# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 4 Discussion
### Literature Analysis
#### 4.1.2 Actuation Types  
The type of actuation chosen to generate motion in UAVs is highly influenced by many factors such as the power and energy densities required by the application and allowed by the solution, mission requirements, and drone size, shape, flight mode, mission, or endurance [2]. Three main solutions for actuators were adopted: fuel engines, electric motors, and hybrid solutions. Fuel engines include mainly gas turbine engines, jet engines, gas engines, and internal combustion engines; electric motors more commonly used in UAVs are brushless and brushed DC motors; the hybrid solution is based on a fuel engine (an internal combustion engine [33], a gas turbine [53], or a turbojet engine [40]) used as the primary source of energy, while the actuation is based on an electric motor. The main advantages of fuel engine-based solutions are high power and energy densities, long flight autonomy, and large payload range, making them the primary choice for larger drones. The gas turbine engine exhibits the highest performances among fuel engines, in terms of power-to-weight ratio, reliability, and operation time. Fuel engines' performances become worse for small-scale UAV applications due to a lower fuel economy, lower efficiency, and high noise level. Moreover, their acoustic emissions and thermal signature are high, and the theme of environmental sustainability (due to the shortage of fossil fuels, global warming, and increasing pollution) is a factor to their detriment and pushes toward the adoption of electric propulsion [129]. The choice of electric motors in small-to-medium-sized UAVs is the most common [9; 64; 100; 106], due to their multiple advantages: high efficiency, low noise emission, low thermal dissipation, reliability, no pollutant emission, self-starting, controllability, and high maneuverability. The brushless DC motors' increasing diffusion among electrically actuated UAVs was confirmed by the literature review: indeed, for the totality of the analyzed articles with electric propulsion UAV, the motor used was a brushless DC. The significant weight of the power storage system (commonly a battery) is the main disadvantage of purely electric-powered UAVs. The hybrid solution benefits from the advantages of both fuel engines and electric solutions, overcoming some of their most important disadvantages. A hybrid UAV may allow long flights, with a reduced heat signature and acoustic emissions, lower battery weight due to the battery recharging during flight, reduced fuel consumption, and functioningin an electric-alone mode when necessary with reduced noise. In a limited number of cases [7,11], UAVs have an electric actuation solution where solar energy, as the primary energy, is used to recharge the battery. Other studies [25,93] considered fuel-cell/battery hybrid UAV power systems. In [93], a performance comparison was made between the use of a metal hydride hydrogen storage system and a compressed hydrogen system. Finally, actuators based on shape memory alloys, piezoelectric or piezo-ceramic actuators, MEMS, or strain actuation [46,94,95,110,113,114,117,118,120,121,122,126] are used to modify the geometry of some elements of the drone (such as the wings), with the aim to allow some motion adjustments.