Originally Posted by
SplineDrive
Once the rotor is out of a pure hover and has edgewise airflow, there are significant harmonic air loads on each blade. 1 per rev, 2 per rev, 3 per rev and so forth. These forces variously sum or negate themselves at the rotor hub and transmit certain frequencies to the fixed system. 2 per rev vertical blade loads sum to a 2P vertical airframe vibration while 1P and 3P horizontal forces in the blades sum to a 2P horizontal vibration in the airframe. Higher frequency harmonic air loads also sum, but the magnitude of the forces generally drop as the “per rev” increases, unless the blades themselves are poorly tuned and get excited near a resonance.
The 1P and 2P harmonic air loads in a rotor are large enough, that even a very well tuned structure sees large loads without a hinge in the relevant direction, but of course, hinges introduce some new problems, particularly hinges in the rotor plane.
Thx for the detailed reply, but those "vibrations" occurred way before translational lift, even before hovering, they were pratically still on the ground when everything was rocking wildly. So no lateral airflow, everything should be perfectly balanced at that stage.
I find it truly amazing that these vibrations happen early on, at low rpms, stationary, at or before hovering and precisely NOT in horizontal flight with all it asymmetry.