## 3 Results
### Results per EIS  
In the current section, the UAVs are grouped in regard to their EIS in four major periods. The first one corresponds to the early UAV designs, i.e., the few platforms that were in service before 1995. The 1995-2005 and 2005-2015 periods contain the majority of UAVs and, as such, can be used to generate safer conclusions. The last period corresponds to the last decade of UAV design and, even though some operational examples exist, the data in the literature are considerably fewer when compared to the previous two periods. Indicatively, 12, 31 and 79 UAV models belong in the first, second and third periods, respectively, whereas only 16 are available for the fourth period (Appendix A). As also discussed in the introduction, the models considered in this section are 138, since the EIS dates are not available for the rest 64. Figure 21: Cruise speed as a function of GTOW with respect to NATO classification. The color code is identical to the one described in Figure 6. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \(\mathbf{R^{2}}\) & \(\mathbf{A}\) & \(\mathbf{B}\) & \(\boldsymbol{n}\) & **Type of Fitting** \\ \hline All & 0.559 & 45.604 & 0.235 & 202 & Power \\ Micro-Mini & 0.691 & 40.447 & 0.266 & 45 & Power \\ Small & \textless{}0.2 & 33.006 & 0.333 & 60 & Power \\ Tactical & \textless{}0.2 & \textless{}0.131 & 224.25 & 45 & Linear \\ MALE & \textless{}0.2 & 5.009 & 0.493 & 29 & Power \\ HALE-Strike & 0.86 & 6.848 & 0.487 & 23 & Power \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Statistical data regarding the trend fitting for the cruise speed vs. GTOW plots. It must be noted at this point that, especially concerning the last decade, the authors have limited access to fixed-wing UAV data. This is possibly attributed to the fact that, since many of the new UAV projects are still under development, their information released to the general public is limited for safety reasons. However, the scientific approach and analysis methods suggested in this work can potentially be applied to any UAV database, e.g., in a few years' time when more data will be available. For the time being, it was decided to omit the last period altogether, so as not to generate inaccurate conclusions concerning the trends of the examined parameters. Six box plots (Figures 23-28) are used to present the progression of key technical parameters through the years, per NATO category. Note that, in some of these plots, the logarithm of the corresponding parameter is presented so that the results enhance the datagrams visualization and comprehensibility. Figure 23: GTOW EIS progression per NATO category. Figure 22: Cruise speed as a function of GTOW with respect to region of origin. Figure 24: Payload weight over GTOW ratio EIS progression per NATO category. Figure 25: Empty weight ratio EIS progression per NATO category. Figure 26: \(W_{\mathrm{p}}\cdot\mathrm{E}\) product EIS progression per NATO category. Figure 23 shows the GTOW progression for the five NATO categories. In terms of GTOW, the small, tactical and MALE categories have a constant presence throughout the years. However, in the last of the examined decades, these three categories tend to deviate from one another and the boundaries between the three become clearer, at least in terms of GTOW. The micro-mini and HALE categories did not exist before 1995, indicating technology was not mature enough. In the last two decades, there has been a large expansion of the design space in terms of GTOW, with the HALE mean increasing by 50.4% and the micro-mini decreasing by 21%. The whiskers in the last examined decade have a larger dispersion, especially for the MALE and HALE categories, indicating an expansion of the design space regarding these categories. It is evident that there is a potential overlap between NATO categories, particularly in Class III, as discussed in Section 2.2.1. Therefore, it is important to consider the flight altitude as well, to accurately determine the appropriate NATO category for a given UAV.