# Effect of Electric Ducted Fans Structural Arrangement on Their Performance Characteristics
## 4 Discussion  
Obtained results show that the 3D printed prototypes reach worse values compared to the purchased originals, which is mainly due to the deteriorated surface and greater tolerances caused by the production technology. The surface of bought reference EDFs was made by plastic injection technology which produces high-quality finish of the surface with small roughness [29]. This leads to much better aerodynamic properties in comparison with 3D printed prototypes, which has higher roughness of the surface which has negative impact to air flow through the cowling. The lower measured maximum values for the printed prototypes are due to the fact that magnitude of vibrations exceeded safe limit at higher thrust values, which could lead to damage of the test equipment and the power unit itself. For these reasons, it was necessary to use these printed reference prototypes for further comparisons between individual prototypes, instead of purchased ones, so that the individual comparisons would be relevant. The difference between the energy characteristic of 50 mm and 64 mm EDF was small, due to the fact that the electric motor used is not suitable for the larger fan diameter as it has too high kV value, which, in this case, worsens consumption [14]. The results show that as the diameter of the fan increases, the required RPM of the fan decreases, and thus the required kV value of the electric motor used also decreases. Using a motor with a smaller kV value, the 64 mm specification would achieve better energy efficiency. The fundamental finding was that as the diameter of the fan increases, it is possible to use electric motors with a smaller kV value to obtain the greater possible thrust with the least possible energy consumption. The results indicate that for the 50 mm EDF, increasing the outlet diameter compared to the fan diameter improves the EDF energy characteristics. In contrast, a narrowing outlet significantly worsens this characteristic. Similar results can be concluded for 64 mm EDF. The difference was only for \(a_{w}=1.1\), which had slightly worse thrust-to-power ratio, than the 64 mm prototype with \(a_{w}=1\). Anyway, recorded waveforms of the power characteristics are similar for both prototypes, correlation coefficient was R = 0.9993. However, these characteristics were shifted in amplitude by approx. 4 W. Although PCA indicates that differences between these two specific measurement series are statistically different, this has negligible effect to power consumption in real conditions. The reason  
Figure 7: Comparison of power characteristics for two-rotor 3D printed prototypes. for this difference is most likely production imperfection. Such a result is important, as it showed that there is a dependence between the diameter of the fan and the diameter of the outlet cross-section. In other words, as the diameter of the fan increases, it is also necessary to increase the diameter of the outlet opening to maintain a proportional improvement in energy performance. The last specification was the two-rotor, which was tested in 3 prototypes. The first tested prototype (EDF\({}_{50-50/1}\)) with a constant cross-section served only as a reference. The first modification proposed was a gradual expansion of the flow channel from the first 50 mm fan to the outlet with the rear 64 mm fan (EDF\({}_{50-64/1,3}\)). The results showed that the same applies to these two-rotor specifications as in the previous one-rotor cases, i.e., expanding the flow channel improves the energy characteristics of the EDF when generating static thrust. Following theoretical knowledge on these two-rotor specifications, a prototype with a separate channel (EDF\({}_{50-64/1,3/s}\)) was made to bring unaffected air to the rear fan to increase its efficiency. The modification is based on the previous expanding specification, so the first fan has a diameter of 50 mm and the rear one has a diameter of 64 mm.