[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
RE: Car Aerodynamics
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
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.