*3.1. Sensitivity Study on the Vertical Tail's Volume Coefficient*

In order to evaluate the influence of changes to the vertical tail's volume coefficient, the MICADO environment is used to carry out a systematic study on this design parameter. Figure 6 illustrates the resulting aircraft designs of the partial turboelectric aircraft with optimized propeller positions and a volume coefficient of 0.04253 and 0.14253. This also explains why it was not possible to calculate a converged aircraft design for a volume coefficient of 0.16253 or greater, as this leads to a large vertical tail and not physically feasible aircraft. The results of this study, in terms of the operating empty mass and the required trip fuel of the aircraft, can be seen in Figure 7.

**Figure 6.** Partial turboelectric aircraft with optimized propeller positions and a volume coefficient of the vertical tail of 0.04253 (**left**) and 0.14253 (**right**).

**Figure 7.** Influence of the vertical tail's volume coefficient on the OME (blue) and the trip fuel (red) for the PT2025 (left) and PT2025opt (right) configurations.

The OME and trip fuel were chosen to show the cascading effects caused by a change in the volume coefficient. A bigger volume coefficient results in a larger vertical tail, which increases the OME of the aircraft. This leads to a larger wing needed to generate the required lift, further increasing the empty mass. For a constant volume coefficient, the larger wing also means a larger vertical tail. This loop continues until an equilibrium is reached and explains the non-linearity of the OME seen in Figure 7. This effect is even more prominent for the mission fuel, as the fuel burn not only depends on the mass of the aircraft, but also on the drag. A larger vertical tail means a bigger wetted surface and an increased viscous drag.

The slight differences between the on-wing and cross-wiring options result from the different cable lengths and masses. This effect is more visible for the PT2025opt aircraft, as the gas turbines are located further outward on the wing. This increases the distance to the electric engine on the opposite wing, while minimizing the distance to the electric engine on the same wing.
