Drooping Nr for better efficiency
I asked Nick Lappos a technical question, Nick kindly replied and suggested I share the question with other Ppruners.
"With regards to the S-61, Sikorsky recommends drooping the rotor system to 94% after losing an engine after TDP(Take off decision point)/ before LDP (Landing decision point). I do understand that any rotor system is designed to be most efficient at a certain RPM and that this is normally slightly lower than normal operating RPM so that the rotor system will become more efficient as the RPM is drooped in the event of an engine failure. I have seen that it works but dont understand exactly why. Obviously it enables a lot more collective pitch to be used but drag is also increased and lift is lost due to the lower V squared. Coning angle will also increase, reducing disc area and therefore lift. I presume this will be offset by a more efficient lift/drag ratio but is there more to it than that?"
Nick's reply :
"The answer has to do with what we mean by efficiency and at what flight regime. Basically, the higher tip speed is used in high speed cruise, and
actually wastes some power in a hover and at low speed. This compromise is
useful, but when you are OEI, you gain several hundred pounds of performance with lower rpm. It is precisely the same in the S76, and the OEI procedures call for allowing a 5% droop to help the OEI performance."
Anyone else care to contibute?