RobboRider;
Thanks for the scans. It looks like Wagtendonk may have 'blown it' from the start. His first sketch shows an expansion of the streamtube instead of a contraction.
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Crab,
What you are saying makes sense. If we assume that the ground IS the cause of Ground Effect, then we must question HOW the ground does this. The only 'connection' that I can see between the craft and the ground is the air. Air can transmit; heat, vibration and pressure. All of these entail an increased activity amongst the molecules of the air, within the area that is available to them. Heat creates pressure, vibration creates an oscillating, self-canceling pressure, and pressure is pressure.
In ground effect, at a given thrust, I suspect that the induce velocity and the induced drag are less because the angle of attack is less, not that the angle of attack is less because the induced drag is less. I think that a reduced angle of attack result in less "air must shift around the wing (below and forward, and then up).
IF pressure differentiation is the reason, then the "canoe paddle" analogy may be up the creek without a paddle.
Dave J