To: 212 Man
You said," If one blade lags as the other leads ) that is quite a torque on the mast/hub. What actually happens(in an underslung teetering head)(draw a simple diagram) is that as the blades flap about the teetering hinge, the c of g does not move relative to the mast".
This is exactly what happens on a Robinson head. Since the blades are free to flap on the cone hinges the blades will lead and lag. Since there are no lead lag hinges the lead lag action is reacted by the cone hinges which will wear in an elliptical pattern. This load is further reacted by the teeter hinge and transmitted directly to the mast.
While all this is going on, the blades are flexing in plane which in my mind can lead to instability and/or divergence.
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The Cat