## 1 Introduction  
Recently, micro multi-rotor aircraft have developed rapidly due to the continuous improvement of technology in flight control. Micro multi-rotor aircraft have been widely employed in military and civil fields due to their simple configuration, high safety and convenient operation [1; 2; 3; 4; 5]. Compared with the traditional rotorcraft, they have a large increase in payload due to the increase in the number of rotors. In particular, multi-rotor aircraft with coaxial rotor pairs show better hover performance. However, there is strong aerodynamic interference between the adjacent coaxial rotor pairs, especially the upper rotors and lower rotors. Obviously, the existence of those aerodynamic disturbances will seriously affect the aerodynamic performance of micro multi-rotor aircraft [6; 7]. In addition, they will be affected by wind gusts when a micro air vehicle is working outdoors, resulting in more complicated aerodynamic interference between rotors. Thus, it is necessary to study the ability of resisting wind disturbance for micro multi-rotor aircraft to improve their flight efficiency and aerodynamic characteristics. Recent studies involved with aerodynamic performance are only focused on coaxial rotors. [PERSON] et al. [8] analyzed the aerodynamic characteristics of coaxial rotors at a high-speed both in hover and forward flight with the wind tunnel test and numerical method. [PERSON] et al. [9] obtained the flow field distribution characteristics of coaxial rotors in hover with the computational fluid dynamic (CFD) method. [PERSON] et al. [10] proposed an unsteady aerodynamic analysis to analyze the aerodynamic interference between coaxial rotors with a vortex particle method. [PERSON] et al. [11; 12] studied the aerodynamic performance of a coaxial rotor system with different rotor spacing and wind speed by experiments and the CFD method. Multiblock grids and the Multiple Frame of Reference (MFR) method were adopted by [PERSON] et al. for calculating hover characteristics of the coaxial rotor system [13]. In their studies on the aerodynamic characteristics of micro multi-rotor aircraft, [PERSON] et al. [14] studied the vortex formation, turbulent regimes, wake region and tip vortex formulation of the multi-rotor aircraft with various wind conditions using the CFD method. [PERSON] et al. [15] analyzed the characteristics of the downwash airflow for six-rotor UAVs in hover. [PERSON] et al. [16] studied the wake interaction between the rotors of a quadrotor on the hover and forward flight. [PERSON] et al. [17] studied the influence of airflow disturbance and rotor disturbance on Hex-rotor aircraft by designing an inversion sliding mode controller. Through the above studies, it can be found that the effect of the wind disturbance is not considered in the aerodynamic performance yet. However, the wind effect is not ignorable during the flight tests. Therefore, it is crucial to study the wind effect on aerodynamic characteristics of the multi-rotor aircraft to improve the manipulations in the flight control. Considering that the wind effect on the aerodynamics of MAVs is undocumented without much reference, it is a challenge to lead concreteness. To be clear, the main research highlights are summarized as follows: (1) 12 rotors with six pairs of coaxial rotors are promoted to maximize the payload of the vehicle. It has a much wider class than those traditionally used, including the visible results from numerical simulations and higher accuracy from the experimental setup to demonstrate the validity of the present optimization method. (2) Wind effects on the thrust and power of multi rotor pairs are tested to demonstrate that additional thrust with the optimal configuration achieved a good wind resistance. This paper goes further by introducing the effect of wind disturbance as part of the engineering design and shows that it satisfies the design constraints whilst optimizing the combined electric and aerodynamic efficiency for typical hover conditions.