Cross-bispectral analysis for detection and diagnosis of helicopter trail-rotor drive-shaft problems

Based on cross-bispectrum, Cross-Quadratic-Coupling spectrum, CQC(f), is developed and used to assess different health conditions of an AH-64D (Apache) helicopter tail-rotor drive-shafts. CQC(f) is derived as a projection of the three-dimensional cross-bispectrum into two-dimensional spectrum that quantitatively describes quadratic coupling power at certain frequency in single-frequency space. Hence, the proposed CQC(f) simplifies the study of cross-bispectrum and inherits its useful characteristics such as high immunity to additive Gaussian noise and high capability of nonlinear-systems identification. Real-world vibration data are collected from a dedicated research test bed emulating full-scale AH-64D helicopter drive-train. Using the proposed index, different quadratic-nonlinear harmonic frequency patterns are detected and used to uniquely identify different cases of drive-shafts mechanical faults, such as angular misalignment and imbalance, compared to baseline case, which helps in gaining more diagnostic/prognostic capabilities. Magnitude response of the proposed CQC(f) is compared to the magnitude response of the conventional cross-power spectrum in terms of immunity to white Gaussian noise, and both analytical and experimental results shows superior performance of the proposed coupling spectrum.