Relative Rotor Phasing for Vibratory Load Minimization for a Coaxial Multicopter

Abstract:
This study focuses on vibration reduction for a coaxial multicopter with 2-bladed, synchronized RPM, variable-pitch rotors through the use of rotor phasing. The study also examines the effect of aerodynamic interference between the rotors of a coaxial pair on the vibration predictions. A set of seven multi-rotor phase parameters are defined—a crossover azimuth for each the four coaxial pairs along with three aircraft level modes—pitch phasing, roll phasing, and differential phasing. The phase modes are examined in both a cross- and plus-configuration multicopter. Irrespective of whether interference was included or not, crossover azimuth of 0? tends to minimize the 2/rev lateral loads, while moderate values of crossover azimuth reduces the 2/rev longitudinal loads, for a coaxial rotor pair. For minimizing overall vibratory moments at the C.G., crossover azimuth corresponding to minimum 2/rev thrust is chosen for all the coaxial rotor pairs. It was observed that when interference is not included in the model, vibrations at the aircraft level are almost zero for pitch phasing and very low for the roll phase mode. However, when interference is included pitch phase is no longer a free parameter. With interference modeled, the 2/rev forces for the cross-configuration are ? 65% lower than the plus-configuration, while the 2/rev moments are ? 70% lower.

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