@ ShyTorque:
The scenario you point to, and which I understand and have practiced, is at flight NR and
in powered flight. MRH torque is orders of magnitude larger than tail torque, so I don't think your objection fits the scenario. Controlling the direction with slight variations in pitch and airspeed/streamline is in part made possible due to that signal difference in magnitude of torque, and due to aerodynamic forces on the fuselage aiding in direction control ... but thanks for making me think about that anyway. And I may still be missing a point, so have at me if need be!
@henra: (and awblain)
Thanks for the point on stalled blade still having a reaction/torque effect. Appreciate it. Probably changes the balance of forces a bit.
@Chris:
No, Chris, NOT under power. That's the point of inquiry I am pursuing.
The energy bleed comes from the combination of a lack of motive power to the drive train, low inertia rotor head, and a collective position that (for x seconds or parts of seconds) keeps pitch on the blades ... which of course makes for drag ... drag reduces rotor speed ... that changes AoA due to reduced airflow ... this compounding pair of factors rapidly makes for critical AoA and a stall ... (Vicious circle, pun intended, however tragic.)
How do I paint this in fixed wing?
A never ending floating flare with a glider until wing stall for an analogue, with the only real difference being four blades spinning. But it happens in three axes, not one. Would you enter a spin in a glider a few hundred feet over the ground? No. Nobody would.
To make this extra tasty, chances are one rotor blade stalls first so that you get a rolling (and pitching) moment as soon as that one loses it ... and it gets more bizarre as the rest of the egg beater gives up the fight ...
in that way it might have a resemblance to the old retreating blade stall at high speed that we are all taught to avoid.
But, ya see, he's not NOT flying fast, so RBS isn't going to leap to his "what's wrong" screen in the brain, is it?
Where am I going with this?
UPSET! UPSET training.
And, you may smite me for this ... AF 447.
The parallel is bizarre, and it's a reach. A pilot maybe ending up in a condition that he (1) doesn't recognize and (2) is not trained for.
(I don't refer to the UAS, I refer to the actual stall of the A330 as the analogous issue)
In this case, there are immensely good reasons not to train to "lets stall the rotor head and see how she flies!" and I'll leave that to another time.
With the above in mind, let's sit in the captains seat in this EC 135.
For a reason (one I am not sure of) drive train input to rotor ends, rotor RPM decays, and for another reason (one I am not sure of) the response of down collective right now, and a bit of back stick to regain inertia, is either late or applied out of synch or he already had the cyclic back and was slowing down ... or ... something. Don't know.
Anyway, the bird gets to the point henra mentioned previously, wherein the main rotor blades are slow enough to stall, and as I suspect the finestron is for at least a few seconds still providing thrust/yaw, the pilot finds himself in a weird place.
Rotary Wing OCF. Out Of Control Flight. UPSET!
At this point, with the rolling and yawing moments mentioned above due to the force combinations in three axes profoundly changed from the usual proportions, he may well have the collective all the way down expecting it to get him back into the autorotation regime, however hasty, that he knows he can get sorted.
He may not realize that the RH is stalled, and on top of that he's just had serious rolling and pitching moments, at night, low altitude, that he didn't expect. He's trying to get the bird stable to make the best auto he can at the bottom.
What he won't know is that he has just become a test pilot, with precious little time to figure it all out before the bottom of the auto that he thinks he's shooting ... and when it gets to the pitch/flare and pull ... if that MRH is still stalled and NR decaying ... there isn't lift, there's no bite on the blades, neither to pitch nor to make that one last pull at the end that makes most autorotations turn from a rapid descent into a landing one walks away from.
TC's analysis some pages back got my brain whirling.
This combination of events and unexpected aerodynamic forces more or less fits what TC was getting at: it hit hard.
I'll add the estimation that the pilot was both intially disoriented by unexpected pitching and rolling moments, and was playing catch up as fast as he could all the way to the ground .... as he flew what he thought was the last bit of an autorotation ... but, as I understand henra's point,
if the rotor blades are stalled, you can't fly an auto!
In another difference form AF 447, he didn't have two, three, or four minutes to sort it all out. He had seconds.
I confess to you all that I am not on firm ground with all of this. Please, critique and point to where my reasoning is flawed/broken.
I doubt very many RW test pilots have ventured out into this region of aerodynamic performance of main rotor blades, that is, deliberately causing them to stall in flight.
For good reason, I might add.
