Effect of downwash airflow of 8-rotor unmanned aerial vehicle on droplet motion and spatial distribution

To explore the downwash airflow distribution characteristics of the multi-rotor unmanned aerial vehicle (UAV) and its influence on droplets movement and deposition, the spraying comparison experiment between static-state spray and hovering-state spray was carried out with DJI MG-1P eight-rotor plant protection UAV. The results showed that the strength of the downwash airflow of the UAV decreased with the height of the test layer; the direction of the airflow velocity directly under the UAV was almost the vertically downward direction, and was characterized by increase first and then decrease. The airflow on both sides of UAV presented as a “contracts first and then expands” horn shape, the airflow directions of the test points L and M near the ground were outward obliquely with angles in the vertical direction of 71.3° and 81.5°, respectively. In general, the velocity and direction of the downwash airflow on both sides of the UAV were symmetrically distributed. In the static-state spray, the settling velocity of droplets was slow and the droplet velocities of the test points in each test layer were all lower than 1 m/s. While, in the hovering-state spray, the wind field formed by rotors greatly improved the droplet velocity, and the droplet velocity distribution was highly consistent with the airflow field strength. Compared with the static-state spray, the high-speed downwash flow generated by the rotors in the hovering-state spray increased droplet size. The best deposition droplets distribution effect could be seen at a height of 20 cm from the ground, with the average deposition amount and the coefficient of variation (CV) of 4.69 μL/cm² and 51.81% in static-state spray, and 5.85 μL/cm² and 36.87% in hovering-state spray, respectively. Correspondingly, the downwash airflow of the UAV could effectively improve the deposition amount and distribution uniformity of droplets. This study give a reference for the optimization of downwash airflow, the coupling of droplet-airflow field, the improvement of the utilization rate and distribution uniformity of spraying for multi-rotor plant protection UAVs.