## 4 Discussion  
Previous research that has compared the drift control technologies have done so without the rotor wind field or with a small rotor unit in a wind tunnel laboratory and concluded that the wind speed [36], the physical properties of the pesticide [37], nozzle [26], and operation parameters [38] have a significant effect on the spray drift. However, in practice, these conclusions, mainly applicable to ground equipment, cannot directly guide the operation of the UAVs, because they were separated from an important factor of the rotor wind field. A finding of particular interest in our study is that the rotor wind field played a remarkable role in reducing drift. Although the spray liquid was still collected from the sampling lines above the rotors fixed on the 2-m spray tower, the downwind field of the rotors also suppressed the droplet drift, even at a relatively high wind speed. This conclusion is very useful for reducing drift in terms of controlling the trajectory of the droplets. However, previous studies on the influence of the aircraft flow field on drift are mostly focused on fixed-wing aircraft [39]. Compared with the wind tunnel test, the field test using the UAV as the pesticide spraying equipment not only gets rid of the space restriction but also the field environment is in line with the actual work scene, so the research results are more authentic. Yet the field results are difficult to reproduce due to the lack of strict control of the experimental conditions because of the rapid changes to the field environment. Our research is an innovation, which introduces UAVs into the wind tunnel laboratory and realizes the combination of wind tunnel and field research advantages. In the field test, the drift obtained from the UAVs flying at a certain speed must be different from that obtained from the UAVs with a fixed position, continuously spraying for 10 s to ensure sufficient liquid collection on the sampling lines. The drift in this test, in which a pitch angle of 5 degrees was set according to the flight attitude during the actual operation, can be equivalent to that of the field test based on the functional relationship of speed, time, and distance. In addition, many of the regulations relating to spray drift issued by research institutions, such as the Local Environmental Risk Assessment for Pesticides in the UK [40], has relied on the wind tunnel evaluation system. The reliable data used  
Figure 8: Drift deposition with and without the UAV rotors working. (**a**) Drift deposition under a wind speed of 2 m/s, (**b**) Drift deposition under a wind speed of 4 m/s, and (**c**) Drift deposition under a wind speed of 6 m/s. to guide the operation can be obtained through small-scale tests under strictly controlled test conditions. The investigations of the relationship between field data and the wind tunnel [40] and the computer-based drift model [41] have been reported recently, but there is difficulty identifying the changing data sets. How to represent the'real world' is a challenging task, because of the uncontrollable and irregular test conditions which may have a great impact on the drift. Coupled with the UAVs rotor factor, the study of the relationship between wind tunnel measurement and field measurement of the UAVs still has a long way to go. Therefore, this research provides an effective solution for this subject. Our results suggest that droplet size distribution may significantly affect spray drift, as mentioned by [PERSON] [12]. In this study, relative to the flow rate ratio, the deposition ratios at pressures of 3, 4, and 5 bar were elevated for the three wind speeds. This provided evidence that the pesticide drift results from the combined effect of droplet size and wind speed. The volume median diameter of the droplets usually used to describe the droplet size characteristics is not enough to discuss the substantial information of an entire spray, either globally or within some subarea.