Analysis of Rotor Load Harmonic Composition and Load Transference Between Frames

Loads prediction for helicopters is more complex than that for fixed wing aircraft given that the aerodynamic forces that produce the air vehicle’s lift and moments are generated by rotating components (e.g., the main and tail rotors). This paper examines rotor load content as a function of harmonics of the rotor angular speed. Accurate rotor loads prediction is driven by a clear understanding of the underlying physics of the problem. This understanding is important for several reasons. First, additive effects of multiple load components fed from the rotating frame to the fixed frame influence the vibratory response of the hub and fuselage. Second, structural fatigue life is driven by load magnitude as well as the number of applications of each load. The mechanics of load transference between the rotating and fixed frames will also be examined. Examples for 4-bladed and 7-bladed rotors will be presented. Definitions of cyclic/whirl (moment transference), collective (force transference), and reactionless/warp (zero force, zero moment transference) modes will be presented for a generic n-bladed rotor. US Army UH-60A Black Hawk flight test data will be examined. Blade lift, main rotor shaft bending, pushrod axial load, and fixed system servo loads will be derived and compared with measured UH-60A results. The intent of this paper is to provide clear insight into the nature of rotor load harmonic composition and load transference between the fixed and rotating frames for a rotor — two areas not well documented (and never visually described) in the literature.