Autorotation and Ground-Effect
I am not a rotor head, sorry, but I have been reading this thread with interest and there seems to be aerodynamic misconceptions creeping in.
There will be an increase in ground effect during an autoration landing over a powered landing due to the fact that with the former you presumably have a higher rate of descent until just prior to touchdown and therefore require more lift to both support the weight of the helicopter and reduce its vertical speed therefore increasing the pressure below each rotor blade. The proximity of the ground affects the ability of that pressure to escape.
A rotor blade is just a wing and behaves in the same way. It generates lift by a combination of (positive) angle of attack and relative airspeed so in powered flight and autorotation the relative airflow must be coming from below the cord line producing the usual decrease in pressure over the top surface, increase in pressure on the lower surface and trailing edge downwash. In powered flight the rotor blade (wing) is moved forward by a force applied to the root in just the same way as a powered aircraft with a fuselage mounted engine. In autorotation the rotor blade (wing) is allowed to glide in the same way as a gliders wing and keeps moving for exactly the same reason that a glider keeps moving.
Last edited by Nostone; 24th Nov 2002 at 20:56.