Helisphere. i don't get the time to check rotorheads so often these days..
The total pressure under the disk is what is holding you up - this is referred to in textbooks as disk actuator theory. Total pressure is the sum of static and dynamic pressure, so the rotor generates either an increase in downwash velocity (hence wake contraction) or an increase in static pressure (since the actual streamlines are revectored as the aerofoil passes). Since the weight of the heli stays the same so does the total pressure. The rotor is accelerating air down to generate the lift, which just either side of the rotor produces a step in static pressure.
In ground effect you are changing the 3D picture around heli. So for the same static pressure there can be a greater static pressure step and a smaller dynamic pressure step. In other words the air needs be accelerated less. This comes about because the ground "blockage" has the effect of superimposing a "mirror image" of heli below the ground. So heli is operating in climbing air.
The root of my arguement comes from vortex panel theory, which is still used as a pleliminary prediction for aircraft performance. The subject can, and does, still boggle my mind.
