2.1.3. Hybrid-Electric-Aircraft Configurations

First, Figure 2 visualises an overview of a 50 PAX hybrid-electric aircraft with a gas turbine and battery as an energy source and the drivetrain's arrangement, valid for two different reference entry-into-service (EIS) years 2030 and 2040 [10].

**Figure 2.** Schematic view of an HEA (EIS 2030/2040, ICE + Battery) - Reprinted/adapted with permission from Ref. [10]. 2023, Marciello, V.; Di Stasio, M.; Ruocco, M.; Trifari, V.; Nicolosi.

The propulsive architecture adopted for the short- and medium-term scenarios was based on a serial/parallel partial hybrid configuration with two distinct propulsive lines. This choice made it possible to use the distributed electric propulsion during the ground phases to increase the lifting capabilities of the aircraft, compensating for the increased mass due to the advanced powerplant. At cruise, in light of the lower efficiency of distributed propellers, delivering all the shaft power through the primary line is preferable, redirecting the energy from the electric storage. New secondary electric machines were designed based on nominal RPMs equal to 8000 since it was found that it is optimal to delegate to gearboxes the task of adapting the number of revolutions to that of the propellers. The specific fuel consumption reflects the usage of pure hydro-processed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK) as fuel [10].

The second HEA configuration deals with a Proton Exchange Membrane Fuel Cell and a battery (PEMFC + Battery). Figure 3 gives a short introduction and overview of the aircraft design and the arrangement of the components. The main difference concerning the Internal combustion engine (ICE) + Battery scenario is that there is only a single-drivetrain, referred to as primary in the present context, with five engines of equal power attached to each semi-wing. For this reason, the electric machines will generate thrust through the propeller for the aircraft in all phases of flight. Since there is no distinction between primary and secondary propulsion lines, the production costs, as well as the maintenance costs of the aircraft, would benefit from having installed electric machines all rated at the same power. The propulsive architecture with Proton Exchange Membrane Fuel Cell (PEMFC) is based on a full-electric configuration, where part of the electricity is produced directly by the fuel cells through the reaction of hydrogen with air. The atmospheric air is supposed to be supplied through suitable air intakes and compressed up to the operating pressure of the fuel cells using a centrifugal compressor. Based on the power and energy requirements, Li-S batteries were identified to be the best choice for this application.

**Figure 3.** Schematic view of the aircraft (EIS 2040/2050; PEMFC + Battery) - Reprinted/adapted with permission from Ref. [10]. 2023, Marciello, V.; Di Stasio, M.; Ruocco, M.; Trifari, V.; Nicolosi.
