Electromechanical modeling and testing of a novel electrically-driven coaxial, co-rotating rotor system

A novel thrust control method for an electrically-driven coaxial, co-rotating (stacked) rotor system by controlling rotor index angle, or azimuthal spacing is described. The stacked rotor comprises of two 2-bladed rotors spinning in the same direction at the same rotor speed, with a fixed axial spacing and variable azimuthal spacing. Changing the azimuthal spacing by approximately 22 degrees results in a 17% change in the total rotor system thrust. An electromechanical model of the rotor and drive system was developed with a blade element aerodynamic model and field oriented control of two phase-synchronized electric motors, each driving one rotor of the stacked system. The model was validated with measurements on a single, 2 m diameter rotor in hover driven by a single electric motor at constant rotor speed as well as during transient rotor speed changes. The validated model was used to explore the behavior of the system in response to a commanded change in rotor azimuthal spacing. At a blade loading of C [subscript T] / [sigma] = 0.08, and a rotor speed of 1200 RPM, computations indicated that a 5° change in azimuthal spacing could be achieved in less than 0.2 s, or less than five rotor revolutions, requiring a transient power increase of 12% the mean power. This may lead to total thrust variations of 9% at an axial spacing of 0.73 chord. These results indicate the feasibility of achieving small changes in thrust at a high bandwidth with a small increase in motor power output.