Recently, the new Green Deal policy initiative was presented by the European Union. The EU aims to achieve a sustainable future and be the first climate-neutral continent by 2050. It targets all of the continent’s industries, meaning aviation must contribute to these changes as well. By employing a systems engineering approach, this high-level task can be split into different levels to get from the vision to the relevant system or product itself. Part of this iterative process involves the aircraft requirements, which make the goals more achievable on the system level and allow validation of whether the designed systems fulfill these requirements. Within this work, the top-level aircraft requirements (TLARs) for a hybrid-electric regional aircraft for up to 50 passengers are presented. Apart from performance requirements, other requirements, like environmental ones, are also included. To check whether these requirements are fulfilled, different reference missions were defined which challenge various extremes within the requirements. Furthermore, figures of merit are established, providing a way of validating and comparing different aircraft designs. The modular structure of these aircraft designs ensures the possibility of evaluating different architectures and adapting these figures if necessary. Moreover, different criteria can be accounted for, or their calculation methods or weighting can be changed.

Optimized electric motors are lighter and smaller than conventional piston engines. As a result, new airplane configurations are feasible as motors can be placed in unconventional positions. Through careful aircraft design higher aerodynamic efficiencies of airframe, propeller, and propeller integration can be achieved. The energy density of current batteries, however, still limits strongly the range of purely battery powered aircraft. But if the energy is stored in liquid fuel and converted by a generator into electric energy, then the advantages of electric propelled airplanes and conventional combustion engines can be combined. But which combustion engine is optimal for such a serial-hybrid electric aircraft? In this new propulsion chain, other boundary conditions apply to the combustion engine than in conventional aircraft designs. These boundary conditions interact with the characteristics of combustion engines. An example for an engine characteristic is that different kinds of piston engines exist. It can be observed that technologies, which result in lighter piston engines, are associated with lower efficiencies and vice versa. In this paper it will be shown through considerations on aircraft level, that the optimal combustion engine for an electric-hybrid airplane should be heavier and more efficient than the optimal combustion engine for a conventional aircraft.