# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 3 Results
### The Integrated Functional Design Framework  
According to data extracted from literature, the functional design of the propulsion system of UAVs should be grounded on an inclusive framework, which integrates different kinds of requirements. Figure 3 synthesizes the interpretation rationale of the context, which was adopted to outline the integrated functional design framework. The analysis of the mission profile, power supply, and actuation sub-systems allowed determining peculiar requirements for the design of the final device; nonetheless, constraints related to technical limits, as well as normative framework-related requirements can also be identified. For instance, the maximum UAV dimensions and cruise velocity could be considered technical requirements, whereas given the drone's size, the operating altitude is a normative framework-dependent constraint. Further constraints could also be added when considering additional factors potentially affecting the design or the selection criteria for some components, such as the personal preferences of the designer and/or manufacturer (e.g., sustainability in processes or production technologies), as well as management-related attributes (e.g., kind of vendor service contracts, provider reliability, and quality of the supporting channel, or consistency with current infrastructures) [127]; Figure 3 collects them in the custom requirements set. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{3}{c}{**Functional**} & \multirow{2}{*}{ \begin{tabular}{c} **Optimization** \\ **Sizing** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **Identification,** \\ **Methods** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Detailed** \\ **Modeling** \\ \end{tabular}  } & \multirow{2}{*}{**Total**} \\  & & & & & & & & & \\ \hline \multirow{4}{*}{**Model**} & **FxW** & 12 & 24.5\% & 2 & 4.1\% & 12 & 24.5\% & 23 & 46.9\% & 49 \\  & **FIW** & 0 & 0.0\% & 0 & 0.0\% & 1 & 20.0\% & 4 & 80.0\% & 5 \\  & **Mr** & 5 & 15.4\% & 0 & 0.0\% & 14 & 43.8\% & 13 & 40.6\% & 32 \\  & **O** & 1 & 5.9\% & 2 & 11.8\% & 9 & 52.9\% & 5 & 29.4\% & 17 \\ \hline \multirow{4}{*}{**Model**} & **F** & 0 & 0.0\% & 1 & 9.1\% & 4 & 36.4\% & 6 & 54.5\% & 11 \\  & **E** & 9 & 10.8\% & 4 & 4.8\% & 28 & 33.7\% & 42 & 50.6\% & 83 \\  & **H** & 1 & 50.0\% & 1 & 50.0\% & 0 & 0.0\% & 0 & 0.0\% & 2 \\  & **O** & 0 & 0.0\% & 0 & 0.0\% & 1 & 7.7\% & 12 & 92.3\% & 13 \\ \hline \multirow{4}{*}{**Model**} & **Ve** & 4 & 30.8\% & 1 & 7.7\% & 8 & 61.5\% & 0 & 0.0\% & 13 \\  & **PrS** & 11 & 23.4\% & 2 & 4.3\% & 15 & 31.9\% & 19 & 40.4\% & 47 \\ \cline{2-11}  
Figure 4 depicts at a glance the integrated functional design framework for the identification of the propulsion system. More than comparing commercial off-the-shelf products to select the most suitable element, this procedure was meant to support the early stages of the design process, as it aimed to identify the functional characteristics that the further detailed design should include for the selection of the final components. The procedure was composed of three main stages: (i) the identification of the requirements for the overall UAV, (ii) the selection of the actuation subsystem, and (iii) the sizing of the power supply subsystem. Within the first stage, the synthesis of the initial design constraints was driven by technical requirements, such as desired payload, cruise velocity, and round-trip, by normative-related constraints, such as a total mass at take-off higher or lower than the limit of 25 kg stated in the Italian regulations for drones, and by custom requirements, such as the preference for a hybrid or electric propulsion system, composed of an odd or even number of direct-drive outrunner multi-rotor units, evenly distributed on the fuselage or specifically located. These constraints define a first attempt at the actuation category, from which a set of possible actuator candidates can be identified.