###### Abstract
The properties of the spray liquid determined by type and dosage of adjuvants in the formulation affect the spray pattern for all categories of nozzles [13], which occupies an important position in controlling the atomization process to optimize pesticide inputs and minimize the output of drifting contamination [14]. [PERSON] and [PERSON] studied the mechanisms of spray formation by a liquid sheet to be oscillating or perforated and explored how these were influenced by viscosity, surface tension, and density [15; 16; 17]. Air-induction nozzles increase the droplet size by generating droplets with bubbles to achieve the purpose of reducing drift. [PERSON] et al. investigated the effects of 11 kinds of nozzles on droplet drift and results showed that the drift potential index of the IDK series was the lowest [18]. Models, both empirical and mechanistic, have been developed for decades to predict the deposition and drift from aerial pesticide application [19; 20; 21]. The ideal model for setting buffer zones and evaluating the off-target movement of droplets must take mechanical principles and physical phenomena into account, such as gravity, droplet evaporation, and air resistance. The field tests, wind tunnel studies, and models are general methods to investigate spray drift. The study of droplet drift in the field can truly reflect the actual spray in the operating scene. [PERSON] et al. evaluated the field drift potential of three different volume median diameters (100, 150, and 200 \(\upmu\)m) produced by centrifugal nozzles of a commercial quadcopter exposed to different wind speeds [8]. [PERSON] et al. compared the drift potential of oil-powered UAVs and battery-powered UAVs with adjuvant products, the results showed that the drift risk of agricultural spraying could be significantly reduced by declining the number of fine droplets and changing the distribution pattern of droplet spectra [22]. However, the field conditions, such as light intensity, wind direction, speed, humidity, and temperature, are complex, changeable, and uncontrollable. The wind tunnel test can simulate field operation and accurately control parameters [23]. However, due to the limited space in the wind tunnel, previous studies generally spaced a single nozzle or a spray unit consisting of a small rotor and a nozzle at the top of the wind tunnel [24; 25; 26; 27]. In order to facilitate the establishment of the model and obtain the convergence results of the model, some perturbation factors are often ignored, which affects the accuracy of the model. Based on the analysis of the advantages and disadvantages of the above methods, it is expected to study the drift characteristics of droplets in the compound flow field of external airflow and the mutual perturbation between rotors while accurately controlling test conditions. Nevertheless, few current studies have taken into account the compound flow field, especially for multi-rotor UAVs, which occupy a large proportion in China. In this paper, to evaluate the drift under three wind speeds (2, 4, and 6 m/s) and four hydraulic pressures (2, 3, 4, and 5 bar), a drift test of a four-rotor UAV was conducted in the wind tunnel laboratory under strictly controlled test conditions. A UAV test platform was placed at the end of a direct-flow wind tunnel, and polytetrafluoroethylene (PTFE) wire was used to collect the droplets driven by the airflow from the wind tunnel and the disturbance of the rotor. The objective of these tests was to characterize the spray drift based on the strict and controllable test conditions in order to provide a guidance program to reduce pesticide drift in farmers' actual operations.