Electromechanical Modeling and Testing of a Phase-Controlled Stacked Rotor

Abstract:
Electric power has enabled unique propulsion architectures to improve eVTOL performance. This paper discusses independently-controlled electric motors to enable active phase angle maneuvers for thrust variation of stacked, corotating rotors. This can potentially increase the bandwidth of thrust control compared to varying rotor speed. A two-meter diameter phase-controlled stacked rotor system with independent electric drives was tested. Both isolated rotor and stacked rotor tests were performed actively varying the position, or phase angle of the rotors. Blade collective pitch was set to 11° and rotor speed was set to 950 RPM. Dynamic and steady state performance was predicted with an electromechanical model incorporating blade element momentum theory, field oriented control logic, and equations of motion. Tests evaluated the performance of varying both the magnitude of the phase change as well as the rate of the phase change. Isolated rotor tests showed good agreement with the model, with lower phase changes modelled better. For an isolated single rotor at a blade loading CT/σ=0.1175, changing the rotor phase by?5° at a rate of 50°/s could be completed in 0.11 seconds, or less than 2 rotor revolutions, with an additional 49.7% of nominal rotor torque. Stacked rotor testing showed that there is significant coupling during phase angle changes, which limited the total phase angle achieved. At an axial spacing of 0.7 chord, thrust changes of up to 16.5% in 0.4 seconds were observed with an increase in power required by 44.6% for a phase change rate of 25°/s. Faster rates had comparable thrust changes, but increased the power required. Future work will include designing a control architecture to account for the aerodynamic coupling between rotors during angle changes and mitigate the drift each rotor experiences.

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