**1. Introduction**

Following the hybridization or complete electrification strategy of the electric drive pursued on terrestrial vehicles, the aviation industry is considering with great attention the application of electrical technology and power electronics for transport aircraft. The growing interest towards electric application in transport aircraft is driven first by the ambitious

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emissions and external noise reduction targets declared by Europe in the Flightpath 2050 program and the corresponding Strategic Research and Innovation Agenda (SRIA) [1] and by the United-States of America in the NASA Environmentally Responsible Aviation N+ series [2]. It is then motivated by the growing evidence that evolutionary improvement of the technologies might not be sufficient to fulfill these targets. Investigations [3] have shown that the reduction potentials might fall short of the 2035 targets and that the deficit becomes even more substantial towards the 2050 goals. This trend can be best explained by the very high maturity reached by contemporary technologies, in particular the propulsion system technology implemented in transport aircraft. Prospects for technological improvements have consequently reached an asymptote leaving not enough potentials for achieving the aggressive targets. As a result, the introduction of disruptive technologies turns out to be essential in view of meeting the future aviation goals. Finally, the progress and perspective in electrical technology development trigger the initiatives and the extensive research activities deployed by the aeronautical community to investigate the feasibility and the potentials of electric technology application to transport aircraft.

A current application of electrical technology for transport aircraft is the so-called more-electric aircraft [67-71]. The objective of the more-electric aircraft initiative, which targets the aircraft power systems, is basically to replace pneumatic and hydraulic systems by electrical systems. The Boeing B787 is the first aircraft utilizing a more electric power system architecture. A logical future conceivable step is the electrification of the propulsion system of the aircraft which is the topic of this chapter. While aiming for the ultimate goal of an universally-electric aircraft, hybrid-electric approach will be first necessary to match the requirement of aircraft propulsion system and the development pace of the electrical components technology. Hybrid-electric aircraft feature typically a combined conventional and electrical propulsion system. The combinatorial variety of hybrid-electric and universally-electric propulsion system topology considered for transport aircraft application as well as enabling technologies are first discussed in Section 2. A compendium of hybrid and universally-electric advanced aircraft concepts is then proposed in Section 3 to obtain a notion of the cloud of aircraft configurations and electric drive options investigated up to this point in time. The feasibility of hybrid-electric aircraft needs to be established for future market segments. On the basis of selected concepts, the integrated prospects of hybrid-electric aircraft are finally investigated in Section 4.
