Mmmmm, this thread takes me back!
I have made good on my promise and read up on blade element momentum theory (Prouty & Leishman), and am now attempting to model helicopter rotor aerodynamics. This is the best way i know to get fully to grips with the issues discussed here. I did enjoy the discussions, D3.
My current model (BEMT & vortex panel) assumes a number of streamlines that pass through different positions on the rotor plane, to be later deflected by the ground. Each streamline has it's own total pressure which does not have to be the same as any other streamline. The helicopter is then kept in the air because of the sum change in total pressure across the rotor, so sum of static pressure and dynamic pressure increases below the rotor. So far the code appears to produce accurate results - that could be a fluke.
This next part is subject to some contention, but should be explained to complete the model: Near the ground the total pressure is the same as under the rotor but is stagnated due to a direction change. This means that altimeters will register an increase in static pressure. This direction changing happens because the airstream sees a mirror image helicopter under ground plane. The mirror image helicopter causes helicopter to see a slight updraught so collective goes down.
My intention is to keep this discussion offline for the time being until i can establish the validity of my approach with D3.
