A preliminary theoretical study of aerodynamic instability of a two-blade helicopter rotor

Abstract : A theory has been developed in preliminary form which seems capable of predicting the aerodynamic instability phenomena of a two-blade see-saw-type helicopter rotor. In particular, the theory indicates the possibility of unstable vibrations even with the chordwise center of mass at or ahead of the 25-percent-chord position. The stability condition for oscillatory motion is expressed in terms of a small number of composite parameters that are evaluated from the moments of inertia, angle settings, and aerodynamic parameters of a blade. Computed stability results for different coning angle settings, center-of-mass positions, and control-system stiffnesses for one value of blade density and aspect ratio are presented in a chart. It is found that, in addition to parameters analogous to those occurring in wing-flutter theory, the present theory contains a parameter that represents an unstabilizing effect due to the difference between the moments of inertia in flapping and in rotation.