# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 1 Introduction  
In recent years, the use of drones has spread to applications such as monitoring activities or delivery tasks and last-mile logistics. According to Drone Industry Insights, the global drone market in 2020 generated about 22 billion USD, and it is expected to grow at a Compound Annual Growth Rate (CAGR) of 13.8% to reach 42.8 billion USD by 2025. Total investment in 2019 was about 1.2 billion USD, +67% with respect to 2018 [1]. According to this, the attention of industrial and scientific communities towards drones' design and optimization processes has rapidly increased. Many drone classifications can be found in the literature, based on different parameters, such as specific architectures and missions [2, 3]. For instance, considering mass and wingspan, a taxonomy that identifies six classes can be defined, as reported in Table 1[2]. According to this classification, drones present very distinct physical characteristics even within the Unmanned Aerial Vehicles (UAVs) class and are therefore characterized by very different configurations, both in terms of structures and elements dedicated to propulsion. UAVs represent complex systems, integrating various kinds of elements within a single body-frame; arranging them into consistent sub-systems, such as the aerodynamic components, propulsion system, or control architecture, is generally the first step of the design process. According to the literature, the propulsion system is the main element of each unmanned platform [4] and has the highest priority among the design requirements [5]. The propulsion system can constitute a significant part of the UAV mass (in some cases, more than half [6]). It generates the mechanical power necessary for the operations of the drone, but also contributes to energy consumption; therefore, its performance significantly affects the performance and autonomy of the UAV. For these reasons, the design of the propulsion system in UAV applications, from its conceiving and sizing phases to the detailed design, presents almost unique characteristics, given the need for an inclusive multidisciplinary approach from the first design stages. The literature depicts a few examples of design sizing and optimization for UAVs' propulsion systems, and these methods are often dedicated to specific drone typologies or power supply types [5; 7; 8; 9; 10]. For smaller UAVs within their class, a formalization of the procedure leading to the choice of the propulsion group is not even present in the literature, to date. Since the heterogeneous nature of possible application cases and working conditions strongly affects the design framework, a review of the scientific literature needs to be performed to properly outline the current state-of-the-art for the design of UAVs' propulsion systems. Within this context, this paper aimed to provide a comprehensive overview of the possible propulsion system design strategies, investigating frameworks and application examples currently available in the literature. The review analysis was performed with a systematic approach, and the results were evaluated in light of a dual custom classification taxonomy, which investigated the primary purpose of each work and specific contents. Then, literature indications and hints were combined into an integrated framework for the functional design of the propulsion systems of UAVs. The proposed method aimed at representing a support to the designer in the first phases of the design process, offering a preliminary selection strategy for the propulsion candidates and a quick sizing method for the supply system, which allows discarding unsuitable components from the early stages of the optimization process. The paper is organized as follows: Section 2 describes the approach and procedures applied to perform the paper selection, as well as the evaluation criteria selected for the results' analysis, with particular attention to the design requirements.