Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Air foil proportions

Status
Not open for further replies.
motorsportsdesign

motorsportsdesign

Automotive
Jul 23, 2003
90
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
 
Replies continue below

Recommended for you

Sort by date Sort by votes
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.
 
Upvote 0 Downvote
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:
Jonathan T. Schmidt
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

C
  • Question
Energy cost of air displacement? 1
Replies
10
Views
2K
WKTaylor
WKTaylor
Feynman81
  • Locked
  • Question
Air flow optimiyation through radiator
Replies
16
Views
516
3DDave
3DDave
HighPressureProfessor
  • Locked
  • Question
Understanding Repair Limits for Air Seal Serration in Gas Turbine High Pressure Rotor Sections 3
Replies
8
Views
153
WKTaylor
WKTaylor
MCA1983
  • Locked
  • Question
Which Type of Stringers cross section is most preferred in aircraft wing and fuselage 2
Replies
14
Views
500
ArunKumar Bala
ArunKumar Bala
Macm_020171
  • Locked
  • Question
Composite Airframe Design Stress stack up versus design stack up of plies 1
Replies
9
Views
416
Ng2020
Ng2020

Part and Inventory Search

Sponsor

Back
Top Bottom