Condition Monitoring of Helicopter Drivetrain Components Using Bispectral Analysis

In this paper, bispectral analysis of vibration signals is used to assess health conditions of different rotating-components in an AH-64D helicopter tail rotor drive train. First, cross-bispectral analysis is used to investigate drive-shaft faulted conditions -- namely misalignment, imbalance, and a combination of misalignment and imbalance -- with respect to a baseline case. The magnitude of the cross-bispectrum shows high sensitivity to abnormalities in the drive shaft, and phase information can be used to distinguish between different shaft conditions. Auto-bispectral analysis is used to study vibration signals collected from a faulted hanger bearing with simultaneous drive shaft misalignment and imbalance. In the presence of drive-shaft faults, shaft harmonics dominate the power spectrum of the vibration signals, making it hard to detect the bearing's fault using only the power spectrum. Application of bispectral analysis provides information about the fault's characteristic frequency and relates spectral contents in the vibration to their physical root causes.