I actually think that using the wheels to create downforce, ie. by directing the air upward is a bad idea in terms of drag.
The aim of downforce (as we know of course) is to increase the effective weight of the car, and hence increase the lateral frictional force generated by the tyres. This allows the cars to go round corners at a higher speed.
My idea (and Manchild's, he got in there first
) is to provide this lateral force by generating horizontal thrust in the appropriate direction depending on whether you're turning right or left.
The result would be that less downforce by method of flip-ups/wings/underfloor is required, reducing drag. Because the vanes in in the wheels are variable in their angle, they could be trimmed to 0degrees for the straight to reduce drag and reduce engine power consumption. But when you get to a corner, say a right-hand bend, the vanes change their angle so that they thrust air to the left of the car. This would have a number of effects:
- The aforementioned thrust which generates lateral force
- The drag would help slow the car down (ie. helping the brakes)
- It would cool the brakes at the time they most need it-under heavy braking.