## 1 Introduction
Quad copter (Figure 1c), hexa copter (Figure 1d), and octo copters (Figure 1e) are multi-rotors which are lifted and propelled by four, six, and eight rotors, respectively. Current RPAs applications for precision agriculture are mainly developed for small- to medium-sized crops, using rotary-wing RPASs with small range and endurance operation, leaving aside large-sized crops, but farmers with a lot of hectares of land find it difficult to reach each corner of field for inspection each time. The fixed-wing RPAS does this task without any hiccup, as farmers can do regular air monitoring of fields to know the status of their crops at regular intervals of time. Table 3 compares the main features of fixed-wing vs. rotary-wing UAVs. In Table 4, an evaluation was done to compare electric vs. internal combustion engine (ICE) engines in both fixed-wing and rotary-wing UAVs regarding some technical characteristics, like payload, wind resistance, autonomy, and landing distance. It can be seen that fixed-wing electric UAVs are those that present more equitable flying parameters. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Reference** \\ \hline Fixed-wing & [23; 24; 25; 26] \\ Single-rotor helicopter & [27; 28; 29; 30; 31] \\ Quad copter & [32; 33; 34; 35; 36] \\ Hexa copter & [37; 38; 39; 40; 30] \\ Octo copter & [41; 42; 43; 44; 45] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different types of RPAS (remotely piloted aircraft system) models developed in past studies for precision agriculture. Figure 1: RPAS (remotely piloted aircraft system) types: Fixed-wing (**a**). Single-rotor (**b**). Quad Copter (**c**). Hexa Copter (**d**). Octo Copter (**e**). Concerning the power supply system of UAVs, electric UAVs are characterized by great advantages such as reliability, efficiency, low-noise, reduction of heat losses, as well as getting an energy supply free of polluting emissions [49]. However, the energy management of these UAVs requires more advanced control, which also takes into account many storage and power supply options [50]. Although it is usual to find that electric UAVs make use of batteries, among which many types have to be differentiated, these have, in general, low-energy density and very large charging times. However, batteries provide great autonomy and reliability of a technology with much development today. Within the different types of batteries, it is not only necessary to assess their electrical characteristics, but also their weight, since this critically influences the energy consumption of the UAV, and; therefore, its autonomy. Looking at the Table 5[51], the energy comparison of the four main types of batteries used in electric UAVs today can be seen. It is found that lithium polymer (LiPo) batteries have a very large specific power, which is the reason why they are used in 90% of UAVs of micro-aerial vehicles. LiPo batteries also have a good relationship of specific energy and energy density, although it can be found in higher values in Li-S (lithium-sulfur) batteries, at the expense of an obvious lower specific power. In addition to different types of batteries, alternatives for recharging can also be considered. Swapping involves exchanging a battery for another already charged, which avoids great complexity in energy management but reduces the necessary size of the batteries [52]. With the laser-beam charging base, an optimal receiver transforms the received light into electricity, thereby avoiding the risks associated with landings, at the cost of reducing the useful height where it can be operated, as well as the search for a location without obstructions for an effective recharge [53]. Tethered UAVs are the safest form of energy transfer, when performed directly, although they reduce the area of operation in a limited way [54]. When a more balanced operation is wanted, supercapacitors can be used.