## 3 Results and discussion
### Field Test
#### 3.2.2 Spray Droplet Size of Each Test  
The essence of the droplet deposition is that the droplets move in all directions after being released by the nozzle, and the movement trajectories of the droplets of different particle sizes are also different. The research on the droplets size distribution in the spraying area can provide the direct reference for the design and selection of the aviation nozzle to reduce the drift and improve the utilization rate of the pesticide. Figure 7 is the result of the droplet size distribution of each sampling line. As can be seen from Figure 7, the maximum values of \(D_{V0.1}\), \(D_{V0.5}\), and \(D_{V0.9}\) appeared on the collecting line 3\(\#\)-1 and were 406 \(\upmu\)m, 728 \(\upmu\)m, and 1179 \(\upmu\)m, respectively. The minimum values of \(D_{V0.1}\), \(D_{V0.5}\), and \(D_{V0.9}\) were all found on sampling line 1\(\#\)-1 and were 152 \(\upmu\)m, 382 \(\upmu\)m, and 617 \(\upmu\)m, respectively. Figure 7: Droplet size distribution for each sampling line of each test. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Test**} & **Wind Velocity and** & **Temperature** & **Humidity** & **Flight** & **Flight** & **Spray Volume** & **Effective** \\  & **Wind Direction** & **(\(\mathbf{\SIUnitSymbolMicro c}\))** & **(\(\mathbf{\SIUnitSymbolMicro c}\))** & **Height** & **Velocity** & **Application** & **Flight** \\  & **(m/s)** & & **(\(\mathbf{\SIUnitSymbolMicro c}\))** & **(\(\mathbf{\SIUnitSymbolMicro c}\))** & **(m)** & **(m/s)** & **Rate** **(\(\mathbf{\SIUnitSymbolMicro c}\))** & **Distance (m)** \\ \hline
1\(\#\) & 0.5/NE & 17.9 & 70.1 & 12.11 & 16.89 & 12.83 & 135.14 \\
2\(\#\) & 0.9/N & 17.2 & 72.1 & 10.87 & 17.58 & 18.49 & 140.65 \\
3\(\#\) & 1.2/NE & 16.4 & 74.8 & 11.65 & 16.36 & 28.58 & 130.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of meteorological data and flight parameters for each test. Compared the results in Figure 7 with the test results in the wind tunnel in Section 3.1.2, there was a big difference in \(D\
u_{0.1}\), \(D\
u_{0.5}\), and \(D\
u_{0.9}\), and the droplet size of each nozzle in the field test was significantly higher. The reason for this significant difference was due to the droplet size measurement method. The instrument used in the wind tunnel was an LDD, the optical detection method was used to capture the diffraction signal or light scattering signal generated by the laser dispersed particles in the liquid to perform particle statistics, then determining the droplets size. The accuracy of the measurement result was higher [30; 31]. The field test was to analyze the droplet size on the WSP by image processing. However, due to the low flying velocity set in the field test, under the same spraying flow, the medium-low velocity spraying produced more droplets deposition than the high-velocity flying operation, which would lead to the higher density of droplets collected by the WSP. The high drop density was particularly pronounced when using CP04 nozzles with large orifice size. In addition, the WSP itself was extremely hydrophilic, and when the surface was excessively misty, diffusion or spot overlap was likely to occur. However, the existing droplet image processing technology was difficult to realize the droplet segmentation count and overlapping droplet identification, which resulted in a larger particle size value measured in the field test [32]. At the same time, it had been shown that the droplet image processing software DepositScan measured the droplets size to a certain extent larger than the standard value, especially when the droplet coverage on WSP was high, the deviation would be exceptionally obvious [33; 34]. This also revealed the shortcomings of the WSP test application in some special operating environment. In the future, it is necessary to improve the image processing algorithm to promote the measurement accuracy or explore a kind of new type of spray test for medium-low velocity aircraft spraying instead of the current test methods.