To answer ASER's statement;
"I think the rotor is more efficient at NR 90% than 100% at 2.5min power , in the transition to Vtoss. No?"
Next time you have an aircraft for training, sit in a high hover in a high power situation, out of wind, and get the training captain to fail an engine. If you droop the NR down toward 90% you will find the Aircraft will sink. By lowering the collective and allowing the NR to increase toward 100% the aircraft will start to climb, indicating the increase in efficiency of the rotor system at 100%NR.
