*2.7. Assumptions and Restrictions*

As stated in Section 2.4, the assumptions are based on the usability of handbook methods, as validated by Ciliberti et al. [4]. Therefore, the suitability of the correction method described in this paper is only valid for configurations with similar properties. These are a high-wing configuration, a comparable tail-cone geometry and a VTP aspect ratio of around 1.0. Additionally, a VTP shape close to the simplified versions of [25] should be considered. As mentioned in the introduction, according to [6] the additional rolling motion due to the local absence of power augmented lift downstream of the propellers is neglected. As the wing tip propellers reduce the induced drag, their failure will increase the induced drag slightly. As the total reduction in induced drag through the use of a wing tip propulsion is only around 5% for the investigated aircraft [16], this effect is neglected. To reduce the complexity of this study, the propeller-induced side wash interaction on the vertical tail and the possible lift increase on the wing due to the induced velocity, are neglected.

A possible solution to reduce the VTP size for the PT2025 configurations could be the implementation of an automatic thrust control system, which is able to reduce the thrust on the electric engine quickly in case of a failure of the conventional engine, according to CS23.904 [8]. Another possibility would be the interconnection of all electric machines or the implementation of a buffer battery. This way, the electric propellers could still operate if one combustion engine fails. The downside to this would be the increased weight due to the added components and the increased complexity, which is why this case was not considered in this study.
