I'm stumped for words! So there is an increase in static pressure in ground effect. The next trick is to understand whether this effect is the same from ground level to rotor height. Good work, Delta3.
Matthew, i concede - but i'm at a loss to understand what causes this. I guess incompressibility is an assumption to make the maths simpler, but it shows that aerodynamics always finds ways to suprise you...
Shawn, what is your experience of torque or manifold pressure over long grass vs tarmac?