C. Beaty;n1136621 said:
Inplane stiffness of the rotor is only ½ of the story; the other half is mast stiffness. The mast supplies a spring restraint at the center of the rotor, lowering the resonant frequency of the system.
The use of square tubing as a mast is part of the problem.
Bensen selected square tubing as an aid for home builders; bolt holes could be drilled so that bolts ran tangentially to inside walls, eliminating the need for internal spacers. Square tube is much too stiff.
Then along came the designers of Euro Bensens with welded airframes of square steel tubing: Young’s modulus for steel is ~3x as great as aluminum. With a welded airframe, there is no need for square tube.
My first gyro, long before I understood the nature of rotor vibrations used a mast of round, 2024 aluminum tube and never had a 2/rev problem, no matter which rotor. One of my early rotors used Hughes 269 rotor blades which the previous owner had tapered the root ends to mimic Bensen blades. I, not believing there wasn't enough inplane strength remaining for a Young rotor, built an articulated rotor with a coning hinge at its center and outboard drag hinges without undersling. Smooth as silk but a real eye opener when I tried the same scheme on a gyro with stiff rotor pylon. A long, slow learning process.
A vibration analyzer mounted at the rotorhead must show 2/rev motion; otherwise, something will break. That’s another reason for a soft mast.
I’ve never needed a vibration analyzer.