Nick,
You are against using the concept of 'pressure' in the discussion of ground effect. But, you have previously commented that the activity under discussion takes place at the speed of sound. The very definition of the word 'Sound' is "Oscillating pressure waves passing through a medium. I believe that 'pressure' is at the very heart of the discussion.
To me, the question is simply 'how does ground effect cause a change(s) in pressure'. The answer is obviously not as simple as the question.
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As a wild guess, I suggest that the ground restricts the airflow. It causes a congestion of air molecules under the disk and it inhibits the replenishment of air molecules above the disk. Hence a pressure differential between the upper and the lower surfaces of the disk (and the fuselage).
PS Merry Xmas
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Edited to elaborate on the possibility of reduced pressure above the disk.
When out of ground effect, the air that is drawn down through the disk is replaced by air from a very large sphere surrounding the helicopter. Some of this replenishment is originating from below the disk.
In ground effect, the ground will restrict the flow from the large area below the disk. This causes some of the replenishment to come from air that has previously gone down through the disk. This 'used' air that is about to be 'reclaimed' is heading upward at a certain velocity and with a certain inertia. It is now called upon to change its direction 180-degrees and head back down through the disk again. The thing that is 'calling it back down' is partial vacumn.
Last edited by Dave Jackson; 24th Dec 2002 at 23:23.