Drooping rotor rpm decreases drag from both main and tail rotor and give more available power for lift(pitch).
Due to OEI there is much less total power = the need is less for tail rotor power(and in Cat A profile you either desends for landing on or accelerates forward so LTE risk should be less).
If MGB/drive train is considered strong enough its allowed to use all available engine power. When engine power is less then power needed the rpm will droop. A 'clever' fuel control/ECU/FADEC will allow the rpm to reduce in OEI to give maximum power in the same time as drag is reduced to leave more power for lift.
Other MGB's/drive trains may have a limitation for input power on single engine that is less then the engines maximum power. In this case the rpm will not drop(or only a very little droop due to fuel control caracteristics).
@pilot and apprentice: I fly the Super Puma, our rotor speed gauge in rpm. Normalt flight gives 268-270 rpm( 265 is considered 'nominal'), and min rpm in one engine flight is 245. At least in the simlutator(AS532) a rpm of 245 give maximum power and lift. I calculate 245 from 265 as 92%, so close the the other helos.
Made a check flight yesterday, and in this cold climate/low level we needed to pull very much power to get a noticable droop att all. The droop came much later and a bit qiucker than in the sim.
Last edited by AAKEE; 22nd Jan 2013 at 22:30.