Automotive Aerodynamics [WAS:90 90q 20v redline top speed]

["Bertenyi, Tamas" <Tamas.Bertenyi@nrc.ca>]

Ok, well since you asked...I'd love to go on about automotive 
aerodynamics! To begin with, let me give you a few more drag figures. 
The actual list is quite extensive. The only difficulty is that the 
year of the model is not given! The list was compiled around 1987 and 
is German in origin. I'll provide the different model names and let 
you sort out which car is which.

Medium Size
Audi 80 CC , 	Cd=0.38, A=1.86m^2
Audi 90/2.0 E, 	Cd=0.39, A=1.87m^2

Upper Medium Size
Audi 100 1.8,	Cd=0.31, A=2.05m^2
Audi 100 Avant TD	Cd=0.34, A=2.05m^2
Audi 80 Quattro	Cd=0.38, A=1.89m^2
Audi 90 Quattro	Cd=0.39, A=1.90m^2

Luxury Cars
Audi 200 Turbo	Cd=0.33, A=2.07m^2
Audi Quattro	Cd=0.38, A-1.86m^2

Sports Cars
Audi Coupe GT 5E	Cd=0.36, A=1.83m^2

A couple of interesting things to note: as has been mentioned on this 
list before, tires size can have a significant impact on the drag of a 
car. For example, two otherwise identical cars, such as an 80 and a 
90, will have different drag figures just because the 90 has wider 
tires.
Also, frontal area is just as important as drag coefficient. You can 
have a car with a stunningly low Cd but if it has a frontal area of 
3m^2 then it still ain't goin nowhere.

Now for the fun physics! To recap, your drag, D, in Newtons at any 
speed, V, is given by:
D=0.5*rho*V^2*Cd*A
Where rho is the greek letter representing the density of air. 
Typically 1.2kg/m^3.

Actually, the term "0.5*rho*V^2" is called dynamic pressure and is 
often represented by the letter "q". This is the pressure a fluid 
would exert if adiabatically brought to rest from its initial 
velocity. (You might remember this from Bernoulli's equation). Note 
that your drag will actually decrease at higher altitudes (and thus 
lower air density)! So if it weren't for those silly engine guys 
starving for air, we could all drive faster in the mountains!

Tamas
'90 90q20v

*********************************************
Tamas Bertenyi
Aerodynamics Laboratory
Institute for Aerospace Research
National Research Council Canada
Ottawa, Ontario, Canada
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