## 2 Materials and Methods
### Experimental Design
#### 2.2.2 Deposition and Distribution Experiment  
The test device is composed of three parts: the NJS-1 plant protection wind tunnel, spray system, and collection system developed by Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs (Nanjing, China). The test device is shown in Figure 1. The speed of the axial fan in the wind tunnel is controlled by changing the frequency of the frequency converter, so as to obtain different stable wind speed. The size of the test section of the wind tunnel is 1.2 \(\times\) 1.8 \(\times\) 10 m (width \(\times\) height \(\times\) length), and the wind speed adjustment range is 0.5-10 m/s; The spray system is composed of the motor, liquid pump, water tank, pressure gauge, pressure regulating valve, nozzle, solenoid valve, timing relay, etc. The collection system mainly includes collection racks of different heights and distances. The height of the nozzle from the ground is fixed at 0.5 m, and the direction of the nozzle is determined to be vertical downward, on the basis of the principle is: the long axis of the spray plume and the wind direction are perpendicular to each other, and the deposition is collected by a polyethylene wire with a diameter of 2 mm. Five polyethylene lines with an interval of 0.1 m were placed from 0.1 to 0.5 m above the ground at 2 m in the horizontal direction of the nozzle, and artificial grass was laid on the ground to prevent droplets from splashing on the ground and polluting the collection line. These collection lines are used to collect droplets passing through the vertical plane air, named \(V_{1}\), \(V_{2}\), \(V_{3}\), \(V_{4}\) and \(V_{5}\) from bottom to top; in addition, 6 collection lines are placed at a height of 0.1 m from the ground along the horizontal direction and separated by 1 m to collect the horizontal drift of the spray from 2-7 m, named \(H_{1}\), \(H_{2}\), \(H_{3}\), \(H_{4}\), \(H_{5}\) and \(H_{6}\) from the inside to the outside (where \(V_{1}=H_{1}\)). The whole measurement system is shown in Figure 2, and the deposition collection system is shown in Figure 3.  
Figure 2: Measurement system of deposition distribution. 1. Water tank; 2. Return water pressure regulating valve; 3. Liquid pump; 4. Spray pressure regulating valve; 5. Timing relay; 6. Flowmeter; 7. Pressure gauge; 8. Wind tunnel; 9. Nozzle; 10. Vertical collection rack; 11. Horizontal collection rack.  
In daily plant protection operations, when the environmental wind speed is \(\geq\)4 m/s, the plant protection UAV cannot perform effective operations. Therefore, the wind speed is set to 3 m/s when the deposition experiment is carried out. Taking into account the saturation of the deposition on the sampling line, the flow of the nozzle is controlled by an electronic timer to control the opening and closing of the solenoid valve to ensure that the spray time of each test is maintained at 10 s. Fluorescent tracer _BSF_ was added to the spraying medium in the proportion of 0.3 g/L to clear water. After each spray, 10 min are needed to ensure that the droplets on the polyethylene wire are solidified, and then tweezers are used to remove the polyethylene wire from the collection rack. After that put the polyethylene wire into a U-shaped tube with 30 mL distilled water and wash it with full oscillation through an ultrasonic cleaner. The eluent of each test is poured into the prepared test tube, and then brought back to the laboratory to determine the concentration of the fluorescent by a calibrated fluorescence spectrophotometer. At the same time, in order to ensure the consistency of the test, at the beginning and end of each test, calibration is carried out under the conditions of spray pressure 0.3 MPa and wind speed 3 m/s. If the results of two tests are within a 90% confidence interval, then they would be considered acceptable; otherwise, the test parameters should be recalibrated. Each experiment was repeated 3 times, and the average value was taken as the final data.