The amount of lift created by a wing , (or down-force in the case of a wing mounted upside down), can be estimated by using the following formula:
Lbs of lift = Lift Coefficient times Dynamic Pressure times Area of Wing (in square feet)
This is a simplified formula that will provide you with a quick estimate the lift (down-force) available with wings of different sizes.
Definitions:
Lift Coefficient - this is a number that ranges from 0 to 1.6 with the type of airfoils and speeds we typically deal with.
The Lift Coefficient for a given airfoil varies with the angle of attack, that is, how the wing is angled relative to it's movement through the air. Steeper angles provide more lift - up to a point. See the included graph of the Clark Y Airfoil.
Dynamic Pressure - for this discussion Dynamic Pressure equals 33 lbs per square foot of wing area at 100 mph.
Using the formula:
Suppose you are using an airfoil for which there is published data, like the Clark Y in the graph. And suppose it measures 14 inches leading edge to trailing edge and is 18 inches wide.
14 X 18 = 252 sq. in. Divide by 144 to convert to square feet = 1.75 sq. ft.
Suppose you mount it at 12 degrees angle of attack. The graph shows a coefficient of lift of 1.25.
1.25 X 33lbs per square ft. @ 100 mph X 1.75 square ft. = 72.1875 lbs of down-force.
That doesn't sound like much, does it - especially for such a large wing. But that is what it is at 100 mph.
Aerodynamic lift increases as the square of the increase in speed. Double the speed (factor of 2) and the lift increases by a factor of 4 (2 X 2). The same wing used in the example above would produce
288.75 lbs at 200 mph. That same phenomenon works in reverse, cut the speed in half and the lift decreases by a factor of 4. The same wing at 50 mph would only have 18 lbs of lift.
At 170 mph (1.7 times 100) the wing should produce 208 lbs. (1.7 times 1.7 = 2.89 times 72.1875)
Bottom line: the wing should be sized according to the speed you run as well as the amount of down-force you want. You may need to make the wing bigger to get the desired down-force before you reach top speed at the finish line.
Pete Robinson had a well deserved reputation for never putting anything on his car that wasn't absolutely necessary. His 160 to 170 mph gas dragsters had the biggest wings of anyone out there.
