Theory of Self-excited Mechanical Oscillations of Helicopter Rotors with Hinged Blades

The theory includes the effects of unequal stiffness of the pylon for deflections in different directions and the effect of damping in the hinges and in the pylon. Both the derivation of the characteristics equation and the methods of application of the theory are given. In particular, the theory predicts the so-called "odd frequency" self-excited speed range as well as the shaft critical speed. Charts are presented from which the shaft critical and the self-excited instabilities can be predicted for a great variety of cases. The influence of each physical parameter upon the instabilities has been obtained. The comprehensive treatment applies to a rotor that has any number of blades greater than two. Only a brief discussion and the formula for shaft critical speed are given for the one- or two-blade rotor.