Effects of Terrain Slope and Flight Patterns on Downwash Airflow and Droplet Deposition of UASS Spraying in Hilly Orchards

The application of unmanned aerial spraying systems (UASS) in hilly orchards is challenged by terrain-induced airflow variability, which affects droplet transport and deposition. This study investigated the effects of terrain slope and flight patterns on rotor downwash airflow and droplet deposition using airflow measurements, computational fluid dynamics (CFD) simulations, and field experiments. Adjustable slope platforms (0°, 10°, 20°, and 30°) were used to characterize airflow behavior, while droplet deposition was evaluated under flat, uphill, downhill, and contour-parallel flight conditions, both outside and within citrus canopies. Results showed that increasing slope transformed the downwash airflow from an axisymmetric structure to a downslope-biased asymmetric pattern. Flight patterns significantly influenced deposition distribution. Uphill flight enhanced deposition in upslope and upper-canopy regions, whereas downhill flight increased deposition in rear and lower-canopy regions due to stronger recirculation. During contour-parallel flight, airflow shifted downslope, resulting in higher deposition on the downslope side of the canopy. Under the experimental conditions investigated in this study, the effective spray swath width during single-flight-line operations perpendicular to the contour lines (uphill and downhill flights) was approximately equivalent to the width of one individual tree canopy, whereas contour-parallel flight resulted in a narrower effective spray swath width due to terrain-induced airflow redistribution. An upslope route offset of 0.3–0.5 m improved droplet deposition uniformity between the upslope and downslope canopy regions. These findings provide guidance for optimizing UASS spraying strategies by adjusting flight trajectories, route offsets, and operational parameters according to terrain slope and canopy position.