# Preliminary Propulsion and Power System Design of a Tandem-Wing Long-Range eVTOL Aircraft
## 3 Results and Discussion of the Propulsion System Design  
The main design goal is to achieve a propeller that is optimised for cruising, since cruising represents the majority of the mission time for the Wigeon eVTOL investigated herein. This approach does not necessarily result in the propeller design that will minimise the energy consumption over the whole mission but allows one to select a single design-point which will be reasonably close to the optimum without the need for an optimisation for the whole mission profile. However, sizing the propellers with only the cruise requirements results in small propellers that do not allow one to reach the necessary thrust requirements of other mission phases, such as hover. Thus, an extra factor is applied to the target design thrust to obtain a propeller whose thrust range is wide enough to satisfy all mission phases. The procedure to select this factor is to first try a factor of 1 that multiplies the cruise thrust, and then check the maximum available thrust with the resulting propeller geometry for the other mission phases, and accept the design if the design meets the thrust requirements. If not, the factor is increased and the procedure is iteratively repeated. With this, a factor of 2.6 is obtained as the minimum factor to achieve the necessary thrust for all mission stages other than cruising. This factor, applied to the cruise thrust of 153.63 N per propeller, results in a design thrust of 400 N per propeller. The flight conditions at the design point are a cruise speed of 72.19 m/s at a height of 1 km. The atmospheric values for density, temperature, and dynamic viscosity were obtained from the International Standard Atmosphere. Before designing the blades, the propeller dimensions need to be defined. For the Wigeon, all clearances are assumed to be 0.3 m. This value was selected to be in the order of magnitude of the expected propeller radius, but it has not been a result of a detailed analysis since the effects of propeller clearance are not accounted for in this design framework; this is further explained in Section 3.3. The effect that these clearances have on the propeller performance and noise are very complex and cannot be calculated with the tools presented in this preliminary design framework, and they remain as future work for further design stages of the aircraft. The half span and fuselage width came from aerodynamic and cabin design requirements detailed in the original project report [3]. Only the number of propellers remains to be chosen in order to obtain the propeller radius. From the shortlisted possibilities of the number of propellers (8, 12, and 16), having eight propellers was ruled out because maximising the radius would lead to propellers with a 1 m radius, which would not be acceptable for ground clearance requirements when the wings are in the horizontal position. Having eight propellers with smaller (non-maximised) radius would be a solution to this, but it would eliminate the benefits of having less propellers. Having 16 propellers would lead to propellers with only 0.3 m of radius and a hover disk loading above 600 kg/m\({}^{2}\). The small radius would already pose problems with the wing and nacelle blocking the slipstream. Moreover, a disk loading of 600 kg/m\({}^{2}\) is above the desired value for the Wigeon, as it is closer to the expected disk loadings of reference lift-fan architectures than to that of tilt-wing aircraft [15]. Therefore, the selected number of propellers is 12, meaning 3 propellers per half wing. For 3 propellers per half wing--with clearances of 0.3 m, a fuselage width of 1.38 m and a span of 8.2 m--the final radius of the propellers is 0.5029 m, resulting in a disk loading of 317.25 kg/m\({}^{2}\). These dimensions also define the spanwise positioning of the propellers, as defined in Figure 5.  
The chosen number of blades of the propeller is 6.