## 1 Introduction  
A Hex-rotor unmanned aerial vehicle (UAV) is designed with the advantages of a simple structure, a stable system, and strong flight adaptability, which has wide civil and military applications [1; 2; 3; 4; 5; 6]. Considering the manipulation of the Hex-rotor UAV, the horizontal movement can only change the rotational speed of the different rotors of the Hex-rotor UAV through a controller, thereby completing tasks such as acceleration and hovering. Also, the external airflow disturbance and the mutual interference between the rotors affects the aerodynamic performance of the Hex-rotor UAV. The vortices shed the rotor of the Hex-rotor UAV and form a wide range of mutual interference areas under the UAV, and the aerodynamic performance of the Hex-rotor UAV will be affected. Therefore, it is very important to the Hex-rotor UAV to analyze the aerodynamic characteristics in the horizontal airflow. In addition, when the horizontal airflow is introduced, the incoming airflow will interfere with the aerodynamic characteristics between the adjacent rotors, making the flow field of the Hex-rotor UAV more complicated, affecting the stability and reliability of the Hex-rotor UAV.  
However, most studies on the Hex-rotor UAV are aimed at attitude control and path planning. [PERSON] et al. describes a control strategy on UAV vertical takeoff and landing (VTOL) [7]. [PERSON] et al. studied a nonlinear dynamic model of a miniature Hex-rotor UAV using MATLAB simulation (Matlab2012, MathWorks, Natick, MA, 2012) as the control algorithm [8]. [PERSON] et al. studied the aerial manipulation system of the Hex-rotor UAV [9]. [PERSON] et al. performed attitude control on the Hex-rotor unmanned aerial vehicle (UAV) [10]. [PERSON] et al. studied Hex-rotor UAV indoor flight path planning [11], [PERSON] et al. studied the nonlinear control of a Hex-rotor UAV [12]. [PERSON] et al. researched a control system designed using neural network algorithms [13].  
Currently, only a few researchers pay attention to the aerodynamic characteristics of a Multi-rotor UAV in the incoming airflow conditions. [PERSON] et al. studied the general characteristics of the distribution of the downwash airflow and the rotation flow fields around the rotor tip [14]. [PERSON] studied flight performance tests of MAVs under wing [15]. [PERSON] et al. applied the controller to obtain the height error and attitude error in the environment of airflow disturbance and verified the effectiveness through aerodynamic analysis [16]. [PERSON] et al. studied the aerodynamic performance of a non-planar Quadrotor UAV [17]. These results are all introduced to understand the aerodynamic characteristics of the Hex-rotor UAV. However, the aerodynamics of a Hex-rotor UAV under external airflow and rotor interference have not yet been studied. Above all, it is necessary to study the aerodynamic characteristics of the Hex-rotor UAV considering the horizontal wind effect.  
This paper is structured as follows: the aerodynamic analysis of the Hex-rotor UAV is presented in Section 2. In Section 3, experiments with different horizontal airflow through a custom-designed experiments platform are introduced in detail. In Section 4, numerical simulations of aerodynamics of a Hex-rotor UAV are performed. Finally, Section 5 gives the conclusions.