Effects of droplet size and spray width settings on drone spot spray deposition

Abstract

The rapid adoption of spray drones in agriculture has outpaced the scientific research needed to support their effective and safe use. While current research on spray drones has examined parameters like flight speed, flight height, application rate, and droplet sizes, the combined effects of spray width and droplet size settings remain unstudied, especially for spot spray applications. This study employed a three by three randomized complete block design to evaluate deposition and drift responses using a DJI Agras T40 drone. Water-sensitive cards were sprayed with water in a single pass under three spray width settings (3.0, 4.6, and 6.1 m) and three droplet size settings (220, 280, and 340 µm). Wider spray widths (4.6 and 6.1 m) and larger droplet size settings (340 µm) significantly increased application volume observed on water-sensitive cards, reflecting higher flow rates and lesser susceptibility of droplets to downwash effects. Increased flow rates reduce centrifugal nozzle spin, which resulted in significantly larger average droplet diameters at the 6.1-m setting. Effective swath width (ESW) measurements varied by method. The bilateral method produced the widest ESWs, whereas the 50% deposition and ESW30 methods produced ESWs at or below intended spray widths. Despite uniform droplet distribution (diameter variation coefficient <20%), substantial drift occurred, extending up to eight times the intended spray width. Overall, our data suggests that increasing spray width settings to 4.6 m and 6.1 m and droplet size settings to 340 μm improved deposition and reduced drift due to greater droplet sizes; however, programmed settings did not consistently reflect field outcomes. This shows the need to (1) account for high drift potential in prescription mapping, (2) conduct field calibrations to optimize ESW selection and overlap, and (3) modify flight plans and spray formulations to better align programmed and actual spray performance.