**3. Compendium of hybrid- and universally-electric aircraft**

Breakthrough in battery technology could be achieved through the development of innovative battery concept as indicated by open battery systems like zinc-air, aluminium-air and lithium-air [32, 34]. Lithium-air batteries are considered with attention for aircraft application with estimated theoretical gravimetric specific energy from 1000 Wh/kg [33, 35] up to 2000 Wh/kg [36] at cell-level. Not currently commercially available, market readiness

Superconductivity is considered as an enabling technology for electric drive application to transport aircraft [13, 38, 39]. Through the utilization of High-Temperature Superconducting (HTS) materials, the gravimetric specific power and the efficiency of the electrical component can be significantly improved. The most common application of HTS materials at this point in time is for electric motors and generators [38, 40, 41]. The application of HTS technology to the transmission cable is also considered [10, 11]. The necessity of developing fully superconducting network (including the fault management, protection and switching implications) was argued by Malkin and Pagonis [42]. The challenge of HTS application for transport aircraft lies essentially on the requirement to operate at cryogenic temperature and the resulting complex integration of the cooling system. Instead of using fossil fuels to operate the aircraft, the utilization of cryogenic fuel such as liquid hydrogen can result in strong synergies with the layout of HTS electrical system as the coolant is already available [10, 13, 39]. Handling safety related issues [43], the negative integration impacts of the cryogenic tank on aircraft design [44] and the infrastructural challenges to supply the liquid

hydrogen to the operated airports all are issues that remain unresolved [45, 46].

as most promising and was the center of several investigations [49–53].

The investigation of hybrid-electric or universally-electric system is often coupled with distributed propulsion technology [72-75]. This combination is explained by the nature of the electric energy, which can be easily distributed and by the search for aero-structural benefits through higher integration of the propulsive device with the airframe. The field of Boundary Layer Ingestion (BLI) [47, 75] consisting of re-energizing the low momentum boundary layer in view of aerodynamic efficiency improvement, becomes central. This is mainly the reason why distributed propulsion has been intensively investigated on Blended Wing Body (BWB) configuration [12, 48] as it offers large potential for application of BLI by distributing buried propulsion devices along the trailing-edge of the fuselage (see Section 3). For BLI application on tube and wing configurations, the Propulsive Fuselage configuration, which is characterized by a large fan encircling the rear end of the fuselage, was evaluated

Electric distributed propulsion technology is expected to disrupt the traditional aircraft design paradigms [54–56]. A salient example is the redesign of the wing for optimum efficiency in cruise enabled by distributing propellers along the leading-edge as initiated in the LeapTech Project [57] (see Section 3). Usually, wing design is constrained by low-speed operations in order to achieve according to the properties of the high-lift system acceptable takeoff and landing field performance. Benefiting from the propeller slip-stream effect on the wing, the low-speed requirement on high-lift devices and wing design could be reduced,

of lithium-air is expected for a time-line horizon of 2030 [37].

*2.2.2. Superconductivity*

122 New Applications of Electric Drives

*2.2.3. Distributed propulsion*

While the first fixed-wing electric flight took place for over 9 min in 1973 with the Brditschka s MB-E124, it took around 30 years before reconsidering seriously electric propulsion system for transport aircraft application. This time lapse observed in the interest revival for electric drive application to aircraft propulsion is explained first by the time required for electrical component technology to evolve to a level applicable to the propulsion system requirement of transport aircraft. Secondly, it is driven by the growing environmental awareness and the search for an alternative to a fossil fuel economy. The late 1990 witnessed the birth of several electric experimental and commercial aircraft targeting the general aviation sector in the one or two seater category. While mainly motivated by engineering curiosity through the impulse of pioneers, the industry is currently demonstrating a strong growing interest in the development of hybrid-electric and universally-electric aircraft. A compendium of experimental, commercial as well as advanced hybrid-electric and universally-electric aircraft concepts, was proposed by Pornet and Isikveren [23]. By illustrating the cloud of concepts in Figure 3 evaluating the number of passenger (PAX) versus the design range (evaluated in nautical miles [nm]) certain clusters and trends in design can be identified.

This compendium comprises universally-electric aircraft concepts using battery as energy source with the four seater Airbus E-Fan [18], the four seater LeapTech concept [56, 57], the Dornier Do328-LBM [60] , the Voltair concept [61] and the BHL Ce-Liner [4] as well as hybrid-electric aircraft, integrating a battery-fuel system either in serial or parallel topology, with the NXG-50 [62], the Sugar-Volt [19], the Bauhaus Luftfahrt Twin-Fan [22] (see Section 4.3) and the Bauhaus Luftfahrt Quad-Fan [23] (see Section 4.4). On the upper right corner, BWB configuration using turboelectric distributed approach are represented with the BW-11 [48] and the N3-X [12].

As highlighted by [54], electric propulsion technology will emerge first in general aviation as it provides benefit advantages for early market success and will evolve with respect to the maturation and the development of electric technology towards application for commuter, regional and narrow-body transport aircraft. A noticeable design implication of utilizing

**Figure 3.** Compendium of advanced hybrid- and universally-electric aircraft. Adapted from [23]

an electric drive relying on battery technology is the achievable design range as indicated by the low range end on the left corner of the chart which reflects more of the regional market segment. With regards to morphology, besides the BWB configuration, no large departure from the traditional "tube and wing" were foreseen due to the implementation of hybrid-electric propulsion system. Outcome of pre-design studies investigated by Isikveren et al. [63] indicated that unless significant departure in the propulsion system integration is entertained, considering for instance distributed propulsion technology, the contemporary tube-and-wing morphology was still considered to be appropriate. Distributed propulsion technology could considerably disrupt contemporary aircraft design paradigm. This was exemplified by the LeapTech concept through distribution of multiple propellers on the wing leading-edge (see Section 2.2.3) and the Propulsive Fuselage configuration selected for the Voltair concept (see Section 2.2.3). The aviation community is in the midst of a pioneering era with regards to electro-mobility and faces an explosion of combinatorial possibilities at system level and aircraft level. Innovative approaches to electrically driven propulsion system needs to be further analyzed and thoughtfully integrated at aircraft level to determine the full potentials. With growing understanding of the technology and its implication at aircraft level, innovative advanced aircraft configurations designed through more ambitious holistically integrated electric propulsion system are expected to emerge.
