Aeroelastic Instabilities of Hingeless Helicopter Blades

In this study the stability boundaries of a hingeless helicopter blade are studied using a system of coupled flap-lag-pitch equations of motion. Divergence and flutter boundaries for the linearized system of equations are presented. Flap-lag, flap-pitch and coupled flap-lag-pitcn instabilities in hovering flight are studied. The effect of the torsional degree of freedom on the flap-lag type of instability is investigated. Similarly the effect of lag on the flap-pitch type of instability is also considered. Results illustrating these effects, together with the effect of various important blade parameters on blade stability are presented. Nomenclature a _ - two-dimensional lift curve slope A,B = tip loss coefficients b = half-chord, nondimensionalized with respect _ to/2 Bl = generalized masses defined in Appendix A Cdo = profile-drag coefficient C/ = element of matrix {C| Ci = quantity defined in Appendix A ei = quantity defined in Fig. 1 ~e = e/l offset used in representing the blade by Young's model Fl = flapping coefficients defined in Appendix A F'x*2),Fx(2) ..,etc. = flutter derivatives associated with the flap equation, defined in Appendix C gi = generalized coordinate, first normal flapping mode !JDi,gD2,gD3 — equivalent damping terms, defined in Appendix C