Multichannel Spectral and Coherence Analysis of Servoflap Main Rotor Blade

Abstract

Flight performance of servoflap rotor helicopters is linked closely with the relationship between the servoflap and the rotor blade; rotor performance can be improved and fuselage vibration can be reduced when the feedback control for a servoflap is properly configured at multiple harmonics of the rotating (1/rev) frequency. In this paper, we have examined helicopter servoflap and main rotor blade parameters via multichannel spectral and coherence analysis. In particular, we have examined servoflap bending, rotor blade flatwise bending at 17 and 24% of the blade radius, and blade torsion at 38% of the blade radius using helicopter vibratory data collected during four representative flight conditions: hover, transition, minimum-power and cruising-speed forward flights. The analysis includes two steps: 1) identifying vibratory energy concentrations of servoflap bending and rotor blade flatwise bending and torsion at multiple harmonics of 1/rev frequency, and 2) quantifying the linear relationships of these spectral energy concentrations between servoflap bending and rotor blade flatwise bending and torsion with measures of magnitude squared coherence function and phase (time) delay. Using these quantitative descriptors, we analyzed helicopter vibratory data during four different flight conditions, summarized spectral energies of the first eight harmonics of 1/rev frequency, and delineated the linear relationships between the servoflap and rotating system parameters. The multichannel spectral and coherence analysis results have generated useful baseline data to assist continual improvement in servoflap rotor design.