Blade Passage Loads and Deformation of a Coaxial Rotor System in Hover

Abstract

In a closely spaced, coaxial counter-rotating (CCR) rotor, the passage of upper and lower rotor blades over each other results in transient loads and blade deformation. This paper describes the measurement and numerical modeling of these transient loads and blade deformations in a single-bladed, 2-m-diam, rigid CCR rotor system in hover. Three-dimensional rotor blade deformation was measured using a time-resolved digital image correlation technique, simultaneously with the rotor thrust and pitch link loads. A numerical model of the rotor was developed using the comprehensive analysis CAMRAD II. This model was validated by measurements on an isolated single rotor and was used to explore the aerodynamic nature of the transient blade passage loads in the CCR rotor system. Rotating-frame natural frequencies were measured, and mode shapes of the first and second flap mode as well as the first lag mode of the rotor blade were extracted using the Complexity Pursuit algorithm. The 2-per-revolution tip displacement of the coaxial lower rotor due to the blade passages was found to be approximately 6% of the mean tip displacement. Vibratory thrust loads of the coaxial lower rotor were nearly 10% of the mean rotor thrust, whereas the vibratory pitch link loads were approximately 30% of the mean value. The numerical model predicted the vibratory hub and pitch link loads as well as out-of-plane deformations satisfactorily within the measurement uncertainties. With the validated analysis, 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.