**4.4. Distributed hybrid-electric clean sheet design**

Motivated by the search for higher synergies in the integration of the hybrid-electric propulsion system at aircraft level and by the interest of investigating the influence of increasing Hp on overall aircraft sizing, integrated performance and flight technique optimality, a hybrid-electric narrow-body transport aircraft employing a quad-fan arrangement was investigated by Pornet and Isikveren [23]. Featuring two conventional Geared-TurboFans (GTF) and two Electric Fans (EF), this rudimentary form of distributed propulsion offers numerous advantages compared to previously investigated hybrid-electric architecture and potentials for further evolution as enumerated in Section 2.1. Due to the greater mean time between failure of electric motor compared to gas-turbine, the electric fans are positioned on the outboard to reduce the one engine inoperative implications on performance and sizing. The sizing strategy of the hybrid-electric propulsion system was analyzed as a function of Hp*use* [23]. The operational strategy selected was to operate the EFs at maximum thrust during the mission segments. Operational phases covering taxi-in/out, descent, landing and hold are performed only with the GTFs. The GTFs are throttled back during cruise to adjust to the instantaneous thrust requirement. This operation was assessed to be suitable up to an Hp*use* of 45%. Above this value, the efficiency of the gas-turbine in cruise is impaired because of part-load operation resulting from the GTF thrust throttling. By sizing the aircraft for interval design ranges between 900 nm (1667 km) and 2100 nm (3889 km) and for increasing Hp*use*, the prospects were investigated in terms of potential fuel burn reduction (Figure 7), change in vehicular efficiency (Figure 9) and change in aircraft size (Figure 11). The integrated performance are contrasted against an advanced twin-engine transport aircraft. As indicated in Figure 7, by increasing Hp*use*, a large reduction in block fuel can be achieved due to the greater utilization of electrical energy and the improvement in overall propulsion system efficiency. Due to the utilization of batteries and because of the electrical system weight, growing Hp*use* leads to large increase in aircraft weight as illustrated in Figure 11. This effect comes at the detriment of the vehicular efficiency which reduces with increasing Hp*use* as indicated in Figure 9. The amplification of the sizing cascading effects, linked to the higher electrical energy requirement, explains the degradation of the integrated performance at higher design ranges. From this analysis it can be concluded that the regional market segment is the most suited segment for the application of this hybrid-electric quad-fan transport aircraft with design range between 900 nm (1667 km) to 1300 nm (2408 km). The volumetric constraint for the housing of the investigated battery

The analysis of this clean sheet design reveals shifts in aircraft design paradigm due to the nature of the hybrid-electric propulsion system as fuel burn reduction can be achieved while the MTOW of the aircraft is increased and moreover fuel burn reduction does not mean

While this investigation was aimed to get first insights into the sizing impact of hybrid-electric propulsion system at aircraft level, the full benefit of hybrid-electric technology will be reached through holistic integration of the propulsion system at aircraft level. As defended by Moore and Fredericks [54], the full potential of hybrid-electric aircraft will be demonstrated only once the synergistic benefits from the integration of the hybrid-electric propulsion system at aircraft level are fully understood. In this regard, an innovative hybrid-electric approach is proposed in the next section with the assessment of a

Motivated by the search for higher synergies in the integration of the hybrid-electric propulsion system at aircraft level and by the interest of investigating the influence of increasing Hp on overall aircraft sizing, integrated performance and flight technique optimality, a hybrid-electric narrow-body transport aircraft employing a quad-fan arrangement was investigated by Pornet and Isikveren [23]. Featuring two conventional Geared-TurboFans (GTF) and two Electric Fans (EF), this rudimentary form of distributed propulsion offers numerous advantages compared to previously investigated hybrid-electric architecture and potentials for further evolution as enumerated in Section 2.1. Due to the greater mean time between failure of electric motor compared to gas-turbine, the electric fans are positioned on the outboard to reduce the one engine inoperative implications on performance and sizing. The sizing strategy of the hybrid-electric propulsion system was analyzed as a function of Hp*use* [23]. The operational strategy selected was to operate the EFs at maximum thrust during the mission segments. Operational phases covering taxi-in/out, descent, landing and hold are performed only with the GTFs. The GTFs are throttled back during cruise to adjust to the instantaneous thrust requirement. This operation was assessed to be suitable up to an Hp*use* of 45%. Above this value, the efficiency of the gas-turbine in cruise is impaired because of part-load operation resulting from the GTF thrust throttling. By sizing the aircraft for interval design ranges between 900 nm (1667 km) and 2100 nm (3889 km) and for increasing Hp*use*, the prospects were investigated in terms of potential fuel burn reduction (Figure 7), change in vehicular efficiency (Figure 9) and change in aircraft size (Figure 11). The integrated performance are contrasted against an advanced twin-engine transport aircraft. As indicated in Figure 7, by increasing Hp*use*, a large reduction in block fuel can be achieved due to the greater utilization of electrical energy and the improvement in overall propulsion system efficiency. Due to the utilization of batteries and because of the electrical system weight, growing Hp*use* leads to large increase in aircraft weight as illustrated in Figure 11. This effect comes at the detriment of the vehicular efficiency which reduces with increasing Hp*use* as indicated in Figure 9. The amplification of the sizing cascading effects, linked to the higher electrical energy requirement, explains the degradation of the integrated performance at higher design ranges. From this analysis it can be concluded that the regional market segment is the most suited segment for the application of this hybrid-electric quad-fan transport aircraft with design range between 900 nm (1667 km) to 1300 nm (2408 km). The volumetric constraint for the housing of the investigated battery

automatically an improvement in vehicular efficiency.

130 New Applications of Electric Drives

distributed hybrid-electric clean sheet design.

**4.4. Distributed hybrid-electric clean sheet design**

**Figure 7.** Relative change in block fuel versus block degree-of-hybridization for energy He*block* . Geared-turbofan cruise throttling [23]

indicates that the cross-section of the narrow-fuselage might limit the design space. With respect to a cargo volumetric constraint of 0.14 m3 per PAX, a standard volume allocation for regional aircraft, it was found that assuming a gravimetric specific energy of 1500 Wh/kg at cell-level, a block fuel reduction of 15% could be achieved at a design range of 1300 nm (2408 km) and an Hp*use* of 30% while the vehicular efficiency is degraded by 6%.

To gain insights into the sensitivity with regards to battery technology, integrated performance was investigated for a battery specific energy of 1000 Wh/kg [23]. As indicated in Section 4.3, sizing effects are considerably amplified for lower battery specific energies due to the higher battery mass required for a given energy demand. It results in a diminution of the potential block fuel reduction and a stronger degradation of the vehicular efficiency. The increase in energy demand with growing design range leads to more pronounced degradation of the integrated performance at greater stage lengths. Assuming a gravimetric specific energy of 1000 Wh/kg, no significant block fuel reduction was achieved for design range above 1300 nm (2408 km), whereas, correspondingly for a specific energy of 1500 Wh/kg it is above a design range of 1900 nm (3519 km).

In the outlook proposed in [23], the analysis of a different operational strategy of the hybrid-electric propulsion system was highlighted. The strategy which consists of throttling the EFs during cruise while the GTFs are operated closed to their peak efficiency is investigated in this section. The comparison of the implication of the different strategies is based on the degree-of-hybridization for block energy He*block*. As this integrated metric includes the overall efficiency chain of the propulsion system and characterizes the integrated block value of the energy split, it is of particular relevance in order to compare the two different operational strategies during cruise. The integrated performance is compared to the identical advanced twin-engine narrow-body aircraft. The potential block fuel reduction, the change in vehicular efficiency and the change in aircraft weight are indicated in Figures 8, 10 and 12 respectively.

**Figure 8.** Relative change in block fuel versus block degree-of-hybridization for energy He*block* . Electric fan cruise throttling

**Figure 9.** Relative change in block ESAR versus block degree-of-hybridization for energy He*block* . Geared-turbofan cruise throttling [23]

Interestingly, for a given He*block* the benefit in terms of block fuel reduction achieved are about identical for both strategies (see Figure 7 and Figure 8). However, it is highlighted that for an identical He*block* the level of Hp*use* is larger in the case of throttling the EF during cruise. It means basically that for achieving the same block energy split, a larger electric motor power needs to be installed. This trend is understandable as less electrical energy is consumed during cruise when the EF is throttled back, compared to the first strategy for an identical Hp*use*. In order to achieve the same He*block*, a larger electric motor needs consequently to be installed to achieve the same block energy split for an identical block mission. This is the reason which explains the more "compact" carpet plots obtained when selecting the strategy of throttling the EF during cruise. Indeed, for the same variation of Hp*use* less electrical energy is utilized during cruise resulting in lower value of He*block*.

**Figure 10.** Relative change in block ESAR versus block degree-of-hybridization for energy He*block* . Electric fan cruise throttling

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> −45

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> −45

**Degree of hybridization for block energy Heblock [%]**

**Figure 9.** Relative change in block ESAR versus block degree-of-hybridization for energy He*block* . Geared-turbofan

Interestingly, for a given He*block* the benefit in terms of block fuel reduction achieved are about identical for both strategies (see Figure 7 and Figure 8). However, it is highlighted that for an identical He*block* the level of Hp*use* is larger in the case of throttling the EF during cruise. It means basically that for achieving the same block energy split, a larger electric motor power needs to be installed. This trend is understandable as less electrical energy is consumed during cruise when the EF is throttled back, compared to the first strategy for an identical Hp*use*. In order to achieve the same He*block*, a larger electric motor needs consequently to be installed to achieve the same block energy split for an identical block mission. This is the reason which explains the more "compact" carpet plots obtained when selecting the strategy of throttling the EF during cruise. Indeed, for the same variation of Hp*use* less electrical energy is utilized during cruise resulting in lower value of He*block*.

Hpuse [%]

Cargo volume/PAX = 0.14m³

**Degree of hybridization for block energy Heblock [%]**

**Figure 8.** Relative change in block fuel versus block degree-of-hybridization for energy He*block* . Electric fan cruise

<sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>25</sup> <sup>30</sup> <sup>35</sup> <sup>40</sup> <sup>45</sup> <sup>50</sup> <sup>55</sup>

Cargo volume/PAX = 0.14m³

Hpuse [%]

1700

Design Range [nm]

900 1100 1300

2100

1500 1700 1900

Design Range [nm]

1900 2100

−40 −35 −30 −25 −20 −15 −10 −5 0 5 10

−40 −35 −30 −25 −20 −15 −10 −5 0 5 10

**Study settings:** ebattery = 1.5kWh/kg GTF cruise throttling

**Relative change in block ESAR [%]**

throttling

cruise throttling [23]

**Relative change in block fuel [%]**

132 New Applications of Electric Drives

15 20 25 30 35 40 45 50 55

**Study settings:** ebattery = 1.5kWh/kg Electric fan cruise throttling

**Figure 11.** Relative change in MTOW versus block degree-of-hybridization for energy He*block* . Geared-turbofan cruise throttling [23]

The analysis of the relative change in block ESAR versus He*block* in Figures 9 and 10 reveals interesting trends resulting from the system implications of the different strategies. As highlighted in the beginning of this section considering the first strategy, an increase in Hp*use* results in a higher level of thrust throttling of the GTFs during cruise due to the installation of larger EF. This effect contributes to a reduction of the GTF efficiency due to stronger part load operations. This operational consequence can be observed through the noticeable degradation in ESAR at higher levels of Hp*use* indicated in Figure 7. In the second strategy, the EFs are throttled back and the GTFs run close to their maximum efficiency during cruise. As a result, block ESAR increases slightly with Hp*use* as the overall propulsion system efficiency is improved through the use of the efficient electrical system and it remains almost independent of Hp*use* for short design ranges. The decrease in block ESAR at higher

**Figure 12.** Relative change in MTOW versus block degree-of-hybridization for energy He*block* . Electric fan cruise throttling

design ranges with increasing levels of Hp*use* is attributable to sizing cascade effects resulting from the higher electric energy requirement which leads to large increase in aircraft mass (see Figure 12). However, it must be noted that for a given He*block* the difference in delta block ESAR is small when comparing the different strategies. It is important to highlight at this point that in the current implemented electrical system model, the efficiency of the electrical components, with the exception of the battery, is assumed invariant with respect to the operational conditions and operating time. This assumption is made under the premise of an appropriate thermal management of the electric components and a thoughtful layout of the propulsion architecture. The efficiency of the electrical propulsion system chain in the model depends consequently only on the variation of the battery efficiency with respect to its discharge characteristic and upon the ducted-fan efficiency according to the flight state and the power setting. Moreover, in the current model the speciï ˇn ˛Ac weight of the electrical components wereconsidered independent of any scale effect. With the availability of more detailed electrical system models, the dependance of the electrical components efficiency with respect to the altitude-temperature envelope and power load conditions as well as possible variations of the specific weight with scaling effects would be considered.

The impact of the hybrid-electric propulsion on aircraft size according to the EF cruise throttling strategy is illustrated by the change in MTOW versus the change in He*block* in Figure 12. Similar trends in MTOW change between the different strategies with respect to increase in design range and growing Hp*use* were identified. For an identical He*block*, similar values in relative change in MTOW were observed.

In summary, a similar level of reduction in block fuel can be achieved when selecting the throttling of the EFs during cruise. This second operational strategy results in similar change in block fuel and block ESAR as well as in MTOW for an identical He*block*. However, to reach the same potential in block fuel reduction, a higher level of Hp*use* (in other words a higher useful electric power relative to the total useful power) needs to be achieved. This translates into the installation of a larger electric motor power. This system implication is rooted in the nature of the operational strategy. When throttling the EFs during cruise less electrical energy is required at an identical level of Hp*use*.
