2.2.2. Propeller and Wing Tip Propulsion

In the regional aircraft sector, the most prominent propulsion method is the propeller, while jets are more often used for longer ranges than the given TLAR for this design. The propeller has the advantage of possible higher efficiencies at low speed [19] combined with lower complexity and easy conversion to an electric drivetrain when compared to jet engines [20,21]. With a small hybridization ratio, i.e., the ratio between electric power and total propulsive power, it is possible to use motor–generator units within a jet turbine.

With power electrification beyond 50%, the power has to be provided by an electric motor driving a propeller. Further, the electrification enables an almost free positioning of the propellers and motors. The current trends are the use of wingtip propulsion (WTP), boundary layer ingestion (BLI) and distributed electric propulsion (DEP) [22,23]. Research regarding BLI showed that the positive effects increased with speed, making the use at relatively low speeds of the aircraft less suitable. DEP showed promising results in an isolated setting; however, further analysis on aircraft level showed no effect or even negative effects for this application [24].

The advantage of using WTP is the reduction of induced drag [25]. Considering the WTP concept individually, it promises an improvement of up to 50% less induced drag during climb and 25% less during cruise [26] regarding the wing and up to 15% less total drag on the aircraft level [27]. Secondary effects are the decrease of the wingtip vortex intensity through the WTP [26], which leads to a significant noise reduction [24] as well as to a reduced wing bending moment, relieving the wing by mounting a mass with a long lever arm on the wing tip.

Concerning the aircraft, the reduction in induced drag is derived as an average of the values of Sinnige et al. [28]. The reduction in drag is measured for a single engine per wing configuration. To account for a possible twin engine per wing architecture, the drag decrease value is assumed to be halved for that setup. In that case, the inside propeller is spinning in the same direction as the wing tip propeller, which could also help in the vortex reduction; however, this requires further investigations. In Table 2, the chosen reference drag decrease values from Sinnige et al. [28] for the single engine case are listed, as well as the assumed correction factor to account for a possible twin engine setup. These reference values are used to reduce the induced drag at the wing level for the calculation within the design code.

**Table 2.** Drag reduction through wing tip propellers [27].

