# Preliminary Propulsion and Power System Design of a Tandem-Wing Long-Range eVTOL Aircraft
## 3 Results and Discussion of the Propulsion System Design
### Future Recommendations on the Propulsion Design  
The proposed design methodology was intended as a preliminary design tool, and as such, several assumptions were made. While some of them were necessary to keep the method simple and sufficiently fast, there are points on the methodology which warrant further research. **Stall of blade sections:** The aerodynamic data for the airfoil was computed in XFOIL, which loses accuracy in the post stall region of the lift polar. Since parts of the blades might be stalled during operation due to the wide range of operating conditions, it is important to ensure that the post-stall aerodynamic data of the airfoil is available and accurate. This is one of the points where the procedure needs improvement, and next steps could for example include the CFD simulations of the airfoil to obtain more accurate post-stall data. **Blade design method:** The method used to design the blade geometry is not robust when the propellers are too highly loaded, where the iterative blade design procedure given in Section 2.1 can fail to converge. The momentum theory approximation that assumes that  
Figure 12: Sensitivity of the efficiency to changes in rpm and pitch angle of the blade in the off-design analysis procedure. Figure 13: Sensitivity of the thrust to changes in rpm and pitch angle of the blade in the off-design analysis procedure. the increase in flow velocity with respect to the freestream velocity at the disk plane is half the total increase at the wake is not always accurate and can result in convergence issues [5]. Propellers with the moderate-to-high disk loading adopted for the Wigeon (317 kg/m\({}^{2}\)) can still be designed with this method, but a different approach would be needed if the chosen architecture features very highly loaded propellers or fans. **Effects of interactions:** Due to the complexity of their analysis, the effects of propeller-wing and propeller-propeller interaction were not modelled in the present study. Since these interactions can have an effect on propeller performance, quantifying them is important for a more accurate design. For the Wigeon, a propeller lateral separation of 0.3 m was chosen, which corresponds to 30% of the radius. A study by [PERSON] et al. shows that, in horizontal flight, a distributed propulsion system using open rotors sees a drop in propeller efficiency of 1.5% for a separation between propeller tips of 2% of the propeller diameter [8]. In a different study carried out by [PERSON] et al., it was shown that the thrust coefficient of two propellers with a tip separation of 5% of the propeller diameter is within 2% of that of a single propeller under static thrust conditions, although a significant increase in thrust fluctuation is seen [9]. [PERSON] et al. also showed that an increase in noise was observed with decreasing separation [9]. Based on the results from these studies, it is expected that the interaction effects will not cause significant deviation from the predicted performance with the chosen clearance; however, the sensitivity analysis on the effect of this clearance could not be performed with the available tools. In transition, these effects can be more significant [7]. Another effect of propeller-propeller interaction is slipstreamingestion in the second row of propellers, located in the aft wing, and not considered in the present study. Quantifying this interaction is important because it can lead to more significant losses in the calculated thrust [7], while yet negatively affecting noise. Finally, the effect of the propeller blockage due to the wing was not modelled, which can also affect propeller performance, while also affecting the positioning of the propellers with respect to the wing in terms of height and depth. **Airfoil selection:** For the present study, a NACA4412 airfoil was assumed, but improved airfoil selection and optimisation were expected in further stages of the design.