Ground Effect
Nick,
Thanks for the interesting and informative comments in your last posting. All that you say makes sense, but the third paragraph raises a debatable point.
Some consider Ground Effect as a 'cushion of denser air' located under the lifting surface(s). This 'cushion' results in additional lift, caused by an effective increase in the angle of attack.
Your third paragraph suggests that this additional lift is the result of the sonic wave bouncing back off the ground at the speed of sound. This action is the basis of tuned exhausts, which are used in racing motorcycles and some recreational helicopters. A tuned exhaust causes the sonic wave from the opening exhaust port to be bounced back to this port, at the speed of sound, to increase the compression in the cylinder. It has the advantage of increasing the engine's torque, but this higher torque is limited to the very narrow range of rpm for which the exhaust pipe was tuned. Outside this narrow range, the torque is now lower than it would be with a conventional exhaust.
Using this analogy, an interesting concern, or question, is raised. Airplane's wings have large chords. This allows sufficient time for the 'shock wave' to bounce back to the wing and produce additional lift. Helicopters have a very low solidity ratio. The mathematics appears to suggest that roughly 90% of the 'shock waves' from the blades will rebound up between the blades. This, of course, would result in negligible additional lift.
Just a thought for consideration.
Dave J.