AERODYNAMICS - Derby Tech, February, 1986

By Leeon Davis

(Reprinted from June 73 Lane Two)

From the time I first became acquainted with the derby I could see that one of the most misunderstood factors of winning was airflow. I am not schooled in the subject but from my flying and aircraft designing expericnce (See May 1973 AIR PROGRESS) I do have a basic knowledge.

The importance of aerodynamic drag depends on the speed of the hill and wind direction and velocity. I attended one race on a slow track with a tail wind where the big cars had a field day. Our Midland track is not fast but we can always count on a good headwind. The only year that I have seen a headwind at Akron was 1969. Steve really moved out on the last one-third of the track.

Many cars which I have seen with pointed noses have either blunt or squared off backs. If you have to make a choice between one end or the other make the back pointed. All lines of the car should be smoothe and flowing.

Let's discuss boundary layer control, BLC. I'm sure you have seen attempts to use this but maybe didn't know what it was. It's application and form is widely varied. Some aircraft use vortex generators to stir up the air next to the skin and keep it flowing and improve control.

Small holes can be drilled in the skin and air sucked or blown out to improve BL flow. This can be used to reduce drag or increase lift. You might have seen racers with holes, openings or air foils to try to improve air flow. I believe that the possibilities of getting help from BLC are verv slight. The drag produced in trying to improve flow would be greater than the drag reduction from the improved flow. In other words you can't get something for nothing.

Ncxt let's discuss area rule. About 20 years ago when jet Fighters were flying near the speed of sound several new designs had fallen far short of engineering data. A young engineer named, Whitcomb, did extensive work in a wind tunnel and solved the problem. He determined that there was a connection between drag and total cross section area. Even though this was discovered at high speed it still applies at low speed. It is best understood by making a graph. This can be done mentally. Take the car and saw it in pieces, every two inches. Figure the area at every cut and put this area into a column two inches wide. See figure 1. This area should include wheels and axles. Now round off corners and compare with figure 2 which would be ideal. it's impossible to achieve this shape but it can be strived for. This might not seem like much but it does play a part. Sometimes it's the little things that hurt. You can sit on a mountain but not on a tack.

You say that you have seen good winning cars at Akron that did not conform to the forementioned dissertation? YES, BUT, they didn't do it against a headwind. Next year there might be a headwind and don't you want to be prepared?

Return to the Derby Tech Page