updated 8/20/2011
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Aerodynamic Drag
You Don’t Get Something for Nothing
Mount a wing upside down on your racecar and, when the car
moves through the air, the wing will press the car to the track
surface, giving more grip. But it’s not free. Just as airplane
engines are needed to overcome the drag of the plane, a racecar
engine has to overcome the drag of the car, including the wings.
There is a basic equation for the force it takes to push something
through air:
Aerodynamic drag = 1/2 D x A x Vsquared
In this equation, D is the density of the air, A is the frontal
area of the moving shape, and V is its velocity relative to the
air.
For real body shapes, air at standard conditions, A in square
ft., V in mph, and drag in pounds of force, this equation becomes:
Drag = 1/391 x Cd x A x Vsquared
This equation shows that to calculate drag you need to know
three things: Cd, the drag coefficient; A, the frontal area of
whatever you’re driving through the air; and the speed of
air past it. This equation shows an important point—aerodynamic
forces are proportional to the square of the speed. That means
you quadruple the drag or lift when you double the speed.
The drag coefficient, Cd, is important because, in concert
with frontal area, it determines the power cost of pushing a
shape through air at a certain speed. A small, lowCd road car
will have a higher top speed than a larger, boxier car with the
same engine power.
Here are measured drag coefficients for some basic shapes.
These numbers come from tests of shapes with known cross sectional
areas. You blow air over them and measure the force on the shape.
That’s what wind tunnels do. The arrow in front of the shape
gives the direction of the air blowing over the shape. The cone
shape, for example, would have a lower Cd if it were rotated
so the air saw the flat end first.
Notice the difference in the Cd of a long and short cylinder.
You probably know that a slippery road car has a Cd of about
0.32. A chunky one is 0.38.
