In (harry) writes:

Hi:
I was just looking over drag coefficient's of cars.. I found a
site with Cd's of many different vehicles, like semis, blimps,
helicopters, etc. It looked like a blimp has a lower Cd than a car,
which leads me to believe the Cd has a function of the vehicles weight
and/or size. I'm looking for a gas driven car with the least
aerodynamic drag. Can someone tell me the function to Cd?
Thanks!
Harry

You probably want the car with the lowest drag (force), not the lowest
drag coefficient (dimensionless).

The drag force D = 1/2 rho Cd v^2 A, where rho is the air density, A is
the vehicle frontal area exposed to the flow, and v is the speed through
the air.

A blimp has a lower Cd because it is shaped like a (fat) bullet, whereas
a car has a windshield that faces the flow.

So for each car you should compute the product of the drag coefficient
and the frontal area exposed to the flow, and choose the smallest value.

But if you're actually interested in fuel efficiency, there are other
major factors involved, such as the weight of the vehicle.
--
John E. Prussing
University of Illinois at Urbana-Champaign
Department of Aerospace Engineering
http://www.uiuc.edu/~prussing

John E. Prussing wrote:


A blimp has a lower Cd because it is shaped like a (fat) bullet, whereas
a car has a windshield that faces the flow.

A blimp is not shaped like a bullet, which usually has a flat
back end. In fact, the great advantage a blimp has is that
it tapers towards the rear.

http://images.google.com/images?q=bl...=Google+Search

So for each car you should compute the product of the drag coefficient
and the frontal area exposed to the flow, and choose the smallest value.

Frontal area is not quite the great sin many suppose, but
rather turbulance cause by lousy streamlining. Most cars
would fare better being driven backwards at speed than
they do forward.