Effective Rear Wing
Comments by The Baron Rockwell
I don't mean to say I am expert, but I have studied fluid dynamics, specifically sub-sonic laminar and turbulent flows . Here is what I know about rear wings with respect to aerodynamics.
First, to be effect (i.e. create down force) a rear wing must be just that; a wing. It must have largely non-turbulent air flowing over it and must have a negative angle of attack (i.e angled down towards the front of the car) In this configuration, the air flowing over the wing will act just like one on an airplane but instead of creating lift it will create down-force.
With respect to cars, the important aspect is non-turbulent flow. For most cars, when the air flowing over the top of the car reaches the rear window, the downward slope is too large to preserve the semi-laminar flow. As a result, the air separates from the surface of the car and becomes extremely turbulent (not good for wings). Therefore, in order for a wing to be effect it must be positioned so that it is above the area of highest turbulence. Hence the reason for high tails on F1, Indy and Touring Cars, Supra Turbo. Most stock wings are really more of a spoiler than a wings. The other alternative it to make a large enough wing so that it will have an effect even if the air is turbulent for example the Porsche Turbo whale-tail.
As I noted, most factory wings act more as spoilers. Spoilers do not really create down force, while they do create drag. The purpose of a spoiler is to have an effect on the air as it flows over the back of the car and meets the air flowing around or under the car. A spoiler attempts to limit the turbulent characteristics of the air behind the car, with the goal of preventing body lift and buffeting due to vortices in the turbulent flow. In most cases a factory rear wing or spoiler is purely for looks.
A front spoiler is one of the best ways to add high speed stability. An "effective" ( stock is not quite large enough) front spoiler, ( also lip, air dam, chin spoiler, etc) will prevent the front of the car from lifting by directing air flow around the car instead of under. Air flowing under the car tends to turn the entire car into a lifting body (wing).