An Exploration of Rotor System Component Loads of the NRC Bell412 Advanced Systems Research Aircraft due to Variations in Flight Control Augmentation

Abstract

Supporting military rotorcraft life cycle research, the National Research Council of Canada - Flight Research Laboratory integrated a flight loads measurement suite (FLMS) into the NRC-FRL Bell412 Advanced Systems Research Aircraft (ASRA). In the rotor system’s rotating frame, main rotor blade, pitch link, and swashplate link loads were captured. In the rotor control system’s non-rotating frame, swashplate support, gimbal ring, and boost tube cylinder loads were captured. To examine inter-relationships between control augmentation and rotor system loading, the aircraft was configured with variations of six rate and attitude -based inner-loop control response types spanning Level 1 to 2 ADS-33 handling qualities. Data was analyzed from 3 Hz limited roll-axis frequency sweep tests performed in hover and forward flight. Results highlighted coherence between piloted inputs, fly-by-wire actuation, rotor control component loading, rotor hub dynamics, and vehicular rates. Component load frequency spectra due to rate, attitude, and hold-based responses differed in modal content. Rate- and attitude-based response types associated with the highest and lowest rotor system loading, respectively. Attitude command and -hold (attitude, velocity), response types associated with the greatest change in maximum oscillatory loads between hover and forward flight. Results project critical requirements for ongoing higher-order rotor state measurement research as next generation unconventional, agile, augmented, and pilot-optional vertical lift vehicles are developed.