Linear Flap‐Lag Dynamics of Hingeless Helicopter Rotor Blades in Hover

The linear stability characteristics of rotor blade flap‐lag oscillations in the hovering flight condition are examined. The present study is focused on the effects of pre‐cone, variable elastic coupling, pitch‐lag coupling, and the aerodynamics of induced inflow. Together with an improved perturbation analysis for deriving the equations these factors are shown to significantly influenee the flap‐lag stability characteristics of hingeless rotor bladea. Routh's criteria are used to derive several fundamental flap‐lag stability relations, and emphasize the utility of simplified rotor blade modeling for understanding complex dynamic phenomena. In order to validate the approximate rigid blade equations, elastic blade modal equations are presented together with comparative solutions. The results indicate a high degree of accuracy for the approximate equations when the elastic coupling is included. Finally, the approximate equations are used to illustrate the influence of elastic coupling on the response characteristics of hingeless rotors.