Handling Qualities Considerations in Control Allocation for Multicopters

Due to multicopters' role in advanced air mobility, there has been an increasing interest in their handling qualities. This paper investigates the effect of rotor speed control allocation of multicopters on handling qualities. Typically, multicopter aircraft are trimmed with equal speed on each rotor. The force generated by each rotor can be modeled, where thrust and torque of the rotor are related to the square of the rotor speed. The dynamic equations indicate that trimming pairs of motors at higher speeds results in an increase of control effectiveness in the associated axis, improving bandwidth and disturbance rejection. A tradeoff occurs where power usage is no longer minimal at trim, but handling qualities are improved in that control axis. A blade element model for multicopter, RMAC, was used to simulate how this off-nominal control allocation would affect the dynamics of a multicopter. System identification flight tests were used to validate the trends seen in RMAC. Control systems were optimized to provide improved disturbance rejection bandwidth in the axis associated with increased rotor speed. Unmanned mission task elements were performed in flight test, showing improved tracking in the lateral axis by using off-nominal mixing to increase trim speed on the rolling motors. The method was also investigated on a full-sale UAM hexacopter, which showed similar trends, but only small changes in the dynamics were achievable because of significantly smaller power margin as compared to the small scale hexacopter.