ORIGINAL: Tall Paul
Ground effect is due to the air's inability to move downward as the wing passes, so the air underneath the wing is compressed.
High velocities over the upper surface aren't needed. The lower surface experiences laminar flow in the boundary layer. This results in a lower Cl for the same lift, and a lower Cd.
It's as if the wing's AR is increased.
The Shuttle and SR-71 land softly because of the compression of the air.
Your wings see the same thing.
The problem with exploiting the condition is turning. Any bank changes the distribution of the air pressure in an exponential manner, the lower wing seeing the most, the rising wing experiencing a large change.
This is why those planes built to use the condition have short spans. Ekranoplan, Lippisch, etc..
Paul,
I agree in principle but disagree in the details of your analysis.
By definition in subsonic aerodynamics, air is considered to be an incompressible fluid, and compression is a factor only as a wing approaches Mach 1 (the "compressibility factor" first experienced at the end of WWII as airplanes were approaching the speed of sound in a dive).
At subsonic speeds, the reduction in drag (and increase in lift) in ground effect is caused by the reduction of the downwash angle (the angle the air departs the trailing edge relative to the chordline).
In free-stream flight (out of ground effect), the downwash angle develops based on a number of structural and environmental factors.
As the wing is brought closer to the ground, the downwash flow cannot compress since the air is incompressible at these speeds, so it interacts with the ground by reducing its departure angle from the trailing edge of the wing.
The "mathematical" effect of this downwash angle reduction is a tilting forward of the total aerodamic force vector, while the practical effect is a reduction in apparent drag and increasing apparent lift components...
Cheers!
Jim