# A Review of Flying Ad Hoc Networks: Key Characteristics, Applications, and Wireless Technologies
## 2 UAV Classification  
There are different types of UAV classifications in the literature. As shown in Figure 1, in this SLR, a comprehensive UAV classification is provided based on different factors such as UAV size and weight, altitude, range, application, flying mechanism, ownership, airspace class, level of control autonomy, and engine type [21], all of which are shown in Figure 1. Size is often a major factor in deciding which UAV is appropriate for a mission. As shown in Figure 1, box \(A\), UAVs can be classified into _Very small_ sizes with very light weight, such as Micro Aerial Vehicles (mUAVs) and Nano Aerial Vehicles (NAVs), _Small_ sizes such as Mini-UAVs, and _Medium_ and _Large_ UAVs that are heavy or super heavy [22]. The classification of UAVs based on weight, endurance, and altitude is provided in Figure 1, boxes \(B\), \(C\), and \(D\), respectively. Table 1 classifies types of UAVs by their different sizes, altitudes [23; 24; 25; 26; 27; 28], weight, range, and Eendurance. Depending on their flying mechanisms, as shown in Figure 1, box \(E\), UAVs can be classified into Fixed-wing drones, Multi-rotor UAVs (rotary-wing), and Hybrid fixed/rotary wing UAVs [16]. Fixed-wing UAVs that can be controlled either autonomously using onboard computers or remotely by the pilot can fly using wings in the same way as aeroplanes. Multi-rotor UAVs have at least four rotors to keep them flying, and hybrid models combines the benefits of two. Another possible classification of UAVs is that shown in Figure 1, box \(F\), which is based on the airspace class; very low altitude UAVs operate in Class G airspace, with the possibility of the aircraft being flown beyond the line of sight of the operator. Medium and very high altitude UAVs operate in Class A through E airspace and Class E airspace, respectively. The International Civil Aviation Organization (ICAO) classifies UAVs based on the degree of autonomy in their flight operations into two main categories, namely, fully autonomous and remotely piloted aircraft, as shown in Figure 1--box \(G\). As shown in Figure 1, box \(H\), UAVs can be classified based on their ownership as public or private, that is, whether they are operated and owned by public parties such as local law enforcement and federal agencies or by private entities or industry. Figure 1: UAV Classification based on different features such as size, weight, range and endurance, altitude, flying mechanism, airspace class, degree of autonomy, ownership, type of engine, and application. In addition, according to Figure 1, box \(I\), UAVs can be categorized based on the different types of engines they have, including turbofan, two-strike, electric, piston-driven internal combustion, rotary, turboprop, push-pull, and propeller. Out of these engine types, piston and electric engines are the most commonly used types, especially for small UAVs and heavier UAVs, respectively. According to Figure 1, box \(J\), UAVs can be classified based on their applications. Figure 2 presents several applications in which UAVs are employed that can be used for classification, as detailed below. * _Military (Figure 2, box A)_: With continuing advancements in UAVs technology, defence forces around the world increasingly use UAVs for a variety of applications, including logistics, surveillance, communications, attack, and combat. Figure 2, box \(A\) shows military applications of UAVs. Famous drone types used in military applications include the RQ-4 Global Hawk, RQ-2A Pioneer RQ-2A Pioneer, QF-4 Aerial TargetQF-4 Aerial Target, R-MQ-8 Fire ScoutR/MQ-8 Fire Scout, RQ-7B ShadowRQ-7B Shadow, RQ-11B Raven RQ-11B Raven, MQ-9 ReaperMQ-9 Reaper, and MQ-1B Predator ([[https://www.military.com/equipment/drones](https://www.military.com/equipment/drones)]([https://www.military.com/equipment/drones](https://www.military.com/equipment/drones)) accessed on 21 July 2022). * _Medical Applications (Figure 2, box B)_: Recently, UAVs have begun to be employed in medical startups.