# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 4 Discussion  
### Literature Analysis  
Focusing on the distribution of the papers among the proposed thematic categories, the results suggested a higher interest of the scientific community in the design of specific elements of UAVs, as the high rate of studies in the detailed design category demonstrated. As Table 12 describes, the highest percentage values coincided with problems related to well-defined technical areas, such as innovative or unconventional components, especially actuators and propellers. Furthermore, the control, identification, modeling category presented a high number of papers: works collected within this class mainly focused on optimized and robust control techniques, as well as mission-planning procedures. Fewer papers were devoted to optimization methods instead. From a general perspective, traditional methods for the propulsion system definition were mostly based on design experience or trial-and-error strategies [38]. These techniques often are grounded on identification procedures for the main subset components; aiming at comparing potential candidates and dynamically selecting the best solution according to the evolution of the design process, large and constantly updated datasets are necessary in order for those selection processes to be effective. In 2019, [PERSON] et al. proposed to simplify and decouple the whole optimization design problem into several analytically solved sub-problems [38]. In 2021, [PERSON] at al. depicted on the contrary an overall optimization, described as a multidisciplinary approach, since it concurrently evaluates analysis models peculiar to multiple disciplines within a common optimization framework [27]. Comparing these last methods, the decoupling approach drastically reduces the computational burden for the implementation of the optimization procedure, whereas the multidisciplinary strategy assures the optimum matching of the propulsion system components. Nonetheless, in order to identify the best solution, this kind of approach unflinchingly evaluates at the same time a potentially huge amount of candidates; therefore, the optimization process should be likely repeated in case new candidates would emerge. Moreover, these kinds of methods are based on cost functions, which only allow considering quantitative technical constraints for the definition of the requirements for the components' selection. As a consequence, these approaches could be hardly adopted in the very early stages of the design process, since they expect for the main strategic aspects of the design concept, such as type of actuation system or drone size, to have been already settled. Besides, additional constraints can also be considered, such as: a specific application, the normative framework, or designer preferences. Finally, nineteen products were classified within the functional design, sizing category. According to the literature, in the choice and subsequent sizing of the propulsion system, some parameters can be considered fundamental: efficiency, reliability, size, mass, power, energy density, complexity, and cost-effectiveness. For UAVs presenting a size consistent with our case study, two possible technologies can provide power generation: electric motors (with energy stored in batteries and, possibly, together with supercapacitors) and thermal engines (with gasoline, methane, or hydrogen fossil fuel). The energy density of fossil fuels is higher than that of batteries, but engines available for UAV applications have low efficiency, a high thermal and acoustic signature, and complex control, and these can cause stability problems. Electric motors have features that make them the most suitable choice for smaller UAVs, such as high efficiency, reliability, low noise, a low thermal signature, advanced control devices and high maneuverability. Moreover, they have the advantage of being more environmentally friendly devices as they do not emit pollutants. In some cases, electric and thermal motors are both used on the same UAV to combine the advantages of the two technologies.