Aeroelastic Instability of Torsionally Rigid Helicopter Blades

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Abstract
This paper is a contribution to the problem of predicting rotor instabilities in maneuvers and gusts during powered flight, based on an analysis of second order flap‐lag coupling effects of torsionally rigid blades. In order to focus on the essentials of the problem, a simple analytical model is selected in the present study where the torsionally rigid blades are also rigid in flapwise and chordwise bending and where they are provided with elastic flexures located at the rigidly supported rotor center. The study includes the effects of low advance ratios and pertains to flight conditions with maximum blade torque for a given collective blade pitch angle. The stability boundary for vertical flight conditions is obtained from linearized equations of blade motion. Examples of response time histories following an effective blade angle of attack input are given based on numerical integration of the non linear equations of motion, both for vertical and forward flight. The example rotors showed stability in vertical flight but became unstable in forward flight. Response time histories computed with the linearized equations agree reasonably well with those from the non linear equations. Because of the simplifications of the analyzed model, the results may not be directly applicable to actual rotor designs where higher blade bending and hub modes could affect the stability limits. One may conclude, however, from the study that in a rotor without lag hinges, second order flap‐lag coupling is an important issue and must be carefully evaluated in any rational dynamic rotor design. The results of this analysis can be compared with s study by M. I. Young who originally raised the issue of second order flap‐lag coupling effects of torsionally rigid blades on the stability of lifting rotors in maneuvers. Significant differences are evident.

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