# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 4 Discussion
### Literature Analysis
#### 4.1.3 Design Level  
For the classification of the papers based on the design level, few works were included in the class vehicle (12 among 123): 46 were focused on the Propulsion System (PFS) and 69 on a specific propulsion component. The papers of this last class were further subdivided into sub-classes, by considering the motor (35), the propeller (8), the battery (3), or another component (23). The papers in the class vehicle were related to the design of the whole vehicle at different levels, for example: [PERSON] in [7] presented a general optimal design method for solar-powered UAVs; [PERSON] in [24] described the design of a quadrotor UAV, which was used as a navigation sensor testbed; [PERSON] in [44] presented a preliminary study on a new concept of a fully actuated UAV, which can simultaneously modify the tilting angle of all the propellers; [PERSON] in [69] presented a three-meter scale model aircraft, named Hyperion, developed by student engineering teams whose design concept included a novel hybrid gas/biodiesel-electric power train as a green aircraft technology. In the works that dealt with the propulsion system as a whole, very different themes were treated, and some examples are summarized below. [PERSON] et al. [11] provided an overview of a propulsion system optimization tool and system power model applied to a long-endurance solar-powered UAV; [PERSON] et al. [16] addressed practical guidelines for propulsion system design and implementation in a quadrotor micro air vehicle; [PERSON] et al. [53] presented the design of a gasoline-electric hybrid propulsion system implemented in a micro triple tilt-rotor VTOL; [PERSON] et al. [22] discussed the conceptual design of a high-endurance UAV and the preliminary design and experimental tests on a hybrid propulsion system; [PERSON] et al. [15] presented an impulsive thruster propulsion system that could be used in a multi-modal UAV capable of sustained aerial flight, locomotion in water, and deployment from a tube filled with compressed air; [PERSON] et al. [57] presented the experimental verification of the performance of a X8 quadcopter, which extended the original quadrotor concept to eight motors, arranged in four coaxial pairs; [PERSON] et al. [88] described a turbo-compression system design and a performance analysis procedure for a high-altitude long-endurance UAV; [PERSON] et al. [115] compared four hybrid propulsion models; [PERSON] et al. [124] described the propulsion system and integration design of a subscale turbojet UAV. With reference to this specific theme, it emerged that for the functional project of the propulsion system in the literature, there were some works focused on specific types of UAVs, but a procedural synthesis was not present for medium-sized electric-propelled multirotor UAVs. The papers in the sub-class propulsion were mainly focused on the motor, and almost all were related to the design or characterization of brushless DC motors, as in the following cases. [PERSON] et al. [42] put forward the optimal design method for UAVs and interior permanent magnet motors for drones that require high thrust of high rotational speed; [PERSON] et al. [78] investigated the design of a multiphase coreless axial flux permanent magnet machine with concentrated windings, to obtain very high power and torque densities; [PERSON] et al. [84] presented an analytical design method to develop a ring motor that was wrapped around the outer diameter of a propeller; [PERSON] et al. [91] presented a novel parameter design method for a DC brushless motor; [PERSON] et al. [98] aimed at the design and development of a small-scale test rig setup for the measurement of the thrust of any brushless DC motor; [PERSON] et al. [100] studied the characteristics of a brushless direct current motor for the purpose of multicopter design. A very limited number of works dealt with actuators based on other technologies, such as the work of [PERSON] et al.