DAMPING IDENTIFICATION IN HELICOPTER ROTOR SYSTEMS

To study damping augmentation in helicopter rotor systems, accurate and reliable 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, error in identified frequency, and spectrally close modes 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 via a Fourier series decomposition based analysis and a Hilbert transform based analysis. These latter two techniques are applied to experimentally obtained transient data from both vacuum chamber tests of passively damped composite beams and hover stand tests of a bearingless rotor employing chordwise bending torsion coupled composite flexbeams. Excellent correlation was found between the two techniques, as well as between the damping identification techniques and analytical predictions in the hover stand tests.