Pitch-Flap-Lag Instability of Elastic Modes of an Articulated Rotor Blade

Details of a main rotor blade instability encountered during flight test of an instrumented experimental helicopter are presented in this paper. The unstable mode was observed to be the first elastic chordwise mode coupled with the second elastic flapwise mode and with the first reactionless torsion mode. An analytical simulation using the DART structural analysis program gave good correlation with the observed frequency and damping, and was therefore used to define a modified blade configuration that eliminated the instability throughout the flight envelope of the helicopter. Flight tests confirmed the stability of the modified blade design. Details of the analytical investigation, including the effects of significant design factors such as weight, stiffness, and chordwise c.g., and of rotor trim, are presented. Particular attention is given to inter-relations between chordwise bending, flapwise bending, and torsional frequencies and chordwise c.g. distribution. The analytical procedures developed during the investigation discussed above were subsequently applied to the design of a new rotor for a similar helicopter. Flight tests confirmed that the design was aeroelastically stable over the entire flight envelope. Details of this program are also presented.