Electromechanical Modeling and Testing of a Novel Electrically Driven Stacked Rotor System

A novel thrust control method for an electrically-driven stacked rotor system is described. The stacked rotor comprises of two two-bladed rotors spinning in the same direction at the same speed, with a fixed axial spacing and variable azimuthal spacing. Changing the azimuthal spacing by around 22° results in a 17% change in the total rotor system thrust. An electromechanical model of the rotor and drive system is developed incorporating 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 is validated with measurements on a single, 2m diameter rotor in hover driven by a single electric motor at constant speed as well as during transient rotor speed changes. The validated model is used to explore the behavior of the system in response to a commanded change in rotor azimuthal spacing. At a blade loading of 0.08, and a rotor speed of 1200 RPM, computations indicated that a 5° change in azimuthal spacing could be achieved in less than 0.2s, or less than five rotor revolutions, requiring a transient power increase of 12% the mean power. These results indicate the feasibility of achieving small changes in thrust at a high bandwidth with a small increase in motor power output.