###### Abstract  
Background: Unmanned Aerial Vehicles (UAVs) applied to agricultural plant protection is widely used, and the field of operation is expanding due to their high efficiency and pesticide application reduction. However, the work on pesticide drift lags behind the development of the UAV spraying device. Methods: We compared the spray drift potential at four liquid pressures of 2, 3, 4, and 5 bar ejected from the hydraulic nozzles mounted on a UAV test platform exposed to different wind speeds of 2, 4, and 6 m/s produced by a wind tunnel. The combination of the wind tunnel and the UAV test platform was used to obtain strict test conditions. The droplet size distribution under spray drift pressures was measured by a laser diffraction instrument. Results: Increasing the pressure leads to smaller droplet volume diameters and produced fine droplets of less than 100 \(\upmu\)m. The deposition in the drift area was elevated at most of the sampling locations by setting higher pressure and faster wind speed. The deposition ratios were all higher than the flow ratios under three wind speeds after the adjustment of pressures. For most samples within a short drift distance (2-8 m), the drift with the rotor motor off was more than an order of magnitude higher than that with the rotor motor on at a pressure of 3 bar. Conclusions: In this study, the wind speed and liquid pressure all had a significant effect on the UAV spray drift, and the rotor wind significantly inhibited a large number of droplets from drifting further. unmanned aerial vehicle; liquid pressure; drift deposition; wind speed; rotor wind field; droplet size +
Footnote †: journal: Applied Sciences  
**Publications:**  
* **1**:  
sprayers are equipped with several nozzles mounted linearly. They are suitable for application on vast land with high work efficiency [8]. Developed countries such as the United States and Australia have been at the forefront of pesticide applications by manned fixed-wing aircraft. Although manned aircraft are equipped with advanced pesticide spraying technology, after decades of development there are still many limitations to them. The flight height is generally more than 5 m to ensure flight safety, which causes serious drift, especially when the wind speed is high in the field. The high cost of pilot training and the need for a dedicated airport make manned fixed-wing aircraft more expensive to operate. Unmanned aerial vehicle sprayers integrate many excellent characteristics of the above application technologies. UAVs overcome terrain difficulties by spraying above the crop without contact [9]. Because of the UAV's rotors, a vortex formed on the crop branches and leaves enhances the penetration ability of droplets through the crop canopy with greater inertial force [10]. According to the report 'global engineering frontier 2020' released by the Chinese Academy of Engineering, the research on precise target application technology of plant protection includes UAVs as one of them. Subject to the report from the Association of Unmanned Vehicles Systems International, more than USD 80 billion from 2015 to 2025 could be created due to the legalization of commercial drones and 80% of that total could be contributed by agriculture drones [11]. Although UAVs drift less than fixed-wing aircraft because of their low flight height and rotor advantages, spray drift is still a concern for scientists. In order to reduce drift, researchers have conducted a variety of studies on the factors that influence droplet drift. The droplet size is one of the significant elements affecting the movement of the droplets. Droplets below 200 \(\upmu\)m can significantly drift off-target when they are exposed to a crosswind of 5 km per hour [12].