## 1 Introduction  
Micro unmanned aerial vehicles (MAVs) have been predicted to be the next, emerging generation of aerospace systems, with many potential applications in both civilian and military missions [1; 2; 3; 4; 5]. Although MAVs offer numerous inherent utilities and exciting new capabilities, they also offer numerous challenges in design, stemming from their small scale. From aerodynamic performance to the number of moving parts, each MAV configuration faces design challenges. Especially for the typical rotary MAVs-quadrotor, rotor distribution can have a large impact on the vehicle's capabilities. The main difficulty in achieving a better performance with multi-rotors comes from the large hover power requirements caused by strong rotor interference. Hover is an intrinsically high-power flight state with considerably larger energy requirements than cruise. If hover extends for a significant fraction of the mission duration, hover efficiency becomes a key vehicle characteristic that must be carefully addressed. Since rotor wakes will produce the phenomena of distortion, winding and crushing due to the induction between wakes, which affects the aerodynamic performance of the quadrotor aircraft, therefore, appropriate rotor spacing should be selected wisely to reduce aerodynamic interference for the structural design of the quadrotor aircraft [6; 7]. Although much work involved with control ability has been performed on quadrotors, there existed scarce data on the aerodynamic performance of quadrotor. For the theoretical research, [PERSON] et al. [PERSON] et al. (2018) studied the propeller design of micro quadrotor aircraft by using computational fluid dynamics (CFD) method, and optimized the aerodynamic performance of the aircraft through the shear stress transfer K-Omega (SST K-Omega) turbulence model. [PERSON] et al. [PERSON] et al. (2018) used N-S equation and free wake method to solve the flow field of the rotor, and obtained a better simulation result with a relatively small calculation cost. [PERSON] et al. [PERSON] et al. (2018) conducted the numerical simulations both in hover and forward flight for the quadrotor aircraft. They found that the inflow caused by interaction between rotors during hover was higher than that of a single rotor, and the tip vortex and downwash flow of the upstream rotors would have a strong impact on the downstream rotors during forward flight. [PERSON] et al. [PERSON] et al. (2018, 2019) analyzed and calculated the aerodynamic interference of the quadrotor aircraft in hover and forward flight through numerical simulation method, which provided theoretical basis for the overall aerodynamic layout design of the quadrotor aircraft. In addition, as for experimental studies, [PERSON] et al. [PERSON] et al. (2019) studied the aerodynamic characteristics of the quadrotor through wind tunnel tests. They found that the lift coefficient of the rear rotor was significantly reduced due to the interference effect of the front rotor on the rear rotor during forward flight. [PERSON] et al. [PERSON] et al. (2019) studied unsteady flow field and aerodynamic characteristics of tilting quadrotor aircraft based on sliding grid technology, and verified the reliability of the method through experiments. [PERSON] et al. [PERSON] et al. (2019) used momentum and blade element theory to deduce the expressions of forces and moments generated by the rotor, and verified them through wind tunnel tests. [PERSON] et al. [PERSON] et al. (2019) analyzed the aerodynamic interference between rotors at different rotor spacing by experiment and CFD simulation, but the influence of aerodynamic interference on power is not considered. [PERSON] et al. [PERSON] et al. (2019) studied the aerodynamic performance of coaxial rotor with different spacing and wind disturbance by wind tunnel tests. In [PERSON] et al. (2019), a test platform was designed to measure the thrust of the quadrotor under the influence of wind.