Evaluation of fibre optic sensing techniques for helicopter rotor blades during ground run and whirl tower test

Abstract

Optical fibre strain and shape measurement sensors were deployed on two bearingless main rotor systems, Airbus Helicopters H135 and Airbus Helicopters H145 (or BK117 D-3) during ground runs with controlled pilot inputs and during a whirl tower test. The sensing capabilities of two optical fibre-based strain sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), and direct fibre optic shape sensing (DFOSS), a shape measurement based on the FSI approach, were benchmarked against conventional strain gauge measurements. Signal-to-noise ratios and modal properties were determined from the collected strain and displacement signatures using an improved operational modal analysis suitable for the removal of amplitude-modulated rotor harmonics and large rotor speed variations. It was shown not only that all fibre-optic based sensing techniques provide detailed understanding into the dynamic properties of the blade, but the measurements also offer insights into couplings from the airframe to the rotor and couplings from the drive train to the rotor. Results and discussions of the analysis of the measurements from the DFOSS system highlight its benefits over strain gauges or of the FBG strain sensing approach.