Unrecoverable Nr first.
Peter Gillies wrote:
Very few helicopter pilots realize that if, with a total engine failure, the rotor rpm is allowed to fall more than about 5% below low green, the flight is over. OVER. There is no recovery possible regardless of what actions the pilot may take or how high the helicopter is above ground. This fact is not mentioned in the sales literature for helicopters nor in the approved rotorcraft flight manuals. It is not mentioned in any of our FAA publications having to do with how helicopters fly and how to fly them. It is treated as a deep dark secret, unfortunately.
Then, in a second post, reiterated:
Easy. As I said in my earlier post, allowing the rotor rpm to drop below the critical point with no engine power to bring it back into the operating range will cause this to happen every time. No exceptions. And as I said before, there is no recovery from this situation. None.
By virtue of several affirmations in this thread, it seems that Peter Gillies has wide experience in the operational side of this business, thus I conclude that this is a case of his being misinformed (by someone equally misinformed).
Truth is, that, though the test standards for US military and civil qualification are somewhat different in the area of transition to power off flight and the testing required to substantiate the H-V plots, one thing is true, and that is, during the course of flight testing any of the machines I have been involved with, this area of testing has always resulted in Nr excursions below the low end of the published green arc ( into the low 80% range ), and I am sure that the test pilot group at the other manufacturers will attest similarly. Thus one's flight is not " over " as Mr. Gillies suggests.
The aero guys at SA in answer to an internal query on an accident investigation some decades ago, did some simulation and opined that there is an Nr number, where at least from the lift vector and angle of attack calculations, the Nr can reach a point where the decay is unrecoverable, and hence ( and this was assumed at full low collective ) you need to put the throttle(s) forward or use your chute. Number was in the 60% range. So there is some margin between where the qualification test crews operate and the point at which the situation turns really dangerous. With an Nr decay rate of 10%/second, though, one does not have all day.
Since either the FAA or the cognizant military test organization gets copies or witnesses all of this testing, the actual Nr behavior is anything but a " deep dark secret ".
As to the H-V subject, I think Nick has said it all. Hard to comment on other manufacturers/models H-V diagrams, but I and Nick have more than a passing relationship with the UH-60, SH-60, all S-76 models and the S-92 models, and for those, I will pass on that the H-V tests were flown to the required structural limits of the machine by pilots who had experience doing that stuff. If one wants to ignore their validity, as at least one poster suggests, have at it, but do have an adequate supply of band-aids on hand.
Thanks,
John Dixson