Rotor Stall Behavior and Loads
SAS, the modern multi-bladed rotors that I have experience with at Sikorsky do not exhibit the severe roll left tendency that I and perhaps others of the S-55/H-19 generation were schooled in.
I can only speak to the S-65,S-70, S-76 and S-92 series. If you drive these rotors into stall, the N/rev vibrations go up dramatically accompanied by non-linear ( with load factor ) increases in both the rotating ( push rods e.g. ) and stationary ( MR servo e.g. ) vibratory loads. A flight test team exploring stall limitations will of course be utilizing telemetry ( TM ) and the relevant loads will be a priority data stream for real time monitoring. Typically, the test team will work to a do-not-exceed limit ( DNE ) for these loads. The DNE is established based upon accelerated fatigue load testing of said components in the test lab, and are based on a very short component life, based in turn usually on a three sigma reduction from the component failure data. It is common for test teams to cycle count fatigue damage on these parts and they are replaced, discarded as required.
As you can see, this approach and taking into account that the published report is that the subject 525 was using TM, and against the personal experience just covered, makes it seem unlikely the cause was as simple as an entry into a rotor stall condition. But I hasten to add that I don't know anything about the 525 rotor, thus I'm out on a limb even going that far. It would be fair to add that during the 39 years at SA, there were a few rotor stability issues that cropped up, each with a different cause and a different solution, and I'd guess that other OEM's are no stranger to those occurrences.
Henra had it right: let the pro's at Bell who know their rotor and new machine do what they alone are equipped to do.
Last edited by JohnDixson; 11th Jul 2016 at 00:12.
Reason: Word left out/grammar correction