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Re: Car Aerodynamics

On Fri, 9 Aug 1996, Eric Thorstenson wrote:

> >> 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.
Aha, so you just proved that it does change with speed. It just doesn't 
chamge with speed^2 (quantity more related to energy). But this is only 
an approximation. Remember that in real world it is much more complivated. 

> >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.
I knew you'd mention that, but the wings can do up to three things:

- Change Cd.
- Enhance air-flow into the engine (Porshe)
- Produce downward force.
Zero angle of attack maybe good for some speeds and bad for others.

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