However, due to the lack of consideration of the aerodynamics of the fuselage and rotor aerodynamic changes, the forward flight performance assessments on the two sites are relatively different from the actual flight. ECalc 1 and Flyeval 2 are two online UAV performance evaluation websites where researchers and hobbyists can evaluate UAV performance by selecting different propulsion systems and G T O W. This reference ignored the changes in the rotor aerodynamics in tilt flow and therefore resulted in approximation errors in the performance evaluation of the UAV. Shi proposed a balance equation to estimate the maximum speed and the maximum pitch angle of a multi-rotor UAV. In order to resolve this issue, Derya established a rotor model in the forward state by ignoring the pitching moment which is remarkably large based on the wind tunnel test. However, these studies have not considered the forward flight modes, and hence can easily lead to the failure of reaching the optimal state when flying forward. Chen, Harrington, Bohorquez, Magnussen and others, ,, ,, ,, conducted relevant studies on the performance evaluation, weight design, optimization of the propulsion system and control design of rotor aircraft. Over the past few years, most of the previous researches about multi-rotor UAV propulsion design only focused on the hovering state. Note that, the flight performance of multirotor UAVs is mainly determined by the propulsion system. These differences bring new challenges to all aspects of the UAV design, especially for the propulsion system (rotor, motor, and battery) design. Although the fuselage aerodynamic forces in hovering mode can be ignored, the aerodynamic drag and lift forces acting on the fuselage increase greatly with the increase of forward speed. Unlike the hovering mode, the rotor will produce pitch and roll moments in the forward mode. According to, ,, , when the multi-rotor UAV moves from hovering to flying forward, the thrust and torque generated by the rotor change greatly. This increases the complexity of the rotor aerodynamics.

There exists an angle between the rotation plane of the rotor and the incoming flow direction when the multi-rotor UAVs are flying forward. Naturally, the characteristics of UAVs in hovering mode are different from those of forward flight mode. With the increasingly diversified application requirements of UAVs, multi-rotor UAVs are likely to be operated more in the forward mode than in the hovering mode during a long-range mission such as emergency rescue and logistics transportation. In recent decades, rotor-wing unmanned aerial vehicles (UAVs) have been widely applied in civil and military fields.

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