Abstract: An analytical model is presented for helicopter rotor/airframe coupled dynamic stability in forward flight. Extended Pitt/Peters dynamic inflow model is employed in perturbed motion in order to integrate hovering and forward flight cases. A method of implicit multi-blade coordinate transformation （MCT） is presented and applied in the derivations of linear differential equations of rotor/airframe coupled system, and the order schemes as well as the hypothesis of low displacement of blade steady flap and lead-lag motion are retracted. By Floquet theory and transition matrix resolution,the dynamic instabilities of coupled rotor/airframe system are analyzed. The analytical model is then applied in the predictions of hingeless rotor helicopter ground resonance and isolated rotor instabilities in forward flight. The predictions correlate well with the test data of references.

Key words: helicopter dynamics, ground resonance, air resonance, dynamic stability