SCCAForums.com

There are some people that think that at autocross speeds the only down force added to the car buy the wings is the weight of the wings. Most of us know better. Even a flat piece of plywood will create some down force, but the angle of attack would be critical and it would cost a lot of drag. The correct airfoil can make all the difference. You can buy wings from someone’s shop or better yet make your own. Because the airfoil so critical, making your own allows you to pick or design a better airfoil. Before you spend too much time chasing the right airfoil, you should decide what Reynolds number (RN or Re) range you will be operating in. You should compare airfoil results for airfoils of similar RNs. RN approximately =Velocity in MPH x Chord in Feet x 10,000. Someone once told me just the copy the DC-3 wing shape, a slow heavy plane with lots of lift. If a DC3 cruises at 150 mph and has an avg. chord of 6', then the RN for that airfoil is 9,000,000. On the other hand, if you want your 6" chord race car wing to start working at 30 mph, the RN is 150,000. So you can forget DC3 foils and for that matter, most if not all NACA airfoils. If you are looking at making your own wings here is a couple sites on airfoils and airfoil data:

www.ae.uiuc.edu/m-selig/a...craft.html
www.ae.uiuc.edu/m-selig/pub/LSATs/

Stanley

 

lightning:

www.ae.uiuc.edu/m-selig/a...craft.html
www.ae.uiuc.edu/m-selig/pub/LSATs/

Stanley

 

weird,  your links look right,  but they don't take me to the listed address. It must part of that microsnot exploder illogic.

 

Hmmm triple weird,  I click on the link and it takes me to a url sans the www,  which of course ends up in a DNS error.  I add the www back to the url manually and I am there.  Somebody smarter than me will have to figure out a way to fix this so it links directly.

 

 

---

lamoureaux
BSP 2004 STi
Wichita, Ks

It's a bug in the forum software, which is redirecting the URLs through SCCAForums, presumably to garner some ad revenue.  It's got nothing to do with Internet Explorer -- it doesn't work in Firefox either.  So, speaking unofficially as a Microsoft employee, thanks for the unwarranted, uninformed, and undeserved insult.  We love you too.

PedalFaster:

It's a bug in the forum software, which is redirecting the URLs through SCCAForums, presumably to garner some ad revenue.  It's got nothing to do with Internet Explorer -- it doesn't work in Firefox either.  So, speaking unofficially as a Microsoft employee, thanks for the unwarranted, uninformed, and undeserved insult.  We love you too.

Sorry to step on your toes buddy. 

 

---

lamoureaux
BSP 2004 STi
Wichita, Ks

I agree in principle to your tactics however all the traditional air foil designs  ( NACA and selig as well as eppler, etc ) all try to minimize cruise speed drag at the expense of  low speed  lift   as well as always try to optimize  the Lift to drag ratios. In any type of  flight application or for high speed vehicles drag is a major limiter , but  in solo and at our top speeds  it almost never is,  we almost always have more power than we can use with the available tire and normal force , a different optimization of the air foil  based on down force and pitch insensitivity  come up with a radically different main body shape for the airfoil, In addition one finds that turbulent behavior and agumented lift devices  can dramatically increase the lift potential  of  the high camber thick  main bodies that come from the new optimization requirements without having to use a powered blowing system.    By using different interelement spacings and overlaps  than traditional wings  have,  one can get significant gains in lift over laminar flow systems because of coanda effect flow attachment improvements. Wing end plates that function at high speed usually can benefit from very subtile edge feature changes that prevent or cancel the high to low pressure vortices  that are called spill over  and can produce up to 20% improvement in down force over straight end plates. Both of these create a larger and further back on the chord low pressure peak and thus a larger low pressure area on the wing than traditional wings do.   I have to agree that if one does their own work  one can build a better wing than they can buy for solo. All the traditional wing manufacturers employ  high speed high reynolds number , non turbulent flow designs that one sees on planes and indy cars. I have spent at least 6 years and countless hours of computer time and wind tunnel testing as well as towed trailer mounted testing  on thousands of wing shapes. I find that air foil shapes modified from those more common to ship sails and wind turbines work much better at speeds from 10 mph to 65 mph than the traditional  ones for airplanes do.

John Garcia

 

 

My car accelerates in a straight line much faster without the wings. So I think some attempt should be made to reduce drag. This may best be done by reducing the stagnation line along the leading edge and reducing parasitic forces. But the main goal is still to produce downforce. Leading edge slats just might be the next step for some wing designs, but leading edge slats could increase the drag to a unacceptable level. Vortex generators could also be the way to go. But vortex generators might not work that well at the lower speeds we drive at. Most of us don’t have resources or access to the equipment to test for the best size and location of the generators to produce the needed effect. If not done right, all they would do is add to the cost and work to make the wing. Basically, end plates will make a 60 inch wing act like a 60 inch wing. I have a hard time thinking that subtle edge features can make a 20% improvement.  Because good testing data has been done for wings with and without endplates. With endplates the down force was increased by 7%. The total drag of the wing + endplates was increased by 1.4%, primarily due to the increase of skin friction of 34%.  Endplates have some improvements that can be done, but still, most endplate improvements will influence the wing at higher air speeds. At Solo speeds we just might not see a lot of improvement with endplates alone.

If you use any kind of airfoil program  then you are aware of the big numbers of lift/down force that can be achieved when you turn off any of the stall calulations.  It is most likely the wing would need to work at a higher angle of attack then we currently use. Maybe, for the greatest development, it just might be in airfoils that can alter, delay and/or reduce the separation and still increase the circulation. There is some research going on for low speed high lift airfoils. So far I have seen wing surfaces that use a small bump about 1/3 back from the leading edge or a surface that looks like a ruffle potato chip.

Most of us have the same problem we try to decipher information from all kinds of places, most all of it applies to cars that are tested at a much higher speed then we use.  It is true that Aerodynamic down force is always affected by the square of the airspeed, but sometimes you work the numbers back words.  Lets say for a under tray tunnel that produces 700 lbs. of down force at 150 MPH, if you work the numbers backwards you will find there is not much down force under 70 MPH. 

Most of us do not have the resources or access to the equipment to make wings to the close tolerances needed and/or really test the wings once done.  But we will try!  Best of all, it is fun!

Neal

I use a true 3 dimensional  CFd program with solution of navier stokes approximations and add ons for  turbulent flow and conservation of energy boundary conversion routines  , The programs are  specifically used to solve any and all generic hydrodynamic flow problems regardless of shape , Run times  are still long even though  I run them on a 4 peta flop vectorizing machine  and include moving road in the cfd  mesh approximations. 

 If you just look into work done by overseas universities on contract to F1 teams you will find that certain  end plates with proper features  do improve cl up to 20 % over no end plates but increase Cd by 40% or more for the same cl improvement .  ( ref Cranfield college of aeronautics, influence of endplates  in ground effect for a 2004 formula one wing ) .  

Next for solo one must use higher angle of attack with the flow attachment modifiers to gain  the improvements,  I have this with the leading edge slat data as well and it can also  allow  flow attachment and  the extension of stall to either  lower speeds with the same angle of attack or increases in angle of attack  with no stall . Using blowing or carefully placed turbulators  the onset of stall can be pushed lower in speed by huge margins  thus make a wing work at much slower speeds as well as numericaly make a wing look longer than it really is.

lastly  I have a home made  collapsable wind tunnel capable of  running   a f atlantic  car up to 100 mph ( 60 with good straight flow )   then a technique of backing out  the wall and  proximity effects   by  using data from  numerically easy  to solve bluff bodies in the flow . . Lastly I use the old technique of a 32 foot flat bed trailer  with  instruments  that is towed behind a pickup with  flow  modifiers on the pickup to straighten the air going to the test subject on the flat bed trailer . All give numbers that correlate to within about 10 to 12%  for most situations.   Some of the cfd actually predict less down force than either an instrumented car or a tow rig measures.  And for the final coup de gra I have a 30 foot water tow rig that can take  small models  and allow low speed testing at higher effective reynolds numbers  to allow  very slow speed video's of the flow issues at 20 to 45 mph in air using dye tracking for flow visualization. (visualization only )

John Garcia

BSEE, BSME,MSEE, MSME, PHD,

 

It appear to me that we are saying to a great extent the same things. We may differ in what we think the results might be. We both agree that to get more down force with the wings we need to find a way the run with more angle of attack. We agree that we need to find ways the reduce the separation and also increase the circulation. I think that you have a good understanding of aero because you mentioned the Coanda Effect and not Bernoulli’s Law (which is mistaken). I am sure that you would agree with this “end plates should be designed differently for the front and rear wings.” I will agree that one might get up to 40% increase in down force with properly done endplates on front wings. But when installing larger, full depth endplates on rear wings the endplates will have to be matched properly with the flow around the rear of the car. Simply extending the endplates will cause excessive drag, as the endplate geometry is highly depend on the geometry on the car to which it is applied.

Now, I am in the process of designing a new wing with a different leading edge. They will take some time to build.  But, I do not have the resources to fully test, it sounds like maybe you do. Would you be willing to test this wing. Send me a Email if so.

Neal

send me your info off the forum , I will do some simulations for you , currently i am booked with other peoples work until end of january .

 

jg

Very interesting reading...I have an A-mod car that is decent regionally (won SEDIV thisyear) but isnt a Bowland caliber car by any stretch. I do not have wings but am currently working to fab some (with some insight from George) Hopefully I will have some downforce this year. I am runing a tube chassis modified from a FSAE car from a few years ago. It is powered by a 900cc Kawasaki Ninja bike engine. It is a low buck deal but an absolute blast to drive.

 

 

 

Just to throw my hat into the ring:

    I am currently pursuing my Ph.D. in Mechanical Engineering, focusing on Computational Fluid Dynamics (CFD).  I participate in the Formula SAE (Society of Automotive Engineers).  As a side project (more than likely, a long, drawn out one), I am developing an aero package for our FSAE car.  Having been to competition and seen the "winged-wonders" of University of Texas-Arlington, University of Missouri-Rolla, and University of Oklahoma, among others, simply dominate by nearly two seconds a lap both the autocross and endurance portions of the events, it is hard to argue that wings are not important at these speeds.  The autocross track for the '07 FSAE competition in Detroit, MI, was a little tighter than normally experience in the average weekend autocross, but that is because the general spirit of the FSAE series is that of engineering, and not driving skill.  (As a counter-example, the University of Australia team with the trick pneumatic suspension was only a second behind UTA in Autocross and UTA did not complete the Endurance race due to mechanical failure)

    But, it is important to know what you're doing, as the autocross and endurance race/fuel economy events provide nearly half of the total points available.  So, you must focus on driver training with an emphasis on car/tire management, as well as reliability.  Also, it all boils down to manpower and time.  You, as a team, must prioritize the different aspects of the car, and decided where you feel the most gains can come from.  Wings, while impressive, must be fully designed and explained during design judging.  Simply throwing something on the car that looks like it works will not garnish respect from the judges.  Fabrication is another issue.  Moldmaking is a labor intensive process.  When it comes down to the last two months before competition, do you want to be sanding a mold, or testing the car?

    With all of that said, I really enjoyed the Racecar Engineering article by Simon McBeath and would also thoroughly recommend his book Competition Car Aerodynamics: A Practical Handbook to anyone thinking about designing wings.  Eventually, I plan on building an A-mod car, so it is very neat to see this topic being discussed in an open forum.  Keep up the good info!

 Matt Bell

Mississippi State University

MSMotorsports FSAE Aerodynamics Group Leader