Linear and nonlinear damping identification in helicopter rotor systems

Abstract

To study damping augmentation in helicopter rotor systems, accurate and reliable linear and nonlinear damping identification techniques are needed. This paper focuses on three analyses for identifying damping from transient test data; an FFT-based moving block analysis, an analysis based on a periodic Fourier series decomposition, and a Hilbert transform based technique. Analytical studies are used to determine the effects of block length, noise, and error in identified frequency on the accuracy of the identified damping level. These analyses are applied to transient data associated with single degree of freedom (SDOF) systems having linear viscous damping, as well as nonlinear Coulomb and quadratic damping. The FFT-based moving block analysis performs well for linear viscous damping, but has unacceptable performance for systems with nonlinear damping. These problems are remedied in the Fourier series based analysis and acceptable performance is obtained for nonlinear damping identification from both this technique and the Hilbert transform based method.