Hello, John. The information about there being a point of no return came to me from Bell Helicopter years ago. Where is this point? The manufacturers won't tell you and don't want you to try and find it for obvious reasons. Just "keep it in the green." My research says that our FAA requires that the rotor rpm be recoverable within 5% below the lower red line. Some rotor systems may be recoverable below that point. I don't know, and I'm not about to experiment with the engine off line. In our light turbines, the idling engine normally keeps the rotor rpm within the green with the collective full down, so the only way to experiment would be to force the engine to idle more slowly or to flame it out (dumb).
The real world many of us fly in every day is often light years away from the finely tuned world of the test pilot. I admire all of you that possess this fine, sharply tuned knowledge, skill and understanding of rotor dynamics and flying on the edge. My comments, thoughts and suggestions to this forum are meant for all the rest of us who simply fly the aircraft and hope we never have to deal with engine failures and the like.
Pete Gillies