"B Lacovara" wrote in message
...
I've been under the impression that the biggest gain in ground
effect is
cutting off the tip vortices thus significantly reducing drag.

Could be dangerous for me to try to interpret Hoerner (in "Fluid
Dynamic Lift"), but the effect on the vortices is indirect, as it's
more relevantly the downwash behind the wing which is being altered
and therefore induced angle of attack, and hence drag. He thus
measures and graphs the height above the ground as the height from the
trailing edge. There's really no "cushion" of air involved, as the
pressure of the air under the wing can be zero or negative with
respect to free-stream air, per normal except at high lift
coefficients, and wing can still exhibit ground effect.

Fred F.