As far as I remember all who had flown the Sportster experienced the same problem with stick shakes, C. Beaty.I remember Don Farrington relating his experience after going for a ride with Martin Hollmann in his “Sportster”at Rockford Illinois many years ago; “Two/rev vibration level was terrifying; I had already seen it fly the pattern so prayed that it would hold together for one more circuit.”
Rigid 2”x3” mast before it was widely understood that see-saw rotors required a compliant mount.
Thank you, Chuck. That does make perfect sense. I was under the impression that a round mast was preferred over square because it allowed some flexibility in all radial directions. If sideways movement (beyond normal, unbraced tube deflection) isn't needed then I'll not deviate from what's tried and true.The primary flexibility requirement for a seesaw rotor is in the fore-aft direction. With the rotor broadside to the flight path, drag is higher than when end wise. It’s really no different than tossing a whirling broomstick.
The control of the tips plane requires then an individual pitch of the blades, which is also a serious drawback.Unfortunately, the real solution lies in a rotor with more than 2 blades...hingless, floating hub rotor; 3 blades with feathering bearings but no flap or drag hinges. The hub was a triangular aluminum plate with a blade/feathering bearing attached to each apex and with the hub attached to the rotorhead via a universal joint.