Research

Source3Part3.pdf

As discussed in Chapter 2, fuel consumption is a major cost driver in aircraft operations. In order to quantify its impact, understanding the cost structure of airlines is fundamental.

10.1 Airline Cost Structure When airlines operate their fleet, operating costs are incurred, such as fuel, maintenance, flight personnel, and aircraft leasing/ownership, among others. Additionally, other charges borne on ground have to be considered, such as servicing and ticketing. Finally, airlines have to pick up the tab for so-called system operating costs. Generally speaking, the following breakdown gives a rough order of magnitude of the cost segments at play:

FLIGHT (DIRECT) OPERATING COSTS (DOC) = 50% • All costs related to aircraft flying operations • Includes pilots, flight crew, fuel, maintenance, and aircraft ownership

GROUND OPERATING COSTS = 30% • Servicing of passengers and aircraft at airport stations • Includes aircraft landing fees and reservations/sales charges

SYSTEM OPERATING COSTS = 20% • Marketing, administrative, and general overhead items • Includes in-flight services and ground equipment ownership

Performance and Business Value of Electric Aircraft Pascal Thalin Chair and Member - SAE Electric Aircraft Steering Group

C H A P T E R

Thalin, Pascal, et al. Fundamentals of Electric Aircraft, SAE International, 2018. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/dcccd-ebooks/detail.action?docID=28983708. Created from dcccd-ebooks on 2023-02-05 20:17:09.

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For relative comparisons, Figure 10.1 shows a rough breakdown of airline direct operating costs (DOC).

As shown, fuel consumption could account for up to one-third of typical airline DOC. Simply put, any reduction made possible in this expense could potentially benefit airline profitability.

Figure 10.2 portrays International Air Transport Association (IATA) data on fuel costs over time of the airline industry. Just to give an idea, it was forecasted that worldwide fuel expenses in 2017 would top out at above a staggering USD 131 billion.

FIGURE 10.1 Airline direct operating costs [10.1].

FIGURE 10.2 Worldwide airline industry fuel costs (USD) [10.2].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 201

The chart in Figure 10.3 shows the price variation over time of jet fuel used to operate conventional aircraft. Actually, jet fuel price variations mirror the price variations of crude oil price. Unfortunately, airlines have no way to influence these price fluctuations.

Now, let us take a closer look at the share of fuel costs in the total airline operating costs. Figure 10.4 shows how fuel-related costs of the airline industry tend to vary over time. The evolution of the ratio of fuel costs to total operating costs is also highlighted.

As the chart shows, from 1997 to 2001, with relatively low fuel prices of around USD 0.6/gallon, the overall fuel cost on average was a mere 12% of the airline operating cost.

FIGURE 10.3 Jet fuel cost (USD/gallon) [10.3].

FIGURE 10.4 Worldwide fuel expense (USD) and ratio (%) [10.4, 10.5, 10.6].

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But in 2008, at the height of skyrocketing fuel prices (USD 3.8/gallon), the ratio of fuel cost to airline total operating expense topped out at 36%. Ever since, following two dips in fuel price, the same ratio returned to 19% in 2016.

Now, with regard to DOC, Figure 10.5 also helps us understand the orders of magni- tude at the airline level. The chart uses two industry metrics, namely, the block hour and the Cost per Available Seat Mile (CASM). It is worthwhile defining them here. The block hour is the industry standard measure for aircraft utilization. For a given flight, block hours take into account the time from the moment the aircraft door closes at departure until the moment the aircraft door opens at the arrival gate following its landing. The CASM is a common unit of measurement used to compare the efficiency of various airlines. It is obtained by dividing the operating costs of an airline by available seat miles (ASM). Generally, the lower the CASM the more profitable and efficient the airline.

10.2 Aircraft Fuel Costs Obviously, there is a direct link between the type and size of aircraft being operated and fuel costs incurred. This also holds true for the ratio of fuel costs to DOC which include fuel, maintenance, crew, and aircraft ownership/leasing costs.

The variation of fuel costs across different aircraft segments is illustrated in Figure 10.6, a snapshot for the year 2013 of large certified U.S. passenger air carriers. Costs are expressed in USD per block hour of operation.

Looking at the proportion of fuel in the aircraft DOC, it can be concluded that the larger the aircraft size the more expensive the fuel bill and higher the ratio between fuel costs and operating costs.

This chart shows that, in 2013, for the wide-body aircraft segment, for instance, the fuel ratio was above half of the total costs, just when oil prices stagnated at extremely high levels.

FIGURE 10.5 Direct operating costs across aircraft segments (U.S. carriers—2013) [10.7].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 203

10.3 Airline Fuel Efficiency Fuel costs of an airline are of course directly linked to jet fuel price in the first place, but there is only so much an airline can do about this. Secondly, fuel costs are tied to the fuel efficiency inherent to the type and age of aircraft making up the fleet, besides the actual efficiency of airline operations. Finally, fuel consumption takes a hit from airspace and airport inefficiencies in the form of tarmac delays and holding delays for instance. But, once again, airlines cannot control these situations.

The impact of jet fuel price could be allayed if the airline opts for shrewd hedging strategies to minimize exposure to price fluctuations. Better operational practices could also help reduce fuel costs further. Last but not least, fuel cost mitigation can be achieved by introducing newer and more fuel-efficient aircraft into the fleet.

Let us see what is behind the fuel efficiency of an aircraft and how it has evolved over time from an airline perspective. For a given type of aircraft and a set of parameters (payload, range, speed), fuel burn is directly linked to the compounded efficiencies of the following:

• Propulsion • Systems • Airframe

Figure 10.7 shows for the commercial jet aircraft segment how, notwithstanding fuel price fluctuations, the introduction of new aircraft has helped cut back on fuel consumption. Additional fuel efficiency improvements are expected over the short term

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FIGURE 10.6 Fuel cost variation across aircraft segments in 2013 (U.S. carriers) [10.8].

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with the entry into service of re-engined versions of conventional aircraft (e.g., Airbus A320neo and Boeing 737 MAX/777X).

One other advantage of aircraft fuel-efficiency gains resides in the improved range performance, that is, the ability of the aircraft to fly farther over longer distances without having to stop to refuel.

Figure 10.8 retraces range improvement of new aircraft over the span of the past 40 years [10.9]. Since 1988, the range has increased by 40%, while operating empty weight

FIGURE 10.7 Fuel burn for new aircraft and fuel prices [10.9].

FIGURE 10.8 Structural efficiency and range for new aircraft [10.9].

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per unit aircraft floor area increased modestly, about 6%. Normally, on a given mission, the longer the aircraft range the lower the fuel efficiency because of more weight. Hence, for the range increase over time shown in Figure 10.8, the fuel efficiency improvement, rather than degradation, is attributable to airframe technology advances (e.g., composite) having been able to offset weight penalties, as noted in ICAO’s 2010 fuel efficiency tech- nology review.

All in all, fuel prices alone may not provide a consistent, long-term motivation for fuel efficiency improvements in the aviation sector. The drive to curb carbon emissions through the reduction of fuel consumption has become another key industry driver. Having no handle on fuel price fluctuations, faced with highly demanding environmental targets, global research in the aerospace industry is focusing on an array of solutions.

Simply put, decreasing fuel, maintenance, and other costs with no compromise on performance at the very least is the minimum expectation.

Thankfully, there are solutions ranging from incremental enhancements to all-out aircraft redesigns. An out-of-the-box approach parlaying advancements in architectures and technologies, leveraging innovations drawn from other industries such as automo- tive, and including cutbacks in operational costs combined with better environ- ment-friendly performance and services could be delivered in the coming years.

The push towards sustainable aviation ushering in cleaner and quieter aircraft technology entails drastic performance requirements that even the most advanced technology on conventional aircraft can hardly cope with. Therefore, rethinking the overall aircraft design and a departure from conventional architectures, technologies, and integration is unavoidable. The move towards the electric aircraft becomes all the more relevant in this context, given the significant added value it could bring in key performance drivers such as fuel efficiency, carbon footprint, and noise while enhancing overall cost effectiveness for the airlines.

In this technology race, industry players are spearheading research to get the picture straight as to how to make the electric aircraft a reality and meet the aggressive challenges of air transport facing the industry.

10.4 Business Aviation Previous analyses show that major benefits could stem from the electrification of ice protection provided weights are not altered, or (even better) reduced, at the aircraft level. In order to see the benefits of electrification, let us take a look at how some conventional systems perform on a business jet, and what lost performance can be retrieved through electrification and the strings attached to it [10.10].

The conventional pneumatic ice protection system operates from hot bleed air derived from the engines. However, this process impacts engine thrust and the aircraft performance gets negatively affected. Figure 10.9 shows the degradation of engine thrust with the conventional system.

Switching on bleed air decreases thrust by 5%-7% when all engines are operating, and 10%-12% in the case where one engine is inoperative. This thrust degradation not only

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needs additional fuel but, in the latter most critical case, ends up restricting aircraft range performance (Figure 10.10).

On a Cessna Citation Jet 2 type of business aircraft with pneumatic ice protection, in order to comply with the minimal climb gradient (2.5% net) of the standard instru- ment procedure in icing conditions with one engine inop- erative, the aircraft takeoff weight has to be reduced by removing almost half of the mission fuel. This drastically degrades range.

Electric ice protection could thwart this drawback and bring real benefit by recovering range performance lost with the conventional system (Figure 10.10). Actually, compared

to pneumatic ice protection, the electric version would produce only about one-fifth as much decrease (2%) in thrust.

It is clear that the advantage of electric ice protection lies in lower thrust degradation that allows climbout with more fuel on board. Therefore, in icing conditions, an electric ice protection system can double the usable range of a business jet, compared to a bleed air system.

Let us now consider another pneumatic system, the Environmental Control System (ECS). On business jets, compared to the baseline pneumatic version, although the electric version draws less power from the engines, the positive impact on thrust remains only marginal. This suggests that the greatest energy-saving opportunity of electric ECS, as opposed to the case of ice protection, might not lie in the energy consumed by the system, but rather in the weight reduction of downsizing or eliminating the bleed air components. Reduction in life cycle costs is another area where electrification can add value, thanks to reductions in part count enabling savings on both inventory and maintenance.

On top of these examples, significant performance enhancement could only stem from system weight reductions within electrical architectures, when transitioning from conventional non-electrical systems to their electrified versions.

FIGURE 10.10 Business jet ice protection—range impact [10.10].

FIGURE 10.9 Thrust impact of conventional ice protection [10.10].

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Weight reductions on an aircraft can allow additional fuel to be carried, and therefore extend the range capability. For instance, on the three-passenger Cessna Citation Jet 2, a 100 lb systems weight reduction can take the aircraft farther by around 60 nm.

It is also interesting to check the fuel-burn reduction that may be obtained by parlaying system weight gains into a scaled-down (wing, tail, and engine) redesign to obtain a more fuel-efficient aircraft. In other words, since the aircraft now has 100 lbs less payload, the takeoff thrust requirement is lower, which can translate into smaller tail structure, etc. In this approach, the same 100 lb systems weight reduction, for instance, could help shave 300 lbs off the max takeoff weight on a downsized Citation Jet 2 design, therefore helping reduce the fuel load by around 1.6% on a 1700 nm flight.

But bringing down the weight of systems that undergo electrification is hardly an easy task due to the fact that ongoing research is yet to reach the weight-reduction targets. As explained in Chapter 2, on a more electric version of this type of aircraft, a drastic reduction in the global engine power offtake for systems is possible, even though the electric part is drastically increased. Without improvements in the state-of-the-art electrical components, the overall weight of the airplane can go up, negating any benefit brought upon by electrification. This added weight would then cause a fuel burn penalty or decrease the aircraft’s range.

Incidentally, any system-level weight increase gets amplified at the aircraft level. In fact, carrying additional system weight onboard comes down to additional fuel load and volume to be carried up to the destination. This in turn calls for extra fuel, fuel reserves, and space, equating to an overall amplified weight increase. Consequently, the fuel burn also gets amplified in the same manner. This ampli- fication effect is called “spiral” or “snowball effect.”

The hurdles behind weight reduction are not trivial. Let us consider a larger, longer-range legacy business jet such as the Falcon 2000 manufactured by Dassault Aviation [10.11]. The analysis in [10.11] uses scaled-up systems and does not take into account weight-optimized engines and systems. Figures 10.11 and 10.12 compare the conventional Falcon 2000 to its more-electric and all-electric versions: partial elec- trification whereby only hydraulic systems are switched to electrical ones and total electrification of all systems. In Figure 10.11, it can be seen that system weight increase for the all-electric aircraft total is north of 300 kg, which at the aircraft level gets amplified to above a ton due to the snowball effect (Figure 10.12).

These weight penalties hurt fuel efficiency and also the range performance.

In the partial electrification case, even though suppressing hydraulic systems reduces the fuel consumption needed for system power delivery, the reaped benefit is hardly sufficient to counter the overconsumption from weight increase with the electrical replacement.

FIGURE 10.11 System weight [10.11].

FIGURE 10.12 Max takeoff weight [10.11].

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The all-electric version, for a given “engine cycle,” does not bring efficiency improve- ments and, therefore, leads to an impressive fuel burn increase compared to the conven- tional baseline.

Even in the case of partial aircraft electrification (hydraulic systems going electrical), capable of better fuel efficiency and cost of ownership with only a minor range penalty, fuel savings get drowned by the fuel penalty of additional system weight.

In summary, based on the above analysis of the Falcon 2000, fuel penalties stemming from weight increases during electrification may get exacerbated by the snowball effect, and end up compromising the expected performance and cost benefits of electrification.

This view shows that the stakes are high in the research of power-dense and fuel- efficient solutions when transitioning from legacy aircraft to their electrified versions, whether incrementally or through a complete redesign. The power-to-weight ratio has to be drastically enhanced in order to make the electric business aircraft competitive.

When sticking to conventionally designed fixed-wing aircraft and turbofan engines, the way forward proposed by several business jet manufacturers is to develop newer and more efficient engines and systems and implement well-thought-out integration strategies.

For instance, engine integration of upsized electric power offtake could be dealt with either by embedding power generators into the engine core or, for that matter, by encapsulating electric power generators into the gearboxes that drive them. Another approach could involve sharing oil cooling functions between the engine and the gener- ator (s) installed on them.

One other strategy could consist of seeking alternatives to the conventional sourcing of electric power from the engines. To this end, resorting to fuel-efficient and power- dense sources available on board may alleviate the burden on the engines while bringing economies of scale in fuel consumption and helping resolve the weight penalty issue. When retooled to that purpose, complementary sources such as the Auxiliary Power Unit (APU) could be up to the job and allow the aircraft to benefit from better fuel efficiencies and power-to-weight ratios.

With regard to the electrical network, 28 VDC or 115 VAC electrical networks are found on many legacy business jets for the power supply of systems. If intensive system electrification is carried out, legacy networks ought to be transformed into high-voltage networks. In fact, with high-voltage DC, the levels of current carried by cables and used by various equipment are lower; thereby weight reduction is possible through the down- sizing of cabling onboard. Moreover, the fact that high voltage is centrally created and distributed to the end systems downstream relieves the burden on these systems to locally create their own HVDC bus. Lastly, moving up in speed, the power generation for a HVDC network is more weight-optimized compared to legacy networks.

As recent research on business aircraft suggests [10.12], the legacy electrical network can be switched to a lighter 270 DC network thanks to high-speed starter-generators and power-dense power conversion equipment. From thereon, the electrification of pneumatic systems helps garner estimated weight savings of more than 100 kg. As explained previ- ously, this could be translated into either range enhancement or fuel cost savings.

As developed in Chapter 2, let us bear in mind that on an electric aircraft the power electronics needed for the power conversion and motor control constitute a large portion of the overall weight. In the process of electrification, there is a tendency to dedicate one

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CHAPTER 10 Performance and Business Value of Electric Aircraft 209

motor controller to each system. Unfortunately, this worsens the dead weight being carried by the aircraft because of the dead time during which controllers do not have to operate.

Logically, multi-purpose motor controllers could be the panacea for reducing dead time. During flight, such a controller can be switched from one system to another when these systems are operated in a sequential manner. But their coverage usually is limited to two dedicated applications.

Taking this approach one step further, dead time and dead weight can be drastically reduced by using standardized power electronic modules addressing multiple systems, as explained in Chapter 2. This modular paradigm, on top of the versatility it brings, helps eliminate unnecessary margins that have to be built into dedicated individual or multi-purpose motor controllers.

10.5 Short-Range Aircraft Similar to the analysis conducted on the Falcon 2000 business jet [10.11] and developed in the previous paragraph, let us now consider another study [10.13] performed on a short-range aircraft.

The benchmark aircraft for this comparative study is a conventional 165-passenger, short-range (3500 nm) twin jet, with a baseline A320 design but including technologies derived from programs not too far in the past like A318, A380, and A350. In this aircraft, pneumatic systems are a mainstay, whereas hydraulic systems are limitedly used.

The study compares the benchmark to a more-electric version based on the removal of pneumatic systems, and corresponding engine offtakes while sticking to reduced usage of hydraulic power. The turbofan engine technology and performance are kept unchanged between the two versions. The benefits of “more-electric” systems have also been assessed at the aircraft level from the perspectives of maintenance and environ- mental impact based on fuel consumption.

The weight breakdown of systems analyzed on the benchmark aircraft is reflected in Figure 10.13. This can be compared to the weight picture shown in Figure 10.14 for

FIGURE 10.13 Systems weight (benchmark) [10.13]. FIGURE 10.14 Systems weight (more-electric) [10.13].

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the more-electric version. In short, apart from the bleed air part of the pneumatic systems, a general weight increase is noticed on the more-electric aircraft.

The weight assessment of systems shows that the more-electric short-range aircraft is heavier than the reference short-range aircraft. As shown in Figure 10.15, when shifting to the more-electric aircraft by way of system electrification, the weight reduction benefits, reaped thanks to several systems, are “outweighed” by the weight penalties encountered in the remaining systems. As illustrated in Figure 10.15, major penalties trouncing weight benefits are attributable to the following consequences of electrification, their contributors, and key impacting factors:

• Scale-up of electrical power needs on board: electrical power generation/ distribution technology and power density

• Replacement of conventional systems with electrical versions: ice protection and ECS technology and power density as well as additional cooling technologies and their power density

• Integrating electrified systems: weight- and volume-related additional impacts on airframe linked to the above contributors

Let us focus on how electrification may impact aircraft performance from a drag perspective. On the reference aircraft, fresh air supply needed for the operation of the ECS is ensured by the engines thanks to the engine bleed system. On the contrary, the same engines when fitted to the more-electric aircraft cannot deliver the air required by the ECS because the bleed system is removed in the electrification process. Therefore, alternative air supply has to be sourced from the outside of the aircraft, requiring the implementation of the following design features:

• Cabin air re-circulation: supply of fresh cabin air from outside the aircraft using scoops • Electrical ECS and cooling system: ram air from outside the aircraft

FIGURE 10.15 Aircraft weight (more-electric vs. conventional) [10.13].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 211

As a result, compared to the reference aircraft, these additional implementations on the more-electric aircraft wind up creating a drag penalty. Nevertheless, research of possible mitigation means could help alleviate this drawback.

Fuel burn comparisons in [10.13] are based on a 500 nm mission. Firstly, the more-electric aircraft has reduced engine-specific fuel consumption compared to the reference aircraft. This stems from engine efficiency gains brought by the removal of bleed components.

Based on this and after further computation, notwithstanding the weight disad- vantage of the more electric aircraft, the study in [10.13] arrives to the conclusion that there is quite no difference in fuel burn between the short-range reference aircraft and its more-electric version.

In addition to the fuel burn assessment, the study evaluates the Direct Maintenance Cost (DMC) for a system perimeter limited to the APU, electric, bleed, ECS, and cooling systems.

The result shows a slight decrease in DMC in favor of the short-range more-electric aircraft.

Ever since the study was performed, leveraging latest breakthroughs, the aerospace industry has developed and tested a full suite of technologies that could be readily inte- grated on a more-electric aircraft, provided the required maturity levels are reached. Even though system weight and integration remain an outstanding challenge, conceptual design studies conclude that the more-electric aircraft may readily deliver airplane benefits in terms of maintenance, operational flexibility, and technology growth potential without, at the very least, any fuel burn penalty. Better, system simplification and weight savings could lead to potential fuel efficiency enhancement. Furthermore, as more-electric tech- nologies are friendlier to the environment than conventional solutions, they appear as key enablers for sustainable growth of the aerospace industry.

But on the flip side, due to its smaller size, the weight and integration stakes facing the short-range more-electric aircraft are far more challenging compared to a long-range more-electric aircraft.

In summary, compared to its more-electric version, the conventional metallic narrow body has a head start from a weight standpoint, whereas both versions are on a par when it comes to fuel efficiency. Therefore, weight reductions on the more-electric version could make it more competitive by reducing fuel costs. Hence the deep focus in ongoing research on power-dense solutions. Yet, the situation is not that negative. Global research being carried out targets not only the alleviation of system weight penalties found in initial studies, but also addresses weight cutbacks with respect to conventional systems.

To make the case for the more-electric aircraft, one may think of resorting to a lighter composite airframe. But, keep in mind that weighty corrections would have to be implemented in order to render such an airframe compatible with the consequences of system electrification, such as incorporating missing features like noise barrier and lightning protection that come free with a metallic airframe. Once again, the size of the aircraft influences the level of optimization that composites could offer.

While the industry supplier base is scrambling to get power-dense more-electric system options on the table, aircraft manufacturers took the approach of investigating the potential of quick-win incremental changes on existing aircraft. In order to deliver

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step reductions in fuel burn, they continued to work with system suppliers, and also turned to engine manufacturers and put the onus on them as well.

Actually, while the industry waited for technology promises to spring up on the systems side, engine manufacturers invested heavily in research to improve drastically the fuel efficiency of turbofan engines. Ultimately, engine manufacturers ended up offering fuel-efficient turbofan engine alternatives very close in form, fit, and function to legacy ones. On a secondary level, rework of some systems and structural areas allowed extra efficiency gains.

The re-engining approach also became compelling in the face of sky-high fuel prices. Aircraft manufacturers heeded market calls and resorted to incremental re-engining developments on existing platforms, allowing lower risks and costs and shorter time to market compared to a full-blown aircraft development. The re-engining solution for legacy short-range aircraft platforms, despite a higher price tag, was readily endorsed by airlines under pressure from hurting fuel prices. Moreover, in such an approach, airlines continue to benefit from proven maturity in service for all the other parts of the re-engined aircraft except, of course, the engines themselves.

In such newly developed turbofan engines, still comprising the bleed system, tremen- dous improvement in fuel efficiency has been possible thanks to leapfrog technologies and materials in the engine redesign, based on following two competing architectures with comparable performance:

• Traditional turbofan architecture on the LEAP engine from CFM International • Disruptive turbofan called “Geared Turbofan (GTF)” on the PurePower® engine

from Pratt & Whitney

Both developments have materialized into ready-for-service engines that can be purchased by airlines as replacements for engines hitherto offered by aircraft manufacturers. They offer drastic fuel-burn reductions at aircraft level. Figure 10.16 shows

FIGURE 10.16 Aircraft fuel burn (re-engined vs. conventional) [10.14].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 213

how, compared to the original aircraft, on the re-engined version the various fuel efficiency gains stack up.

All in all, simply re-engining the short-range aircraft brings a steep 15% reduction in aircraft fuel burn.

Helped by the technology overhaul that conventional turbofan engines have gone through in the re-engining strategy, fuel efficiency wise the situation tips in favor of the re-engined version of the conventional short-range aircraft. Logically, this puts into question the performance assumptions, mainly fuel efficiency, behind the reference aircraft used for benchmarking its more-electric version.

Nothing to do with a more electric aircraft, the advent of re-engining options has all but challenged the development of narrow-body more-electric aircraft by raising the bar higher with regard to the reference aircraft against which the more-electric version has to compete.

Factoring in the newly available engine performance into the conventional aircraft architecture serving as the benchmark widens the fuel burn gap that the more-electric aircraft would have to offset first, and prior to offering additional benefits in its own right.

Therefore, making the case for the more-electric short-range aircraft gets trickier from a timeline and technology perspective. This is the main reason why decisions on the more-electric versions of the short-range aircraft segment are ever more tied to power-to-weight and efficiency improvements that aircraft systems research can deliver.

In the meantime, out-of-the-box approaches have gone full swing in the research of alternatives to both more-electric and re-engined versions of the conventional short- range aircraft. When observing the trends behind, both power-dense electrified systems and electric propulsion, whether hybrid-electric or totally electric, appear to be mainstays in the next paradigm shift in electric aircraft design.

As discussed previously, hybrid-electric propulsion will require onboard recharge- able energy storage devices such as battery packs. Global leapfrog research advances are speeding up large-scale shrinking of batteries while making them as energy dense as called for by the electric aircraft design. In the meantime, sweet spots are under inves- tigation using downsized conventional turbine engines, matched and operated hand in hand with a certain degree of electric propulsion. This gives birth to the concept of hybrid-electric propulsion, wherein a parallel may be drawn with HEV powertrains already commonplace in the automotive industry.

Isikveren [10.15] provides performance comparison between a future medium-range 180-passenger reference aircraft with advanced electrical systems and its hybrid-electric version using 1500 Wh/kg batteries. This means a tenfold increase in specific energy, compared to the most recent battery technology available on the Tesla electric car. Results obtained for a range of 1100 nm show that hybrid-electric propulsion, utilizing electric energy during 50% of cruise, could theoretically wind up delivering a 20% reduction in fuel burn compared to the reference aircraft fitted with turbine engines.

Hornung [10.16] analyzes hybrid-electric propulsion using 2000 Wh/kg batteries on a reference 189-passenger short-range aircraft with a self-trimming nonplanar C-wing (Figure 10.17).

FIGURE 10.17 C-wing hybrid-electric aircraft [10.16].

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Moreover, aircraft systems are also fully electric and powered solely by batteries. Batteries, lodged in cargo containers, would not require recharge during turnaround because used battery containers would simply get swapped with pre-charged ones during turnaround. The electric propulsion system comprises large high-temperature super- conducting electric motors with an integrated cryocooler and high-voltage power electronics dedicated to motor control. On the systems side, high-voltage DC is the adopted standard for the electrical network supplying loads via power electronics or power supplies.

For a range of 900 nm, Figure 10.18 shows the performance of such an aircraft depending on the degree (or ratio) of propulsion electrification. A ratio of “1” stands for fully electric propulsion, a ratio of “0” applies to the most recent bleedless turbofans; any value in between pertains to hybrid-electric operations using both fuel and battery energy.

Because no jet fuel is used by the fully electric aircraft, its operation entails zero emissions. When the electrification ratio is set to 50%, efficiency gains are around 30%, even though aircraft weight is increased by around 25%. This weight increase is mainly due to the following factors:

• Battery system, despite a challenging specific energy assumption of 2000 Wh/kg. • Wing redesign to keep wing loading constant. In fact, wing structural weight

increases when compensating for lower wing bending moment relief from lower quantity of fuel onboard.

• Lower amount of fuel mass burn-off during mission.

Yet, still sticking to conventional “wing-and-tube” aircraft shapes, another study in [10.17] shows that a tri-fan morphology, comprising two under-wing podded gas- turbines (GT) and one aft-fuselage mounted serially configured motor (M) driven by

FIGURE 10.18 Hybrid-electric aircraft performance vs. degree of electrification [10.16].

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batteries, could be an appropriate choice for a 180-passenger hybrid-electric aircraft (Figure 10.19).

The study concludes that, even though resorting to a high-temperature superconducting motor could deliver 2%-4% reduction in aircraft weights, a normal conducting motor is deemed to be a pragmatic choice. In the latter case, the tri-fan morphology motor would have to deliver 8.5 MW of shaft power for a 180-passenger aircraft. The reference aircraft used for performance comparisons is a year 2000 A320-200 aircraft with evolutionary technologies: advanced ultra-high bypass (~20) geared turbofan, all-electric systems, high wing aspect ratio (~12%), reduced zero-lift drag, and advanced structural materials. It achieves a 39% reduction in block fuel, compared to the A320-200, demonstrating the high level of optimization taken into account for the reference.

Over a range of 1100 nm, Figure 10.20 shows that in order for the hybrid-electric aircraft to achieve 15% block fuel reduction compared to the reference aircraft, batteries with 940 Wh/kg specific energy would be necessary. Likewise, a 20% block fuel reduction would be possible over ranges of 900, 1100, and 1300 nm, with a battery specific energy of 920, 1100, and 1290 Wh/kg, respectively.

Let us consider the operating costs, excluding certain costs linked to cost of ownership (depreciation, interests, and insurance) and including additional noise and emissions- related charges. From the perspective of operating costs, the hybrid-electric aircraft is ~10% more expensive than the reference aircraft. Concerning energy costs, that is, fuel and electricity, they go up by ~6%. These figures were estimated using a price of USD 3.30/g

FIGURE 10.19 “Wing-and-tube” hybrid-electric aircraft [10.17].

FIGURE 10.20 Hybrid-electric aircraft fuel savings vs. range and battery specific energy [10.17].

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for kerosene and USD 0.1109/kWh for electricity required for battery charging. Fuel price fluctuations considered in the sensitivity analysis show that with USD 2/g, the operating costs go up by 4-5%, compared to the reference architecture, whereas the same costs are cut back by 5%-7% with USD 6/g. Given these cost variations, achieving a cost-neutral situation, thanks to further optimizations, is deemed realistic. For a 1100 nm mission, using 940 Wh/kg batteries for hybrid cruise, the operating cost breakdown for both the reference and the hybrid-electric aircraft is shown in Figure 10.21 [10.17].

Now, building from a baseline Boeing 737 type of aircraft fitted with CFM56 engines (not the LEAP-1B ones), the study in [10.18] has devised a reference aircraft design called SUGAR High operating solely from fuel. From a structural standpoint, the reference design incorporates various improvements based on high-span truss-braced tube and wing morphology allowing high L/D ratio. The engines of the reference aircraft incorporate engine technology enhancements, spanning from the baseline CFM56 to the latest avail- able engine options for the short-range aircraft segment.

The study goes further by checking the potential of hybrid-electric propulsion combined with 750 Wh/kg batteries on the SUGAR High reference aircraft leading to its hybrid-elec- tric version called SUGAR Volt (Figure 10.22).

Results obtained in [10.18] for a 900 nm mission demon- strate in the first place that the future reference aircraft would per se deliver block fuel reduction of up to 54% compared to the baseline 737 (Figure 10.23). Secondly, the hybrid-electric variant brings a fuel-burn reduction of 14% compared to the reference which in reality amounts to a 60% reduction when compared to the conventional short range with baseline turbofan engines (not including the most recent advances available on the 737 MAX or the A320neo). These conclusions are illustrated in Figure 10.23 showing comparisons with respect to the baseline 737 of the future all-fuel reference (SUGAR High) on the one hand and, on the other, its hybrid-electric variants. The latter concerns two different electric motor sizings (1380 and 1750 hp),

FIGURE 10.21 Trip operating cost breakdown—reference vs. hybrid-electric aircraft [10.17].

FIGURE 10.22 High-span truss-braced tube and wing hybrid-electric aircraft [10.19].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 217

both being utilized in balanced hybrid operation throughout the duration of the mission.

For hybrid designs, it is important to look at reductions in fuel, but also reductions in total energy use in order to take into account energy expended in charging the batteries if they are charged on ground prior to flight. In this way, and by examining the costs of each type of energy source, the airliners can make informed choices. For emissions benefits, one must look at the fuel burned while flying and also go deeper and look at any carbon expended during the manufacturing of batteries and production of the electricity used to charge them.

10.6 Long-Range Aircraft As discussed, re-engined versions of short range are already in service. With regard to legacy long-range aircraft, re- engining with minor structural and system modifications is also in the making.

But, compared to what happened with the electrification efforts on short-range aircraft, the story has been quite different on the long-range aircraft. In fact, prior to the race towards power- dense systems solutions and the re-engining strategy, cleansheet more-electric versions have been successfully brought to market through the Boeing 787 and Airbus A350 long-range aircraft developments. These programs also benefited from more efficient engine developments and, in the particular case of the Boeing 787, were afforded a bleedless version to match the large scale of system electrification.

On the Boeing 787, the bleedless system architecture brings fuel burn reduction in the order of 3%. The bleedless engines allow for circa 15% reduction in specific fuel consumption. Structural changes bring their share of efficiency gains thanks to decreased weight and lower maintenance. In fact, massive usage of lighter composite structures helps leverage these benefits at the aircraft level, and also compensate weight penalties caused by system electrification.

In essence, economies of scale in the operation of the more-electric Boeing 787 could only be delivered through the balanced combination of system electrification, bleedless engines, and composite airframe structures.

When targeting better operational efficiency with the more-electric design, there are differences in the relative constraints of the short-range and long-range aircraft, the latter being largely helped by its bigger size, longer range, and larger engines:

• Larger size facilitates the integration and thermal management of large-scale electrification.

• Longer flight cycle durations allow better fuel optimization. • Larger engines are more fuel efficient than smaller ones.

Once again, without resorting to composite materials on the structural side, these benefits may not have materialized.

FIGURE 10.23 Hybrid-electric aircraft performance vs. reference and baseline 737 aircraft [10.18].

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In summary, with a long-range more-electric composite aircraft with bleedless systems architecture and turbofan engines, such as the Boeing 787, airlines may expect fuel burn reduction in the order of 15%-20%, depending on route distances. Lower costs for base checks, thanks to the composite structure and its monitoring, among other factors, allow for at least 15% reduction in maintenance costs. All this contributes to the Boeing 787’s ~20% operating cost advantage over similar legacy aircraft.

With this new type of more-electric long-range aircraft, airlines expect to cash in on the opportunity to reduce the fuel and maintenance portions of the operating costs. But, for a given trip, the total trip cost has to take into account aircraft financing charges as well. However, airlines often still find that aircraft financing charges could make their total unit costs higher than the older aircraft they are replacing. Thanks to carbon fiber technology, composite aircraft life is expected to double, and so its financing terms and lease rate may be lower than for conventional aircraft. Luckily, newly purchased aircraft enjoy maintenance cost write-offs during the initial years of operation thanks to manu- facturer warranties helping bring down operational costs. All in all, when using more-electric aircraft to replace older conventional aircraft, financing cost burden could be allayed by large-enough reductions in cash operating costs [10.20].

The cost performance estimations presented in Figure 10.24 corroborate the competitive edge of the more-electric Boeing 787 over other legacy Boeing aircraft in the long-range segment. The comparison is based on a 4000 nm mission, like on a trans- atlantic Paris-New York trip, for a fuel cost of USD 2/gallon.

From an operational standpoint, this analysis of Boeing aircraft shows that the baseline more-electric B787-8 stacks up against the conventional B767-200 with a steep 24% cut in the unit cost per available seat-mile (CASM) for a barely higher trip cost (+3.3%). Similarly, when comparing the larger B787-9 to the legacy B767-400ER, the CASM plummets by 17% along with a 4% dip in the trip cost. Incidentally, with the B787, airlines not only benefit from the operating savings but could potentially offer new routes where the B767 doesn’t go and where the B777 is deemed too big. Cases in point for such ultra-long-haul

FIGURE 10.24 Trip cost and CASM (more-electric vs. conventional) [10.21].

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operations are longer transatlantic (e.g., London-San Francisco or Europe–South America), trans-Pacific or Europe–Asia Pacific flights with an average range of 5500 nm.

Therefore, it is obvious that the long-range more-electric aircraft sets a new standard in operating economics by providing airlines with an aircraft that has a significantly improved CASM compared to their current fleet. In addition, thanks to its ultra-long- range capability, it provides the bonus of an opportunity to transform their long-haul networks.

The more-electric Boeing 787-8 has been in service since 2011. Airline operational data for the year ending in 2013 [10.7] shows that the DOC per seat per mile for this aircraft is ¢7.2. This is almost 8% below the average ¢7.8/unit cost registered for the overall long-range aircraft segment.

Revamped system architectures, more efficient engines relieved of system constraints (bleedless), and composite airframes have been the most recent key enablers in the design of more-electric aircraft all the way up to entry into service in the long-range market. Depending on the aircraft segment, when shifting conventional systems to the electric domain, performance could either be leveraged as shown above or get under- mined by weight issues stemming from electrification in the absence of design solutions alleviating them.

Economies of scale possible on a large aircraft may get watered down when the aircraft size goes down. The more-electric short-range or business aircraft requires ever more stringent optimizations and ups the ante when it comes to integration. While power-dense system research is in full swing to meet these expectations, engine manu- facturers have been able to drastically improve turbine engine performance. This allows existing aircraft to be re-engined at reduced development costs within a shorter time- to-market, thereby undercutting fuel cost impacts on operational economics.

Nevertheless, the re-engining wild card may have reached its limits. Any further attempt to sizably enhance fuel efficiency of turbine engines might prove unsuccessful, for all their potential and margins may already have been squeezed out.

Therefore, aircraft and engine manufacturers may have run out of viable incremental optimization opportunities on existing aircraft platforms. Even though operational cost and carbon footprint reductions do make the case for more efficient conventional aircraft and engines, they still come with noise and greenhouse gas emissions. Industry consensus for business growth, competitiveness, and environmental friendliness is grounded in the prospects of radically different aircraft designs resorting to alternatives to jet fuel. This longer-term approach simultaneously targets steep cutbacks in operational costs, driven not only by fuel efficiency but also by the cost of ownership, and environmental impacts of aircraft design and operations.

Taking stock of the performance ceiling reached with turbine engines using jet fuel, research is doubling down on the electric propulsion paradigm, breaking new ground in energy efficiency and emissions by getting rid of the reliance on fuel altogether. This opens new avenues for innovative configurations where energy is transmitted advanta- geously around the aircraft to achieve aerodynamic advantages, and reduce cost of operations and opens up avenues for using other sources of ground power.

This entails the replacement of fuel by an alternative electric energy source or storage carried on board. When the replacement is carried out only partially, we end up having

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a hybrid-electric aircraft powered by hybrid-electric engines running on both fuel and electricity. A direct parallel can be made with the hybrid-electric vehicle (HEV) revo- lutionizing the automotive industry. In this case, the original fuel load gets split into two parts, a downsized fuel part and another part made up of batteries. The hybrid-elec- tric engine can be schematized as a downsized turbine engine working in tandem with a highly efficient electric motor. Similar to the more-electric aircraft, the electric and hybrid-electric aircraft systems come in their electrified versions.

It is important to note that though electric motors are more efficient than turbofans, the aircraft system also has to take into account how the energy sources are stored on board: jet fuel versus battery cells, typically. Similar as well to the power-to-weight ratio in systems electrification, gravimetric energy density (Wh/kg) and volumetric energy density (Wh/L), commonly named “mass specific energy” and “volume specific energy,” respectively, are key characteristics of energy storage on electric or hybrid-electric aircraft. Unfortunately, these parameters for even today’s most advanced battery storage systems fall well short of those obtained with kerosene: by a factor of 18 for the volume ratio and a factor of 60 for the weight ratio. This core problem tends to restrict the extent to which batteries may be utilized for powering systems and engines.

Space and volume issues previously raised on smaller-sized aircraft may come in the way of battery implementation. Nevertheless, the lower volumetric ratio may be less critical as long as the aircraft is not limited in space available for integration. Otherwise, the aircraft would require larger wings, fuselage, or additional external “energy pods” which would lead to losses in overall aircraft efficiency due to larger wetted surface. Ideas such as tightly integrating batteries and other storage systems with structural elements, like in the Tesla electric vehicle, are also coming through in the aerospace sector.

In summary, electric propulsion could help ratchet up overall aircraft performance, provided the specific energy of batteries replacing fuel is up to the challenge. Therefore, at the aircraft level, efficiency of turbine engines using jet fuel, though with highly competitive specific energy, may be outstripped by novel hybrid-electric or electric propulsion architec- tures powered by a new generation of batteries with dramatically increased specific energy.

10.7 Regional Aircraft Regional aircraft are powered by either turbofan or more fuel-efficient turbo-propeller engines. Let us see how battery supplied electric propulsion stacks up against the latter type of fuel-dependent turbine engine.

The Dornier 328, a regional aircraft fitted with turboprop engines, can transport 32 passengers over a range of 1200 km. On such an aircraft, if fuel is simply replaced by baseline battery technology (180 Wh/kg), and engines are switched to electric ones, range capability would plummet to 202 km [10.22] for the same payload. But, if the mass-specific energy of batteries were to be pushed higher to, for instance, 720 kW/h, range performance would get extended to 800 km, but still fall short of the conventional aircraft capability of 1200 km.

Figure 10.25 portrays how, depending on the specific energy of batteries, payload variations impact range performance for both the conventional aircraft (Dornier 328 TP)

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and its electrical version (Dornier 328 E) with drop-in battery and electric propulsion replacement of fuel and turboprop engines.

Due to the comparatively higher specific energy of kerosene, from Figure 10.25, it can be concluded that when it comes down to range performance, trading payload for fuel is much more beneficial than trading payload for batteries, the payload-range gradient depending on the specific energy of batteries.

Thereupon, without taking advantage from worthwhile structural modifications, the battery-powered aircraft might seem less flexible. Nevertheless, payload-range capa- bility equivalent to that of the baseline aircraft is reached when batteries with specific energy exceeding about 1500 Wh/kg are used.

This puts the bar very high for battery performance when resorting to electric propulsion on large aircraft. In order to power large aircraft, a dramatic improvement in battery technology would be required. Hence, the success of scaled-up electric aircraft hinges on lofty levels of specific energy for battery technology that ongoing research is yet to deliver. To attract commercial interest for larger (regional) aircraft, the specific energy of today’s battery technology, in the range of 150-200 Wh/kg, would have to be increased by a factor of around ten.

Nevertheless, practically speaking, the specific energy of current battery technology is such that airplanes powered by electric propulsion are today limited in size to small planes carrying up to two passengers over rather short ranges and limited endurance. Neglecting costs, the current technology is suitable for small ultra-light aircraft, but not yet for commercial aviation.

10.8 General Aviation As with previously discussed aircraft segments, the drive for electrification in general aviation is once again motivated by technical and cost performance enhancements. Reducing carbon emissions through transformative aircraft and airspace operations is among the industry goals, together with the alleviation of specific general aviation’s

FIGURE 10.25 Regional aircraft performance (electric vs. conventional) [10.22].

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concerns like the far higher accident rate compared to other aircraft segments and road transportation.

Electric propulsion offers a threefold improvement in efficiency compared to turbine engines, whereas the leap in energy efficiency can top out at 4% compared to piston engines commonly used in general aviation. Incidentally, high efficiency of electric propulsion is achieved across more than half of the operating speed range. Additionally, with electric motors being six times more power-dense compared to piston engines, electric propulsion could deliver power-to-weight ratings drastically improved by more than 500%. All the above add up to lower energy consumed and incurred costs. Electric propulsion in general aviation, thanks to the lower passenger count, size, and range, may help cut costs by up to 10%.

Electric propulsion, be it hybrid-electric or totally electric, offers lower community noise. Since recourse to air breathing is either alleviated or totally suppressed, greenhouse gas emissions are either cut back or, better, reduced to zero. Moreover, operational pitfalls of general aviation such as power lapses with altitude or hot weather conditions, directly related to the reliance on air-breathing, may also be circumvented by electric propulsion.

Fewer moving parts with electric propulsion wind up offering more reliable designs. Also, the inherent integration benefits of electric propulsion allow compact aircraft sizing applicable to all aircraft segments, including the small scales in play in general aviation. On top of this, when Distributed Electric Propulsion (DEP) is implemented, the additional integration benefits enable closely coupled synergies across aerodynamics, propulsion, control, acoustics, and structures.

Conventional general aviation aircraft are only aerodynamically efficient at low speeds in cruise due to the wing oversizing necessary to meet constraints related to stall conditions and airfield lengths. This unfortunately compromises the lift-to-drag ratio. When converting to an electric aircraft, wing downsizing in conjunction with distributed-electric propulsion offers better wing loading, more resilient aerodynamics, lower drag, and higher lift; therefore allowing higher speeds during cruise (Figure 10.26) [10.23].

In summary, to the extent that required battery specific energy and cost reductions are made viable, with the help of bespoke certification and standardization procedures yet to come, electric aircraft in general aviation may become a reality in the near future. Ongoing research targets both Conventional TakeOff and Landing (CTOL) and Vertical Takeoff and Landing (VTOL) types of aircraft, carrying 4-9 passengers and one or more passengers, respectively, with tentative entry into service slated for 2025.

FIGURE 10.26 Conventional GA aircraft redesigned into an electric aircraft with DEP [10.23].

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CHAPTER 10 Performance and Business Value of Electric Aircraft 223

Although aircraft range performance is independent of speed, up to now many general aviation flight demonstrators have concentrated on low speeds only. Nevertheless, research focus is now being devoted to the proof of concept of higher speed electric aircraft, paving the way for upward scalability in electric aircraft design. Therefore, the general aviation electric aircraft is poised to become the stepping stone for scaled-up aircraft designs, thereby allowing the advent of larger electric-aircraft platforms in the future (Figure 10.27) [10.23].

A tentative entry into service timeline of CTOL electric aircraft could be drawn up as follows:

2020: General aviation 2025: Commuters (~9 passengers) 2030: Regional airliners 2035: Large aviation Distributed electric propulsion when applied to vertical flight (VTOL) could

dramatically improve key performance characteristics, thereby allowing attractive value propositions (Figure 10.28) for mobility solutions including future autonomous air travel.

Just like with CTOL, the VTOL aircraft incurs low marginal costs of operation by cutting energy costs by a factor of more than ten, and alleviating maintenance costs by more than half. The increase in speed by more than half allows quicker travel times and high productivity at fleet level, thanks to high-utilization rates (>1500 h/year).

FIGURE 10.27 Upward scalability of CTOL electric aircraft design [10.23].

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The expected total vehicle cost per mile for distributed electric propulsion applied to vertical flight is compared across various on-demand transportation choices in Figure 10.29.

Enabled by its quieter operation, the electric VTOL aircraft may therefore open new markets for more competitive on-demand urban air travel, compared to current ground transportation services offered by networks such as Uber or Lyft. Nevertheless, such an air travel solution cannot become a reality without bespoke air traffic control and infrastructure yet to come. Provided adequate air traffic management is in place ensuring safe, seamless, fluid, and quick turnarounds, VTOL urban air travel may

FIGURE 10.28 Distributed electric propulsion VTOL vs. conventional aircraft [10.23].

FIGURE 10.29 Total vehicle operating cost per mile vs. cruise speed [10.23].

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offer quicker and hassle-free journeys by circumventing road congestions, totally upending travel experience with zero emissions over the city.

10.9 Cost of Ownership On top of trip cost and CASM comparisons, airlines use the metric of cost of ownership to evaluate the economics of newly marketed aircraft. This parameter allows the bench- marking of all costs incurred by this or that aircraft an airline intends to purchase. This information can be fed into the value analyses it usually performs in parallel. The partic- ularity of cost of ownership, also going by the name of “Life Cycle Cost (LCC),” as opposed to trip cost or CASM, resides in the fact that it is estimated not just at a given point of time but over the lifetime of an aircraft. It is based on cost projections starting from entry into service until the disposal of the aircraft.

Obviously, the electric aircraft, which puts together more expensive newly developed technologies and materials, comes with a higher price tag when it enters the market. This is similar, absent the government incentives attached, to what is observed in the automotive sector.

As discussed previously, maintenance cost savings can potentially contribute to an overall reduction in cost of ownership. The more the aircraft gets electrified the more its maintenance costs could be cut back, especially when health monitoring is imple- mented on board. Nevertheless, prior to the fruition of the benefits of the latter down the line, a certain amount of time lag is induced by factors such as an initial learning period, recoup time of health monitoring overheads, etc. As presented in [10.11], Figure 10.30 compares the health monitoring and aircraft downtime costs of a conventional, more-electric, and an all-electric business jet, whereas Figure 10.31 displays the relative break even times of the cost of ownership index.

FIGURE 10.30 Health Monitoring (HM) and downtime cost comparison (business jet) [10.11].

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10.10 Environmental Footprint From an environmental perspective, a lot of progress has been made thanks to the more-electric aircraft in service, such as the Boeing 787.

The Boeing 787 bleedless systems, more fuel-efficient and easier to maintain, are also good news for passengers. Since bleed air, used in the air-conditioning of conven- tional aircraft, is a byproduct of the engines, it might still contain traces of combustion products which at very, very low concentrations don’t pose a health risk. On the contrary, thanks to its electrified air-conditioning system without reliance on engine bleed air, the Boeing 787 uses cleaner outside air that has never been in contact with the engines.

Noise is also a major concern, whether perceived by passengers seated inside the aircraft or by communities along the flight routes.

Passenger aircraft noise is hardly attributable to engines only, for it is also generated by the aerodynamics of some structural and system components. During approach conditions, with landing gears deployed and flaps extended, airframe noise predomi- nates; whereas during other flight phases, the engines are to be blamed for the majority of the noise.

On conventional aircraft, bleed air is also used for de-icing: hot air is simply blown over the wings via exhaust holes but create an unpleasant “whistling” noise. In the bleedless system, an “electric blanket” is built directly into the wings, which heats the wings to keep them free from ice. This is a completely silent process, providing a nicer experience for passengers, but also for communities living near flight routes.

A lot more of these “more electric” optimizations are possible by replacing other pneumatic and hydraulic systems with electromechanical ones. Pneumatics and hydrau- lics are responsible for a lot of the weird noises that passengers hear during a flight; the switch to electrical solutions would result in quieter operations.

As concluded by the study in [10.18], considering a 154-passenger short-range aircraft, the total aircraft noise footprint hardly changes between the reference aircraft and its hybrid-electric variant. This is no surprise considering that similar noise perfor- mance is obtained for both reference turbine engines and their hybrid-electric variants.

FIGURE 10.31 Cost of Ownership index comparison (business jet) [10.11].

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Nevertheless, there is still room for further noise reductions by tackling the root causes of dominant subcomponents in noise generation. As it turns out, this approach comes down to rethinking the design of landing gear and flap systems on the airframe side, along with jet and fan optimizations on the engine side.

All-electric aircraft allow outright elimination of combustion engine noises offering drastic noise reductions. Figure 10.32 shows how, in the case of general aviation, distrib- uted electric propulsion helps alleviate noise issues by enabling sound reduction methods such as “reduced propulsor tip speed” and “spread spectrum,” among others [10.25].

Let us now consider the situation concerning the nitrous oxide emissions (NOx). Bradley and Droney [10.18] assessed NOx emissions for hybrid-electric engine variants designed to power the “high span truss-braced” short-range aircraft reference described in Section 10.5. Results obtained show that during takeoff and landing phases, hybrid-electric engines could achieve, thanks to propulsive and thermal efficiency gains in combination with improved thrust lapse characteristics, diminished NOx emissions in comparison with the CFM-56, the baseline turbofan powering the Boeing 737. So much so that NOx levels achieved by hybrid-electric engines, estimated in the range of 7.5% to 11% of CAEP1/6 levels, literally outperform the “not-to-exceed” goal of 20% of the same levels.

As far as cruise conditions are considered, for the hybrid-electric aircraft with engines operating in the “balanced” hybrid mode, wherein both turbine engine and electric motor sections are operated in a balanced manner throughout the mission, NOx emissions are close to the 80% reduction goal. When resorting to a hybrid-electric engine capable of the “core shutdown” mode, meaning that the turbine engine section gets cut off at a certain point during the mission leaving the electric motor to operate on its own, the aircraft has essentially no NOx emissions over approximately 50% of the cruise segment of a 900 nm mission.

As of 2010, direct greenhouse gas emissions from aviation account for more than 2% of global emissions. Nevertheless, this hardly represents a significant share of the 14% of global emissions attributable to the entire transportation sector.

1 Committee on Aviation Environmental Protection of ICAO.

FIGURE 10.32 General Aviation noise performance (turboprop vs. DEP) [10.25].

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Climate change is a global issue that needs to be tackled from many fronts. In the aerospace industry aircraft and engine manufacturers, their supply chain, airlines, airports, air traffic management services, research institutes, and civil aviation author- ities have been working towards a common objective of reducing the overall impact of aviation on the environment.

In order to reach ambitious environmental goals in a limited timeframe, government policy [10.26] and funding have been concentrated on research in this area. As such, Europe is at the forefront of atmospheric research and has taken the lead in the formulation of a prioritized environmental action plan, and the establishment of global environmental standards.

In 2011, a European group of experts set out a vision of European aviation with the publication of Flightpath 2050. In response to this, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) produced a Strategic Research and Innovation Agenda (SRIA) in 2012 that defined the path to reach these ambitious goals. This vision would draw upon airframe, engine, system, ATM/infrastructure, and airline operation optimizations. To spur this into action, the ACARE council set the following steps for the industry to follow (Figure 10.33) [10.27]:

• An average improvement of 1.5% per year in terms of fuel efficiency to reach a carbon-neutral situation by 2020 in the first place as an intermediate goal.

• From 2020 onwards, ensure a carbon-neutral growth, assisted by economic measures, to ultimately achieve in 2050 the following reductions compared to 2005 levels:

■ 50% reduction in CO2 (Figure 10.33) ■ 80% reduction in NOx emissions ■ 50% reduction in noise

FIGURE 10.33 Initial aviation carbon emissions reduction roadmap [10.27].

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Recently the European Commission has established a 2050 target goal in its highly ambitious Flightpath 2050 Vision for Aviation. Pushing the environmental agenda even more aggressively further down the line, Flightpath 2050 sets forth steeper industry goals for 2050 in comparison to capabilities of typical new aircraft available in 2000:

• 75% reduction in CO2 emissions per passenger kilometer • 90% reduction in NOx emissions • 65% reduction in perceived noise emissions of flying aircraft

This sets the bar very high on additional new-generation technology to come on top of advances in existing technology including significant shifts in the design approach of both aircraft and engine systems.

Also included are goals calling for aircraft movements to be emission-free when taxiing.

As discussed in Chapter 9, taxiing is an area where manufacturers have already been looking to increase fuel efficiency and reduce emissions and noise through elec- trification. Currently, aircraft use their main engines to move around on the tarmac. As you can imagine, this consumes a lot of fuel. By adding electric motors to the wheels of the plane and powering them from the onboard Auxiliary Power Unit (APU), significant fuel burn reduction can be achieved on ground operations. This allows an aircraft to be able to taxi without using its main engines, offering an attractive fuel saving of around ~3% on ground operations for a short-/medium-haul aircraft.

Sometimes, green solutions do come with economic benefits, but this may be tied to the type of energy used and the related price levels. One such instance is the electric green taxiing system, an incremental electrification solution. Unluckily, relatively low oil prices are not helping the entry into service of such solutions. Indeed, the development of the electric taxiing system described above is currently on hold, despite wide industry support, the reason being that oil prices are so low that it doesn’t make economic sense, at least for now.

On one hand, airlines may undeniably get more profitable in case of oil price drops. But on the other hand, low oil prices weaken the economic prospects of incremental developments based on conventional aircraft platforms, irrespective of whether they use electric or conventional technology, as long as their focus is on fuel-burn reduction. In line with that rationale, in the face of low oil prices, re-engined aircraft platforms may suffer weakened sales growth due to their higher price tag.

On conventional aircraft, as fuel is a major cost driver, bringing fuel consumption down helps indirectly to reduce gas emission levels. But we can’t rely on the industry’s natural desire to decrease fuel consumption in order to reduce emissions, because during periods of low fuel prices the impetus may slacken, or worse, simply grind to a halt.

For the future of aviation, the electric aircraft, whether fully electric or in a hybrid-electric version, is the centerpiece in finding the sweet spot between reining in emissions, boosting energy efficiency, and offering a compelling business case.

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