Nick,
All your comments are totally reasonable, except for
"The [ground] effect is 'felt' at the speed of sound". I find two problems with this.
1/ By definition, sonic waves can be heard by the human ear. The rotor blades make relatively little noise, particularly when compared to an engine's un-muffled exhaust port. It therefor seems that the sonic forces propagated from the rotor blades will be quite insignificant.
2/ Any sonic forces that are produced by the blades will be done at all heights AGL. The ground will not be involved in the production of these sonic waves. All that the ground can do is reflect the waves back up towards the rotor disk. In addition, the ground will diffuse these waves. Also, since the blade area is only a small percentage of the disk area, very little 'sonic lifting force' will be received by the blades.
I suspect that a good analogy to hovering in ground effect would be to consider a helicopter that is OGE and maintaining its vertical height while experiencing an updraft. In both the above situations, the collective is lower than it would be for hovering in still air OGE.
The implication is that in ground effect less air is going down through the disk, just as in an updraft.
PPRUNE FAN#1
What you say make sense ~ I think.
Forgetting the helicopter's and the gyrocopter's rotational inertia for a moment. Lets say that a plane, a gyrocopter and a helicopter have forward linear inertia. While these craft have forward velocity, they are all producing vertical lift. When the (cyclic) stick is pulled further back for flare the crafts' overall pitch angles are increased and lift is maintained while forward velocity is reduced. In all three cases, at some reduced forward velocity, their airfoils will finally stall out.
I think that planes, gyrocopters and helicopters will all have to have some forward velocity at touchdown. The only exception to this is the helicopter which can also use up it rotational velocity by use of the collective, for a touchdown without any forward velocity.
Now to bend over and assume the Lu position
Dave J.