*2.2. Reference and Concept Aircraft*

Based on the outcome of a market analysis, the Beechcraft 1900D was chosen as the reference aircraft. According to the results of an initial technology identification and selection process (with respect to technologies associated with aircraft propulsion system electrification), a partial turboelectric propulsion system featuring two gas turbines supplemented with two electrically driven propellers on the wing tips for a reduction in induced drag was selected as the most promising concept for the evaluation in a year-2025-scenario [21]. The basic assumption in the technology selection process was that an aircraft with electrified propulsion systems needs to fulfil identical mission requirements as current conventional aircraft in terms of its range, speed and passenger capacity in order to be recognized as a viable alternative.

Based on data of the Beechcraft 1900D and the PT6A-67D turboprop engines available to the public, such as the Pilot Operating Handbook [22] and the EASA-issued engine type certificate [23], a redesign of this aircraft was conducted using the MICADO environment [15], see Figure 3 left. In order to obtain a comparable conventional reference aircraft, some adjustments were made prior to the electrification of the Beechcraft 1900D. First, the wing was moved from a low to a high position to ensure sufficient ground clearance for the wing tip propellers. Corresponding to this, the position of the landing gear was moved from the wing to the fuselage. Lastly, the engine performance and the scaling factors of the mass estimation methodologies were adjusted according to the description in Section 2.1. The resulting aircraft from a further execution of the aircraft design loop after these changes served as the conventional reference for subsequent comparisons, depicted in the centre-left of Figure 3.

**Figure 3.** Redesign of a Beechcraft 1900D (**left**), modified redesign (**centre-left**), as well as nonoptimized PT2025 (**centre-right**) and optimized PT2025opt (**right**) versions of the partial turboelectric concept aircraft.

In a second major modification step, the conventional propulsion system was exchanged with the partial turboelectric propulsion system, leading to the PT2025 aircraft configuration, see Figure 3 centre-right. The positions of the gas turbines of this aircraft do not change compared to the conventional reference aircraft. They are supplemented by two electric wing tip propellers that are powered directly from two generators mechanically connected to the gas turbines. Subsequently, an aerodynamic optimization of the propeller positions revealed a further outboard location of the conventionally driven propellers, leading to the derivative aircraft design PT2025opt [16]. Since MICADO considers only static loads, the estimation of the wing mass was conducted based on the sizing of the wing box structure considering the three quasi-static load cases, pull-up, gust and landing. Aeroelastic investigations were conducted by numerical flutter analysis and showed that no critical flutter instabilities occur up to 1.2 times the dive speed. Figure 3 shows the optimized aircraft configuration on the right-hand side.

For all aircraft configurations the same design mission, 510 NM trip with 100 NM diversion distance reserve and a 45 min holding, was used. Cruise altitude was set to 23,000 ft at a cruise speed of Mach 0.4.
