Abstract: Nonlinear characteristics of a hydraulic lag damper with pressure relief valves and a filling-oil device are analyzed. Numerical simulated investigations are conducted for the ground resonance stability of a hingeless scaled rotor. Influences of two different types of nonlinear hydraulic lag dampers on ground resonance are analyzed. The results show that the equivalent damping of the lag damper at low velocities decreases significantly due to the free-play of the filling-oil valve. As a result, at each rotor angular velocity where the basic damperless rotor remains stable, the lag damper cannot improve the system dynamic stability at low perturbations. During the basic damperless rotor instability margin, stable limit cycles are present due to the nonlinear hydraulic lag dampers. The limit cycle amplitude is increased as the free-play of the filling-oil valve increases. The present investigation is of value for hydraulic lag damper design.

Key words: helicopter, ground resonance, nonlinearity, hydraulic lag damper, dynamic instability, limit cycle, numerical simulation