Originally Posted by
SplineDrive
It’s a long video so I’m not sure which minute mark you’re referring to, but I did scrub through and see the cockpit video when the ship was at some ground idle condition with the crew clearly bouncing in their seats. At that condition, the rotor isn’t tuned well to that condition and quite possibly the airframe/gear/ground system isn’t either. Often, a helicopter is tuned for only a few specific rpm/operating conditions and away from those targeted conditions structure is nearer or in resonance, etc. The UH-1 is a tad before my time so I don’t know much about the airframe/gear frequency placement but I do know something about the rotor.
Tune? It should be balanced right. If something is balanced it is so at 100 rpms as well as 400, not?
Considering that the centrifugal force increases proportional to rotational speed squared (!), I'd be very nervous when something is already rocking visibly at 100 rpms to go on and crank it up 400ish rpms, as that would create 16 times the force working against bearings and frame than one could witness rocking the scene at 100 rpms.