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*To*: Eric Thorstenson <erict@cta.com>*Subject*: Re: Car Aerodynamics*From*: Alexei M Voloshin <Alexei.M.Voloshin-1@tc.umn.edu>*Date*: Sun, 11 Aug 1996 14:28:28 -0500 (CDT)*cc*: quattro@coimbra.ans.net*In-Reply-To*: <9608091939.AA28071@fester.cta.com>*Sender*: owner-quattro@coimbra.ans.net

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 >

**References**:**Re: Car Aerodynamics***From:*erict@cta.com (Eric Thorstenson)

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