Originally Posted by
wrench1
Just to add, I've found if a 206 mast is under power it will either twist or shear vs bend.
You'll find that 63 year service history does apply as Bell blades were known to "bounce" as well. However, there are a couple other differences between the Electra whirl mode vibrations and this helicopter. For one is the difference in the cycle rate of the "harmonic vibration coupling," to use your term, between the Electra and a 206 or any other helicopter. While the Electra destructive wing flutter happened over a period of flights or time, any similar "coupling" or bounce would have destroyed this 206 within that single event and probably within the 6 bounces. Its somewhat similar when a helicopter goes into ground resonance where the "harmonic vibration coupling" can and will self-disassemble the helicopter within a dozen or less blade rotations. The other difference, the "bounce" discussed here is induced by the pilot which in turn is usually due to a system discrepancy like an improper collective minimum friction setting vs a design issue like with the Electra. Because of the destructive nature of "harmonic vibration coupling" in helicopters, any design flaws are usually worked out long before the aircraft is even flying.
The Electras were destroyed by a single catastrophic coupling of two frequencies that converged, the propellor shaft wobble and turbulence flexing the wing. Even people who should know fail to see it has nothing to do with anything but converging frequencies. If there is a rotor bounce at a certain frequency and a wobble at the tail rotor gearbox nothing will happen if those frequencies remain discreet. Matched up, they destroy the helicopter simply because they are on the same frequency. Voodoo, witchcraft, whatever. It cannot possibly have anything to do with engineering.