## 3 Classification of UAVs  
The growing interest in UAVs in recent years has led to the strong emergence of various types of aircraft with varying configurations and components in terms of shape and size. UAVs are divided into four types: single-rotor, multi-rotor, fixed-wing, and hybrid [33], as shown in Figure 4.  
1. **Single-rotor**[50] (or helicopter): this category of UAVs takes off and lands vertically. Generally, they use a main rotor for attitude control (roll, pitch, and yaw) and a tail rotor to control the direction. Its main advantage is the ability to carry heavy payloads over a longer flight time. However, the complexity of their mechanical systems, as well as the large size and high cost of the rotors constitute a danger for the uncrewed versions. It is more unstable in bad weather. 2. **Multi-rotor**[51] (or multicopter): this is a UAV having more than two rotors. This UAV category is further subdivided into five sub-categories, which are birotor, trirotor,  
Figure 3: Overview of a multi-UAV ecosystem. quadrotor, hexarotor, and eight-rotor (octocopter) [13; 15]. As with single-rotor UAVs, multi-rotor UAVs also ensure a vertical takeoff and landing. They are fast and agile in flight, allowing them to perform complex maneuvers and flights in confined spaces. However, the short flight time is the main weakness of these types of aircraft. 3. **Fixed-wing**[52]: the navigation principle of this category of UAVs is based on a simple structure of a fixed rigid wing. The classification of these drones is not only based on the type of wing, but also on the body and the power system (Li-ion, Li-Po batteries, or gas-powered). They are subdivided into four subcategories: normal, swept back, swept forward, and delta. Furthermore, they can carry heavier payloads than multi-rotors [53]. The disadvantage of these UAVs is that they have limited agility in flight, which does not allow them to perform complex maneuvers and fly over confined spaces, as well as the necessity for a runway for takeoff and landing. 4. **Hybrid**[54]: this last category is still under development. It is an improved version that takes advantage of both multi-rotor and fixed-wing UAVs. They offer good agility and velocity on long-distance flights. They can carry large payloads and do not require a runway. The main disadvantages are the high price, the complicated mechanics, and the lower performance in terms of flight stability and the restrictive speed ranges. Table 1 presents a brief comparative study of the advantages and disadvantages of each category of UAV. This comparison focuses on payload capacity, endurance, agility, speed, and area of coverage. Generally, the classification is made according to the weight, size, range, and endurance [32]. We will focus on the classification according to the size, which will be useful for this discussion. As shown in Table 2, four classes of UAVs, organized as very small, small, medium, and large, are usually defined. Very small UAVs refer to micro- or nano-UAVs. These UAVs resemble insects or birds with wings, and their dimensions generally vary between 5 and 50 cm. The components are extremely small and lightweight. They can reach a top speed of >10 km/h and are limited to a maximum altitude of ca. 120 m by law. Typical examples of very small UAVs include the German TobyRich SmartPlane Pro ([[https://www.drohnen.de/tag/tobyrich/](https://www.drohnen.de/tag/tobyrich/)]([https://www.drohnen.de/tag/tobyrich/](https://www.drohnen.de/tag/tobyrich/)), accessed on 15 March 2023) with a wing length of 30 cm; it is ultra-stable and equipped with a VGA camera of 640 \(\times\) 480 pixels. Moreover, the DJI Mavic 3 ([[https://store.dji.com/de/product/dji-mavic-3](https://store.dji.com/de/product/dji-mavic-3)]([https://store.dji.com/de/product/dji-mavic-3](https://store.dji.com/de/product/dji-mavic-3)), accessed on 15 March 2023), configured with dimensions of 28.3 \(\times\) 10.7 cm\({}^{2}\), is  
Figure 4: Different categories of UAVs.