The explanations given above may be true initially but as the aircraft passes minimum power speed the rotor speed will decrease again assuming the engine is at fixed power. rjmp, to which aircraft are you referring?
Sure about this? My understanding is that induced drag decreases throughout the speed range of the helicopter, so other things being equal and ungoverned, the RRPM will increase with airspeed. Type shouldn't have any bearing.
Good explanations given by Autoranger, Rotortorque and Shawn Coyle already.
TT