ne charachteristic has become apparent and I wonder why it occurs. If the thing is flown up and hovered very close to the ceiling (lets call this ICE - In Ceiling Effect) it can remain there despite a very large decrease in RPM.
This is probably a venturi effect but I'd be interested to hear thoughts on the blade element theory behind it.
Interesting question...
Recalling the "venturi effect" you probably mean the "ground effect" - i.e:
A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'.
Now... when hovering close to the ceiling your heli should require more power, because
ground effect (ceiling effect!) reduces the lift generated by the rotor. If your heli remains close to the ceiling despite a very large decrease in RPM i guess that's probably because of temperature raise of the air close to the ceiling (and density decrease). Am I wrong?