Rather than leave these ideas dangling...
I think the simplest explanation of a wing in ground effect is via "Lifting Line Theory", although nobody would claim that was simple. The trick is to put a mirror image vortex system "underground", which simulates the ground by eliminating all vertical airflow at ground level.
The mirror image "bound vortices" (i.e. mirroring those along the wing) reduce airspeed above and below the wing, i.e. increase pressure above and below the wing, with a slight reduction in lift. There is no cushion of air.
The mirror image "trailing vortices" (i.e. mirroring those in the wake) reduce downwash behind the wing, but increase upwash ahead of the wing. This makes the lift vector more vertical, reducing induced drag.
A hand-waving physical explanation, (which does at least predict these effects), is:
Air can't pass through an aircraft so it has to go around it. In sub-sonic flight the air starts getting out of the way some way ahead of the aircraft. It is reacting to a pressure wave ahead of the aircraft.
The most important thing to note here is that the pressure wave starts many chord lengths ahead of the aircraft.
At height, there is plenty of room above and below the aircraft for the air to get out of the way, and each route takes about half the air. The pressure wave is symmetrical vertically. The airflow approaches the aircraft more or less horizontally and (after the wake vortices have cancelled themselves out) we see a residual flow deflected downwards.
In effect the wing has 'bent' horizontal air downwards slightly to achieve lift. The lift vector must be tilted backwards, and we have induced drag.
In Ground Effect, most of the air must pass above the aircraft (the pressure wave is shaped more like a giant flattened hemisphere), and so the wing 'bends' rising air back down again. The lift vector is more vertical, with less induced drag.
Another (small) Ground Effect is that the smaller one-sided pressure wave has to be twice as strong as the two-sided version, and so the air approaching the aircraft has a slightly lower airspeed, reducing lift slightly.
Ground Effect is all about reduced drag. It is often perceived as extra lift, because losing drag means you are soon faster than you would have been, which does allow you more lift.
Last edited by 24Carrot; 23rd Jun 2011 at 15:27.