Handling Qualities Assessment of Large Variable-RPM Multi-Rotor Aircraft for Urban Air Mobility

Optimization-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
(1200 lb) and total disk areas (producing a 6 lb/ft2 disk loading) are compared to one another 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 15% weight fraction, but the quadcopter’s motors are heavier, requiring 17% 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 14.6% weight fraction for motors (compared to 8-9% weight fraction from hover
torque requirements).