Author Topic: Formula for aero drag.  (Read 16694 times)

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Offline 7800ebs

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Re: Formula for aero drag.
« Reply #15 on: November 05, 2013, 11:13:31 AM »
This is fun..

Reminds me of  what Rich Fox told me years ago. I was telling him of this computer program that predicted acceleration rates, I went on telling him how FAST I was going to go…. He listened quietly.. then casually said…”A… Dalton… why don’t we all meet at the Motel 6…. And RACE our computers… it WOULD be so much CHEAPER..”

Similarly,  I was speaking to Charley Markley..  Again.. there I was..”yea Charlie.. I need 300 mph in the first mile, then 400 in the quarter, and we will go out the back at 500….”  A long pause….  And he said.. “ well don’t you think you should go 400 first…”   

You know … I’m a slow learner…  As my partner Jackson says…” we will be smarter 12 months from now” and we are learning..   The basics..  get to the other end … faster than the record twice.. in a row..
Be sure your tires aren’t rubbing the frame..  only dial up hp needed.. and don’t overpower the given course condition .. it ruins $1500 of tires.. fast.

I heard rumor Jack Costella was quoted as saying… My cars don’t go through the wind, they go under it..    that’s probably true..  :-)

Well .. “ My car won’t be going through or under the air..  When the wind hears us coming, it will get the heck out of the way”   :cheers:

Bob


All should watch jack’s intro video at      http://www.jackcostella.com/ 



Offline bbarn

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Re: Formula for aero drag.
« Reply #16 on: November 05, 2013, 11:17:52 AM »
Let us discuss a little further, I think IO and Rob are on to something in their dialog.

Fa in and of itself is not a useful number, it doesn't convey any meaning or measurement other than ft2. It doesn't describe a shape or how it interacts with the air, it only conveys a number.

Here is an example that illustrate the issue:

Let us assume we have two objects that have an Fa of 1937.5 ft2, which one of them is more aerodynamic?

You can't tell from these numbers alone...or can you? (<-Legit question, I don't think you can, but maybe I missed something)

The two objects I have used here is a 747 and a building where the dimensions are 20ft wide and 100ft tall. Both of these can fly, but only one can do so efficiently!

It is not until you introduce the Cd with the Fa that you get something useful. The Cd describes how the air is moved around the object with an Fa(x), and from that you can determine the general aerodynamic properties of the shape. (FYI, the drag coefficient of that 747 I used above is 0.03 (http://www.withouthotair.com/cC/page_273.shtml).

IO is correct in his statement about FPE as well: "the difficulty is in determining each of their drags and how they arise out of the different modes of drag creation--shape, skin friction, etc.". It may be a completely cost prohibitive endeavor to remove and measure each component of a standard vehicle to measure it's contribution to drag. Think about the challenge of measuring the frontal area and the drag coefficient of a fender, hood, bumper, windshield, door handle, tires, hub caps, antenna, mirror... not to mention the fact that you are missing the aerodynamic interaction of one part against another.

Here I think is where the real differentiators are: In the world of aviation, you are not incumbered with certain details like, will it fit under existing bridges, how well will it parallel park, will it fit in the average parking garage, I don't need a rear view mirror, tires only need to be sticking out when I am on the ground, all other times they are out of the airstream....

The world aviation engineers have to work in have different requirements because they have different goals. You don't see many passenger cars with a ~2000 ft2 frontal area that seat 400+ and drive cross country in 4 hours. Conversely, you don't see many airplanes that you can hop in and go 3 miles from home and run through the drive-thru to get your kids french fries either!

Back to IO's comment. I think in a door-slammer situation, you are not likely going to do an entire buildup using the FPE method to determine total drag, it is not economically feasible. I do think though that if you are talking about "Specialty construction" or building adaptations for your door-slammer, there is great value in knowing the FPE, if for no other reason, you can get your drag, Reynolds, wetted... if you base your build on known shapes.

If you are like me, looking at this link http://en.wikipedia.org/wiki/Drag_coefficient will yield nothing but an aggravation -- unless the subject interests me, then I'll devour it as best as I can. You can skip reading the whole thing, just look at the top right side and the pictures/shapes they have. I am not sure the accuracy of these numbers, nor am I implying they are reliable, I am only illustrating that your basic shapes can be estimated, some work has already been done, there may be more accurate sources for Cd's as well.

I know for sure that the numbers presented in that link are not precise, because there are many "streamlined" shapes (foils) to choose from, velocity, size, AOA... all contribute to the drag. Some foils are more effect at different velocities and AOAs and produce less drag than represented in that table. If you are talking about a foil, get a copy of "The Theory of Wing Sections", it has what you need to calculate foils, lift, drag, by AOA, by velocity...







« Last Edit: November 05, 2013, 11:29:47 AM by bbarn »
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Offline 7800ebs

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Re: Formula for aero drag.
« Reply #17 on: November 05, 2013, 11:42:26 AM »
Don't forget...

   We don't have all day to get to the perfect speed. We aren’t racing to terminal velocity.  Well ok some are. Fast cars not so much. The dragsters in NHRA are going 320 mph in 1000ft today, and we are trying for 500 in 5 miles.  Hmm.  We are racing on an island of salt.  We don't have more than 6 miles tops ... if that.   Fyi , my own calculations put the terminal velocity of the Turbinator at over 700 mph, but then again, we only have 6 miles.

This is fun stuff.  Interesting and NOT Nascar.. bs. 

Real Innovation    Real Racing    Let the Innovators loose.

 :cheers:


Robin UK

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Re: Formula for aero drag.
« Reply #18 on: November 05, 2013, 12:10:11 PM »
When I'm trying to understand this I too think like Sparky in terms of pools or areas of drag. Frontal area, overall surface area (skin drag), coefficient of friction for wheels/tyres and so on. Anything that you can categorise (even the density of the air) that will stop you going as fast as you think you ought to. I've been lucky enough to work with Richard Noble's aero guru Ron Ayers over the years and he's helped to clarify my understanding of this area - at least to some extent for a layman like me. I've learnt one general and one very specific lesson from Ron.

One is that when you move into the whole computer model, CFD, wind tunnel correlation world for vehicle design then you need to keep an open mind and understand what the numbers tell you rather than simply using them to subjectively support your original theory. That's because as you learn more about what's really going on as opposed to what you assume is going on, some things are counter-intuitive and at first you think "that can't be right". An example. Prior to SSC almost everybody talked about how the whole subsonic/transonic/supersonic airflows at Mach 1 would lift the nose of the car. You only have to look at a pic of a shockwave on the front of a fast jet to understand why that was a common belief. While that is a danger, it also turns out that at around Mach 0.9 (670ish mph) the compressed and accelerated air shooting out of the back of the car underneath at getting on for Mach2 tries to lift the "back" of the car. So if you haven't got a method of producing down force (ie a tail) then you are in for a spot of low level aerobatics. Who would have guessed that let alone been sure of it without CFD and rocket model test data allied to an open mind? The Budweiser Rocket and SoA had rear end lift off at around these speeds. Coincidence? Maybe.

The more specific lesson is one I've mentioned before and that's wake drag. Literally the drag caused by the plume of salt or dirt kicked up by a vehicle traveling fast. Look at the latest pics of NAE at Alvord (or SSC at Black Rock for that matter) and you'll see huge plumes of dirt. That plume - still attached to the vehicle and getting bigger until the stuff at the back settle down again - effectively increases the overall vehicle aero envelope and drag. Which is why on JCBDieselmax Ron used carefully crafted tunnels underneath the body and around the wheels to stop the plumes forming in the first place. Looking at the latest Mormon Missile and a couple of others it's seems they too have picked up on this but I wonder how many carefully crafted aero envelopes designed with low CDs to slip through the air ignore it. That's now another pool or area of drag on my list.

As I said, I'm no aero expert but I know a man who is so it's a subject that is as fascinating as it can be baffling until you slowly fill in the gaps in your knowledge. I'll stick to being a keyboard jockey I think.

Cheers
Robin
  

Offline bbarn

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Re:
« Reply #19 on: November 05, 2013, 12:34:24 PM »
Robin UK - I think the wake drag you are referring to is the separation drag Rob is referring to. I think a good example of this is a canoe vs. a row boat. Think about giving two good stokes in a canoe and the same two strokes in a traditional row boat (blunt/flat rear), which one will go further on the same thrust?

The difference is the canoes shape lowers the separation (wake) drag to be less than the row boats drag.

Mental exercise: Take the row boat and the canoe, scale them so they have the exact same frontal area and repeat the test...what would the difference be?
« Last Edit: November 05, 2013, 12:48:21 PM by bbarn »
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Offline Rex Schimmer

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Re: Formula for aero drag.
« Reply #20 on: November 05, 2013, 07:55:55 PM »
Good reference books for this discussion.
"Race Car Aerodynamics, Design for Speed" by Charles Katz: Get the second addition, good book on everything from streamlining, wings, ground affects, all applied to cars.
"Competition Car Aerodynamics" by Simon McBeath: Mostly related to formula and sports racing cars but good reading. Mr. McBeath also has a monthly article on aero in "Racecar Engineering" magazine.
"The Leading Edge" by Goro Tamai: My favorite aero book, I know it is about solar powered car aero but it all applies to what we are interested in and it is very informative and well written.
"Fluid Dynamic Drag" by S.F. Hoerner: Not available in a current printing (I think) I down loaded from the net, over 450 pages on almost anything aero you can think about. Well done but heavy reading.
" Theory of Wing Sections" by Abbott and Von Doenhoff: This is "the" book on wing design and aero, very technical but a great reference for picking wing sections.
"Personal Aircraft Drag Reduction" by Bruce Carmichael: This is the book that the "flat plate" aero design comes from that Rob is talking about. Carmichael is one of the leaders in the development of the NLF, Natural Laminar Flow, school of aero design. This is the basis of the design for Rob's Project NACA 6600 car. Great reading with discussion of many projects using NLF.
Plus there is a ton of NACA , NASA and SAE papers on aero that are available on the web.

Read all or any of these and your "ignorance of aero will be diluted"

Rex
Rex

Not much matters and the rest doesn't matter at all.

Offline robfrey

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Re: Formula for aero drag.
« Reply #21 on: November 06, 2013, 12:00:56 AM »
Carmicheal's book is just plain awesome.
It's a great compilation of his work and many others. I just can't say enough about it. Highly recommended reading.
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Offline SPARKY

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Re: Formula for aero drag.
« Reply #22 on: November 06, 2013, 12:08:21 AM »
I think I subscribe to the Comic Book of Aero---like BB example---show me the pictures  :roll:
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Offline robfrey

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Re: Formula for aero drag.
« Reply #23 on: November 06, 2013, 08:31:32 AM »
Same here Sparky, luckily Carmicheal's book is full of illustration.
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Offline Sumner

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Re: Formula for aero drag.
« Reply #24 on: November 06, 2013, 08:49:58 AM »
Good reference books for this discussion......"Personal Aircraft Drag Reduction" by Bruce Carmichael: This is the book that the "flat plate" aero design comes from that Rob is talking about. Carmichael is one of the leaders in the development of the NLF, Natural Laminar Flow, school of aero design. This is the basis of the design for Rob's Project NACA 6600 car. Great reading with discussion of many projects using NLF....

I have a number of the books listed, but not the Carmichael one.  If I remember right all give equal weight to A (in our case frontal area) and Cd (the total Cd of the object) such as in the reference mentioned above...

http://en.wikipedia.org/wiki/Drag_coefficient

Does Carmichael not give the frontal area equal weight in figuring drag force or does he use a different method to figure the total Cd of the object and then gives equal weight to frontal area when figuring the drag force. 

I also think the Leading Edge is one of the best sources although some of the math is over my head,

Sum
« Last Edit: November 06, 2013, 08:53:15 AM by Sumner »

Offline 7800ebs

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Re: Formula for aero drag.
« Reply #25 on: November 06, 2013, 09:34:55 AM »
 Masters of the Wind..

How many thousands of pounds of jet thrust does it take to make the dirt of the earth, blow in the wind?

The answer my friends, is blowing in wind, the answer is blowing in the wind.


Offline Richard 2

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Re: Formula for aero drag.
« Reply #26 on: November 06, 2013, 03:21:15 PM »
       "Race Car Aerodynamics, Design for Speed" by Charles Katz----Joseph Katz--- Maybe
219.648 mph F/BFMR 2010 Record
4 cylinder Esslinger
Could of had a V8

Offline Richard 2

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Re: Formula for aero drag.
« Reply #27 on: November 06, 2013, 03:28:51 PM »
At SW 2013 I was in the 911 pit talking to Donny and I said something about how slick, lose the salt was and his team mate turned and said "That's why 911 weighs 6400 lb." 

No wing of coarse
Richard
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Offline tauruck

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Re: Formula for aero drag.
« Reply #28 on: November 06, 2013, 05:04:20 PM »
Prof. Joseph Katz. Car guy. Helped me with my project. He's at SDSU. Thanks Joe. :cheers:

Offline robfrey

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Re: Formula for aero drag.
« Reply #29 on: November 06, 2013, 10:44:20 PM »
       "Race Car Aerodynamics, Design for Speed" by Charles Katz----Joseph Katz--- Maybe

This is a great book and I've always wanted to meet Dr Katz as he has been a great asset to so many racers. I did think his book focused more on generating downforce than minimizing drag. I thought it to be a better book for SCCA racers than SCTA racers. Learned a lot from this book though.
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