# Review of Propulsion System Design Strategies for Unmanned Aerial Vehicles
## 3 Results
### The Integrated Functional Design Framework
Within the second stage, the main design constraints related to working conditions and tasks were outlined. Once having selected the desired actuator type (e.g., AC or DC, brushed or brushless), the analysis of the mission profile allowed defining the main kinematic constraints required for the UAV. For a first sizing of the actuation components, the worst working condition should be considered. Besides, starting from a free-body analysis of the UAV, the generalized forces (meant as linear forces and torques) acting on the device were depicted, and the characteristics equations of the system were extracted. Expected generated thrust forces \(T_{adv}\) and \(F_{Climb}\) for advancement and climb respectively were identified for each possible actuator unit, whereas lift and drag coefficients were evaluated with respect to the angle of attack \(\alpha\) for each possible propeller, as well as voltage, current, and energy consumption \(E_{C}\). If the values of \(T_{R}(t)\) and \(a_{R}(t)\) can be considered the main design constraints according to the expected mission profile, \(T_{P}(t)\) and \(a_{P}(t)\) can be then defined as the corresponding parameters for the propulsion system candidates, distinctive for the specific solution. Those values can be computed by solving the characteristic equilibrium equations of the UAV's free-body diagram. In order to be suitable, the actuation candidate must assure an acceleration \(a_{P}(t)\) higher than the maximum value of the required acceleration \(a_{R}\) for at least one configuration of the throttle percentage (or instant \(t\)). The best candidate for the actuation system reaches the required acceleration at lower throttle percentage values, the other characteristics being equal. Several methods can be adopted to evaluate the introduced quantities: for instance, at this design stage, the lift and drag coefficients can be estimated as approximate values through  
Figure 3: Schematic of the integrated functional design framework: the propulsion system is modeled as the set of the power supply and actuator sub-systems, interacting each other and with the mission profile module. All the blocks are affected by constraints imposed by technical, normative framework-related, or custom requirements. numerical computation (e.g., in the Xfoil 6.97 environment), or different kinematic and dynamic models can be adopted to analyze the vehicle aerodynamics (such as the particle model or a system of rigid bodies). Therefore, the same quantities can be defined according to different methodological hypotheses and described by distinct formal expressions. Figure 4: Synthetic road-map of the integrated functional design framework for the propulsion system identification. The third stage of the procedure allowed sizing the power supply subsystem. As custom requirements, the preference for an even number of Li-ion batteries could be considered, and a maximum value of the Depth of Discharge (\(DoD\)) should be settled. Given the selected actuation, the energy consumption \(E_{TOL}\) and \(E_{\textit{cruise}}\), required for the take-off or landing and cruise phase of a mission, respectively, can be evaluated. Those values depend on the kind of mission, for instance: A monitoring mission would require a take-off phase, then loitering until at last the maximum allowed range \(d_{M}\), and a final landing back at the base. On the contrary, in a delivery mission, the drone is expected to transfer a good from the starting point to a defined target and then return to the base, so the mission profile should foresee a take-off phase, the cruise until at least the maximum allowed distance \(d_{T}\), the landing at the target position, then the return trip composed once again of the take-off, cruise, and landing phases. For delivery missions, even more trips could be conceived of within a single route: in this case, the energy consumption corresponding to an additional landing and take-off pair should be also considered for each additional target.