RE: Car Aerodynamics

[Alexei M Voloshin <Alexei.M.Voloshin-1@tc.umn.edu>]

On Thu, 8 Aug 1996, Roland Broberg wrote:

> I think there is a misunderstanding here if I remember way back to my dynamics 
> courses.  The Cd is measured in force/area.  It is not the drag, but the drag 
> coefficient.  To do a true comparison between vehicles you'd have to multiply 
> the Cd x the frontal area which is where the Q is lower since it has a smaller 
> frontal area than the 5K.  
> Roland
> V8
> 100
> 5KTQ former

I really don't think you remember your dynamic courses very well.

A simple test - compare the physical units:
First of all, lets us consider the units of coefficient of drag: NO units 
here! (it's a ratio-> a coefficient)
Then let's consider what you're saying -> force/area

force is (mass * length)/(time^2).
area is (length^2).
devide one by the other, you get: (mass)/(time^2 * length).
Somehow units of Cd and untis in you definition do not help!
Booohhh....
The drag coefficient is the ration between two forces.

Now another thing:
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... call non-linear turbulance. How fast it shoots up 
depends on the aerodynamics of the car. However when talking about the 
frontal area -> sure at pays a role, but I would be careful how I include 
it in there. Remember, frontal area is a vector in these calculations, it 
is not a scalar.
I kind of busy right now, but if you're interested I'll tell you more. 
Anyway, Dick Feynman is hell of a lot better at this than I'm, that is 
why they gave him the Nobel Prize. Dig up his Freshman Lectures in 
Physics, I think you'll like it.

Alex