## 4 Discussion and Conclusions  
In this work, an analysis and evaluation of fixed-wing UAV correlations with respect to NATO classification, region of production, entry-into-service (EIS) date and other key operational specifications is conducted. An in-house database is populated using a set of 202 existing fixed-wing UAVs and a statistical analysis is performed, using a combination of charts and coefficients. The key conclusions are presented as follows:  
* As of today, the majority of UAVs currently in operation are used to perform missions related to intelligence, surveillance, target acquisition and reconnaissance. * The statistical analysis of the key design parameters and ratios indicated that the NATO classification should not be omitted when generating the corresponding trends. More specifically, it was observed that, when examining the general dataset, the trendlines have a considerably better fitting than the trendlines calculated for each category separately. However, new design projects are usually based on a given set of requirements that points towards a specific NATO category (or a range of adjacent categories). Therefore, as interesting as these general dataset fittings may be from a statistical point of view, they are of little practical use. The calculations for the corresponding category should be employed instead of the \"nice\", but misleading, fittings calculated for the general dataset. * As discussed in the previous point, the data for several design parameters and ratios present a considerable dispersion. Therefore, apart from considering the corresponding trendlines and coefficients, the design engineers are advised to refer to charts such as dot and box plots, where the data dispersion can be quantified and the overall design space is better defined for each variable and NATO category. * Considering the EIS date parameter for the statistical analysis has provided some interesting conclusions. The empty weight trends indicate that better materials and processes are employed for fixed-wing UAV manufacturing, leading to decreased empty weight ratios over time. Moreover, the payload-weight-times-endurance product, Wp-E, increases over time for most categories. Combining the above observation with the fact that the ratios of payload weight over GTOW remain practically constant, it can be suggested that the UAV market prioritizes increasing flight endurance. * Concerning the operational parameters, the majority of fixed-wing UAVs that support satellite communication belong either to the HALE or to the MALE categories, followed by the tactical one. When it comes to low observability, the list is even more confined, since only UAVs that belong to the HALE and MALE categories are identified with such features. Moreover, even though the European legislation dictates that all fixed-wing UAVs operating in the continent should be airworthy, no such platform exists yet. There are several areas for future work. For example, further research could be conducted to expand the database to include additional UAVs and parameters, such as cost, reliability and maintainability, as well as to further populate the existing ones. The  
Figure 31: UAVs designed for low observability per NATO category. authors would like to deepen their research activities in an attempt to better capture the correlations and provide better guidelines to designers. To that end, the current database can be used to train machine learning algorithms capable of providing any underlying trends. Finally, the authors suggest that a corresponding study should also be conducted in approximately ten years' time so that reliable data can also be extracted for the current age. Conceptualization, P.P., D.M. and S.K. ; methodology, D.M., S.K. and D.T. ; software, S.K. and D.T. ; validation, P.P., S.K. and D.T. ; formal analysis, S.K., D.T. and D.M. ; investigation, D.M., S.K. and D.T. ; resources, P.P. ; data curation, D.M. and D.T. ; writing--original draft preparation, D.M., S.K. and D.T; writing--review and editing, P.P. ; visualization, S.K.