Landracing Forum
Tech Information => Aerodynamics => Topic started by: CarGuy250 on January 25, 2016, 01:27:57 PM
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Hello, I'm brand new to this site.
I have been wondering about ways to "suck" the bottom down at high speed.
I want to figure out a way how to create "negative pressure" under the car to suck the bottom of a car down. I was thinking about making a "small-scale model" version for testing purposes only. One thought was possibly using electric High-suction fans (similar to the 1970 Chaparral 2J Can-Am racing car ("sucker car").
I has some questions regarding drag, lift and top speed, etc. with doing something like this:
1.) Will creating a vacuum (negative pressure) underneath the car cause a "drag" on the car when it's moving?
2.) How would I figure out the "correct amount of vacuum" to create underneath the car to keep it from "lifting" at very high speeds?
3.) Would too much "Negative Pressure" slow the car's acceleration & reduce the potential top speed?
4.) What is more important in regard to "negative pressure" sucking the car down...a large surface area or a large volume?
5.) Would creating a "negative pressure" be any different than just any weight to the car?
6.) Would you have to increase the vacuum under the car as the car increases speed and lower the vacuum at lower speeds?
Any formulas that are related to weight & down-force, acceleration, speed, lift and drag as related to "negative-pressure" would be great!!!
My main goal is I'm trying to increase the down-force without increasing drag (like using a wing).
Any direction of how to figure some of this stuff out before I build a "scale model" of it would be very helpful.
Thanks a ton!
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There is no magic or free rides just less expensive ones!
Weight, Splitters on air dams in the front and diffusers or spoilers in the back YMMV
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Carguy 250,
You are asking a lot of aero questions that really cannot be answered by a lot of "rules of thumb". My suggestion is to get some of the better aero books that are available today and find out what the answers "may" be. A couple of my favorites are: " Competition Car Aerodynamics" and "Competition Car Down Force" both by Simon McBeath and both are very readable and not to encumbered with excessive math. "Race Car Aerodynamics" by Joseph Katz is also a good start and my favorite is "The Leading Edge" by Goro Tamai which pretty technical and is about streamlining sun racer cars but the basics and concepts of aero are very well done. Start with one or all of these and then I think you will find what you are looking for and also find out that aero study is very interesting and not very "cut and dryed"
Rex
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LOL
Rex what a diplomat :cheers: :cheers:
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Just as Sparky said, there is no free ride when it comes to aerodynamics. Just as lift creates drag, so does negative lift. The drag increases logarithmic-ally the faster you go unless you can somehow control it using adjustable wings. But gravity is free and does not care how fast you go. Using the right tires and adding weight is the best way to get traction.
Don
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To what Don said about traction from weight, not aero: When I was running the nitrous bike we got about a 7MPH gain -- by putting sixty pounds of lead on the swingarm. It cut way down on wheelspin -- and the aero "cost" was negligible.
Aero works, lead works, YMMV. What else can I say? :-D
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As stated, there is no free lunch. Any lift (positive or negative) causes Drag, the horsepower of which to overcome goes up by cube of the speed. ie a lot and fast. Rolling resistance does go up by adding weight. This resistance is proportional to speed in one part and to the square of speed in another but is much smaller even so. Rolling resistance on the salt is certainly considerably higher than on pavement but the advantage of not adding aero drag with tunnels, wings etc is considerable when adding weight. Of course, acceleration rate is slower with weight but we are not in a typical drag race and HP exceeds traction many times. Be careful where you put the weight. Tail heavy vehicles are not happy when they loose traction (can you say spin?)
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I don't even want to THINK of a spin while I'm on a bike. :cheers:
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Rex talking about Tamai :roll:
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I want to figure out a way how to create "negative pressure" under the car to suck the bottom of a car down. I was thinking about making a "small-scale model" version for testing purposes only. One thought was possibly using electric High-suction fans (similar to the 1970 Chaparral 2J Can-Am racing car ("sucker car").
What class are you thinking of entering? Before you expend a lot of time and effort be sure you're working on an approved concept.
Sliding skirts have been used on road race cars with a fair degree of success when legal.
I'm not sure how effective sucker technology would be at higher speeds. I would think a fairly effective seal is essential.
I have my doubts about how effective a scale model would be as it would be difficult to scale the amount of seal and The suction would probably too little to measure accurately on a small model.
Pete
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One thing to consider in the design is what happens if you go sideways. Does all downforce cease?
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CarGuy250, I agree with all of the above replies. You didn't mention what
application (Street driving? Road racing? Drag strip? Airport runway? Bonneville?)
you have in mind. Here are some things to think about:
For street use, using sticky and large-enough tires should give you all the traction
you need for all 4 tires. But using airflow could help. If this is provided by motor(s)
or engine(s), it adds a significant amount of weight and cost. And controlling/adjusting
the amount of this pumped air could be quite challenging. If the airflow is automatically
provided by your forward motion, the effect of this airflow will vary a LOT, based on
your speed. At relatively low speeds, you won't notice much difference.
At higher speeds you will, but achieving this will require more horsepower to push
the vehicle. Do you have this power available?
For road racing, the same applies.
Drag racing is mainly about acceleration. You could refer to what others have
done to succeed. Unless you are going REALLY fast, you could probably benefit
more from shaving weight and using sticky tires and a smooth overall vehicle
shape, compared to the power-hungry aero tricks (such as "wings", "spoilers",
"flaps", etc.). And using suction, provided by motor(s) or engine(s) adds a lot
of weight. Not likely to provide a net benefit.
On an airport runway, which already provides good traction, you won't have as much
distance available for acceleration. So mass won't be your friend there. This
suggests that you don't use suction provided by motor(s) or engine(s). Downforce
provided by aero could help if your vehicle is powerful enough to need this. Many
top fuel dragsters have more power than they can put to the ground unless they
use the spoilers to create aero downforce. And this applies to funny cars as
well. But they all have available horsepower to use for the aero downforce.
At Bonneville and other dry lake events, you have a long distance in which to
accelerate. So the mass of your vehicle is not the only important factor. It may
(based on the weight on your drive wheels) actually help by increasing traction,
thereby providing better acceleration.
Many, many vehicles have so much power available to the drive wheels that
spinning those wheels becomes a HUGE issue. Not only because spinning them
limits forward acceleration, but perhaps more importantly (especially from a
SAFETY and CONTROL viewpoint), spinning wheels don't do a good job of
keeping a vehicle aimed where you want it to go. Spinning rear wheels have
a tendency to push the rear of a vehicle sideways and forward at the same
time. This causes the vehicle to get sideways, frequently causing it to roll.
This should be prevented/avoided as if your life depends on it, as it well might!
Generally, I suggest that you err on the side of caution, and make only small
changes at a time, gradually improving the results when these small changes
are successful. Safety first! And good luck to you.
My suggestion is to keep the aerodynamic shape of your vehicle as smooth
as you can. And make as much (dependable!) horsepower as you can. Then
take any necessary steps to get it all to the ground. The methods for this
are likely to depend on things like your budget, the class of your vehicle
(in competition).
Good luck, and welcome to the discussion.
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I want to figure out a way how to create "negative pressure" under the car to suck the bottom of a car down. I was thinking about making a "small-scale model" version for testing purposes only. One thought was possibly using electric High-suction fans (similar to the 1970 Chaparral 2J Can-Am racing car ("sucker car").
What class are you thinking of entering? Before you expend a lot of time and effort be sure you're working on an approved concept.
Sliding skirts have been used on road race cars with a fair degree of success when legal.
I'm not sure how effective sucker technology would be at higher speeds. I would think a fairly effective seal is essential.
I have my doubts about how effective a scale model would be as it would be difficult to scale the amount of seal and The suction would probably too little to measure accurately on a small model.
Pete
Pete. the skirts don't have to slide. :-D Remember what Gordon Murray did on the Brabham???
The never could find a scrutineer that could run fast enough to measure. :cheers:
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The Chaparral car and concept is illegal for a couple of reasons. Can't have more than one engine in anything but streamliners and diesel trucks. (assuming that is not what you are building). Fans pointing out the back as that car had would add thrust of some amount- not legal (IMO). Electric motors with enough grunt to accomplish what you want would be sizeable. Where is that power coming from? Driving such fans from the primary engine would be complicated to say the least. The other changes to make such a car work puts you in CC at least since belly pans, skirts etc are necessary to make the concept work. Non Chaparral like tunnels for down force which are used in many other racing venues also would fall into the belly pan concept so class again would be Comp Coupe/Modified sports. Complicated to build unless doing it from scratch as in not just adding such devices to an existing vehicle.
You were looking for formulas. Basic ones include:
HP= D x V/375
D=drag force in lb. =qACd where q= dynamic pressure lb/sqft= .00256(V^2) lbs/sqft at standard temp and pressue. A=frontal area, Cd= drag coefficient.
Formula for lift is similar substitute Cl for Cd where this is coefficient of lift and is range of +.400 to -.250 (positive or negative lift) so L=qACl.
Total aero drag would then be D+L. ( if there is no lift or down force (negative lift) the result would be aero drag (D) only.
Road load: F (total drag)=WCf + WCvV +D where W is weight of car, Cf is coefficient of frictional resistance, Cv is coefficient of viscous resistance and D is aero drag which in this case includes L.
Approximate values of Cf are .015lb/lb vehicle weight and Cv= .0001lb/lb weight.
These formulas include numbers for rolling resistance of tires (likely higher for on the salt as I mentioned before) and viscous resistance which is at least part of drivetrain losses. Figuring the coefficients are not easy and may require empirical testing. Just look at all the arguments about the relationship of flywheel HP and wheel HP on various dynos and how to relate them. A percentage or a fixed number? Very hard to do.
The above formulas come from the Chevrolet Power book. Don't shoot me if you disagree. :)
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The idea of negative pressure under a car is nothing new. There have been some excellent comments here from others on the topic. For it to work the specifics of the application need considered such as speed, clearance, body shape etc.
Look for the general topic car down force. High speed cars such as F1 and Nascar use surfaces to generate downforce like the body or wings. Another method they use is what you are talking about which is low pressure under the car. Part of this approach is by means of diffusers. Also you might consider the topic of "ground effects". These are topics you might read about. This is a method that is studied and used widely. I also expect that some of the faster land speed cars are using this or at least studying it.
As far as formulas go, one that applies is Bernoulli's principal. Wikipedia has a general explanation of Bernoulli's principal and various forms of the equation that might apply in some way. You can work out what variation applies to the specific element you are considering. Do not expect general equations that apply to your specific application, even to your specific fender, wheel, diffuser, scoop etc. There are plenty of other equations in their specific form that applies to your specific element such as drag, lift, Newton's second law, static pressure etc and they must all be accounted for if you are taking a purely analytical approach. I am not saying this to discourage you but this is a topic that has been rigorously studied very much.
A general knowledge of the principals, experimentation and other's experience can go a long way without a purely analytical approach.
As far as getting downforce thru a low pressure zone under the vehicle without using additional horsepower due to additional aerodynamic drag, this is not entirely out of the question. Most vehicles have a low pressure zone behind them. To go fast (Nascar) or to maximize gas mileage (car manufacturers) many smart people are trying to minimize this pressure differential and decrease its size. If your body shape already makes this low pressure zone inevitable (you have already sacrificed power to it) then make use of it by sucking on the bottom of the car with it. A diffuser may take part in this air exit function. Some diffuser/body relationships actually decrease aerodynamic drag.
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I has some questions regarding drag, lift and top speed, etc. with doing something like this:
1.) Will creating a vacuum (negative pressure) underneath the car cause a "drag" on the car when it's moving?
Additional negative pressure will cause at a minimum an increase in rolling resistance by the increase in downforce. How the downforce is applied can have an external influence that could increase drag. It depends on this is accomplished.
2.) How would I figure out the "correct amount of vacuum" to create underneath the car to keep it from "lifting" at very high speeds?
At very high speeds the force caused by the vacuum has to overcome lift. Lift or downforce, depending on the shape of the vehicle, increases with the speed squared.
3.) Would too much "Negative Pressure" slow the car's acceleration & reduce the potential top speed?
Same as for 1.)
4.) What is more important in regard to "negative pressure" sucking the car down...a large surface area or a large volume?
The surface of the underside facing downward. The negative pressure times the area equals force. The pressure may not be constant along the length or width of underbody. Depends on how this done.
5.) Would creating a "negative pressure" be any different than just any weight to the car?
Same as 1) and 3.
6.) Would you have to increase the vacuum under the car as the car increases speed and lower the vacuum at lower speeds?
Lift and drag will increase with the square of speed. So suction on the underbody may also have to increase to overcome that lift increase. How you do this will influence the external shape and contribute to lift.
Any formulas that are related to weight & down-force, acceleration, speed, lift and drag as related to "negative-pressure" would be great!!!
My main goal is I'm trying to increase the down-force without increasing drag (like using a wing).
Any direction of how to figure some of this stuff out before I build a "scale model" of it would be very helpful.
Thanks a ton!
[/quote]
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Down Force = V^2 x A x RH / CD + HP x RPM / AC^3
ULC
RH relative humidity
CD coefficient of drag
A area
HP horsepower
AC acceleration
V velocity
RPM rev/min
ULC universal L Constant
The ULC is an experimentally determined constant. Sometimes 42 is applied but this is not suggested. Please read Douglas Adams' research on 42.
The fictitious equation above should be applied under adiabatic conditions.
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DaveB,
Interesting formula, any chance you can point me to were I can see where and how it was derived?
Rex
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Quite funny really.... If I can get the adiabatic part working I could do anything.
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The proper background of the fictitious and adiabatic equation can be quickly determined by checking the Douglas Adams derivation of 42 on Google.
You may also notice that I did not specify units for any of the variables. Using any units you prefer yields the same quality of answer.
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But you would need the Infinite Improbability Drive to apply it
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Absolutely! :-D
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Well the real key to all of this is now do they make SFI rated towels.
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DaveB said "You may also notice that I did not specify units for any of the variables. Using any units you prefer yields the same quality of answer." I did really miss the units part and you are so correct regardless of the units used the answer is BS! 42 is obviously the universal fudge constant, it is all so clear now!
Perfect!
Rex
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:cheers: 8-)
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No one pointed out that properly solving this equation also requires a firm understanding of bistromathics. :cheers:
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You guys are wacked out on goofballs. LOL
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Thank you :-D
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Down Force = V^2 x A x RH / CD + HP x RPM / AC^3
ULC
RH relative humidity
CD coefficient of drag
A area
HP horsepower
AC acceleration
V velocity
RPM rev/min
ULC universal L Constant
The ULC is an experimentally determined constant. Sometimes 42 is applied but this is not suggested. Please read Douglas Adams' research on 42.
The fictitious equation above should be applied under adiabatic conditions.
ROTFLMAO!!!
Seriously, the equations expressed before this post are so far off of reality they defy description.
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I hope that everybody by now realized this equation was a joke. I did call it fictitious and others also joined in on the silliness in their posts. :-) It seemed like an appropriate way to answer the same repeated questions. My post #14 did show that any equation to determine down force would have to be derived for the specific application. Because of this, any equation that did it in terms of generalities would be a joke. Come on, engine RPM directly relating directly to down force? :roll: 8-)
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Dave we like to :dhorse: :dhorse: around here. :cheers:
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Just as Sparky said, there is no free ride when it comes to aerodynamics. Just as lift creates drag, so does negative lift. The drag increases logarithmic-ally the faster you go unless you can somehow control it using adjustable wings. But gravity is free and does not care how fast you go. Using the right tires and adding weight is the best way to get traction.
Don
I don't want to open a can of worms here but two top guys who have good reputations, one being Joe Katz and one other who has been well over 400 disagree with your statement. :wink:
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Someone may correct me (memory not what it used to be) but I think Roger Lessmans liner was a ground effects car = negative pressure for downforce. If or when you get out of shape in yaw, you lose downforce = many problems. :cry: :cry: Not saying negative pressure is bad, just don't depend on it entirely.
Ron
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Just as Sparky said, there is no free ride when it comes to aerodynamics. Just as lift creates drag, so does negative lift. The drag increases logarithmic-ally the faster you go unless you can somehow control it using adjustable wings. But gravity is free and does not care how fast you go. Using the right tires and adding weight is the best way to get traction.
Don
I don't want to open a can of worms here but two top guys who have good reputations, one being Joe Katz and one other who has been well over 400 disagree with your statement. :wink:
Perhaps we can say that if a given car has lift naturally, and that if it is modified/designed further to generate down force (negative lift) to counteract the positive that is present (ie making it neutral) that there is (may) be no drag penalty?
Gravity is free but it does care as rolling resistance is proportional to velocity. From that stand point, lighter is better. Traction and staying straight may make the penalty worthwhile though. (put the weight in the right place)
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the downside to weight---it is still a drag race
for the 3+ cars especially the 4+ ones and the want-a-b's
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Perhaps we can say that if a given car has lift naturally, and that if it is modified/designed further to generate down force (negative lift) to counteract the positive that is present (ie making it neutral) that there is (may) be no drag penalty?[/quote]
Jack, as an exercise, I think it quite possible to create neutral lift by generating negative lift to counteract positive lift but would likely try reducing positive lift first. But drag will still be a question how you went about generating that negative lift. If you use a wing you may generate much more drag than using ground effects. Ground effects can be tricky to get right, especially on a long car.
This reminds me of an old Jimmy Stewart movie about Charles Lindbergh's flight across the Atlantic. The story tells of a fly in the cockpit and Lindbergh asking himself quizzically if the fly landed on the airplane, would it add weight or if it flew around in the airplane, would it not add weight. I hadn't thought about this until reminded of a similar experience when at Lake Gairdner. I had a fly in the cockpit with me and wondered if he would get into the 200 MPH club too.
John
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It is my understanding, and sort of what I was trying to say, is that if you have a body that is neutral, any changes that produce lift- positive or negative-will also produce some drag as well. There is no free lunch, it all costs horsepower. Wings may be very efficient at lift production but as you say are more complicated and may have a higher penalty. Seem to work well when horsepower is in excess- and traction is good too. All a trade off.
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Having two separate forces (positive lift and negative live will cause two sources of drag. Wayno
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There is at least one ground effects car out on the salt I can think of.
Ground effects is a good way to create downforce (negative lift) but it is subject to some limitations and here are a coupe of things to bear in mind:
- Increased Pitch Sensitivity
- Increased Yaw Sensitivity (in relation to downforce created at larger cross wind angles)
- Limits Suspension Geometry
- Can impact upper body flow
Unless you have access to a rolling road wind tunnel (I wouldn't trust CFD for absolute lift values - especially not steady state) I would tread very carefully with developing grounds effects. Make speed increments slowly, check live downforce data (easiest way is tracking suspension loading at speed) for signs of the ground effects "stalling" (not really the right term - but it makes the point).
Teams with big budgets have backflipped cars into tree lines when getting ground effects wrong (cough... Mercedes).
You can build a scale model to test the effects.... however, you will need to flow much faster than the full scale car to get in the correct reynolds regime. Once at the right reynolds number you can essentially scale the force values. Then you will need to consider the design of the scale tunnel you are constructing carefully, incorrect boundary layer (i.e. anything above a 1mm displacement thickness) will give false results.
This is a very tricky subject, F1 teams invest billions on this question alone.
Don't want to discourage you, just trying to keep you safe. Please try it, but increment the design of the ground effects slowly, use the great white dyno and read the data carefully.