Air foil proportions

[motorsportsdesign]

I have entered a gravity powered car race like soap box derby for adults. The size of the car is a maximum of 4 x 4 x 9 feet.

I would like to know what the lowest drag airfoil shape is that I can try to apply wherever possible.

The average speed will be about 40mph with max at about 60mph.

I am struggling with a design decision, please let me know if you have any ideas about this.

If the driver lays on his back looking forward the body is widest about 75% from the front, so the taper near the rear of the body will be more severe than most air foils but the frontal area will be small.

If the driver has feet forward and head forward near his knees like old soap box derby. The body will be widest at about 40% from the front but the frontal area will be larger.

Which is likely to have less drag?




Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[sreid]

Here's what I would try. You have two things under your control, the frontal area and the drag coefficient.

Getting the smallest frontal area is critical. I would lay the driver down on his back with his head/helmet proped up just enough that he can see (a one mirror periscope should be considered).

To minimize lift induced drag, have symetrical airflow around the sides of the car. Provide top and bottom flat "End Fences" to channel the air aaround the sides. In plan view, the sides are "Elipcally" shaped and chopped at the rear at the 9 feet limit. Use turning vanes to help pressure recovery behind the car.

 

[motorsportsdesign]

sreid,

I'm not familiar with end fences.

Do you mean a plate parralel to the ground fit to the top and bottom of the side surfaces?

In drag racing they use something called "spill plates" on the ends of the wings, is the design something like that?

I am suprised about the suggestion "chopped at the end" would it be better to make it to a point?

You can see other cars at:

http://www.gravityseries.com

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[sreid]

Motorsportdesign,

Yes, exactly, flat plates on the top and botttom of the side surfaces. This is to help stop the negative pressures along the sides from entraining air from the top and bottom of the car. This creates energy draining vortexes.

Chopping the car off at the rear; to keep the air attached at the rear, the sides cannot have an angle of more than 7 degrees. This would make the tail of the car much longer than the rules allow. Von Karmen's theory says that if you can't keep the air attached you may as well simply chop the tail off. Turning vanes can help the pressure recovery at the rear.

 

[sreid]

Another thing you could do is to send an email to Yoshi Suzuka at Susuka Racing Services and get his opinion.

http://alcatraz.net/ysuzuka/