RE: Car Aerodynamics

[Alexei M Voloshin <Alexei.M.Voloshin-1@tc.umn.edu>]

On Fri, 9 Aug 1996, Eric Thorstenson wrote:

> 
> Alex said:
> -----
> Cd is dependent on speed once above about 30 MPH. Cars are optimised for
> different speeds. Normally the drag coefficient shoots up for any car
> after above 70 MPH.
> -----
> 
> Cd as a function of speed? Hmmmm.....
> 
> Sorta flies in the face of the SAE standard practice of determining
> a (constant) Cd using acceleration and coast-down tests, which by
> their nature involve a wide range of speeds.
> 
> Drag *force* as a function of speed, yep. Drag *coefficient* as a
> function of speed, well, that would be news to me...
> 
> Eric T.     '86 5kS
> erict@ra.cta.com
> 
It's really simple. If the formula for drag force was: Speed * C = Drad 
force, where C is some experimentally measured constant then Cd would be 
independent of speed so would be air density etc... Also note that C is 
not dependent on speed in that equation.
 
 Cd would be part of that constant.
 This is precisely the case at low speeds. 

However, at higher speeds the realtionship between 
speed and drag force is not linear! Hence, Cd would have to change.
A good approximation for any objectat high speeds is drag force = speed^2 
* C, but this is for a round ball! Normally, aeroefficient objects that 
are designed for high speeds beat it.
Why do you think F1's, Porshes ... have a movable wing -> to optimize Cd for 
different speed ranges. 

Alex