Airfoil analysis with JavaFoil

[PaulL]

Came across a cool (and free!) airfoil analysis program a few weeks ago .. JavaFoil;
http://www.mh-aerotools.de/airfoils/javafoil.htm

I used it to model the 2 element wing on the rear of my DSR, and might have found a higher downforce, lower drag configuration!

Refer to the flow field pictures attached ...

Previously, I had my main plane about 5 deg down into the flow, and the upper element about 15 deg down.
I had Cl of -2.4 and Cd of 0.044 (based on whatever scale I happened to be running the program at).

After experimenting with the program for a bit, I discovered that actually pointing the main plane UP into the flow 1 deg, and the upper element down 28 deg, my Cl went up to -3.0 and Cd went down to 0.033

It will be interesting to try out this config at the track to see if it's an improvement!

[nulrich]

These 2D airfoil analysis programs (there are several out there) are fun to play with, but they won't give you any useful results for race car applications.

As it says on the home page: "Because JavaFoil does not model laminar separation bubbles and flow separation, the results will be incorrect if either of these occur."

Panel codes like JavaFlow uses are inviscid, so they are generally valid for airfoils with thickness less than 15% at angles of attack less than ~8 degrees, where everything is linear (due to the absence of separation). Some account for separation using experimental correlations but that just slightly extends their capability. Very useful for aircraft but not for automotive applications.

Also, the flow field (direction and energy of the air flow) around the rear wing has to be taken into account, it isn't like the wing is sitting in clean air.

That said, I wouldn't discourage you from trying different configurations of your rear wing, just don't expect on track results to match what's predicted by JavaFlow.

Nathan

[Mike B]

Would Javafoil be useful to determine relative performance? If one was to model his current wing/flap configuration (ignoring the quality/quantity of airflow in real life), could you try different profiles to see if they're better and how much better on a percentage basis? If a given wing is 5-10% better in Javafoil, would you see 5-10% improvement in the real world?

[nulrich]

Unfortunately, no, you would not expect any sort of correlation.

One reason that modern race car airfoils have such "strange" shapes.

Nathan

[Larry]

Help me out, Nathan. I'm trying to overcome more than 40 years experience as a pilot. There is no optimum design for a wing for all airplanes, as each is optimized for a particular mission, parameter or environment. It is therefore a compromise when out of the limits of the design environment. Wouldn't the same be true for a race car? To get the ultimate performance, shouldn't I optimize the wing for the particular track AND conditions at the track? I can easily see why a basic airfoil analysis doesn't give you an accurate picture because of all the disturbances and interference caused by the rest of the vehicle, but it seems that to achieve the optimum, I would have to select a different airfoil for each track and for the conditions.

Larry Oliver
International Racing Products

[nulrich]

You are absolutely correct, Larry. Each race track is different, and therefore the optimum aero package would be different for each track. That's why (where permitted by the rules) F1 cars have different parts for each track.

That's just not realistic for our level of racing, obviously. The parts cost is one thing, but the development expense is really the biggest obstacle. Just to calibrate what I mean, the aero development on the Rn.10 would have cost seven figures had we paid someone like MIRA to do it (well over 1000 hours of aero design work and more than a hundred full CFD runs). And that includes only two front wing packages and two upper rear wings (although most of the work was on the body and undertray shape). Even though our cost was much less than "retail," it only makes sense when amortized over many cars and wing packages, and then just barely. Designing and tooling up several different aero packages just doesn't make sense.

Fortunately, we can get pretty close to optimum by adjusting wing angles and, occasionally, changing parts out for especially fast or especially slow tracks. And, at this level, I think that is good enough.

Nathan

[ziig]

One very important thing to note here guys is that what aerodynamically occurs on a aircraft will NOT be the same on a race car....the priciples will remain the same except a plane uses aerodynamic in a much more 3 D enviroment , and a car is more of a 2D world...hence the reason for floor effects.
To gain the most out a aerofoil in use on a race car , much more thought is needed as to what shape will offer the most negative lift with least drag, many times engineers have found that using mulitple foils and a diffuser at the rear is the most effective method of acheiving this....
I have played with several CFD programs and made comprehensive comparissons using a 1/5 windtunnel and computaional methods on paper....and I have found that using a good CFD is almost as accurate but no substitute for actual windtunel testing....

my best advice is start with a good known configuration, and take records as you make changes , what will work for you on a straight may in fact create issues in a fast corner or even lose grip in a slow corner, every track is different and will affect your set up as a consiquence..

there is no substitute for actual testing, no matter how expensive your equipment , software or wind tunnel may be...the race track is where it will be proven to work or fail...

any help I can offer email me at henrybaboolal@gmail.com

henry