Originally Posted by Owain Glyndwr
A&E calculate that the deflected air might be drawn from as much as a semispan above the wing, but this is based on some assumptions rather than any definite scheme of things.
According to A&E, L = mdot*vv. Assuming that all the mass captured in the "virtual scoop" uniformly gets a downwash velocity vv, the power required for this is Pi=0.5*mdot*vv^2. Equating this to the power Pi=Di*V to overcome the induced drag Di, it can be shown for an elliptic lift distribution that the area of the scoop is equal to a circle with a diameter equal to the wing span.