Dynamic Stability Analysis for Helicopter Rotor/Fuselage Coupled Nonlinear Systems

In order to accurately predict the dynamic instabilities of a helicopter rotor/fuselage coupled system, nonlinear differential equations are derived and integrated in the time domain to yield responses of rotor blade flapping, lead lag and fuselage motions to simulate the behavior of the system numerically. To obtain quantitative instabilities, Fast Fourier Transform (FFT) is conducted to estimate the modal frequencies, and Fourier series based moving block analysis is employed in the predictions of the modal damping in terms of the response time history. Study on the helicopter ground resonance exhibits excellent correlation among the time domain (TD) analytical results, eigenvalues and wind tunnel test data, thus validating the methodology of the paper. With a large collective pitch set, the predictions of regressive lag modal damping from TD analysis correlate with the experimental data better than from eigen analysis. TD analysis can be applied in the dynamic stability analysis of helicopter rotor/fuselage coupled systems incorporated with nonlinear blade lag dampers.