Re: Car Aerodynamics

[erict@cta.com (Eric Thorstenson)]

>> Drag *force* as a function of speed, yep. Drag *coefficient* as a
>> function of speed, well, that would be news to me...

>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.

Huh?? Who ever said the drag force was proportional to the speed?
"is a function of speed" does not in any way imply "is proportional
to speed".

>However, at higher speeds the realtionship between
>speed and drag force is not linear! Hence, Cd would have to change.

Of course it isn't linear, but that doesn't mean Cd changes, it simply
means proportional to something other than speed^1. In this case,
speed^2.

>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.

And hence have Cd lower than that of said sphere... Doesn't make it a
variable.

>Why do you think F1's, Porshes ... have a movable wing -> to optimize Cd for
>different speed ranges.

Sorry, adjustable wings are used to control the overall amount of downforce
and the front-to-rear distribution of that downforce. If the intent were
"to optimize Cd", you'd simply set them to zero angle of attack, or 
remove them entirely, as in the case of a FV or FF.

Eric T.     '86 5kS
erict@ra.cta.com