Objective  To explore droplet deposition distribution patterns from aerial spraying and the application prospect of small plant protection unmanned aerial vehicle (UAV) for fruit trees, and study the influence of spraying parameters of UAV on droplets deposition distribution in citrus canopy.

Method  Spray test with six-rotor plant protection UAV was arranged by an orthogonal test of three factors (flight height, flight velocity, nozzle flow rate).

Result  Preferred spraying operation parameters for small plant protection UAV were 2.5 m flight height, 4.0 m·s^–1 flight speed and 1.0 L·min^–1 nozzle flow rate based on the test results of density and uniformity of deposited droplets. The factors that affected the density of deposited droplets were in order of flight velocity, flight height, and nozzle flow rate. Test No.2 with 2.0 flight height, 4 m·s^–1 flight speed, 0.6 L·min^–1 nozzle flow rate and No.8 with 2.0 m flight height, 1 m·s^–1 flight speed, 1.0 L·min^–1 nozzle flow rate resulted in relatively high density and penetrability of deposited droplets, and the penetrability of droplets were 22.21% and 22.41% respectively. The factors that affected the penetrability of deposited droplets were in order of flight height, flight velocity, and nozzle flow rate.

Conclusion  Due to the influence of the wind field of UAV rotor and the unique structure of citrus, the operating parameters of plant protection UAV should be optimized to ensure effective deposition and distrubition of droplets in citrus canopy from aerial spraying. This research can provide reference and guidance for reasonable spraying using small UAV on fruit trees, for improving the spraying efficiency.