Measurement of transient blade passage loads of a coaxial counter-rotating rotor in hover

Abstract

This study describes the measurement of transient hub loads and blade deformations due to blade passage in a 1x1-bladed, 2m-diameter rigid coaxial counter-rotating rotor system in hover. The experimental results were analyzed and evaluated in combination with a comprehensive analysis model using CAMRAD II. Lower rotor blade deformations were measured using a time-resolved digital image correlation technique. Rotating modal parameters including natural frequencies and mode shapes were identified from measured time histories of rotor blade deformations using the Complexity Pursuit algorithm. Experimentally identified modal characteristics correlated well with numerical results for the first three modes. The numerical model also predicted the vibratory hub and pitch link loads as well as out-of-plane deformations satisfactorily well and within the measurement uncertainties. The2/rev blade tip displacement due to blade passage was found to be 6% of the mean tip displacement, and the transient blade motion of the lower rotor blade was found to be larger than that of the upperrotor blade. Additional numerical studies on the aerodynamic angles of attack, the inflow velocities from rotor�rotor interaction, and sectional lift distributions over the upper and lower coaxial rotor disks provided further insight into the sources of transient loads due to blade passage. For example, it was found that the angle of attack on the lower rotor induced by the upper rotor had a maximum at 15? azimuth before the blade passage.