## 1 Introduction
These allow a very high-energy density with a rapid response dynamic, which facilitates the start-up of the equipment, as well as maneuvers that require a greater instantaneous energy supply. It is also possible to make use of fuel cells, where hydrogen as an energy vector provides electricity [55], with great advantages in terms of weight and equipment maintenance, although with a slow dynamic. Additionally, fixed-wing UAVs have the possibility to carry solar cells and use solar energy, so fuel consumption can be reduced [56; 57]. In Figure 2, it can be seen how each technology has very differentiating characteristics in terms of its energy supply capacity. That is why more and more electric UAVs makes use of hybridization \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Fixed-wing**} & \multicolumn{2}{c}{**Rotary-wing**} \\ \hline  & **Electric** & **ICE (Internal Combustion Engine) Engine** & **Electric** & **ICE Engine** \\ \hline Payload & 3 & 4 & 2 & 4 \\ Wind resistance & 2 & 3 & 2 & 4 \\ Minimum speed & 2 & 2 & 4 & 4 \\ Flying autonomy & 3 & 5 & 2 & 4 \\ Portability & 2 & 2 & 3 & 3 \\ Landing distance & 3 & 2 & 4 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Evaluation of some UAV (unmanned aerial vehicle) platforms, according to the literature [46; 47; 48] and the authors’ experience. The evaluation is from 1 (low) to 5 (high). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Ni-Cd** & **Ni-Mh (Nickel-Cadmium)** & **LiPo (Nickel Hydride)** & **LiPo (Lithium Polymer)** & **Li-S** \\ \hline Specific energy (Wh/kg) & 40 & 80 & 180 & 350 \\ Energy density (Wh/L) & 100 & 300 & 300 & 350 \\ Specific power (W/kg) & 300 & 900 & 2800 & 600 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Batteries electric characteristics comparison. among the different technologies [58], taking the benefits of the advantages of each one of them, and adapting to the application for which the electric UAV is designed for [59]. The hybrid combination of different types of energy storage also requires the implementation of larger control systems [60], so that they must reach a balance between energy management alternatives and the size required for computer control equipment, together with the different sensors and actuators that make up the plant balance [61]. After this review of the state-of-the-art of electric UAVs, it is realized that one of the solutions, that the literature shows as the most promising in terms of flight autonomy, is the use of hydrogen and fuel cells in electric RPASs [62; 63; 64; 65]. As a first step for this ambitious ultimate goal, a fixed-wing electric RPA was developed. This work shows how a fixed-wing RPA with an internal combustion engine, used as a target for military applications, was deeply modified in order to transform it into an electric RPA, equipped with a pack of LiPo batteries with the proper battery management system (BMS) and an electric engine with suitable controller. A propeller was also changed, but the main airworthiness characteristics of the RPA were maintained. Among the main advantages of electric engine is that they are of universal use, widely understood, simple in concept and operation, large efficiency, low cost due to economies of scale, and very low-noise level [66].