Handling Qualities of Fixed-Pitch, Variable-Speed Multicopters for Urban Air Mobility

Optimisation-based control design techniques are applied to multicopters with variable-RPM rotors. The handling
qualities and motor current requirements of a quadcopter, hexacopter and octocopter with equal gross weights
(5,360N) and total disk areas (producing a 287N/m² disk loading) are compared in hover. For axes that rely on
the rotor thrust (all except yaw), the increased inertia of the larger rotors on the quadcopter increase the current
requirement, relative to vehicles with fewer, smaller rotors. Both the quadcopter and hexacopter have maximum
current margin requirements (relative to hover) during a step command in longitudinal velocity. In yaw, rotor inertia
is irrelevant, as the reaction torque of the motor is the same whether the rotor is accelerating or overcoming drag.
This, combined with the octocopter’s greater inertia as well as the fact that it requires 30% less current to drive its
motors in hover, results in the octocopter requiring the greatest current margin, relative to hover conditions. To meet
handling qualities requirements, the total weight of the motors of the octocopter and hexacopter is comparable at
13.5% weight fraction, but the quadcopter’s motors are heavier, requiring 16% weight fraction. If the longitudinal
and lateral axes were flown in ACAH mode, rather than TRC mode, the total motor weight of all configurations
would be nearly identical, requiring about 13.5% weight fraction for motors (compared to 7–9% weight fraction
from hover torque requirements).