Limitations on RPM Control for Large Multi-Rotor Aircraft

Typical multirotor aircraft are designed with arrays of fixed-pitch, variable speed rotors. This control strategy requires changes in the rotational speeds of different rotors to impart the forces and moments necessary to control aircraft motion. At small scale, where the rotor inertia is small and the available motor torque is large enough, controls can be input to the aircraft quickly and with relatively low energy expenditure. This, along with the mechanical simplicity resulting from the lack of a swashplate mechanism for each rotor, makes rotor speed control attractive for smaller multirotor aircraft. As aircraft size increases, rotor inertia will also increase to the extent that available motor torque and battery power may be insufficient in providing the required rate of change in rotor speed. This in turn will slow the rate at which forces and moments generated by the rotors will change, resulting in a sluggish performance and a reduction in the handling qualities for the aircraft as a whole. By investigating the scaling of rotor inertia and typical motor/battery requirements with aircraft size, the point at which rotor speed control becomes undesirable can be identified and different hybrid control schemes can be applied.

Research Scientist