Spatial Distribution of UAV Droplet Sizes Based on the Asymmetric Characteristics of the Effective Swath in Mountainous Nanguo Pear Orchards

Plant protection unmanned aerial vehicles (UAVs) provide an effective approach for spray application in mountain orchards, yet droplet deposition distribution and spray swath stability under complex terrain remain unpredictable. Based on previous findings on asymmetric effective swath width in mountain Nanguo pear orchards, this study investigated the spatial distribution of droplet size under different operational parameters and clarified the formation and displacement mechanisms of the effective swath from a droplet-size perspective. Under unidirectional flat conditions, small droplets (≤100 μm) decreased and then increased across the swath, whereas large droplets (>150 μm) and DV0.5 showed opposite trends, with larger droplets concentrated in the central region. Similar patterns occurred in mountain conditions, but asymmetry was more pronounced, especially during inter-row flight. Peak DV0.5 values generally occurred 7–8 m from the flight path. In reciprocating operations, nozzle atomization mainly affected droplet size beneath the UAV. Inter-row flight shifted the effective swath downslope by approximately 3–4 m, whereas treetop-following flight improved swath stability. These findings provide guidance for nozzle selection, flight path optimization, and precision spraying in mountain orchards.