I try to proof read as much as I can, but I usually skip over the simple things and concentrate on the more complicated things. I hope slim will switch the 2 words around if not, oh well. Some of the posts are so long and I try to make them as understandable as possible so I repeat things as I see necessary to make certain areas more understandable to the less initiated or to new members that want to learn about such things.
Some of the things I am working on I do because there simply isn’t good sources on line, the NACA submerged inlet as example, I simply could not find good information on them, so I spent considerable time on the subject and I think a lot of people got a better understanding of them as a result and I have gotten several E mails thanking me for my work.
As the old saying goes you can’t please all of the people all of the time, but I do what I can.
If at any time there is a lot of clamor wanting me to stop I will be more than happy to do so.
As I complete 1 post I start 2 others and sometimes it gets a bit overwhelming trying to keep it all organized and straight.
Thank you and I am so sorry for the slip up I will do my best to make sure it doesn’t happen again.
IO thank you for your input.
We all make mistakes, to err is only human, such as SSS last post, thanking IB when we all knew he was refering to IO.
Sometimes I wish I could copy and paste or plagiarize someone else, it would keep me from making mistakes like this and save me a LOT of time, research and thought.
I enjoy doing this and helping to improve other peoples knowledge also and I am increasing my understanding as I delve into some of these subjects also.
Superford317 - sorry - it really wasn't a complaint just a comment. When we talk about complex phenomenon we can sometimes forget to reinforce the basics and then we tend to wander off topic like some of these threads! :-D
I have my own Superford317 file that I read and reread. This stuff is not easy to cogitate or verbalize! :-(
But to IO's point, some useful references or illustrations would help everyone.
To all - keep it coming - these are some of the best forums on the net! :cheers:
-
Ahmen Brother PREACH ON!! as the hung over Southern boy might say from the back bench on Sunday morning!!!! :cheers:
-
Me make pointy car, exhaust out back, drive fast. :-D
-
"The mushroom cloud formed from a nuclear explosion is a vortice also. High flying jet aircraft leaving visible contrails is due to vortices."
Out of Superford's whole post, the first thing that caught my eye was this. Dunno why. BS! I said, a mushroom cloud is just like a cumulus, a simple rising column of (extremely) hot air. Visible high-altitude contrails are condensation from the exhaust, not the same as the teeny 'contrails' pictured in a previous post that occur under the right temperature and humidity conditions by vortex generators or simple high-to-low (I can understand the mixup there) flow at a wingtip.
Then I read the post (a good place to start, eh?). And my fluid dynamics education is way too rusty to say yay or nay on some of it, but the aerodynamics as it relates to airplanes is spot on. I'll give it a thumbs-up.
-
Amen brother “sparky”, I’m preaching to the congregation but that old devil rears his ugly head, aint that right brother “interested observer”
As hard as it is to believe, maybe I should have made my last post on vortices lengthier, so there would not be so much confusion, I could have explained more things in detail I suppose.
“bobc” you being from New Mexico you may be eligible for part of “the radiation compensation fund” since 1990 more than 1.4 billion dollars have been paid out to people that were exposed to radiation from government atomic bomb testing. The first atomic bomb ever detonated was named “the gadget” and the test site was named “trinity”. The location was near Alamogordo, New Mexico.
It was detonated 100 feet above the ground on a tower July 16 1945 at 5:29 mountain time and left a crater of glass 10 feet deep and over 1000 feet wide, the shock wave was felt over 100 miles away and the mushroom cloud reached 7 miles high. “The Gadget” only had an explosive yield of 20,000 tons of TNT, later you will see why I said “only” 20,000 tons.
The mushroom cloud is actually called a pyrocumulus cloud, “pyro” meaning explosion.
Mushroom clouds can be formed from any large explosion. I have witnessed them personally several times here in Iraq.
During the explosion a large mass of low density hot gas is created. The hot gas will rise very quickly.
The cap or crown of the mushroom, the round distinctive cloud on top of the stem, is caused by the rise and rapid expansion of the super-heated gasses forming vortices.
The entire cap or crown of the mushroom cloud is referred to as a vortex ring, if you are looking at the cloud, the right side is clockwise rotation and the left side is counter clockwise rotation. The massive vortices form a tremendous suction between them, drawing up dust, debris and smoke, that is what forms the stem of the mushroom, extending from the ground to between the counter rotating vortices.
Similar to the NACA duct, forming counter rotating vortices on its side walls, which greatly enhance airflow into the duct. The vortices draw in the airflow, down the diffuser ramp.
Nuclear explosions are detonated above the ground to get the maximum effect from the shockwave and the fireball, if they were detonated at the ground surface like most conventional bombs, the ground would dissipate a large majority of the explosion.
Nuclear explosions in space or very high in the atmosphere where there is little air, would only have a round circular shape, No stem and no vortices or vortex ring.
The most powerful weapon ever tested by mankind was detonated by the Russians on October 30 1961.
The AN602 was a 3 stage hydrogen bomb nicknamed the “Tsar Bomba” its explosive yield was 50 megatons, equivalent to 50 million tons of TNT, 10 times the power of all conventional explosives used in WWII combined. Dropped from an airplane, it detonated 2.5 miles above the earth’s surface using barometric pressure sensors.
The fireball was seen 620 miles away. The mushroom cloud was 40 miles high and 25 miles wide. Brick buildings 34 miles away were totally destroyed and 3rd degree burns would have happened 62 miles away. A shockwave was seen in the air 430 miles away. The shockwave was still measured on seismographs on its 3rd passage around the earth. Windows were broken in Norway and Finland.
As built, the AN602 “Tsar Bomba” was designed to have double its as tested power, the test was at 50 megatons and it was built to be 100 megatons. Part of its Uranium 238 was replaced with lead to limit the power of the detonation, to limit the nuclear fallout. Lead is useful for other things than ballast and fishing sinkers I suppose.
It is unknown if any were ever built, but it was theorized and could have been built in the 1960s, known as the “Doomsday” bomb, A single large hydrogen bomb impregnated with cobalt, if detonated it would have killed every living thing on earth, unless you were buried deep underground and remained there for 5 years, essentially sterilizing the Earth from massive amounts of radiation.
Knowing the military and the intelligence of world leaders, what do you think, were they built?
An atomic bomb is very limited to the explosive yield it can be built to, where as a hydrogen bomb has no limit and can be built as powerful as desired, it takes an atomic bomb to trigger a hydrogen bomb. An atomic bomb is the splitting of atoms or “Fission” as in nuclear power plants and a hydrogen bomb is the combining of atoms or “Fusion” as powers the sun.
Our nuclear strategy was known as “MAD” mutually assured destruction. MAD indeed.
A major portion of the “Manhattan Project”, the development of the American atomic bomb in WWII, was carried out not far from my home, in Oak Ridge Tennessee.
Well, with the mushroom clouds and vortices taken care of, now on to jet contrails and vortices.
As was discussed in my last post on the vortices, the wing tip vortices can last for a considerable time behind the airplane.
The temperature and air pressure is reduced in the vortices and can cause moisture in the air to condense and cause the vortices to become visible.
Perhaps, in my post on vortices, I should have said, at certain times jet contrails at high altitude are visible, due to vortices.
I enjoy watching F1 races, that are ran in the rain and watching the vortices from the rear wing trailing behind the vehicles.
Now, back to the 6 other posts I am presently working on, in my meager spare time, for land racing. I think they will be very liked and informative to a LOT of people.
-
:cheers: how do you spell waiting with baited breath????
-
I'll agree with Sparky, Mr. Ford. While your essay is interesting -- it doesn't have much to do with land speed racing other than getting the subject from another post. A few months ago we had quite a long thread going about a non-racing event - the tsunami and following issues with the nuclear generating station in Japan, and I let it go on since there was such widespread interest and many of our Forum members had cogent information to share. But your post, being history, isn't all that pertinent to this Forum. I'd therefore thank you to keep such posts short (if you elect to make any off-topic posts at all), and if you must go on at such length as you have (above) -- post a precis here and put the entire thing someplace where interested Forum members may find and read it.
Thanks, nonetheless, for the full story. Too bad it isn't racing stuff. . . :-(
-
Thank you for this wonderful site “SSS”.
When I attempted to keep the subject short and mention 2 examples in my post on vortices and aerodynamics I was called out, “BS”, so I attempted to give the full story so that it could be understood much better by our members.
I will try to refrain from quite so much detail in the future I suppose, if this has to be done again.
I need to get a portable scanner to use here in Iraq I suppose, I can include hand drawn pictures, one picture is worth a thousand words, as they say :-D
-
Because the airflow patterns are so different between open wheel and closed wheel vehicles, this series of posts will deal with open wheel vehicles, specifically the air flow around the tires. Hopefully at a later date we can talk about airflow patterns around the tires on closed wheel vehicles.
A French inventor and engineer Nicolas Cugnot built the first known self-propelled mechanical vehicle in 1769. His creation was steam powered and made use of one of the first known methods to convert reciprocating motion to rotary motion.
Viktor Schauberger from Austria was the first person to recognize vortices in nature.
Frederick Lanchester from Great Britain, made the first full descriptions of lift and drag and made models of vortices that occur behind wings.
Charles Goodyear invented the rubber vulcanization process in 1844.
Robert Thompson invented the pneumatic tire in 1846.
Tires with beaded edges were first used in the United States in 1892.
The Michelin brothers made the first pneumatic automobile tire in 1895.
Wheels for land speed racers have been everything from stock tires and rims to specific built race tires and rims to forged aluminum alloy rims with carbon fiber wound around them.
The thrust driven vehicles will usually have their tires and rims made of billet aluminum with carbon fiber wound around them, most rubber tires would disintegrate at the speeds these vehicles operate at.
Wheels and tires ran in open airflow will have very unstable airflow that moves from side to side as the tire rotates and will have large areas of airflow unsteadiness and separation. The turbulent airflow behind the tire is controlled by counter rotating vortices.
The tire sidewall profile, shape of the contact patch and if the wheel tire combination is running covers over the axle side and the outer side of the rim, will have a large influence on the airflow around the tire. The tire air pressure will influence the airflow around the tire also, as it will affect the shape of the tire and the contact patch. Wheel camber angle will affect the airflow and the vortices around the tire due to changing the tire deformation.
Most compressed air sources will contain moisture that will increase the expansion rate of the compressed air in the tires as they are heated. Dry nitrogen will expand at a lower rate than the moist air and hold the set tire pressure longer than the air. A drop of 25% in tire pressure will reduce the tire life by 15%. Nitrogen makes up 78% of the earth’s atmosphere by volume. Increasing the pressure in the tire will decrease rolling resistance and on hard surfaces will decrease stopping distance. Tires are permeable to air and will lose pressure over time. Severely overheated tires can give off flammable gasses and it can react with the oxygen in the air used to pressurize the tire and cause an explosion to occur, the use of nitrogen to pressurize the tire will stop this from happening.
Tires have a fixed shape and bad aerodynamics. The main function of a tire is to put the power to the ground and not aerodynamic properties. On an open wheel car the tires will typically account for 30% to 60% of the total drag on the vehicle and generate a large amount of unwanted lift also. The amount of lift the tire generates will depend on the size of the tire. The drag of the tire is proportional to the tire frontal area, so reducing the tire size will reduce the amount of lift and drag generated. Tires and wheels on open wheel cars will generate more lift than drag. The tires can also greatly influence the airflow around the rest of the vehicle also, including the underbody and brake cooling as well as wings and upper body surfaces.
The atmospheric air pressure at sea level is 14.7PSI. Water vapor weighs less than dry air. Moist air has a lower density than dry air. Drag and down force will increase as the air density increases. As air temperature increases air viscosity will increase also. As air density increases its viscous effects will increase.
Be sure the tires are spaced far enough away from the body, so the turbulence they produce will not interfere with the airflow along the body. By keeping the tires spaced at an adequate distance from the body, the air will have enough room to smoothly flow between the body and the tires, without the two air layers interfering with each other. If the two air layers were allowed to meet, the total drag generated would be greater than the sum of the two. The air turbulence from the tire, will usually cause the wake to spread out from 14in to 18in on each side, behind the tire, depending on the size of the tire and the speed. Because of interference from the axle-spindle side of the tire, the vortice and ensuing wake generated from the inner side of the tire will be less than the outside.
As the track width of the wheels is set wider it will give better stability but will affect spring rate and dampening. The narrower the track width of the wheels it will firm the suspension and the wider the track width it will soften the suspension.
Having the same track width for the tires, front and back, will aid in reducing drag. The front tire will break the air and the rear tire will pass through the hole the front tire made more easily, reducing the drag on the rear tire. Like cars drafting or bicyclists following behind each other, the airflow is only disturbed once. In bicycle racing the rider produces 70% to 80% of the drag. A bicycle racer drafting the racer in front of him will exert 30% to 40% less energy.
If it is a grooved tire running on a hard surface, there will be very little airflow passing between the tire grooves and the ground.
At high speeds, aerodynamics will affect the vehicle speed and acceleration rate more than any single factor in a LSR vehicle. The front surface of the tires is pushing against the air. The forward acceleration of the air generates drag. As the tire passes through the air it leaves a void behind the tire that that air moves into from all directions to fill. The air can’t move backwards through the tire and the air can’t move upwards through the ground. This results in forward drag and due to the forward and downward acceleration of the air in front of the tire, lift is generated because of a high pressure area at the base of the leading edge of the tire, just in front of the contact patch at the ground, as the surface of the tire moves toward the ground and the contact patch it moves energy to the stagnation point and increases the pressure as the speed increases. Because of the tire rotation it will cause separation of the airflow at the top of the tire, earlier than would normally be expected. Due to the separation of the airflow at the top of the tire generating a low pressure area and the high pressure area at the base of the tire, lift will be increased more than normal. There will be a negative pressure at the backside of the tire, due to the void. At the rear of the tire, at ground level, the airflow will stagnate at the contact patch near the center of the tire and move to the left and right, splitting to each side of the tire. The airflow in front of the tire, then moves up the tire surface and towards the center of the tire due to the low pressure area located there. The high pressure at the front of the tire and the negative pressure at the rear of the tire will generate a pressure drag due to the differential pressure.
Because the tire is a circular shape, the air will treat the tire as if it were 2 different halves. The air hitting the front of the tire will move down from the center and the top half will move up from the center and over the top of the tire and separate from the tire surface. The air moving down from the center will strike the tire ground contact patch area and build up pressure.
Due to the rotation of the tire, the air flow in front of the tire goes down towards the ground and as a result, the stagnation point will be lowered closer to the ground and will continue to be lowered as the tire rotation speed increases.
Airflow around the tire and through the rim will depend on if the tire and rim combination are running wheel covers on the front and backside of the rim. Using wheel covers on both sides of the wheel, will reduce the drag by up to 25%. The wheel covers will stop the airflow from passing through the wheel from one side to the other and make the aerodynamics better around the tire also.
There will normally be 3 sets of vortices coming off of the tire, trailing in the wake behind the tire, they will be counter rotating.
There will be a very small counter rotating vortice generated from each leading side of the contact patch and wrapping around the bottom of the tire and down each side, at ground level, following the shear layer into the wake behind the tire. The high pressure in front of the tire at the contact patch and the lower pressure at the sides and rear of the tire will cause the air to move laterally to the side of the tire and speed up, this is called “jetting”.
The second set of vortices are the largest and most persistent in the wake behind the tire. They will be generated at the center front of the tire, at the leading edge, where the airflow splits and travels down the front and up across the top. The vortices will wrap around each side of the tire. One vortice from the front center to the axle side and one from the front center to the outer side. Each vortice will be counter rotating, traveling from the center of the tire around the rear sidewall and into the wake behind the tire. The outer vortice will have a clockwise rotation and the inner vortice will have a counter clockwise rotation. The vortice on the outside of the tire will be larger and longer because it is in open airflow. The vortice on the inner side of the tire will be smaller in size and shorter in length because of the axle tube and the spindle affecting the airflow around the inner side of the tire. These 2 vortices will dissipate slowly as they travel downstream in the wake, and will last for a considerable time. The Separation of the airflow at the leading edge of the tire causes an area of reverse airflow on the axle side and the outer side also. The separation will follow the curvature of the tire and will cover about 30% of the axle side and the outer side of the tire and rim surface. The airflow from the leading edge of the tire accelerates as it crosses over the face of the tire and moves toward the sidewall. Because of the airflow acceleration it will bypass the edge of the tire and create an area of reverse airflow with a weak velocity.
The third set of vortices will come off of the top of the tire, travel partially down the back of the tire before separating and traveling off into the wake behind the tire. The airflow forming the vortices off of the top of the tire will separate, because the air is slowing as it crosses the top of the tire and starts down the backside. These vortices will dissipate quickly, as they merge with the other vortices in the lower wake.
On a tire and rim with no wheel covers on either side, running on an open wheel vehicle, air will enter on the wheel axle side because of the closure of the airflow separation and circulate through the holes in the rim and then move downstream on the outside of the rim because of the low pressure area being generated in the wake on the trailing edge of the tire, creating a suction. Some of the air flowing from the outer, center of the rim, will partially go towards the area of reverse flow on the leading edge of the tire, but most of the airflow will go into the low pressure area behind the tire at the trailing edge.
Starting in 1991 air deflectors were used on F1 cars to blow air into the low pressure area behind the front tires, to lower the drag generated by the tires. In F1 the open tires generate 50% of the drag for the entire vehicle. A good example to look at, for the front air deflectors, would be the 1993 McLaren MP4/8 F1 car.
Because of the rotation of the tires, the vortices they generate and the pressure differentials created, the tires can have a large influence on airflow around and underneath the vehicle if they are not kept at a distance from the vehicle body.
The tires generating a wake at their backside, creates a negative pressure, which can pull airflow from under the vehicle. This can lower down force, if the vehicle was set up to generate down force from the underbody.
The tires generate so much drag it would be worthwhile to add some air deflecting devices to channel some airflow into the wake behind them, to reduce the negative pressure and reduce the drag being generated. Some engine exhaust, if rules permit, can be redirected into the wake behind the rear tires to lower the size of their void. The air deflectors will add some drag but at a cost of having a lower overall drag due to the tire drag being decreased.
-
Is the greater gain above or below the rear axle????? Inquiring minds need to know????
well at least a go faster want-a-B :roll:
-
Gains can be made from above center or below center of the wheel, but the largest gains will be made below center as that is the area of highest pressure and lift.
-
Is the greater gain above or below the rear axle????? Inquiring minds need to know????
well at least a go faster want-a-B :roll:
I think not :roll:
Nice post on tires....keep it coming, would be even better with hand drawn pictures....please :-D
-
but I do want to go faster :-P
-
Thank you SF for the tire/wheel lessons. I run a small displacement motorcycle and reasoned that the narrowest front tire would have the best chance of slipping through the air. I opted for a 21-inch Avon with ribs thinking the downforce air on the tread would have some amount of clearance to slip through the contact patch. It seems to be working but I have yet to surpass 84mph!
Your info is supporting an improved front fender design. My current fender is from a 26-inch bicycle to match the tire width. As I add skirts, I will try to direct air 'through' the fork clearance toward the lower rear of the tire. All suggestions are welcome as I do intend to gain a few miles per hour as I lose weight and build motor power.
-
but I do want to go faster :-P
I ment you are NOT a want a B.... of course you WILL go faster, it is inevitable :-D
-
Ol Scrambler, the AMA rules and a lot of others limit the amount of wheel circumference that the front fender can cover. A wedge shape can help to split the air and to direct it around the sides of the bike. It can be an asset in partial streamlining.
-
My last post was about the surrounding airflow and aerodynamics of an open exposed wheel mounted on a 4 wheel vehicle, its effects and how it interacted with the vehicle. Due to the size and shape of a motorcycle and the size and shape of its tire and rim, its wheels will act much differently and the surrounding airflow will be influenced in different ways than a 4 wheel vehicle. It would help to review my earlier post on exposed vehicle wheels before reading this post.
This post will pertain to a motorcycle in straight running with the following conditions, a front wheel with spokes, no front fender, telescopic front fork and a rear suspension. We will look at the way aerodynamic forces interact with the front wheel, how it affects braking and the stability of the vehicle.
The world’s fastest single person, human powered, unpaced vehicle record belongs to a Canadian cyclist, designer and builder, Sam Whittingham. Sam is the holder of several world records, on September-18-2009 he pedaled a streamliner recumbent bicycle named Varna Tempest, 82.8mph for 660ft with a flying start. Sam also holds the 1 mile flying start record set on June-10-2001 at 78.6mph. Thus far, Sam is the first and only person to break 1/10 the speed of sound, human powered, when he pedaled to 82.3 mph on September-18-2008.
On a motorcycle the main contributors to overall drag are the rider, the cooling radiator, the fairing and the wheels.
Aerodynamics are more important at high speeds than power, because the power required increases with the cube of the speed and increases in speed can be more easily attained by improving aerodynamics rather than adding horsepower. At high speeds the aerodynamic forces on the motorcycle have the largest influence on its top speed and acceleration. These forces are the pitching moment (nose-up and nose-down rotation of the bike), yawing moment (rotation to the left or right about a central vertical axis) and rolling moment (roll to the left or right in a crosswind) as well as the side force, lift and drag are the combined forces that determine the aerodynamic loads on the motorcycle and will influence top speed, acceleration, crosswind handling and stability. Normally, a motorcycle with less rolling and yawing will move around less in crosswinds and if it has less front end lift will normally respond better to steering movement at high speed and be more stable. A motorcycle that is taller will normally have more front end lift and an increased rolling moment.
The motorcycle will have 3 to 6 times the power to weight ratio as most vehicles. The tire and wheel is the most important part of the entire vehicle, everything else is built around them. Most of the braking force is thru the front tire and the acceleration forces are thru the rear tire. If a tube is installed inside the wheel tire combination it will cause an increase in the rolling resistance and build up more heat inside the tire, if possible a tubeless tire would be much better. A radial ply tire will generate less heat and normally be lighter weight than a comparable diagonal cross ply tire. Under acceleration power is applied thru a forward direction in the tire and applied in a rearward direction under braking conditions. Due to sidewall flexibility the tire can make up 8% to 10% of the absorber and suspension system and this can be tuned with air pressure to increase or decrease the flexibility. A new tire will have more rolling resistance than a used tire, if the tire is worn 50% to 60%, it will have about 20% less rolling resistance than when new. Tire cord stresses account for about 30% of the rolling resistance and rubber compound elasticity accounts for about 10% and tread compression and flexing make up about 40% of the resistance. Rolling resistance of a cross ply tire will be greater than that of a radial tire. Lighter wheels and tires will improve steering and suspension response because they are part of the unsprung weight. There will be a self-aligning force generated at the center of the contact patch due to the force trailing behind the pivot point. Depending on speed and lift forces generated on the front tire the amount of trail will change and this will influence the characteristics of the steering. Under normal operating conditions the tire will bulge at the side wall at the ground contact patch, due to this, the side wall will approach the ground at an angle and this will cause the greatest force to be applied at the edge of the tire and the leading edge of the contact patch while the forces will be less at the center of the contact patch. To help them bank around curves, the tires have a rounded cross section. Compared to the average car, a motorcycle will apply double the power to the ground for the same size contact patch with much less weight to generate traction, but the motorcycle has a small fraction of the frontal area as a car and thus generates much less aerodynamic drag for the tire contact patch to have to overcome at high speeds. Due to the motorcycle tire being smaller and very rounded, as compared to a car tire, it is much more aerodynamic than a car tire, it will generate much less lift from the base of the leading edge, at the contact patch, because there is less surface area for the pressure to act against and generate the upward lift, but mainly due to the tire being convex and very rounded, it will bleed the high pressure air off to the outside edges of the tire before it could build up very high and generate very much lift. Because the tire is convex and very rounded, the high pressure stagnation point at the front leading edge of the tire will be much smaller than normal and not generate as much drag, due to the high pressure air bleeding off around the front and over the sides of the tire.
I do not know if rules would permit it but if motorcycle tires could be fitted to the front of an open wheel car it would lower the amount of lift and lower the drag forces also. If motorcycle tires were placed on the front of a vehicle they would make it more susceptible to side winds and lower the braking force of the tires due to having a much smaller contact patch and the very rounded contour.
To determine how the air flow around a front wheel of a motorcycle can affect performance, the whole vehicle package should be looked at. The front wheel is the part of the vehicle that the oncoming air sees first. The aerodynamic performance of components behind the front wheel will be affected by events occurring upstream at the wheel. The drag associated with the rider, assuming that the riding position has been optimized, is dictated by the fixed size and shape of the rider. The design of the front wheel can be optimized to improve overall aerodynamic performance and handling characteristics, the optimum wheel rim depth and cross sectional profile, spoke count, size and shape, the location and shape of the caliper and the shape and size of the brake rotor, tire air pressure, wheel diameter and tire size will all effect airflow around the wheel. The optimization of the front fork, frame and brake caliper for the individual wheel used on the vehicle would lead to performance improvements also. The rear wheel is designed to withstand a combination of loads and a driving torque that is needed to transfer the force from the hub to the rim of the wheel which propels the vehicle forward, as well as the rim of the wheel also supporting the tire and the weight of the vehicle. During hard acceleration, for maximum tire grip, the rear wheel will have to spin a little faster than the actual road speed; the tire slip will be about 10% to 15%. The spokes of the front wheel, while very small in area, do account for drag forces which are comparable to those of the wheel hub, though the spokes have no impact on vortex shedding from the wheel. The air flow around the center wheel hub will be greatly impacted by the vortices from the upstream leading edge of the wheel. The highest drag occurs on the leading edge of the wheel. At the top of the wheel the drag forces will be close to zero and will actually create lift in this area due to early airflow separation, because of the forward tire rotation, causing a low pressure area. Variations in the track surface will lead to vertical movement of the wheel, thus varying the tire contact patch area and affecting its aerodynamics due to changing the shape of the contact patch.
Tire air pressure can have a big effect on how the vehicle reacts to lateral side winds and gusts, if air pressure is to low it can allow the vehicle to move about quite a bit in side winds. As the air pressure is raised in the tires it will increase the lateral stiffness and help to prevent this unwanted movement. The tire rolling resistance will increase by 1% to 3% for a 1 psi reduction in tire pressure from the recommended standard pressure settings. Rolling resistance increases with tire width and decreases with wheel radius, nearly 50% of the tires rolling resistance come from stresses in the tire belts or cords and the elasticity of the rubber compound. The aerodynamic drag contribution from the wheels will be 15% to 25% of the total drag of the vehicle.
A small rider will have an advantage over a larger rider; if you're big it will be harder to tuck in, causing extra drag, the smaller rider will have an advantage. Wearing tight clothing will help to reduce the "balloon effect," and can provide a reduction in drag. The rider refining and perfecting their riding position will have a big impact on aerodynamic forces of the vehicle, decreasing drag and increasing stability. It is important to work on riding position if you want to go fast, neatly tucking in behind the wind screen or arching the back of your body up in the air a few extra inches to improve airflow and keep it attached to the back of the vehicle, reducing the low-pressure areas behind the fairing will get that last little bit of speed from the chosen combination of parts. Adjustments to the drivers’ position, even minor ones, can result in a decrease of the aerodynamic drag, which can lead to what can be considered as big performance gains once you are approaching 200mph.
Below 60mph to 70mph the tires rolling resistance and friction from the drivetrain are the primary force your vehicle will have to overcome to maintain speed. Over about 70 mph the increasing aerodynamic drag is the primary factor in the top speed and acceleration of your vehicle. To go faster you can add more power, which is not very effective in small amounts at high speeds, reduce the frontal area or reduce its drag co-efficient. If you wanted to double your speed, you need to increase the power of your engine eight times. It is much more effective to try and reduce drag than to add horsepower, especially at very high speeds where added power in small amounts will not be very effective but small changes as far as drag is concerned will see much greater returns on performance.
Side winds will produce a torque that can make the vehicle fall over away from the wind, to maintain balance it will have to be leaned into the wind. The amount of lean will depend on the location of the center of pressure and the center of gravity. The center of pressure is the point through which the wind forces on the side area of the vehicle act. Centre of pressure should be behind the center of gravity, on a motorcycle you will be very limited but the greater the distance between the center of pressure and the center of gravity the more stable it will be in side winds.
The side force contributions by the wheel components will have the hub contributing the least to overall side forces. The aerodynamic side forces of the wheel will usually increase in a near linear fashion with increasing yaw of the wheel. Aerodynamic forces changing with the yaw angle of the wheel and creating more lift will only act on the wheel and do not generate lift on the hub or spokes. The aerodynamic forces will always act downward on the hub at most yaw angles. The aerodynamic side forces are the greatest on the trailing edge of the wheel for all yaw angles and will increase with the speed of the vehicle. These increases in aerodynamic side forces on the trailing, inside part of the wheel will have an effect on the handling and maneuverability of the vehicle at high speeds. There is an area of transition where the aerodynamic forces increase unwanted lift at low yaw angles, leading to problems with control and maneuverability. There will be an area of high pressure generated in the leading edge of the wheel at the ground contact patch that will create lift and due to airflow separation creating a low pressure area at the top of the wheel generating lift also and a low pressure area at the rear trailing edge of the tire generating drag also.
By increasing the yaw angle of the front wheel will have a strong influence with the amount of flow separation occurring on the trailing edge, side of the wheel. The air flow can be attached to the wheel and start to become detached as the yaw angle increases. The drag will increase gradually with increasing yaw angle. There will not be much difference on the drag of the wheel whether the fork is present or not. The passage of the wheel spokes will have a strong effect on the fork drag. The rim will interfere with the drag on the fork but the fork has little effect on the drag of the rim.
as the yaw angle increases, a clockwise (as viewed from behind the wheel) vortex coming off of the outer edge of the wheel, and a counterclockwise vortex coming off of the inner edge of the wheel rim on the suction side. Both vortices, once separated will move in a downstream direction, being carried along by the surrounding flow. These vortices will be carried along with the forward rotation of the wheel. For most yaw angles there will be a vortice generated at the top of the wheel from the impact of the air being dragged forward by the outer edge of the rotating wheel into the oncoming free stream flow. In the upper part of the wheel, airflow passing the outer edge of the wheel will either join the vortices already there, or be dragged along downwards, following the rotation of the wheel. As the yaw angle increases the vortex will roll down along the front of the wheel, extending from the upper area of the wheel down to the ground.
Due to the front tire rotation towards the ground, the differences in the air speed at the leading edge of the wheel will be at its highest at the top of the wheel, and decreases to a minimum at the ground contact patch. At the center leading edge of the wheel the air flow speed will be zero causing a pressure build up and a slight lift generated due to causing a rearward rotation of the motorcycle at high speeds. At the ground contact patch the air flow speed is zero and the pressure will be high generating a lift there. In the upper area of the wheel, airflow shedding from the inner rim occurs sooner due to the greater difference between the wheel speed and the oncoming air. As the difference in speed decreases, the shedding of the vortices from the inner rim is delayed. The timing between the shedding of the structures will be the lowest at low yaw angles and increase shedding as the yaw angle increases.
Along the trailing rear area of the wheel will be another pair of counter-rotating vortices with one being generated by interaction of the flow being pulled along the inner edge of the wheel and the oncoming air, and the other resulting from the flow on the outer edge of the wheel meeting the oncoming air. Flow in this area of the wheel will be highly disrupted, due mainly to the vortices from the upstream areas of the wheel.
The amount of tire wear, tire construction and tire pressure will affect the handling and stability of the vehicle. With the addition of a streamlined fairing it can lead to a high speed instability on what was a stable vehicle. Severally worn tires can cause a handling problem at higher speeds due to the non-uniformity of the tire. As the tire width increases there will only be small changes to the camber stiffness.
Many stability problems of the vehicle will be due to gyroscopic force and how it relates with the steering. Wobble is the rotation of the front steering turning relative to the rear frame, wobble is a steering oscillation and weave is oscillations of the vehicle that involves steering, roll and yaw and is a fishtailing-type motion that is normally a high speed problem. The flexibility of the frame and forks will have an influence on the speed that the wobble will occur at; normally the more flexible the forks and frame are, the lower the speed wobble will occur at. Weave will becomes more likely when the rear wheel loading increases, or with a higher aerodynamic center of pressure, and wobble will be more likely when front wheel loading increases. Under acceleration at high speeds, the vehicle is much less likely to have front end wobble but more likely to weave due to the aerodynamic loading of the rear wheel. Under hard deceleration the chance of wobble increases and weave will normally not be affected by the deceleration. Wobble becomes more likely under braking and the effects become exaggerated as the deceleration rate increases, the driver tensing his arms and upper body will have little effect on controlling the wobble once it starts. Vehicle weave can be influenced by vehicle weight, wheel base, driver posture and weight. The driver shifting his upper body weight forward can reduce weave. A lighter driver will have more instability problems than a heavier driver. A light driver will have more problems with weave caused by the track surface than a heavy driver. Weave will be relatively unaffected by braking and is affected most at very low speed. Wobble will be caused by the design of the front and weave will normally be caused by both wheels. Tuning the front suspension dampers will not have much effect on wobble but a front steering damper will greatly decrease wobble. The stiffer the vehicle frame is, the more prone it will be to high speed wobble. Weave can be decreased by increasing the damping on the front shocks of the vehicle. By increasing the rear spring rate, adding a steering damper or increasing the force of the steering damper will increase the chance of weave occurring.
Not that we need to worry about it, but under high speed cornering conditions both weave and wobble will be more likely to occur due to the increased tire loading.
Under acceleration the top of the drive chain will be in tension and under engine braking deceleration the bottom of the chain will be in tension. The aerodynamic drag will tend to load the rear wheel, while lightening the load on the front tire. The forces in deceleration are equivalent to the gravitational forces of going downhill and will react the same way.
Aerodynamic drag will do most of the high speed braking, but this task is taken over by the brakes as the aerodynamic drag decreases. Aerodynamic drag will tend to lighten the front wheel and load the rear wheel. As the speed of the vehicle decreases the brakes will take over and do the majority of the stopping. As the aerodynamic braking force decreases the load on the front tire will increase and braking force will have to be increased to have the same level of deceleration. Under deceleration, as the aerodynamic drag decreases, the weight on the rear tire decreases so the front will have to do the majority of the braking. With most of the vehicle weight being on the front wheel most of the braking will be accomplished by the front wheel. Braking with the front wheel at low speeds will have a greater chance at causing wobble so rear wheel braking should be used at very low speeds.
-
Superford, not so fast here, I quote from your last post: "Aerodynamics are more important at high speeds than power, because the power required increases with the cube of the speed and increases in speed can be more easily attained by improving aerodynamics rather than adding horsepower." If you look at the basic formula to estimate horse power required for a vehicle with know aerodynamic characteristics you will see it is:
bhp = Cd x A x V(to the third power)/146600; Cd is the coefficient of drag, A is frontal area in square feet, V is velocity in mph and 146600 is the universal factor that makes all of the dimensions (mph, square feet etc) work out. This formula is from "Competition Car Aerodymanics" by Simon McBeath. If you look at the formula you will see that any change in Cd, frontal area or bhp will have exactly the same affect on the maximum calculated velocity. A 50% reduction in either Cd or A will have the same affect as a 50% increase in bhp on the achieved velocity. I am not trying to "nit pick" your dissertation only trying to make sure everyone understands the various importance of the parts of the "required horse power to go fast" formula. Aero is not the most important, it is equal to drag coefficient (Cd) and brake horse power. If you have enough bhp you can go pretty fast regardless of the shape, reference the 911 blown fuel roadster.
Rex
-
Rex your observations are correct but folks also need to consider the limits of what can be accomplished by all of them.
On full sized sedan type bodies it is very difficult to reach a CD below about .25 - .27 on production bodies so you are also faced by a diminishing returns problems. Once you clean up the major contributors to drag significant reductions in CD get very expensive very fast. The Tesla Model S claims a CD of .22 but that has not been verified by independent tests. The Toyota Prius claims a CD of .25 but independent testing puts its real world CD at about .30. The 1984 Pontiac Fire bird with optional aero package claimed a CD of .29, so did the 1958 Lotus Elite. Point being that we have known how to make a .29 CD sedan body for over 50 years at commercially viable pricing, and we are have only gained 16% to a .25 CD sedan over that span of time. The engineers have been hard put to reduce drag below that level given other packaging and drive-ability requirements, not to mention manufacturing costs.
Over that same span production engine power levels for street driven cars has more than doubled with the Chrysler 300 setting the mark for highest production power in its day (hence its name) to production mass production cars now being sold in the 500+ range and if you want to get silly super cars are north of 700 hp.
Likewise frontal area runs into hard packaging limits as it approaches the minimum size required to cover the engine and the driver in a useful driving position.
You can only chop the top so much and still see the road, or narrow or chop the body is limited by other factors.
The important take away should be of the three primary factors, CD frontal area and reliable power, which one is the cheapest for you to accomplish. Don't waste your time trying to squeeze a bit more power out of an engine that is already on the ragged edge of self destruction if you can gain a larger percentage reduction in CD or frontal area for the same or less cost and effort.
Some cars are already making more power than they can deliver to the ground, for them slightly de-tuning the engine to ensure it survives qualification and backup runs and investing the same money and effort into improved traction (ballast) so they can get more power to the ground, lower drag or better stability might be a much smarter investment.
Larry
-
Hot Rod,
I just wanted to make sure that everyone understood the basic formula and how, bhp,Cd and frontal area all have equal weight.
Some guys go fast with big horsepower and some go fast with good aero but the real fast guys have BOTH!
Rex
-
OMG . . . what is the top secret trick to sucessfully posting a 3,517 word thesis to the forum! :-o I can't wait to see the credits on the belly tank when it hits the salt. :wink:
-
Superford317, in Aerodynamics for Racing and Performance Cars, Forbes Aird touches on drag resulting from air pressure against the large area of the firewall (this is air from a relatively small inlet) and reducing this drag by reducing pressure under the open engine compartment by lowering. Okay, I understand that. It's not applicable with my belly pan and rear engine, but the idea of the effect is troubling.
The rules require a forward facing fresh air inlet directed to the driver. That puts it inside, where we have the back of the driver's compartment, essentially the entire cross sectional area of the vehicle, for it to act against. If I understand Bernoulli correctly, that is higher pressure once it leaves the small duct and slows entering the large compartment. We have a closed floor, so there's no analogous relief there. Intuition says dump it out the back window into the low pressure area to help the turbulence there. But it's a mighty small exit relative to the cross section of the compartment. Obviously all the air that comes in will go out, but like the air you discussed under the car, it's the effect it has in the process that I'm concerned about. I'd appreciate your comments. We're talking in the 100 mph range.
-
Bucketlist,
Since the driver’s compartment is essentially a large “room” with no particular restrictions to the free movement of air within it, for all intents and purposes the pressure at any point in the compartment will be the same. And since there is just as much projected area for the pressure to act on in the “front” of the compartment as in the rear, there will be no thrust or drag created by the air pressure inside the compartment.
When you inflate a basketball it doesn’t, as a result, begin to move.
-
Interested -
That room concept was pretty much the way I thought until I read what Forbes said about the firewall.
Nice analogy with the basketball. But I wonder, if we put a hole in the basketball opposite the inlet, so all the air was migrating to go out the opposite side, would the basketball move.
-
The BB won't move if the intake is EXACTLY the same for its constant rate of flow and dimension as that of the exit............
What I like about the SF novel is the point about the reduced pressure at the top of a rolling open wheel that creates lift............and the confirmed suspicion that I should shave my front tire to gain a 'rounded' profile..........I have thought about shaving almost to a V-shape to ride on the center at speed.
Regarding HP.........I know you need it......and more of it to go faster..........but I thought the primary effort was to gear for maximum speed at a point of or just past the peak torque in your motor.
-
TE---Tractive Effort
TQ x tr x ra x tc= TE
tq=torque
tr= trans ratio
Ra= Rear ratio
tc =tire correction
bottom line RPM is just as important in formula as TQ because you can multiply tq with gear choices
3-19-12 update:
The dyno sheet with this formula across the entire RPM range will tell you which tires and rear gears to choose for a given speed----DO NOT FORGET PARASITIC LOSSES on rear end and trans choices
-
You can you use either torque or HP. Bottom line, you decide which parameter your motor produces most efficiently and you gear accordingly. Example, the Phoenix (diesel truck) or any Hayabusa powered vehicle. In one case full RPM is 3,200 and the other is 12,000+.
-
I gear for max speed at or just past peak HP.
-
Spot on Fred, That is usually where the TE starts falling off-- only you and your Dyno Sheet know for sure-- :-D
-
Thanks for the conversations................I am encouraged to go another tooth down on the rear sprocket and tuck my head for more speed.
-
This post will pertain to a full body vehicle with a flat floor underbody, rear diffuser, 3 inches of ground clearance and 4 wheels enclosed in wheel wells. It will cover the airflow through the rims and around the tires and wheel wells and show some of the dynamics of the tires and how they affect other parts of the vehicle, as well as a glimpse into what is occurring at the tire ground contact patch when running on a deformable track surface.
The fastest speed ever attained by a manned rocket-powered aircraft was by the North American X-15, it was part of the X-series of experimental aircraft and eight different X-15 pilots met the USAF spaceflight criteria and became qualified for the status of “astronaut” and gained either USAF astronaut wings or NASA astronaut wings in doing so. The X-15 was air dropped by a B-52 and could burn 15,000 pounds of fuel in 80 seconds. On October 3, 1967, William J. "Pete" Knight, piloting the X-15A-2, set a world speed record for an aircraft, 4,520 mph. He earned his astronaut wings by flying an airplane in space, reaching an altitude of 280,500 feet. He was inducted into the Aerospace Walk of Honor for his bravery and accomplishments.
The effect humidity has on atmospheric density is very small, the average amount of water vapor in the atmosphere by weight is less than 1%. At sea level the air density would change less than 1% even with a 100% humidity reading. Moisture content in the track surface, on the other hand, can have a big influence on vehicle performance.
Before reading this post it would be helpful to review my earlier post, on air flow around the wheels of an open wheel vehicle. In my earlier post on airflow around open exposed car wheels, there were 3 sets of counter rotating vortices generated around the tire, a pair from the top of the tire; a second set from the center of the wheel axis and the final set are the high pressure “jetting” vortices coming from the ground contact patch and following along each side of the tire. When the wheel is enclosed in a wheel well, three out of the six vortices disappear: only the pair of jetting vortices at ground level and the external vortex shedding from the outer wheel axis remains. The vortices from the top of the tire will disappear because the wheel well will interfere with the external air flows ability to attach its self to the upper portion of the wheel. There will be other vortices generated from inside the wheel wells that will travel downstream, but they are not attached to the wheel, only influenced by the wheel.
Applying the engines power to the track surface takes a system of components all working together, the engine, transmission, driveshaft’s, differentials, springs, absorbers and tires, but in the end it all comes down to those 4 small rubber contact patches and how they react with the track surface beneath them.
Due to the interactions between the tire being deformable and the track surface being deformable, it will cause many problems for the performance of the vehicle. Rolling resistance of the tire will produce heat from the mechanical energy passing through it to the track surface, consuming part of the engines power in the process. All major forces and moments affecting the vehicle, other than aerodynamic and gravitational forces, are applied through the tire ground contact patch.
Hysteresis in the tire is caused by the deflection of the tire sidewall and tread. While rolling, a tire with low hysteresis will have reduced internal heat generation and a reduced rolling resistance. A tire with a shorter sidewall will deform less, therefor generating less heat in the tire and less rolling resistance. A tire with high hysteresis will require more energy to initially deform the tire during loading than the unloading; it can be thought of as deformation and recovery occurring at high speeds, that extra energy will be dissipated or "lost" as heat into the tire, and will be more pronounced under high speeds. Hysteresis and tire slip will be what generates most of the heat buildup in the drive tires. The viscoelasticity of the tire rubber causes the hysteresis and it is the main cause of the tires rolling resistance. During the tires construction, the rolling resistance can be lowered by substituting certain compounds into the tire, such as silica can be substituted for some of the carbon black in the tread compound or nano-clay can be added to the mixture during the tires construction to reduce rolling resistance also. Increasing the air pressure in the tire will decrease the flex of the sidewall and the contact patch and lower the rolling resistance. Severally over inflating the tires may not always reduce rolling resistance because the tire may have more of a tendency to hop and skip over the track surface and the wheel slippage and heat buildup may increase due to the rotational speed changes in the tire and lead to premature tire failure and blistering. Rolling resistance will decrease in the tire as the tire temperature increases, within reason. Hysteresis is the stretching and movement of the belts and plies inside the rubber tire that causes the subsequent heat buildup. At speeds of 80mph to 95mph rolling resistance from the tire can be broken down as 90% to 95% due to hysteresis in the tire sidewall and tread and 5% to 10% due to friction in the contact patch. The rolling resistance from hysteresis in the tire structure can be further broken down into a 70% to 75% contribution from the tire contact patch area, 10% to 15% from the sidewall, 10% to 15% from the tire shoulder and 2% to 4% from the beads. A thicker tread and sidewalls will normally increase the rolling resistance due to an increase in hysteresis losses. Generally a tire made of natural rubber will have a lower rolling resistance than one made of synthetic rubber. A rough track surface will have a higher rolling resistance than a smoother track and a moist track surface will have a higher rolling resistance than a dry surface. Radial-ply tires will have less rolling resistance and heat generation as compared to a bias-ply tire. On a radial tire, as the temperature of the shoulder increases, rolling resistance decreases, up to a temperature of between 160’F and 170’F before it levels off, the increased tire temperature alters the stiffness and hysteresis of the tire rubber. A tire with a shorter sidewall will deform less, generating less hysteresis, therefor generating less heat in the tire and a lower rolling resistance due to the decreased hysteresis.
The cords in adjacent plies, in a bias-ply tire, run in opposite directions and the diagonal plies flex and rub on each other, this is the main cause of the high rolling resistance in a bias-ply tire, whereas flexing of the tire involves very little movement and rubbing of the belts in a radial-ply tire. On hard surfaces the proper inflation pressure of a bias-ply tire is more important than on a radial-ply tire, for decreasing rolling resistance. The rolling resistance of a radial-ply tire will be 40% to 60% less than that of a bias-ply tire under similar conditions, and the tire life can be double that of an equivalent bias-ply tire. A radial-ply tire will have a more uniform pressure over the contact patch area, whereas the contact patch pressure for a bias-ply tire will vary greatly as the contact patch area undergoes a wiping motion from the cross plies flexing and rubbing over each other in the bulging side wall and contact patch area.
The forces transferred to the vehicle thru its tires and wheels, from the track surface, are the main influence on the vehicles motion, influencing its stability and maneuverability. Vehicle tire sinkage into the track surface and the distribution of pressure in the contact patch is a function of tire slip and the amount of downforce applied to the tire, rolling resistance will increase with sinkage. Because the tire is deformable, when ran on a soft track surface, it will tear up particles from the track surface and displace the material above the original track surface in the wake of the tire. Under acceleration the tire will remove track material from the contact patch and it will be transferred to the trailing edge of the tire. Moisture content in the track surface above 15% to 20% will cause the tire slip to become less dependent on tire sinkage. On harder surfaces there will be more of the tire bulging at the side wall at the ground contact patch, due to this, the side wall will approach the ground at an angle and this will cause the greatest force to be applied at the edge of the tire and the leading edge of the contact patch while the forces will be less at the center of the contact patch, but on softer surfaces the pressure at the edges of the contact patch will be about 50% of what it is at the center of the contact patch. The compaction of the surface will involve the particles sliding over each other and as the loading and pressure increases it will cause fragmentation of the particles. There will be erosion of the particles due to their low ductility and the shear response will be based on its yield-strength vs. pressure. As the particles fragment they still have the ability to support pressure but its capability to withstand tensile and shear stresses will decrease.
The effect of the rotating wheels, inside a wheel well, will have on the flow past the vehicle will be many and far reaching. The rotating wheels will influence the cooling of the brakes, underbody air flow, track surface deposition on the vehicle, the lift forces and drag acting on the vehicle body.
Tires on open wheel vehicles will generate large amounts of lift and drag, whereas enclosed wheels inside wheel wells will generate an overall increase in downforce, as well as a reduction in drag, and if the wheels are properly optimized for their respective vehicle, even higher downforce and lower drag will occur. The total aerodynamic drag from the wheels is about 25% to 30% for an ordinary passenger car and the rear wheels will contribute about 60% to 65% of that total 30% drag. The airflow from the front wheel can affect the airflow at the rear wheel but the rear does not affect the front. Most of the drag is due to the crossflow through the rims and by optimizing the rim design and size of the wheel wells, will decrease the drag of the vehicle. Wheel covers placed on the outside of the wheel rim only, for all 4 wheels, will decrease wheel drag by 20% to 35% and increase overall vehicle downforce. With the addition of ducts installed inside the wheel wells, to vent the rear wheel well pressure off into the wake behind the vehicle, will lower the rear wheel drag an additional 2% to 3%. Using outer wheel covers in combination with internal wheel well ducting, to vent the high pressure off, will result in a further 2% to 3% decrease in the front wheel drag also. Using moveable underbody panels, attached to the spindle and lower A-arm will help to seal the wheel well from the underbody airflow and reduce the front wheel drag by an additional 1% to 3%. Adding wheel covers to the back side of the rims will decrease the drag and lift even more, especially on the front wheels. A substantial decrease in vehicle drag can be achieved with the proper rim design. The wheels will have a large influence on the overall aerodynamic properties of the vehicle. In the vicinity close to the wheels and wheel housings the airflow is highly dependent of the rim design. When the rims have small spokes and large open areas a significant airflow through the rims will occur, creating large areas of low pressure around the wheels where energy will be lost. By covering the entire rim outer surface with a moon disc or some other aerodynamic cover and blocking the crossflow through the rim, will have positive effects, mainly reducing the drag, but it will have negative effects also. By covering the rims, it will result in smoother airflow along the sides of the vehicle, as well as prevent the airflow underneath the vehicle from being pulled out and spilling towards the sides of the vehicle and therefor maintain a higher flow velocity underneath the vehicle. By blocking the crossflow through the rims, it will result in increased static pressure in the front wheel well, increasing the lift, whereas blocking the crossflow in the rear rim will result in increased downforce due to an increased flow rate through the diffuser, the rear wheel when not running an outer cover will pull air from under the vehicle and away from the diffuser. Blocking the air crossflow through the rims can destabilize the vehicle stability by increasing lift on the front wheels and increasing downforce on the rear wheels, so additional care should be taken to add some downforce on the front wheels when wheel covers are added, such as extra ballast in the front, adding a splitter, inner wheel covers or a front diffuser or underbody vortex generators added to the front underbody. The crossflow through the front rim openings will create large wake structures that will travel downstream, creating turbulence and drag. When the front wheel is covered the wake will be narrower. Installing an aerodynamic cover over the front rim will help to reduce the strength of the jetting vortices coming from the contact patch, thereby reducing lift from that area and the airflow passing the rim on the outside will be less disturbed and be pulled in closer to the side of the vehicle, lowering the drag from the side airflow. Covering the front wheels will usually result in a decrease in drag but at a penalty of an increase in lift on the front axle, covering the rear wheels will reduce the drag even more than only covering the front. The static pressure inside the front wheel well will be higher when the rim is covered and the high pressure will move in front of the wheel close to the contact patch and it will influence the flow around a majority of the side of the vehicle, causing the low pressure that exists there to be partially eliminated. If only the outside of the front wheel is covered, higher pressure inside the back of the rim will occur because the crossflow through the rim that was occuring is stopped, creating higher static pressure and lift, causing a flow reversal, and causing the wake on the inside of the wheel to be slightly wider. By stopping the crossflow through the wheel the flow from inside of the wheel house needs to pass between the wheel and the wheel well. The higher pressure found inside the back of the rim, due to the crossflow being stopped, will increase the lift of the front axle. The higher pressure generated inside the wheel well, will cause more of the air that would normally enter the wheel house, to travel along the outside of the wheel. The drag of the vehicle will increase as the volume of the wheel well increases. The radius of the wheel well will affect drag more than the width of the wheel well. Tire wake thickness and width will increase as the radius of the wheel well increases. Drag will also increase as the wheel well is widened but it has less effect on the drag than the wheel well radius. The air flow and wake from the front wheel will have a big influence on the flow around the rear wheel. By covering the front wheel, it will cause a reduction in drag at the rear wheel of 7% to 21% due to a decrease in the strength of the high pressure region around the contact patch of the rear wheel and a decreased rear wheel wake. If the front wheel is not covered the wake from the front wheel will be larger and will extend all the way back to the rear wheel, this affects the stagnation pressure for the rear wheel. The low pressure area around the rear wheel will become larger when the front wheel is not covered. The difference in pressure at the leading edge and trailing edge of the rear wheel becomes greater, increasing drag and creating a larger wake behind and to the outside of the rear wheel.
Covering the rear wheel will cause the pressure close to the ground, starting from the contact patch, to be slightly lowered. The downforce from the diffuser can be increased by 4% to 10% when the rear wheels are covered because the air underneath the vehicle is more enclosed by the closed rims and the flow through the diffuser is increased, and not being pumped out of the vehicle underbody by the rims crossflow pumping effect. Covering the outer part of the rim will cause the largest reduction in drag. The wake area at the lower part of the wheel will be significantly smaller for this rim due to blockage of crossflow through the rim. In general the drag will decrease with increasing covering area whereas the lift will increase with increasing covering area. Changing the shape or spoke design in small amounts will have little influence on the results. If both front and rear wheels are covered they will have the lowest drag but will be influenced the most by yaw angle, increasing the drag more than if they were not covered. By covering the rear wheels only the vehicle will be the most sensitive to yaw, in terms of lift.
A vehicle with decent aerodynamics will benefit more from the wheel covers, but if the vehicle has very bad aerodynamics and a large base drag, low pressure area behind the vehicle, fabricating and installing a centrifugal fan on the rims to pull air flow through the rear rims, from the outside to the inside of the wheel well, the wheels can be used to pump air into the wheel wells and channel it through ducts, from the wheel well, to exit out the rear of the vehicle, helping to fill the wake and lowering the pressure drag on the rear of the vehicle. This technique can reduce the overall vehicle drag by 15% to 20%. The greater the pressure drag behind the vehicle the better this will work. The installation of the fans on the rear rims will reduce the strength of the jetting vortices exiting the contact patch at ground level also. 60% to 70% of the airflow pulled through the rim by the fan, will attach its self to the tires tread surface, forming a vortice, and be carried forward into the contact patch area where it will split into 2 vortices, reducing the strength of the 2 “jetting” vortices from the contact patch area, thereby reducing the wake behind the wheel and reducing lift that is generated by the “jetting” vortices. About 30% to 40% of the air being pulled through the rim by the fan will exit through the duct and out the back of the vehicle into the low pressure wake area. Fans can be installed on the front rims also, but there effect will not be as dramatic as the rear fans, due to not being able to contribute to the reduction of the rear pressure drag, but they can still reduce the “jetting”, lift and the tire wake size of the front tires.
By virtue of their close proximity to each other, changes in one part of the vehicle will have an effect on other parts. By changing the location or angle of a rear wing or spoiler or the shape of the fender behind the rear wheel will cause significant changes in the drag of the rear wheel. There is an important interaction between the wake of the rear wheel and the wake of the rear body.
A separation of the rubber “sandwiched” between the belts of the tire is a common failure of a tire at high speeds and will be due to a fatigue crack in the rubber between the belts. The rubber that bonds the belts to the tire plies and the tread surface can separate “peel” due to the centrifugal force at high speeds, causing the outer belt and tire tread to separate from the body of the tire and this can become more likely with increased tire age and depending on the use and storage of the tire during its life. As the tire ages it will lose elasticity and cause the growth rate of cracks to increase and start to form at the belts, resulting in tread separation. As the tire ages the strength of the rubber between the belts will decrease and the peel strength decreases accordingly. The aging or “Oxidation” of the internal rubber is very dependent on the temperature that the tire has been maintained at during its life. The tire rubber oxidation will occur from the outside surface in and from the inside pressure surface out, but the oxidation rate from the inside of the tire to the outside will occur much faster due to the high pressure air inside the tire. The use of nitrogen for tire inflation will reduce the oxidation of the rubber between the belts, due to the tire being permeable to the air and the escaping pressurized air accelerating internal tire oxidation. The nitrogen molecules are much larger than the air molecules and will not permeate through the tire as easily and if it does, it will not cause rubber oxidation due to it being an inert gas. Natural rubber is more permeable to air while synthetic rubber has a much lower permeability rate. The innerliner built into the tire is critical in reducing oxidation of the tire rubber; it is made from a special blend of synthetic rubber to reduce the permeability and oxidation from the high pressure air inside the tire. The tire innerliner does not stop the permeation it only slows it down. Time and temperature are the most important factors to internal tire rubber oxidation. Land speed racing would be more susceptible to tire oxidation and failure than any other form of racing, due to the fact our tires can last much longer, in some cases many years, and are normally purchased when a vehicle build is commenced that could take years to complete and when we do race it is normally in climates with extreme temperatures and faster speeds than any other form of racing, putting extreme pressures on our tires. At times there have been shortages of certain tires we require and older tires or any that were available were used. Sometimes a racer may make 2 events in a year, park it for 1 or 2 years dust it off and make another event. The faster the vehicle and the more pressures and loads the tires are subjected to, will make the effects of oxidation more dangerous. Purchasing a tire with a higher speed rating than is required will allow a margin of safety for some oxidation of the rubber. A tire with a taller side wall will have a faster rubber oxidation rate than a tire with a shorter side wall. The oxidation of the tire rubber will increase as the tire inflation pressure is increased and the oxidation will increase with temperature up to about 160’F and then start leveling off. Inflating the tire with nitrogen in place of air will reduce tire rubber oxidation 60% to 75%
If the tires have tread grooves and are running on a hard surface there will be deformation of the tread grooves as they enter and exit the contact patch. Due to the tread deformation as it enters the contact patch the air captured in the grooves will be compressed and then expelled back out of the leading edge of the tire contact patch. At the trailing edge there will be air displacement as well, due to the tread expansion, which will generate an inflow effect. The air squeezing out of the compressed grooves and rushing into the expanded grooves will cause air fluctuations. The air pumping effect can occur due to the cavities in the tire tread, as well as being due to pockets in the track surface that the tire rolls over. As the surface of the tire moves toward the ground contact patch at the leading edge, it will transport energy towards the stagnation point increasing the total pressure. The pressure difference between the high pressure area in front of the ground contact patch, and the sides of the wheel forces the flow laterally and causes it to accelerate rapidly, this is called “jetting” and will cause vortices to form at the ground level, coming out of the front and trailing down the sides of the tire contact patch area. The vortice on the outside of the wheel will be larger than the vortice on the inside of the wheel due to the outward deflection of the approaching air flow down the side of the vehicle.
The friction, which provides the traction, between the tire and ground surface is complex and will depend on many factors, such as the elastic properties of the rubber, camber angle, load pressing on the tire, tire type, tread design, ground surface texture, temperature, tire slip ratio, vehicle speed and tire inflation pressure. The friction between the tire and the track surface will be dependent on the vehicle speed and the slip ratio. On a soft track surface, both the tire and the track surface will be deformable. As the wheel sinks into the track surface under acceleration, it will encounter resistance by the track surface, because it will have to compact the surface as it rolls forward, thereby requiring more energy to overcome. Deformation of a soft track surface, in front of and beneath the tire, as well as lateral movement of the track surface, increases the tire rolling resistance. On very hard surfaces, the effect of the lateral movement of the track does not occur and is of no concern, but on soft surfaces, lateral movement of the track surface will increase the tire rolling resistance. The vehicle tires and track surface having a resistance to deformation will have a resistance to penetration by the tires and therefor generate lift and drag, this effect will increase with vehicle velocity, as the track surface will have less time to react to the movement and at some point the tires of the vehicle can actually be raised up above the surface of the track and start to “plane” on the surface, much like a boat raising out of the water and planing on the surface of the water at high speeds. The amount of lift and drag will be determined by the amount of deflection from the track surface and the tire load deflection. When the vehicle tires start “planing” above the track surface, it will lower the drag and smooth out the ride. There will be a fine line between the amount of tire drag and the down force required, whether from ballast or aerodynamic down force, required to keep enough traction for forward momentum. If too much downward pressure is created it will push the tire into the surface too much and create more drag than required, slowing the acceleration rate and top speed of the vehicle. If there is not enough down force, either ballast or aerodynamic down force, the vehicle will start to “plane” to early and too much and become very loose, spin the tires too much, cause vehicle spin out or even cause the vehicle to become uncontrollable and possibly become airborne. Sinkage into the track surface will be due to the compressibility and shear strength of the track surface in response to the vehicle load. A combination of the track surface, tires and vehicle velocity can combine to form a type of bow wave of particles from the track surface that will be moved along in front of the rolling tires at higher vehicle velocities, this is called “bulldozing”. Because the force of a moving wheel is acting on the tire at a more inclined angle, a rolling wheel on a deformable surface will sink more than a non-moving wheel, with the same vertical force applied to it. Wheel sinkage will decrease as the wheel diameter increases. The softer the track surface, the less internal tire rolling resistance, hysteresis, there will be due to less rolling resistance. Rolling resistance of the tires on a semi soft surface will be mainly due to a compaction force, displacement effect of soft track surface particles and side wall friction. The moisture content of the track surface will play a large part with tire rolling resistance and the tire behavior in these situations.
Jim Hall’s Chaparral-2E, had a driver adjustable rear wing that could be adjusted while the vehicle was moving on the track. The driver would use it to generate maximum down force for cornering and then reduce its angle entering the straights, for maximum top speed and less drag. By controlling the amount of airflow into the underbody, with an adjustable rear diffuser angle or adjustable height side skirts, an adjustable wing or a type of moveable canards on the side of a vehicle that the driver can adjust on the move, down force can be adjusted to suit changing track conditions. If a person wants to use ballast for all of the down force, the moveable aerodynamic downforce can be used for a sort of fine tuning device, which the driver can adjust on the move. If he does not have enough downforce and is spinning the tires, there are only so many runs that can be made, instead of going back to the trailer to add ballast make an adjustment while moving on the track, it could make the difference between an aborted run or a record setting run. 600 pounds of downforce could be dialed in by the driver with the push of a button, the pull of a lever or the turn of a knob. According to Wikipedia, the bodywork of the Target 550 streamliner of Marlo Treit and Les Davenport will provide 1000 lbs. of downforce to help prevent the car from becoming airborne. It seems as though a lot of the newer streamliners are building in aerodynamic downforce, just to name a few Target 550, Speed Demon and Mormon missile either with shaped underbody floors or upper body work. The DOWNFORCE (wmts) from the underbody is virtually drag free though.
I think this post should transition nicely into my next post.
-
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
-
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
That is providing somebody had the stamina and was not blind by the time they got to the bottom of the tome. :-D
-
The calculation Rex showed is one of a few methods I use to figure out how much less slower I will go with more power. The horsepower I enter into the equation is reduced to reflect the loss due to altitude. Is there an adjustment to the CD factor to model the thinner air at B'ville?
-
Drag is classically calculated as equal to 1/2(rho)(velocity squared)CD(area). Rho is air density, and would be assigned the value of the air density at Bonneville. You don’t mess with the CD.
However, to the degree that the power generated and the drag are both proportional to air density, it is generally a wash.
-
I think this thread started talking about exhaust flow? :?
-
I think this thread started talking about exhaust flow? :?
Tman, I have found it exhausting at times! :-D
But sometimes we just have to go where the hot air takes us! :-o
-
lol :-D
-
:roll: :roll: :roll:
-
Tman, I have found it exhausting at times! :-D
But sometimes we just have to go where the hot air takes us! :-o
Good one, Woody! :wink:
-
Hey! What would I know; I drive a roadster with 32 shell. Point the exhaust toward the back if you can and step on the throttle......May the best man win :cheers:
-
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA! That's something for nothing. "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."
IOW, there is no free lunch. Someone a lot smarter than me figured that out.
Drag from the underbody whether it produces downforce or not already exists on many cars. Subtle changes to the underbody may make significant improvements to downforce without increasing drag beyond the drag that is already there. At no point is this downforce "drag free". The fact that this is possible on a high drag design does not in any way mean the downforce is free. The drag is already there due to poor design.
In fact, changes to the underbody can both reduce separation drag, increase downforce, and with it the induced drag. Then the car makes the same total drag with more downforce. This is still not "free". Drag is still present and a detailed drag analysis can separate the drag due to downforce (induced drag) from the rest of the vehicle drag.
On any vehicle, downforce comes with a drag penalty. This is called the lift to drag ratio or L/D. In ground vehicles we would use the term -L/D. Typical ground racing vehicles that generate significant downforce have -L/D ratios of 0.3 to 3 due to large amounts of separation and low aspect ratios. Air vehicles have L/D from 5 to 20 to 50 for fighters to airliners to sailplanes.
Note the progression: higher aspect ratio wings and smaller fuselages have higher L/D. Cars are the opposite. While we might lump all kinds of "drag" together for a given vehicle, it is more accurate to break it down to each piece and each type of drag produced. Then the induced drag specific to the lift (downforce) can be dealt with against all of the rest.
There is no free lunch.
-
:evil: Damm there is that inconvient truth rearing its ugly head again!!!! :-o
-
After becoming an American citizen an old man was asked what he found most interesting about America!
He said, "Free lunch is 99 cents!" :cheers:
[Don't make me explain this!!] :x
-
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA! That's something for nothing. "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."
Since there is no downward motion, downforce isn't perpetual motion. There's no second law violation. No actual work is being done by downforce. No need to overstate the case to make your point.
-
Colin Chapman, the father of modern ground effects, was asked during a news conference about his Lotus Model 78 Formula 1 car on its debut and he replied “we found something for nothing” referring to the downforce from the underbody. The downforce from the underbody is not totally drag free, that was why I said it was virtually drag free, but it is as close as you can get, being much more efficient than any other source of aerodynamic downforce.
-
"Colin Chapman, the father of modern ground effects..."
I think Jim Hall earned that title.
Regards, Neil Tucson, AZ
-
Touche! :-D
Doug :cheers: :cheers: :cheers:
-
F1 ground effects
http://youtu.be/xdqgY9YOaBo
-
Whooops the propster may take this as a wakeup call and try to make a return :-D
-
It's "virtually drag free" because there is a heap of shear drag happening underneath the car already, tidying it up and adding some order to the flow offsets the majority of the ground effects drag.
It's like saying someone gives you $10 and you buy something for $11 is "virtually free", only because you didn't bank the $10, still cost you $11.
My opinion anyway.
jon
-
Colin Chapman, the father of modern ground effects, was asked during a news conference about his Lotus Model 78 Formula 1 car on its debut and he replied “we found something for nothing” referring to the downforce from the underbody. The downforce from the underbody is not totally drag free, that was why I said it was virtually drag free, but it is as close as you can get, being much more efficient than any other source of aerodynamic downforce.
This reflects a 40 year old understanding of race car aero limited by restrictions on wings. The fact is that high aspect ratio wings, well separated from the rest of the vehicle, are far more efficient at creating downforce (-L/D) than ground effects. They are not used because every racing body since the early 70's Can Am days restricted wings area, width, and especially height in an effort to control downforce. The use and science of ground effects are a result of this restriction, they are not the best solution. The best solution has been outlawed everywhere but LSR.
-
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA! That's something for nothing. "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."
Since there is no downward motion, downforce isn't perpetual motion. There's no second law violation. No actual work is being done by downforce. No need to overstate the case to make your point.
If we press up on an immoveable object, there is no work being done. However the air is not immoveable, it is a fluid. The first law of motion is that for every action there is an equal and opposite reaction. When we create downforce by moving through a fluid (air) that force equals moving a mass of air up.
F = m x a
So for a given downforce (F), we must accelerate (a) a mass of air (m) up. This is power, and over time, work.
Similarly, drag is the exchange of momentum from the vehicle to the fluid (air) that we are traveling though. If we do this without any separation or recirculation, the drag is limited to viscous effects. Most LSR vehicles have significant separation, recirculation, and stagnation, leading to high levels of pressure drag and viscous shear momentum exchange. It takes power to create downforce just as it takes power to overcome drag. That power went into all of the wake turbulence behind the car and the salt spray going up and back. We produced the power, we burned the fuel; where else did the work go?.
While this seems esoteric, the fundamental physics are what separate good design decisions from bad. Look at the ALSR programs: they let their (m) get out of hand, and can't make their design speed with the space and fuel designed even if they won't admit it.
-
F1 ground effects
http://youtu.be/xdqgY9YOaBo
The video clearly shows that Peter Wright was the "discoverer" of modern ground effects, not Colin. FYI, I went to high school with Finn O'neil, who's father invented the cylinder cut-out for fuel management in F1 for Lotus working with Colin. Finn taught me how to race motorcycles.
Peter doesn't mention drag, nor the difference between -L/D ratios from wings vs. downforce. No one in the film mentions that their wing size/width/height was being restricted every year. More telling is the later focus on vacuum with fans: clearly a sub-optimum approach dictated by rules, not physics. LSR is much more open.
Wide span wings work better because it is better to push up a little bit on a lot of air with small edges than to push up a lot on a little bit of air with long edges.
-
:cheers: Thanks to BOTH you guys---our grasshopper AERO minds are soaking it up---thanks again for so much to ponder as we work toward enlightment!!
-
Interesting subject.... which I know Very Little...
Although, some things are not on You tube... which doesn't mean they didn't happen.... OR WERE PUBLICIZED..
Like the effect Bob Herda's un-louvered ( NO vents ) under Body on his Liner created...
Speaking with those who were there, the spring rates had to be increased , due to the car "Sucking Down" at speed.
If you take the time... Note in the story below, about the "negative pressure" under the car...
http://bryantfamilyauto.com/my_friends.htm
scroll down...
http://picasaweb.google.com/psychocross/BonnevilleChromeYellow360More#
some pics mixed in with the Markley cars
Bob passed away in his car. But he was a very good aero engineer. Yes in fact that was his day job.
All of this took place ... before most anyone else knew ... .What Bob Herda knew.. in the 1960's....
just because it is not on You Tube... doesn't mean it did not happen..
403 mph .... Single engine car..... First SINGLE engine car over 400 mph ...
Where is that on you tube? and who was it? I know ... as I watched it.... or was I dreaming..
bob dalton
-
If we press up on an immoveable object, there is no work being done. However the air is not immoveable, it is a fluid. The first law of motion is that for every action there is an equal and opposite reaction. When we create downforce by moving through a fluid (air) that force equals moving a mass of air up.
F = m x a
So for a given downforce (F), we must accelerate (a) a mass of air (m) up. This is power, and over time, work.
Consider the suction cup. It exerts downforce without moving a mass of air up. Now consider a perimeter skirted vehicle with a frictionless, airtight seal. Same thing. Of course, the frictionless, airtight seal does not exist. But to the extent you approach it, you get downforce without drag. It is not apparent to me how one would calculate the theoretical lower limits of friction and leakage.
-
You forget that the suction cup had work done on it to force the air out in the first place and create the suction -- no free lunch here.
The energy needed to create the lower pressure comes from some where, it is not free, although it might be lower cost than other forms of equivalent down force.
Larry
-
Hotrod, you support my point. There are other ways of making downforce than moving air upward, and aero drag is not a necessary result. The best example is, of course, ballast.
-
I'm getting in this discussion way late but here’s something to think about. Note: I have not read the last 16 pages in this thread but just the last few about downforce and the drag resultant. When you tape a radiator grill inlet you reduce drag and gain downforce... (More than a free lunch)
-
The air under the car is attached to two surfaces. I think the effect is simple capillary action. Anyway, the velocity of the air attached to the salt is near zero. The velocity of the air attached to underbody parts is also near zero. This is a very old phenomena and is well documented with porting studies over the years. I agree these stagnant zones of air can be minimal and as little as .002-.015” thick. But air velocity is near zero in these areas. Somewhere between the car body and the salt, turbulence develops. Whether it is at atmospheric pressure or less than atmospheric, the shearing action of the air or turbulence causes drag. You get down force but it does have a cost.
-
I'm getting in this discussion way late but here’s something to think about. Note: I have not read the last 16 pages in this thread but just the last few about downforce and the drag resultant. When you tape a radiator grill inlet you reduce drag and gain downforce... (More than a free lunch)
I would submit that you gain no down force at all, but you do reduce lift. Just because the car got lower on the springs and quit darting around in the traps, does not mean you gained down force. By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.
Reducing lift and increasing down force are two very different things, but easy to mix up and ascribe one to the other.
If you reduce lift you naturally will also reduce the induced drag caused by the work performed by that lift. In that case you are getting exactly what you are paying for not a free lunch.
Larry
-
If the car is lower than it used to be at the same speed because you have reduced lift its better than a free lunch, you have stopped paying for lunch for someone you didn't like.
jon
-
. . . By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.Larry
Haven't you just relocated it to the new blocked off section at the front of the radiator?
-
Saltfever,
Just a comment on your statement regarding the velocities of the boundary layers next to the ground plan and also the surface of a moving body above the ground plane. The velocity of the boundary layer is zero relative to the surface that it is next to so the boundary of the ground plane is zero relative to the ground but it is at the speed of the moving body at the surface of the moving body. Now if they are relatively close to each other then if you look at a velocity profile of the air between the ground and the moving body it is zero at ground level but it is traveling at the moving body velocity at the surface of the moving body.
Rex
-
Haven't you just relocated it to the new blocked off section at the front of the radiator?
Yes you have but the peak stagnation pressure is only achieved in small area of the center of the radiator, you have significantly reduced the surface area that pressure is acting on.
Also by adding a smooth front surface you force the air to go around the radiator rather than through it, so you also eliminate a significant amount of "internal drag". Radiators by their very purpose are intentionally built to maximize air stream surface area contact and passage turbulence in between the fins and tubes of the radiator to maximize the heat transfer to the air flow. That means flow drag and pressure drop across the radiator, both of which essentially disappear when you keep the air from going through the radiator core in the first place. Much less flow drag for the air to move across a smooth body panel than to push its way through a nest of closely spaced fins.
Larry
-
Thanks, Rex. I didn’t make that point perfectly clear or maybe I even mis-stated it. Obviously, if the air is stuck to a moving body its velocity will be the same. I think the picture I was trying to paint was looking out at that vast, flat, 32 square miles of salt, the air is hardly moving in relation to the car. When we pass a body through that air mass we are imparting various degrees of acceleration (laminar or turbulent) to the entire mass under, around, and over the car. Accelerating air anywhere takes energy regardless of the dynamic effect on the car.
-
I would submit that you gain no down force at all, but you do reduce lift. Just because the car got lower on the springs and quit darting around in the traps, does not mean you gained down force. By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.
Reducing lift and increasing down force are two very different things, but easy to mix up and ascribe one to the other.
If you reduce lift you naturally will also reduce the induced drag caused by the work performed by that lift. In that case you are getting exactly what you are paying for not a free lunch.
Larry
Larry, this is proven with track testing and wind tunnel testing. By taping grill openings or reducing the inlet size you reduce the under hood pressure and create more downforce. Have you ever wondered why they tape off the nose in NASCAR for qualifying? Because it reduces the drag and gives the cars more downforce (mostly front) and lets the drivers get better lap times then in race trim by providing a few more %front balance. They can only run a few laps before engine cooling becomes an issue, but it is done by 100% of the teams for this reason for the 2 lap qualifying.
I am talking about a car that has -CL already. And IF you have lift this would decrease the lift (more on the nose) but is still the same "free lunch" you guys say can't exist. Lift is a bad thing and to counter act you need to either add ballast or reduce the lift. If you reduce the lift and decrees drag, isn't that the same free lunch?
-
Lift is a bad thing and to counter act you need to either add ballast or reduce the lift. If you reduce the lift and decrees drag, isn't that the same free lunch?
Yes and no -- I am making an important but subtle distinction regarding first principles. Reducing lift is not the same as "creating down force" it is the elimination of an undesirable negative not the creation of a new force pointing in the opposite direction at no cost (ie violation several physical laws)
Cutting loose a boats anchor is not the same as putting up more sail while leaving the anchor out. They may both increase the speed of the boat but different things are going on. In the case of taping the nose you are cutting the anchor chain, reducing a negative that already exists, you are not getting any free energy you are reducing your losses.
I agree with you entirely, NASCAR is an excellent example of reducing drag by cutting internal flow drag by taping off the radiator.
The point several people here are trying to make is that the conception that "down force" in certain situations is some how "free" in terms of drag is a faulty understanding of the physics involved. If that faulty understanding is applied uncritically to other problems it leads to big mistakes both in understanding what is going on and why you should be doing it.
Some folks understand that distinction some do not. Yes the reduction in drag is "free" in the sense that the tape does not cost much and little effort is involved but physically it does not "create" anything new. Lowering front end lift by reducing underhood pressure is not "increasing down force" it is reducing lift. Two very different things that have the same net result in this case.
For example adding a gurney lip on an inverted airfoil substantially increases down force generated by that inverted airfoil at a relatively small cost in drag, but it is still not "free down force" you do pay a small price in additional drag. That penalty may be much smaller than the drag penalty you would pay for creating the same down force with a bigger wing or a wing with more camber or with the same wing at a higher angle of attack but it is still not free, it is just the least costly of the 4 alternatives to get to the same place.
Going back to your analogy, adding ballast actually creates down force and does not increase aero drag in any way, but it will increase rolling resistance, and bearing losses in the loaded wheel bearings. It also certainly slows down your acceleration to the 2. Again it appears free but it is not, you are just paying for it at a different window.
Larry
-
Going back to your analogy, adding ballast . . . certainly slows down your acceleration to the 2.
Not certainly. A powerful, nose-heavy car might be helped, no?
-
The proper application of the throttle may also help that ET.
DW
-
All this talk of a free lunch made me go up to the track today to help some Ferrari challenge cars and get my free lunch. Tony :mrgreen:
-
All this talk of a free lunch made me go up to the track today to help some Ferrari challenge cars and get my free lunch. Tony :mrgreen:
Get any new avatars I mean race car pictures? :-D
-
Larry, I understand what you are trying to say about lift. But, am I to assume that everyone in landspeed racing has lift on their cars? (I think not). In the case of NASCAR they have DOWNFORCE front and rear and adding tape to block internal cooling WILL INCREASE DOWNFORCE and REDUCE DRAG. (PERIOD!) So why do you keep going back to lift for that example? This can be said for production based LS cars too. Maybe I can post some data to help drive this point.
Going back to your analogy, adding ballast actually creates down force and does not increase aero drag in any way, but it will increase rolling resistance, and bearing losses in the loaded wheel bearings. It also certainly slows down your acceleration to the 2. Again it appears free but it is not, you are just paying for it at a different window.
Larry
My analogy is NEVER stated that ballast adds aero downforce as you are saying or did I say it does not come with penalty (rolling resistance & acceleration).. If a car has lift, at some point could be uncontrollable to drive and even cause a spin. In this case you are adding ballast to simply add total weight to the car so it will stay on the ground. And I didn’t say ballast is the best way to balance a car. It’s simply a fix to an aero lift problem and in my opinion a last resort (to fix an aero lift problem).
-
We are getting WAAAAAY far away from the original intent and purpose of my comment.
There is a fun game I used to play with a friend at work when computer CD's came out with a Thesaurus. You start with one word and then take synonyms in the thesaurus and build a chain of them that eventually point in exactly the opposite meaning of the original word.
That is what has happened to this portion of the discussion, each successive response has moved slightly away from the original comment and now we are trying to discuss/defend something totally different from the original. So lets go back to the beginning.
two pages back we started on this discussion of down force with no input of work when Blue made the following comment.
Drag from the underbody whether it produces downforce or not already exists on many cars. Subtle changes to the underbody may make significant improvements to downforce without increasing drag beyond the drag that is already there. At no point is this downforce "drag free". The fact that this is possible on a high drag design does not in any way mean the downforce is free. The drag is already there due to poor design.
My comment about ballast was not even addressed to you but to the folks that recommend ballast as a low cost way to lower the nose of the body back down where it belongs and they commonly assert it is "drag free" down force. I never said it created aero down force it creates gravitational down force and "might" change aero down force due to how it changes the car body attitude.
No it is not drag free! It does not produce additional aero drag ( and the change in body position obviously dramatically lowers aero drag) but the added weight increases other forms of drag. Extra load on the tires increase rolling friction of the tire due to tire flex as it rolls, and the increased rolling resistance caused by making a deeper impression in the salt (when it is soft enough), higher load on the wheel bearings increases rolling resistance due to bearing drag, and the higher inertia of a heavier car is harder to accelerate (force = mass x acceleration). This will slow the cars acceleration during the early part of the run as long as the car is not traction limited. For those cars that need extra traction on the short end of the track while getting up to speed, might gain lots more acceleration due to improved traction but they are still loosing acceleration due to the increased inertia of a car that might weigh 1000# more than it did without the ballast.
One of the big problems with forum discussion is we are not talking in real time, so it is sometimes even with quoting difficult to determine who someone's comment is actually responding too.
We are actually in agreement, just talking about different things. Taping the nose on a NASCAR does increase down force and does reduce drag, which is exactly what I said in an earlier post about blocking off a radiator. You reduce lift because the air pressure under the hood is reduced which also not only lowers the nose (creating a lower drag angle of attack of the body) but you are also reducing the pressure drag caused by the high pressure air pushing on the large surface area of the fire wall.
The car has lift when ever the aerodynamic forces raise the body above its static position it would have when parked. It only has down force when that lift force goes negative and pushes the body downward below its static body position. Technically both lift and down force are lift, the only difference is the sign of the forces. Lift has a positive force vector (pointing upward from the track) and down force has a negative force vector pointing downward toward the track. Even with in that summary you can be developing down force on the very nose of the car due to the dynamic pressure of the air stream on the front body work, while at the same time have substantial lift developed only a foot or two farther back on the hood where the low pressure zone typically forms (area the Ford GT40 placed its radiator exit ducts) then just a foot farther back have strong down force on the cowl and front face of the windshield. Depending on the relative magnitude and areas those forces develop over, the net of them could be either lift or down force. Depending on what sign that force is and how it changes the angle of attack of the car body you might increase total flow drag due to the change in the body's attitude to the air stream, which is a totally different form of aero drag.
I was trained as an engineer and am just trying to get people to recognize that there are lots of bits and pieces here, and over simplifications like that is "drag free" is almost never correct, they are really trading one form of drag for another in most cases and the "drag free" is just lower cost drag but not free.
This was in response to superford317's comment :
Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
Then tortise responed with:
Since there is no downward motion, downforce isn't perpetual motion. There's no second law violation. No actual work is being done by downforce. No need to overstate the case to make your point.
Then Jon commented with:
It's "virtually drag free" because there is a heap of shear drag happening underneath the car already, tidying it up and adding some order to the flow offsets the majority of the ground effects drag.
It's like saying someone gives you $10 and you buy something for $11 is "virtually free", only because you didn't bank the $10, still cost you $11.
My opinion anyway.
jon
Then Blue responded with:
If we press up on an immoveable object, there is no work being done. However the air is not immoveable, it is a fluid. The first law of motion is that for every action there is an equal and opposite reaction. When we create downforce by moving through a fluid (air) that force equals moving a mass of air up.
F = m x a
So for a given downforce (F), we must accelerate (a) a mass of air (m) up. This is power, and over time, work.
Similarly, drag is the exchange of momentum from the vehicle to the fluid (air) that we are traveling though. If we do this without any separation or recirculation, the drag is limited to viscous effects. Most LSR vehicles have significant separation, recirculation, and stagnation, leading to high levels of pressure drag and viscous shear momentum exchange. It takes power to create downforce just as it takes power to overcome drag. That power went into all of the wake turbulence behind the car and the salt spray going up and back. We produced the power, we burned the fuel; where else did the work go?.
Then tortise responded with:
Consider the suction cup. It exerts downforce without moving a mass of air up. Now consider a perimeter skirted vehicle with a frictionless, airtight seal. Same thing. Of course, the frictionless, airtight seal does not exist. But to the extent you approach it, you get downforce without drag. It is not apparent to me how one would calculate the theoretical lower limits of friction and leakage.
and finally I responded with:
You forget that the suction cup had work done on it to force the air out in the first place and create the suction -- no free lunch here.
The energy needed to create the lower pressure comes from some where, it is not free, although it might be lower cost than other forms of equivalent down force.
Larry
Both blue and I are making a simple statement of basic principles that no matter how you get down force it is never free. It always cost something. In the case of the NASCAR front down force they spent fuel to go fast enough to get the dynamic force to develop on the front body surface and for the front valence skirt to pull down to the track (helped a great deal by magic shock absorber packages that ratchet the car downward every time it hits a bump due to very different bounce and rebound stiffness in the shocks) for all that aero stuff to work. That down force is not free, they had to burn fuel and go near 200 mph to make it work. Punch a little hole in the front valence by running over some debris and they instantly run into the same sort of engine compartment pressurization and aero lift on the front end that most of the sedan bodies at Bonneville deal with at speed, as the air pressure under the hood is higher than the air pressure of the air stream over the hood. As a result some of them jack the front end of the car up 6 -12 inches at speed. Many of them do not fully appreciate how much extra drag and lift they are developing until I show them a picture of their car with the nose lifted several inches above its resting position.
In response to your comment:
Larry, I understand what you are trying to say about lift. But, am I to assume that everyone in landspeed racing has lift on their cars? (I think not).
Yes in almost all cases the cars at Bonneville have front end lift (at least most of the folks interested in this thread) there are very very few sedan bodies out there that "suck down" to the track at speed. Even a few of the streamliners and lakesters experience front end lift at speed. That is one of the reasons the Spirit of Rett streamliner changed their nose configuration from a symmetrical nose to a drop point nose. A hand full maintain more or less normal ride height, but the vast majority of the cars that run on the long course have obvious aero lift at speed. I can see it in my photographs as their wheel well gap is almost always greater at speed that when static at the starting line. Some of them have truely stunning lift jacking the front of the car body up so high it is almost on the upper bump stops of the suspension. A few of them could run over a spare tire at 200 mph and not hit any of the front body work -- might clip the transmission or the differential but nothing on the front half of the body would be dinged.
Larry
-
Larry;
Interesting. Lift on the front end can be tough to solve; those net forces act through the front tires and greatly affect handling at high speed.
"...you can be developing down force on the very nose of the car due to the dynamic pressure of the air stream on the front body work, while at the same time have substantial lift developed only a foot or two farther back on the hood where the low pressure zone typically forms (area the Ford GT40 placed its radiator exit ducts) ..."
I have copies of the original GT40 tests done in the MIRA 200 mph wind tunnel on 12/3/71. The GT40 front lift was 295 lbs @ 0 degrees yaw angle. This was in the configuration as raced at La Mans in 1969. The rear had lift as well, 49lbs increasing to 134 lbs @ 10 degrees yaw. The frontal area was 17.58 sq ft and the Cd was 0.31. This resulted in 558 lbs of drag @ 200 mph and required 298 BHP. Ford never did solve the front lift problem in the GT40.
I thought I'd throw in some measured 200 mph data points for reference in case anyone is interested.
Regards, Neil Tucson, AZ
-
one of the MAIN reasons I run solid suspension I do not want any springs helping the nose change its angle of attack! On RATICAL in its final config,. we had a 1 3/4 " spliter plate around the nose to help generate down force at speed.
-
That is one of the reasons that they put a small chin spoiler on the GT-40's as I understand it that reduced the front lift to a manageable level but never did completely eliminate it.
Bob Bondurant
Shelby GT40 By Dave Friedman page 65
"we finally got to where I could go flat out down the Mulsanne Straight by adding some chin spoilers to the front of the car. I was the third fastest qualifier and got one hell of a start. I got the car up to 212 mph on the straight and my balls grew a little bit when I had to brake to 30 mph for the hairpin at the end of the straight."
Larry
-
I just finished a book on Rolls Royce and with lots of info on the Merlin engine, seems the engineers did some work on the exhaust so that the shorty headers they ran would produce the maximum amount of forward thrust. It was good for a 10 mph speed increase. Using 380 mph as the base speed then the speed increase to 390 would require an additional 10% hp which would probably be around 200 hps on that series of Merlin. That was one reason that they were not in a big rush to go to turbo charging.
Rex
-
Larry;
The GT40 that Bondurant is refering to was fitted with a new nose that was desiigned by Shelby American to try to reduce the lift that Ford's original design was generating at high speed. No significant improvement was achieved by that new bodywork but at Ford Advanced Vehicles (FAV), Len Bailey designed a new nose that made a great improvement in both reduced lift and reduced drag.
Ford played politics by having two GT40 efforts, Shelby American in the US and FAV in the UK. For the inside story of what really happened, find a copy of "Racing In The Rain" by John Horsman-- he worked with John Weyer and later became the JWA director. John does not avoid politics in his book-- unlike Friedman. John observed that Ford's racing program should have been run by their engineering department instead of the marketing organization.
Regards, Neil Tucson, AZ
-
Larry, I understand what you are trying to say about lift. But, am I to assume that everyone in landspeed racing has lift on their cars?...
Yes.
Cars are by nature, round on top and flat on the bottom. i.e. they are primitive lifting airfoils. Locally, many areas of typical LSR sedans push air down and therefore make lift.
Negative rake can compensate and even overcome this, as can air dams, spoilers, wings, and especially diffusers that make the air the car is going through go up rather than down. Local effects like radiator flow take air entering the grill and force it out the bottom of the car in front of the firewall: it went down, therefore that air made lift. F=ma there is no getting around this. Taping off the grill limits that mass flow, so lift is reduced.
What is missing from all of this is some suspension deflection data that would show weight-on-wheels and settle the argument. From my own experience, every front engined car I have drive over 100 mph started to lift the front end at some point unless the back was really jacked up. Ride height did not make anywhere near as much difference as the rake angle.
Draw a line where air starts at the front of the car and where it ends. If it is going up we are creating downforce; if it is going down, we are creating lift. Look at ALL of the air, not just the part the smoke is going over. Road racers figured out 30 years ago to vent their engine compartments to the side instead of down to reduce front end lift due to cooling. Rear engine cars did it better by taking air in on the sides and exiting it up.
-
Several posts ago, I made one very large posting on flat underbodies, venturi shaped underbodies, underbody tunnels, diffusers, front air dams and front mounted splitters and there effects on vehicles. I have received a lot of E-mails asking more in depth questions about their different aspects and construction, so to address them I will split them up and delve a LOT deeper into each of them over the next few posts. I will start it off with the flat underbody and the rear diffuser because it is the most common and most recognized of all of the different underbodies used for down force generation or to merely decrease underbody drag. The flat underbody is the one that is the easiest to build, but as we will see in some of the later posts I will be making on the venturi underbodies and the tunnel underbodies, the flat floors with diffusers are not the best for down force generation.
A diffuser is next to useless without being attached to a flat underbody, so I will combine them together here. The vehicle underbody will need to be as smooth and flat as you can make it, to have an effective and operational diffuser. This post will cover a vehicle with a flat floor in close ground proximity, a rear mounted diffuser and 4 tires in wheel wells.
United States Air Force pilot Joe Kittinger holds the world record for the highest, longest and fastest sky dive. On August 16, 1960, leaping from a helium balloon at 102,800 feet, he fell for 4 minutes and 36 seconds and attained a maximum speed of 614 miles per hour. During a previous jump from 76,400 feet, Joe lost consciousness due to equipment problems and went into a flat spin at 120rpm’s that generated 22G’s on his extremities, which set another record. Joe wore a pressure suit for all of his high altitude jumps. At 63,100 feet in altitude the air density is very low and the air pressure is one‑sixteenth that of the standard sea level atmospheric pressure and water will boil at the normal temperature of the human body, 98.6 °F. The water wetting a person’s lungs would boil as well as the saliva in the mouth, this is called the Armstrong limit, and it is named after Harry George Armstrong.
If you were to use a front airdam with a splitter and a rear spoiler to generate the same amount of downforce as a flat floor with a rear diffuser, the airdam and rear spoiler would generate 20% to 30% more drag than the flat floor with the rear diffuser. Something I like to say and I repeat over and over, downforce from the underbody is virtually drag free and when properly implemented will actually lower overall vehicle drag and generate downforce at the same time.
For land speed racing the air can be a pretty viscous and downright brutal enemy. You have to hit it and force it out of the way, making an opening to pass through as it pushes back against you, forming a high pressure stagnation point at the leading edge of the vehicle, and just when you think you have made it past it, it grabs ahold of you, wrapping its arms around your throat and tries to hang on, forming a low pressure area behind the vehicle in the wake that will pull back on you. Both the high pressure at the front and the low pressure at the rear will retard your forward movement, hindering acceleration and top speed.
The air is normally stationary and not moving very much close to the ground and if it does move it’s only as fast as the wind blows. The air does not really flow over your vehicle very well, unless it is a streamliner. The air is stationary and a vehicle under acceleration is hitting the air particles and forcing them out of the way, making an opening for your vehicle to pass through. Air has weight and density and there for viscosity that resists movement. At sea level, air pressure is 14.7psi at 60’F; 1 pound of air occupies a volume of 13.1 cubic feet. As air temperature is reduced, the air pressures decreases and the air density increases. The movement of your vehicle through the air causes several unwanted things to happen around it, such as increased skin friction, a high pressure area at the leading edge called a stagnation point and a low pressure area at the trailing edge of the vehicle called a wake, that has an effect similar to a suction pulling back on the vehicle and slowing its acceleration. The power required to run a given speed goes up with the cube of velocity. Increasing the speed of your vehicle from 100mph to 125mph requires double the horsepower and going from 100mph to 200mph requires 8 times the horsepower. Drag increases with the square of the speed.
For a typical 4 wheel vehicle, the rolling resistance and aerodynamic resistance will be equal around 30mph on a hard surface and as the speed increases the aerodynamic drag will become the main source of resistance and the rolling resistance will stay more or less constant on a hard surface. The rolling resistance of a wheel being run on a deformable surface will be greater than that for a hard surface and will have a greater variance.
Aerodynamic down force will tend to improve vehicle directional stability at high speed. The amount of downforce produced by the vehicle underbody is related to the shape of the underbody. As vehicle ground clearance decreases, the underbody airflow velocity increases and in turn the air pressure decreases. Downforce should be generated with as low a drag as possible. The highest aerodynamic down force with the least amount of drag can be generated at the vehicles underbody. The “ground effect” underbody parts of a vehicle are very efficient; they will contribute less drag with much more downforce than any other part that can be added to the vehicle, such as wings, rear spoilers, splitters or dive plates.
The rear low pressure wake, pressure drag, will help to drive the airflow through the rear diffuser and the diffuser will have an influence on the lower region of the rear wake, near the ground. If there were a rear deck mounted spoiler or wing it would influence the upper part of the rear low pressure wake. Higher static pressure in the wake will reduce the drag of the vehicle somewhat, but also it will reduce the driving force of the diffuser slightly, leaving it less efficient. The diffuser will influence the lower part of the wake behind the vehicle, since that area of the wake will help to drive the flow through the diffuser. The low pressure in the wake will help to pull airflow through the diffuser.
There will be 3 main things to consider when designing and building a proper diffuser, the inlet to outlet area ratio of the diffuser, the ramp angle inside the diffuser and the vehicle ride height.
The role that the diffuser will serve in the generation of downforce, will be to take the accelerating airflow that is moving under the vehicle from the front to the rear, slow it down, which will cause an increase in its pressure. As the air in the diffuser slows and gains pressure it will cause the air particles to move much closer together and in doing so will cause a pumping effect within the diffuser, pulling air from under the flat underbody, which will cause the airflow underneath the vehicle to speed up and lower its pressure even more, reducing the pressure along the path of the airflow on the underbody, resulting in a decrease of the static pressure underneath the vehicle.
By decreasing the forward angle of the vehicle “rake” it will have the same effect as an increase in the effective angle of the diffuser.
If there is a contoured inlet leading into the vehicle underbody, the downforce will increase as the inlet angle increases. The underbody downforce can be moved slightly forwards or backwards with the angle of the inlet but it will have a much smaller influence than the angle of the rear diffuser has on the point of maximum downforce. An increase in the inlet angle will bring some of the downforce closer to the inlet due to the steeper inlet angle allowing the floor area to be larger and the steeper inlet will allow the low pressure in the underbody to begin at a more forward location and thus generate downforce at a more forward location than for a shallower inlet angle.
The rough underbody will contribute about 20% of the total aerodynamic drag of the average passenger vehicle with no modifications done to it. The flat floor can be used in several ways to reduce drag or generate downforce on a vehicle. One example is to seal the gap between the sides of the vehicle and the ground entirely, leaving only the rear portion open, allowing the low pressure in the wake behind the vehicle to generate the low pressure under the vehicle, thereby generating downforce or you can add a rear diffuser to the flat floor to generate more downforce, you would normally want the highest diffuser angle without causing internal flow separation, to generate maximum downforce. Another way of using the flat floor is to seal the vehicle underside off 360° to the ground and use an active means to lower the air pressure under the vehicle, using a powered fan, there are 2 documented cases of it successfully being used, and both were banned, the last well over 30 years ago in formula 1. With careful construction and placement of a rear wing it can be used to increase the pumping effect on the underbody or to increase the effectiveness of the diffuser, I will cover that in more detail on a later post covering rear wings.
The drag penalty is quite small for a diffuser, compared to the downforce generated from it, so a lot of care should be taken to have the diffuser operating at its best. On a race car the diffuser and flat floor with no side skirts, will contribute much more downforce than a 2 tiered high camber rear wing with a small fraction of the drag penalty. If rules were not so strict vehicle underbodies could make 100% of the required downforce, 30% more than the rear wing and front wing combined, but then there would be a balance issue to sort out. Formula 1 is actually considering a reversal on a 30 year old moratorium on venturi shaped underbodies so there can be more overtaking during the races and allow the vehicles to be more stable when following closely.
The downforce from the diffuser can be increased by 4% to 10% when the rear outside portion of the rims are covered because the air underneath the vehicle is more enclosed by the closed rims and the flow through the diffuser is increased instead of being pulled out by the pumping effect of the rear wheels if they are not covered. By covering the front and rear rims, will result in a smoother flow as well as prevent the flow underneath the vehicle from spilling out towards the sides and maintain a high flow velocity underneath the vehicle. By blocking the crossflow in the rear rims will result in increased downforce due to an increased air flow rate through the underbody and the diffuser, downforce can be increased when the rear rims are enclosed. The rims, if they are not covered, will generate a pumping effect, pulling the air from under the vehicle and lowering the amount of airflow thru the diffuser and decreasing the downforce. By covering the rear wheels only, this will be most sensitive to yaw regarding lift. In yaw conditions the flow underneath the vehicle will be greatly disturbed. Since the rear diffuser is the most important device to create downforce the lift will be greatly affected by the yaw condition.
The interaction between the vehicle underbody and the track surface is strongly dependent on the vehicle ground clearance. The air flow that goes under a vehicle that does not have a flat underside will be subjected to many obstructions along its path. The air will strike the oil pan, steering rack, front suspension, transmission, tires, driveshaft, exhaust, rear end, and fuel tank and all of these will cause points of stagnation and high pressure, that will increase the vehicle drag and generate lift, both of which should be avoided. The flat underbody will keep the airflow moving fast and at low pressure, decreasing drag and lift.
As air flows underneath the vehicle there will be a boundary layer generated on the underside of the vehicle, its effects can cause everything from generate down force to make the vehicle uncontrollable and become air born. A vehicle with a flat underside will have a minimum lift and drag at around 5in to 6in of ground clearance and downforce will start to be generated from there, as the ground clearance is reduced, and drag will gradually start to increase also. Under vehicle air velocity will increase as ground clearance is decreased. Velocity varies with area; if you reduce the area by 50% you will double the air velocity. At higher ground clearances the air flowing through the underbody will be affected by viscosity less; as the ground clearance is reduced it will be affected more by the viscosity of the air, as the speed of the airflow is increased. The most downforce will be generated from a flat floor, with a ground clearance of between 1.5in and 2.75in.
On a vehicle with a flat floor and rear diffuser, the point where the greatest aerodynamic downforce occurs will be where the flat floor meets the diffuser ramp; this point can be moved forward or backwards by moving the location of the entrance of the vehicle floor to the diffuser. The highest downforce will be generated at the transition from the vehicle floor to the diffuser entrance. The flat floor area leading into the diffuser entrance should be kept as smooth as possible, no rivet heads or other protrusions so the flow will have the least disturbance as this will be the point of maximum low pressure and greatest downforce. The diffuser entrance can be moved by changing the angle of the diffuser ramp. The angle of the diffuser floor can be between 5deg and 13deg, with an angle of 9deg to 10deg being most effective, diffuser angels over 14deg will cause the pressure to become too great and cause the airflow to separate, reducing downforce. The majority of the downforce from the flat underbody and rear diffuser will be produced at the rear of the vehicle. This will cause a stabilizing effect at higher speeds but it will also cause the vehicle to become less responsive to steering input. A vehicle with nothing more than a flat underbody and a slight forward angle will generate downforce with no rear diffuser, but without the rear diffuser the low pressure, high velocity air exiting the underbody at the rear of the vehicle into the turbulent wake, will only add more turbulence to the wake and increase the drag slightly, it will not be easy for the flow exiting the vehicle underbody to merge with the wake due to low pressure mixing with low pressure, it will hinder the underbody downforce production. A plain flat floor with a few degrees forward angle and an up sweep of the floor at the rear of between 5 degrees and 13 degrees will generate more down force. The more elaborate you make the design the more down force will be created. The diffuser will allow the air to slow down and gain pressure, when it exits the underbody at the rear of the vehicle the higher pressure air will help to reduce the turbulence and decrease the drag. The low pressure in the wake region and the higher pressure flow exiting into the low pressure region will have a slight suction effect and increase the flow in the diffuser.
General wisdom would dictate that the diffuser angle should be between 5° and 7° and normally that would be true, but we have an unseen aerodynamic ally on our side that will help the diffuser function in many ways, vortices. Where the flat floor meets the diffuser entrance there will be vortices created, much the same way as the vortices are created and functions inside a NACA duct, which will travel down the length of the diffuser, adding rotation to the velocity which will decrease the pressure along its length, causing an increase in the airflow into the diffuser. Low pressure areas will form in the corners of the diffuser where the vortices originate from. This vortex flow also adds energy to the flow and will delay internal flow separation inside the diffuser allowing for a greater than normal diffuser angle to be used. There will be counter rotating vortices formed at the sides of the diffuser, which will improve the airflow through the diffuser. If the diffuser angle is too steep, the underbody flow will tend to separate at the sharp diffuser entrance only to be reattached by the counter rotating vortices further into the diffuser. The loss of downforce in a high angle diffuser at low ground clearances; will be the result of vortex breakdown causing the flow separation. The low ground clearance destroying the vortices leads to the flow separation, the flow would become reattached in the diffuser as the ground clearance is increased and the vortices regain their strength. Low angle diffusers will experience some loss of downforce at low ground clearances due to the vortex breakdown decreasing the amount of flow through the diffuser but the shallow diffuser angle will not experience internal flow separation as in the high angle diffuser during the period of vortex breakdown. If the vehicle is to be run with extremely low ground clearances, to the point that the vortices cannot function, the diffuser angle will be best at between 5° and 7° angle.
The air entrance is just as important as the air exit. There will be a tradeoff of drag and downforce with the diffuser, as diffuser angle is decreased the drag will decrease and as the vehicle ground clearance is reduced drag will increase. The main job of the rear diffuser is to slow the speed of the under vehicle low pressure, high velocity airflow and decrease the speed and let the pressure rise to that of the external free stream airflow or as close to free stream flow as possible, before exiting into the free stream air at the rear of the vehicle. The greater the cross sectional area ratio between the entrance and the exit of the diffuser, the greater the suction effect will be on the underbody. Some down force will be generated by the diffuser because normally the pressure in the diffuser will be lower than external pressure. Downforce is created not only in the diffuser but also under the entire floor area. The diffuser drives the airflow for the complete vehicle underbody. Because of the angle of the diffuser floor, its internal volume will increase as the aiflow moves to the back, causing the air from the underbody to expand as it passes through the diffuser, pulling air through the underbody. The diffuser is not sealed to the ground; it has gaps around the edges allowing air leakage into the diffuser. If the angle of the diffuser is too great, the airflow will separate, because it will not be able to overcome the pressure in the diffuser, causing pressure to build up under the vehicle. The angled flat floor will generate downforce by its self; the addition of a diffuser will increase the velocity of the air under the vehicle. The job of the diffuser is to convert the airflow’s kinetic energy or dynamic pressure into pressure rise or static pressure. The expansion of the air from the underbody in the diffuser slows the air, increasing its pressure. As the air is slowed down it is forced to become denser as the pressure increases. The diffuser can be a simple upsweep in the flat floor at the rear of the vehicle, but adding side plates that drop close to the ground and forming a tunnel will increase its effectiveness greatly. The side plates will allow the diffuser to generate more downforce at a lower speed. There will be a counter rotating vortice generated at the inside of the diffuser side plates, enhancing airflow through the diffuser. The attached vortices inside the side plates will trail downstream into the wake behind the vehicle and cause a vortex induced suction. At very low ground clearances, the effect of the counter rotating vortices will be reduced as ground clearance is reduced. The counter rotating vortices will help to keep the airflow attached to the diffuser surface longer than it would be expected to at higher angles. The diffuser angle should be between 9deg and 10deg to be most effective. The smooth flat floor of the vehicle leading to the diffuser will allow a larger area over which the low pressure air can act, creating more downforce. The diffuser itself does not create downforce, it is the area in front of the diffuser that the low pressure acts on that creates the downforce. Another benefit of the diffuser is that by it being located at the very rear of the vehicle, allows all of the high pressure mass airflow to help fill the void behind the vehicle, reducing the wake size and the resulting pressure drop and the induced pressure drag. The diffuser will reduce the turbulence in the wake, thereby decreasing the pressure drag.
The rear diffuser and the angle it is set at will have a big influence on the underbody airflow and the wake behind the vehicle and can contribute to reducing the pressure drag in the rear wake of the vehicle. At the lower angles for the diffuser, the downforce will be gradual as the ride height is reduced and at very low ground clearances the falloff of downforce will be more gradual also. At the higher angles for the diffuser, downforce will build more quickly and falloff quicker at reduced ground clearances.
The larger the floor area can be maintained, the more downforce will be created. Efficiency in the inlet and diffuser can be sacrificed somewhat, for the sake of a larger floor area, for the low pressure to act upon. The amount of downforce generated from the flat floor is proportional to the pressure differential and the size of the vehicle floor. Generally for every 1% increase in underbody airflow velocity, generated by the rear diffuser, will generate 3% of extra downforce. The efficiency of the downforce created by the flat floor is only as good as the efficiency of the diffuser.
A vehicle with nothing but a flat smooth underside installed can be neutral, generate lift or generate downforce, depending on the angle of the underside and the effect will be magnified with reduced ground clearance. Everything being equal though, as ground clearance is reduced, drag will increase; because of the viscous effects of the airflow under the vehicle will be increased as ground clearance is reduced due to the airflow velocity increasing. As ground clearances become smaller air velocity will increase, thus increasing its viscous drag. As the ground clearances fall below 5in to 6in, the effects of the ground will begin to be felt and be magnified as the ground clearance is reduced.
If side plates are used with the diffuser, the thickness of the side plates that drop down close to the ground on each side of the diffuser, that makes the outer walls, will have a big effect on the downforce and drag of the diffuser. The thickness of the side plates will have an influence on the amount of airflow entering the diffuser from the sides. The more high pressure airflow that leaks into the diffuser from the sides, the more the pumping effect of the diffuser is decreased. The thinner the side plate is the less outside airflow from the sides will enter the diffuser, thereby maintaining a lower pressure between the side plate and the diffuser ramp. If airflow thru the flat underbody is maintained and flow in the diffuser is not allowed to separate, the boundary layer on the underbody will remain thin with a high friction drag at the underbody surface.
Diffuser efficiency can be increased with the addition of longitudinal vanes to the diffuser. The longitudinal vanes can be straight or curved in the diffuser. Adding straight longitudinal vanes in the diffuser will increase the friction drag but they will generate an extra mixing effect that will increase the airflow in the diffuser and cause the air flow to remain attached, you can think of the extra longitudinal vanes as turbulators, increasing the airflow in the diffuser. If highly curved vanes are used in the diffuser they maximize the mixing effect inside the diffuser and allow a steeper diffuser angle to be used. With the addition of longitudinal center vanes in the diffuser, it will maintain a higher flow velocity and help to prevent the rear wheel wakes from extending towards the center of the vehicle and decreasing the diffuser efficiency, if the rear wheels do not have covers to prevent the crossflow. This will help to maintain a higher flow rate thru the diffuser and increase the downforce. There will be a lower pressure generated in the diffuser with the center vanes than without them due to the higher flow velocity. The position of the center vanes can be tuned to achieve even better performance from the diffuser. The increased flow rate from the diffuser will decrease the wake at the rear of the vehicle. The center vanes channel the flow and will generate more downforce with only a minor increase in drag. The diffuser with the center vanes will be influenced more by small yaw angle movements of the vehicle than a diffuser with no vanes but the diffuser with vanes will generate more downforce regardless of yaw than with no vanes. At angles where the flow starts to become separated in normal open diffusers, the multiple-channel diffuser will have large improvements in downforce with a minimal increase in the drag, allowing for increased diffuser angles and a decrease in the ride height sensitivity. The improvements in the diffuser will occur through improved diffuser pumping and pressure recovery in the inner and the outer channels of the diffuser. Diffusers with multiple-channels will generate greater downforce per unit of area than conventional open diffusers.
The rear downforce will be most affected by large increases in yaw, because the rear diffuser will lose efficiency in yaw conditions. As vehicle yaw increases the airflow underneath the vehicle is significantly disturbed. Since the rear diffuser is the most important device to create vehicle downforce, as the yaw increases the downforce will decrease.
In an ideal diffuser the pressure at the diffuser exit would be that of the freestream flow. If the design of the diffuser were to cause the airflow pressure inside the diffuser, to reach atmospheric pressure before the diffuser exit, it would hinder the performance of the diffuser due to making the remaining portion of the diffuser behind the point of where atmospheric pressure was reached, to be a waste, because the job of the diffuser is to increase the airflow pressure to that of the freestream. If freestream pressure is reached inside the diffuser too early, before the diffuser exit, it may be due to two reasons, either the diffuser ramp angle is too large or the inlet to outlet area ratio is causing the expansion of the airflow to be completed too early. As the ratio of the inlet to outlet area increases this will generate greater pressure recovery, decreasing the pressure at the inlet.
If a rear wing is mounted to the vehicle, it will influence the upper regions of the base wake whereas the diffuser influence the lower regions of the base wake and since the base wake drives the flow through the diffuser, the suction force created by the vacuum in the wake behind the vehicle will be far more detrimental to the vehicle performance than the high pressure at the leading edge stagnation point.
It will be much harder to construct, but a diffuser with a curved roof will generate more downforce and can be operated at a more extreme angle than the traditional flat roof diffuser. A curved diffuser roof will increase the rate of expansion more gradually as the diffuser cross section area is increased inside the diffuser. As the expanding cross section of a diffuser with a flat roof slows the incoming airflow, the air pressure will increase at a much faster rate as it moves towards the rear of the diffuser. If the diffuser angle is to steep, the airflow velocity in the diffuser will be greatly reduced, to the point of becoming nearly stagnant at the diffuser outlet, thus leading to airflow separation and a dramatic increase in turbulence inside the diffuser. The separation in the diffuser can be decreased and the proper diffuser expansion rate can be maintained with the use of a curved diffuser roof. The use of a curved diffuser roof will have a more gradual and controlled change in the buildup of the air pressure inside the diffuser, reducing the separation and turbulence of a flat ramp high angle diffuser. Optimal diffuser roof angle will be dependent on the vehicle ride height due to the diffuser pressure recovery being highly influenced by the ride height. The diffuser pressure recovery coefficient and diffuser downforce performance will be controlled by the diffuser inlet to outlet area ratio. As the vehicle ride height is decreased the initial expansion rate inside the diffuser can be decreased, so shallower diffuser angles will be required and increased ride heights can use larger diffuser angles.
There are 2 types of vortex generators that can be used on the vehicle underbody; they are classified by their height. The first one is the smallest in height and is referred to as a Sub-Boundary Layer Vortex Generator and it will normally be about 60% to 80% of the height of the boundary layer it is placed in and the vortices it will generate will not be very strong but they will be generated directly in the boundary layer, the Sub-Boundary Layer Vortex Generator does not extend above the boundary layer so its drag will be very low. The Sub-Boundary Layer Vortex Generator will generally have a greater angle than the regular vortex generator has, normally the Sub-Boundary Layer Vortex Generator angle will be around 20° to 25°. The second type of vortex generator is much taller than the Sub-Boundary Layer Vortex Generators and there height extends above the boundary layer and will generate much larger and stronger vortices that will be generated in the airflow above the boundary layer and there drag will be much greater than for the Sub-Boundary Layer Vortex Generators, these type of vortex generators will have shallower angles than the Sub-Boundary Layer Vortex Generators, normally around 10° to 15°. A stable and long vortex will reduce the pressure along its trail, and if used in the vehicle underbody with a flat floor and adequate ground clearance, will increase the downforce generated by the flat floor. If vortex generators are installed on the vehicle flat underbody it would be best to install them very close to the front of the vehicle, so the vortices will have time and the distance for their trails to generate the low pressure in and around them. The size and strength of the vortices will be mainly dependent on the size of the vortex generator. So a very strong and stable vortex can be generated, the vortex generator needs to be taller than the boundary layer it is placed in, on the vehicle underbody. The vortices will increase in strength as the ground clearance is reduced, increasing the suction effect and low pressure but at the same time drag will increase also. The underbody vortex generators can create 15% to 30% more drag but contribute from 30% to 75% more downforce. Vortex Generators increase downforce by increasing the dynamic loads by creating strong vortices near solid surfaces. Unbound vortices trapped in ground effect can be used to increase the downforce, because it increases the vorticity of the airflow. The bottom surfaces of a racing vehicle with a flat underbody, generally moves very close to the ground at almost-zero angle of attack, which makes stability of trapped vortices less of an issue, increasing downforce. If there are openings or breaks in the flat underbody that can make the airflow become detached from the surface of the underbody, such as openings for suspension arms to have vertical movement or frame connectors, vortex generators can be used to delay the separation or can make the airflow reattach to the underbody. Most underbody vortex generators will have wing profile shapes. If single vortex generators are used at trouble areas to help the airflow they should be angled between 10° to 25° with respect to the vehicle’s longitudinal axis.
If multiple vortex generators are placed close together, the optimum vortex generator spacing should be 2 times their height in lateral spacing and the optimum orientation of the pair of inner and outer vortex generator should be between 10° and 20° with respect to the vehicle’s longitudinal axis. The vortices from the vortex generators will breakdown and disappear right after they enter the diffuser inlet. The vortices will remain stable throughout the center section of the vehicle underbody until the moment that they enter the diffuser inlet. The vortex breakdown is due to the vortices not being able to handle the adverse gradient inside the diffuser. A vortex created close to the diffuser will be very strong and will cause a disruption in the diffuser flow. The area behind the vortex generators will produce a very strong suction on the vehicle underbody. As the vortex generators are placed closer to the diffuser inlet they will become less effective due to the suction peak in the diffuser inlet. As the vortex generators are placed closer to the diffuser inlet they will begin to interfere with the diffuser function and cause underbody downforce to begin to drop off. As a rule vortex generators should not be placed on the underbody past the rear edge of the front door when a rear diffuser is used with a flat underbody. Despite the fact that the Vortex Generators are highly cambered the air flow does not separate on the vortex generators. At the Bottom Rear of the underbody, the boundary layer is not able to cope with the diffuser’s adverse pressure gradient, causing it to separate. This severe separation will cause underbody downforce to drop off quickly, causing lift to increase and downforce on the back of the vehicle to decrease. If the vehicle has a rear wing or rear spoiler attached, the separation of flow in the diffuser can result in a decrease of downforce from the rear wing or spoiler also, creating a cascade effect that will increase the vehicle lift even more. If a vortice is created on the vehicle underbody it will be stretched and distorted due to the accelerating flow in the underbody, decreasing the radius of the vortice.
If vortex generators are used at the middle to rear part of the vehicle underbody that has a rear diffuser installed it will lead to a very inefficient way of increasing the downforce of the vehicle, because the drag of the vehicle will increase and downforce will decrease to such a level that the lift-to-drag ratio will be below that of the original vehicle before the vortex generators were installed. Vortex generators should be placed away from the front tires, centered between them if possible and not be placed directly behind the front wheels either as they will have a negative influence on the drag of the Front wheel wells. If cambered vortex generators have to be placed close to the diffuser, decreasing the amount of camber will help to reduce the separation in the diffuser and help to maintain its downforce.
Vortex generators on the vehicle underbody can produce a significant amount of drag, but they can provide the vehicle with a lot of downforce as well. In very high horsepower applications or in situations of extreme loss of rear grip, the drag, unlike the downforce, is not very important. In times of low tire grip, high downforce levels can be achieved from the underbody mounted with vortex generators, the drag coefficient is not much of an issue in high horsepower situations. Vortex generators can be used as a tuning aid, they can be easily modified so there camber, locations and length can be adjusted, as well as removed and installed easily.
-
say whaat :-o
-
Isn't this a thread about the aero considerations of exhaust flow?
-
superford317,
Thank you for taking the time to continue this post. And thanks for everyone's contribution.
Where would you dump the exhaust? The Elise has it exit in the middle between the diffusers. The Ferrari has no intrusion into the underbody or full width diffuser and have not looked into this very far. I could put the exhaust anywhere including out the side or wheel wheelwell, however I think above the diffuser to the rear and perhaps slightly up.
Geo still working the car
-
^^^ same question as above. I am thinking about the flat bottom, diffuser design and its relationship to the exhaust muffler and exit. I am currently going to cut the rear bumper higher (so it doesnt act like a parachute) and the diffuser can extend a little longer. However I need to decide if I want the exhaust to run all the way to the back, and if so, exit out the center or tuck up and maybe come out the sides of the rear bumper...
-
what kind of car?
-
2013 Subaru Sti Hatchback. We are trying for 206 mph in the mile (airstrip attack/mojave mile etc)
-
^^^ same question as above. I am thinking about the flat bottom, diffuser design and its relationship to the exhaust muffler and exit. I am currently going to cut the rear bumper higher (so it doesnt act like a parachute) and the diffuser can extend a little longer. However I need to decide if I want the exhaust to run all the way to the back, and if so, exit out the center or tuck up and maybe come out the sides of the rear bumper...
Here's a Yahoo link for Mike Reichen's Mitsubishi EVO.
https://search.yahoo.com/search;_ylt=A0LEV1pBH0VWiwUAH_tXNyoA;_ylc=X1MDMjc2NjY3OQRfcgMyBGZyA3lzZXRfaWVfc3ljX29yYWNsZQRncHJpZANKNkRqZExDbVRmNnh1WVFqWk1CLmZBBG5fcnNsdAMwBG5fc3VnZwMxBG9yaWdpbgNzZWFyY2gueWFob28uY29tBHBvcwMwBHBxc3RyAwRwcXN0cmwDBHFzdHJsAzIzBHF1ZXJ5A21pa2UgcmVpY2hlbiBtaXRzdWJpc2hpBHRfc3RtcAMxNDQ3MzcwNjI1?p=mike+reichen+mitsubishi&fr2=sb-top-search&fr=yset_ie_syc_oracle&type=orcl_hpset&fp=1
He went 237 at Maxton and IIRC has been in the 215 at Wilmington and over 200 at Loring.
His car should give you some ideas for front end aero. Also, IIRC his exhaust does exit out the back.
HTH,
Gregg
-
You are awesome! Thank you!!!
-
I would put the exhaust straight back under the car aimed back down the track--
I know a blown classic altered that picked up 5+ MPH by changing the exhaust from blowing out to blow back---also you will not have to put a steering correction to offset the side thrust of the single exhaust!
-
If a post is a "cut & paste" it would help to cite your sources.
Paragraphs, paragraphs, paragraphs!
Regards, Neil Tucson, AZ
-
Hey greg, was checking out those links. Apparently the exhaust on that Evo exits the front bumper... man that thing is quick (for us 4 cylinder awd guys that is!)
-
Hey greg, was checking out those links. Apparently the exhaust on that Evo exits the front bumper... man that thing is quick (for us 4 cylinder awd guys that is!)
Those are old pictures when he did have the exhaust out the right side.
Here is a pic of from Loring in July:
(https://lh4.googleusercontent.com/-HWLpF3vhLeA/VbQrt_JL6JI/AAAAAAAAGHE/L154Olvpl9E/w947-h710-no/IMG_1141.JPG)
In this pic you can just barely see vapor coming out the exhaust at the back:
(https://lh4.googleusercontent.com/-1y2l-SYhEa8/VbQrN6QngoI/AAAAAAAAF8E/SU_qXOLmV1M/w947-h710-no/IMG_1055.JPG)
I'll see if I have any better pics.
Gregg
EDITED TO ADD:
Here is my Google+ link to the above pics:
https://plus.google.com/photos/115248360774022016668/albums/6175608284824009201
-
Not really that good of a pic but you can barely see the exhaust. It is below the battery charging leads and is next to the second strake.
(http://i947.photobucket.com/albums/ad318/GKABBT/Wilmington%2005032014/IMG_0028_zps1389d66e.jpg)
HTH,
Gregg
-
Yep! I see it now.'looks like he has it just sticking through the diffuser.