Originally Posted by
Shaft109
Hello- just a fixed wing guy here with a question or 2.
Are the main rotors (or indeed the tail rotor) prone to stalling in the same way as a fixed wing ie critical Alpha?
what sort of symptoms are present?
and if so do you treat the individual blades reaching this angle or does the disc react as a whole if that makes sense ? ( ie it simply won’t bite the air any harder or like a fixed wing it will actually descend the more collective you pull?
So far people have been describing one kind of stall - retreating blade stall, where high speed flight causes excessive flapping and can stall the outer area of the blade when it is at the 9 o'clock position (in a CCW turning rotor). As someone mentioned, Ray Prouty talked in one of his books about how rotor blades can continue to generate high lift/drag in this situation because the AOA is not above the critical AOA long enough for flow to totally separate (I believe he said it separates, but then reattaches). I only had retreating blade stall once, a side effect of high speed flight at high altitude. The aircraft definitely let me know it was unhappy - the cyclic resisted being pushed forward, and the entire aircraft shuddered and vibrated each time I did (until it finally got through my thick skull what was going on). On the other hand, one very high time guy I know mentioned that some aircraft may try to roll inverted if you try to push it into deep retreating blade stall. I don't plan on finding out if he is correct.
I'm surprised that nobody has mentioned stalling the rotor system following an engine failure (at least, it would be one scenario where it could happen). If you allow the rotor RPM to decay, and allow a high sink rate to develop, the relative wind from below can drive the entire rotor disk into AOA above the critical AOA, and in this case the rotor will stall and stop rotating (and the aircraft will drop out of the sky). I believe it will happen in a fraction of a second, i.e. a human isn't fast enough to do anything about it once it starts, and there isn't a recovery. (to do an airplane style recovery of lowering the nose of the aircraft, we would need a collective pitch that could produce large negative pitches on the rotor system. Full size rotor systems simply aren't built or rigged with that capability (some have the ability to produce small negative AOA for special reasons).
So, basically we train helicopter pilots to never let the situation develop in the first place, since there isn't the possibility of recovery.