Manned multi-rotor electric Vertical Takeoff and Landing (eVTOL) aircraft is prone to actuator saturation due to its weak yaw control efficiency. To address this inherent problem, a rotor cross-tilt configuration is applied in this paper, with an optimization method proposed to improve the overall control efficiency of the vehicle. First, a flight dynamics model of a 500-kg manned multi-rotor eVTOL aircraft is established. The accuracy of the co-axial rotor model is verified using a single arm test bench, and the accuracy of the flight dynamics model is verified by the flight test data. Then, an optimization method is designed based on the flight dynamics model to calculate an optimal rotor cross-tilt mounting angle, which not only improves the yaw control efficiency, but also basically maintains the efficiency of other control channels. The ideal rotor cross-tilt mounting angle for the prototype is determined by comprehensively considering the optimal results with different payloads, forward flight speeds, and rotor mounting angle errors. Finally, the feasibility of the rotor cross-tilt mounting angle is proved by analyzing the control derivatives of the flight dynamics model, the test data of a ground three Degree-of-Freedom (3DOF) platform, and the actual flight data of the prototype. The results show that a fixed rotor cross-tilt mounting angle can achieve ideal yaw control effectiveness, improving yaw angle tracking and hold ability, increasing endurance time, and achieving good yaw control performance with different payloads and forward speeds.

The Differential Longitudinal Cyclic Pitch (DLCP) in coaxial compound helicopter is found to be useful in mitigating low-speed rotor interactions and improving flight performance. The complex mutual interaction is simulated by a revised rotor aerodynamics model, where an improved Blade Element Momentum Theory (BEMT) is proposed. Comparisons with the rotor inflow distributions and aircraft trim results from literature validate the accuracy of the model. Then, the influence of the DLCP on the flight dynamics of the aircraft is analysed. The trim characteristics indicate that a negative DLCP can reduce collective and differential collective inputs in low speed forward flight, and the negative longitudinal gradient is alleviated. Moreover, a moderate DLCP can reduce the rotor and total power consumption by 4.68% and 2.9%, respectively. As DLCP further increases, the increased propeller power and unbalanced thrust allocation offset the improvement. In high-speed flight, DLCP does not improve the performance except for extra lateral and heading stick displacements. In addition, the tip clearance is degraded throughout the speed envelope due to the differential pitching moment and the higher thrust from the lower rotor. Meanwhile, the changed rotor efficiency and induced velocity alter low-speed dynamic stability and controllability. The pitch and roll subsidences are slightly degraded with the DLCP, while the heave subsidence, dutch roll and phugoid modes are improved. Lastly, the on-axis controllability, including collective, differential collective pitch, longitudinal and lateral cyclic pitches, varies with DLCP due to its effect on rotor efficiency and inflow distribution. In conclusion, a reasonable DLCP is recommended to adjust the rotor interaction and improve aircraft performance, and further to alter the flight dynamics and aerodynamics of aircraft.