[QUOTE=Seahawk;7855830
Helicopters are vibration nightmares and many design compromises are made to dampen vibs, which substantially impact the airframe structure in terms of fatigue and stress cracks.
The drive train in a helo must be in harmony with respect to vibration...there are instances where low frequencies harmonics in different flight regimes (harmonics in helicopter change depending of hover, forward flight, etc.) can quickly damage the airframe and drive components.
I can refer to any number of airframe and drive component "improvements" to the H-60 that ended up causing unintended stress on the airframe.
You can also cause what is referred to as "ground resonance" by changing drive components:
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I have scratch built and flown my own airplanes... That said, helis scare the crap out of me. As a mechanical guy, I can't get past the small fittings/ linkages/bearings that are involved keeping the whirring blades under control. While I have ridden in quite a few over the years, I view it as playing Russian roulette.
