## 5 Conclusions  
This paper presented the aerodynamic performance of a Hex-rotor UAV considering the horizontal airflow by experiments and numerical simulations. Combined with the results of low-speed wind tunnel experiments, the aerodynamic performance of a Hex-rotor UAV with different horizontal airflow (0 m/s, 2.5 m/s, 4 m/s) is analyzed in detail. Conclusions are as follows:  
Figure 12: Velocity contour distribution (a) 0 m/s (b) 2.5 m/s (c) 4 m/s.  
Figure 13: Vortex distribution of Hex-rotor UAV (a) 0 m/s (b) 2.5 m/s (c) 4 m/s.  
1. The pressure and velocity distribution proved that the deformation of the downwash resulting from the horizontal airflow will increase the power consumption, indicating that the aerodynamic performance of the Hex-rotor UAV is determined by the instant thrust variation. This is because when horizontal airflow is introduced, the streamlines are squeezed and coupled with each other and the maximum pressure region of the rotor may have the maximum thrust and a higher power loading.
2. For a lower rotor speed (1500-1900 RPM), the Hex-rotor UAV showed a good hover efficiency characterized by a higher power loading, especially at 1700-1800 RPM. In fact, the horizontal airflow increased mutual coupling of the vortices and improved the aerodynamic performance by maintaining a higher thrust. This can be useful in the aerodynamic design of a Hex-rotor UAV for wind resistance.
3. For a higher rotor speed (1900-2300 RPM), the hover efficiency decreased with a thrust decrement and power increment as compared to the case at 0 m/s. Actually, the stronger coupling interference began to move along with the wind direction, which caused the rotor vibration, especially around the rotor tip. In this case, reactions will need to include increasing power, increasing angle of attack, sideslip, etc. in a quasi-steady manner to maintain the desired flight path when the rotor speed is higher than 1900 RPM, especially for the work mode at 2200 RPM.
4. The aerodynamic performance of the Hex-rotor UAV is also related to the vortices movement or deformation of the blade tip. The streamline and vortices distribution at 2.5 m/s showed that the proper airflow may improve the aerodynamic performance by increasing the thrust, which also indicated that the Hex-rotor UAV will be simply manipulated by changing the rotor speed in a light breeze. On the other hand, the controller manipulation will be introduced in the gentle breeze with a higher wind speed. In addition, further research will involve a higher wind speed to study the wind resistance of the Hex-rotor UAV.  
[PERSON] carried out experiments and analyzed experimental results; [PERSON] wrote the manuscript with assistance of [PERSON].  
This research was supported by the National Natural Science Foundation of China, grant No. 51505087, the State Scholarship Fund from China Scholarship Council, grant number 201806655008, and Fujian Provincial Industrial Robot Basic Components Technology Research and Development Center, grant number 2014H21010011.  
The authors thank the Key Laboratory of Fluid Power and Intelligent Electro-Hydraulic Control (Fuzhou University), Fujian Province University, and Fuzhou University Jinjiang Science and Education Park for applying the experimental field.  
The authors declare no conflicts of interest.