Study on equivalent damping calculation for hydraulic damper of helicopter rotor

Great amplitude and highly nonlinear damping exists in the hydraulic damper of helicopter rotors. The Fourier series based moving block, the Hilbert transform based technology and the energy balance method are studied in the equivalent damping calculations in terms of the rotating blade lead-lag free oscillations. Influences of the factors, such as the damping amplitude and the logarithmic curve fitting of decaying envelope on estimates are also discussed. The results indicate that, for the linear vibratory or the lightly nonlinear oscillatory systems, the consistent predictions could be obtained with above three deferent equivalent-damping-estimate methods. However the divergence exists in amplitudes of equivalent damping in heavily damped nonlinear oscillatory systems and it needs to be dealt with cautiously.