Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 11. More electric aircraft.

February 28, 2020, ©. Leeham News: We now look at technology developments that make sense, and can deliver real improvements in the near future.

We start in this Corner with what more electric aircraft and engines can bring.

Figure 1. Boeing’s 787, the first more electric airliner. Source: Boeing.

More electric aircraft

The first more electric aircraft is flying our long-haul routes since 2011. Boeing was a pioneer in changing the power distribution systems in an airliner to use electrical and hydraulic power instead of the classical bleed air, electric and hydraulic architecture.

The replacement of the inefficient bleed air system saves energy, which reduces the aircraft’s fuel consumption. The change to two power distribution systems instead of three should also reduce weight, but in this first implementation of a more electrical aircraft, the weight was about equal to a classical approach.

Right now, a more electric aircraft increases the cost of the plane, but this will gradually change as electric power conversion electronics get cheaper and more efficient. Improved power conversion electronics will also reduce the weight of the system.

More electric engine

Research into more electric turbofans has been conducted ongoing since the 1990s. It’s not about changing the main components of the engine to electric ones. A turbofan for a single-aisle airliner develops more than 25MW and for a widebody 100MW. Aeronautical electric motors and generators are at present in the MW class.

Instead, the focus is on the turbofan’s auxiliary gearbox, which drives the hydraulic pumps, fuel pumps, and electric generators, Figure 2. It’s also the place for the engine air starter, using bleed air from a ground source or the APU.

The drive to the auxiliary gearbox is always on whether these units need power or not.

Figure 2. The more electric engine has an embedded starter generator instead of a mechanical gearbox drive. Source: Rolls-Royce and Leeham Co.

In a more electric engine, the generator and bleed air starter is replaced by an embedded starter-generator placed directly on the compressor shaft at the cold end of the engine.

By sizing the starter-generator correctly, it can deliver the aircraft’s electric power but also drive electric pumps for fuel and hydraulics placed off the engine in a convenient place. These pumps can then be scheduled on-demand, reducing the energy consumption of the aircraft.

Additionally, the starter-generator can allow a more optimal design of the engine’s compressors and turbines. Today, engines are designed to function over a broad power spectrum, from ground idle to full take-off power over a wide range of air densities and speeds.

This forces compromises in the compressor and turbine designs, reducing their efficiency. With a powerful starter-generator, the compressors and turbines can be assisted with accelerations and retardations in areas in their operating range where they have stall tendencies. This allows a more aggressive design of the aerodynamics of these parts of the engine, which increases the engine’s efficiency.


The research around more electric aircraft and engines started decades ago. All airframe and engine OEMs are perfecting more electric concepts in different state-sponsored research programs.

Before we see an electric hybrid or full electric airliner, we will see further iterations of the more electric aircraft.

65 Comments on “Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 11. More electric aircraft.

  1. Does the above tally with the facts?
    We have seen two enormous clean sheet aircraft programs costing multiple billions with the expectation that they will become mainstay aircraft for the next circa 40 years.Those being the A220 family and A350 family.They did not choose that inefficient’ route.
    Indeed the new 777x program has a brand new wing and engine but ( as far as I am aware) have not chosen this ‘electric’route either.
    Don’t get me wrong I think it was/is absolutely the way to go and fully in favour of it.
    But right now 10 years on it’s an orphan technology -no?

    • A220 and A350 definition came around 12-15 yrs ago, the 777X maybe 7 yrs ago. Clearly the changes would be for future planes or engines yet to fly.
      It’s all about looking forward – No

      • Kind of an odd way to put it, as the 787 proceeded the A350 and they made the decision to go with bleed air.

        C Series certainly proceeded the 787 but also not proven it would have been a good candidate currently (pun) for more electric due to costs and single aisle vs wide body.

        One area not mentioned is the saving in big tubes with hot air running around an aircraft and the much easier design and routing of wire.

        Frankly I like Pneumatic (applied right – electronics control pneumatic s is a nice mix) but the world is moving to electric and the A350 will be the last new aircraft built that does not do that.

        The 777 was built as bleed air and there were no advantages to making a big change when the fuselage was not going to change to take advantage of it.

        Big costs vs established system in place and the supply chain making the equipment for it.

    • I personally don’t care how an aircraft is anti iced, bleed air or electric, but passengers and crew should not be subjected to turbine oil fumes from failed seals. Turbine oil is a known carcinogen and neurotoxin. We’ll see what legal issues this creates in the future.

      Electric anti icing may allow better anti icing if delays on the ramp are experienced ?

      • Hazard of bleed air though not often a problem. What it does to cabin crews long term???????????.

        MDs with AK had a bad period and I flew one of those. FAA should have shut em down.

        • Swabs of aircraft cabin interiors show that nearly 45% of cabins of turbine oil contamination with the common neurotoxin tricresyl phosphate (TCP) (3%) together with corrosion inhibitors present.

          • Toxic oil fumes is a problem maily from the APU’s, of cause are there filters and activated coal bags but they does not always help. Just look at the Avro RJ-series with the Lycoming ALF engine that was notorious generating oil smoke. Honeywell had to redesign its oil system when buying Lycoming jet engines. Hence fuel cell APU’s might be certified pretty soon.

  2. The gain seems low, as the A330neo is very close to the 787 with an older, less refined airframe, and the same engines, only with bleed air.

    • Correct. The gain for the 787 was low, not the least as the size of that generation of power conversion electronics required an additional equipment room at the front of the rear cargo bay and the heat (all power that doesn’t get converted create heat) has to be cooled off (all cooling intakes cause drag). Let’s assume the efficiency was 90%, meaning 10% of the drawn electric power had to be cooled off.

      The latest power conversion electronics have efficiencies above 95%.

      • Bjorn,

        Do you think we will see electric motors integrated into landing gear powered by the APU for taxi so that main engine start is delayed until just prior to takeoff ?

        I thought that I had seen somewhere there were trials of such systems.

        • There have been trails but they have not been convincing. The operational use of such systems is still not to see. The fuel burn gains and hence CO2/cost reduction is not where it needs to be.

          • I see an application in replacing brakes with bldc generator/motor devices recuperating braking energy or sinking the energy in wing deice surfaces.
            This could be wearless.

        • @JakDak: An outfit called WheelTug has been playing with this for something like 10 years. There have been demonstration taxi tests and they announce LOIs from airlines, but after all this time they don’t have certification. I doubt it will ever fly–so to speak.

          • In general I’m in favor of more electric when the trades all work, but the A350 and A220 entered service after the 787 and the 787 technology was well known to the engine and systems companies years before it went into service. There are multiple benefits from eliminating engine bleed air including engine efficiency and eliminating potential contaminants. However, pipes valves, and filters are reliable and low cost, imposing no extra drag penalty. Perhaps for these reasons even Boeing has not pursued “electric” bleed in subsequent conceptual designs.

            Additionally, the engine gearbox also provides services to the engine, not just the aircraft. Regardless of how they are driven, aircraft primary generators and hydraulic pumps need to be always on, even though the load varies. In flight the load is never zero (hopefully). Embedding a starter/gen into the compression system is doable but a maintainability nightmare. But the idea of combined electric start and gen, eliminating air start equipment, makes great sense and will be standard on new engine designs.

            As pointed out, motors, generators, motor drives, and cables all generate heat that must be removed in some fashion. Then there’s additional lightning, EMI, HIRF mitigations. There are few free rides and everything has trades for weight, cost, reliability, maintainability, packaging, performance, etc. Net unfavorability in the sum of those trades is why the A350 and others don’t use electrically drive hydraulics and high pressure air.

          • Reminds me of open rotor, always a few years down the road.

          • “Net unfavorability in the sum of those trades is why the A350 and others don’t use electrically drive hydraulics and high pressure air.”

            You’d then obviously be surprised what is electric on the A350 ( and the A380 before ).
            IMU Airbus introduced hydraulic/electric backed up units as a first.
            the A350 is a 2 hydraulic + 2 electric fully symmetric fail over system. The 787 ( with 3H 2E ) systems layout shows less orthogonality.

          • The A380 & A350 use what are essentially electric actuators on the primary controls. They are called EHA’s or “Electro Hydraulic Actuators” which are essentially electrical variable speed drives used to drive a reversible hydraulic pump. The pump and servo motor is integrated into the actuator. Should the actuator fail bypass valves are opened which put the actuator into “damping mode” while the EHBA Electro Hydrostatic Backup Actuator is taken out of damping mode. The EHBA is constructed in the same way as the EHA with its own servo motor and reversible pump but also has a hydraulic supply and a servo valve to use hydraulic power. It would have been possible to use geared servo motors but the failure modes and probabilities of gearing and de clutching mechanisms was considered too unknown or risky. No jack screws on ailerons.

      • IMU the A380 started that electric systems approach with its 2H+2E system architecture.

        My personal opinion is that Boeing entered that field with its full scale application too early. 1/2 generations.
        There are “cheaper” industries that push efficiency strongly ( renewables, (terrestrial) automotive ).
        Boeing may have learned a lot about stuff that is superfluous in the next generation of solid state power conversion.

        • I agree but I my guess is their decision to go more electric on the 787 was to Boeing’s expected follow on (737 replacement) as the new design approach on the 797 is/was to its expected follow on (still the 737 repalcement). Basically the only way to gain knowledge, mature the designs and get costs low enough in a commercial aviation, Boeing specific situation, ready for the 737 replacement that never came.

          Probably also partly about spreading moon shot risks across launches.

          • “.. Basically the only way to gain knowledge ..”

            If you work in an intellectual handyman environment: YES.
            If you start from a scientific assessment point of view : NO.

          • I never mentioned anything about scientific assessment. I mentioned “in a commercial aviation, Boeing specific situation”. Engineering. Production. Real people involved. Hands on stuff. Finding issues that no abstracted model can find.

  3. Bjorn,
    Unfortunately the claims that more-electric-aircraft bring efficiency improvements are unfounded. However, there could be other improvements in terms of reliability and design simplicity brought about by such technologies. To address the points you mention, the replacement of a bleed air conditioning system with an electric system does not improve efficiency since the same air-cycle machine is used for both cases and the power losses in the bleed air system are equivalent to an electric system losses once you factor in power electronics, distribution, and the cooling power required for this system. Your second point about more electric engines that a gearbox is on whether aircraft systems need power or not is slightly misleading; an engine air starter typically has a clutch that disconnects after the engine is on therefore it would not extract power from the gearbox in flight. In addition a typical engine-driven variable displacement hydraulic pump has a swash plate that extracts power from the engine gearbox only when needed. So if anything, driving electric generators that in turn drive hydraulic pumps would be less efficient due to chain losses and the need of a minimum electrical power for such pumps to be idling. The last point about using a powerful starter generator to assist an engine compressor sounds like a hybrid electric propulsion system, namely parallel hybrid, correct? are you aware of any such system that has been already implemented or studied and how different it is from hybrid electric propulsion?
    Lastly, thank you for these weekly reads. I now look forward for my Friday mornings.

    • You can have efficient power (LEDs) or you can have less efficient (electric oven vs a Microwave).

      Bleed air has to be cooled off as well.

      But you can also have point of service hydraulic pumps vs running hydraulic lines through an aircraft and you don’t have to run the gear pumps other than when the gear is doing down or up.

      And with the 787 mfg debacle, Boeing has elected to make money rather than upgrades and the cost of those.

    • I agree, the 787 was a bit electrical power for the sake 0f it. Like replacing bleed air with cabin compressors and then feed the air thru cooler and into regular air cycle machines. The 787 Starter/generators system with APU generators require a couple of expensive black Hamiton Sundstrand boxes connected with heavy power cords, systemwise not as bad as Honeywell did it on the MD-90. Next iterations might be a big step forward but maybe not as Honeywell did it on the F-35 aircraft gearbox with “everything” bolted onto it. Maybe the B-21 is the next step forward?

    • Boeing has a different view from yours Ali of the efficiency of its more electric approach for the 787

      Of course the 787 engines arent without bleed air, but its only used for engine functions.
      This gives a more detailed breakdown of the ‘off engine’ replacement electrical systems, these being “The electrified
      functions are wing deicing protection, engine starting, driving the high-capacity hydraulic pumps, and powering the cabin environmental control system.”

      And since you mentioned the cabin air-conditioning system ‘ as being the same as the bleed air system (that would have been )used previously.
      Boeing says:
      “In the 787 electrical architecture, the output of the
      cabin pressurization compressors flows through low-pressure air-conditioning packs for improved efficiency. The adjustable speed feature of electrical motors will allow further optimization of airplane energy usage by not requiring excessive energy from the supplied compressed air and later regulating it down through modulating valvesresulting in energy loss.
      Avoiding the energy waste associated withdown regulation results in improvements in engine fuel consumption, and the environmental-control system air inflow can be adjusted in accordance with the number of airplane occupants to achieve the lowest energy waste while meeting the air-flow requirements.”

      In my own home ac/heating system which is electrical the variable speed motors ( where DC is called ‘inverters’) certainly improve efficiency along with using heat pump principles.

      • Correct, there are losses in bleed air systems due to down regulation of pressurized flow (which can be reduced with proper engine bleed port selection and/or occupant dependent flow schedules), but there are also losses in electrical systems (generation, distribution, control, etc.). Once you also factor in the additional weight of motorized compressors, power electronics, additional cooling systems, electrical wiring, bigger generators, and the list goes on, the impact on aircraft level efficiency would be negligible. But of course Boeing would not say that.
        You home air conditioning system is not comparable to aircraft air conditioning systems, so I would not go there.
        Don’t get me wrong though, I am not advocating against more-electric-systems. I am saying that such systems do not bring major efficiency improvements, if any at all. The focus should be on using these technologies to reduce complexity and to pave the road towards electric propulsion in the future once battery specific energy is adequate for use on aircraft (since conventional aircraft systems would not work with an electric propulsion system).

        • No . You are missing the point . You say “since the same air-cycle machine is used for both cases”
          They are saying there are differences and its not that same. My home unit is more efficient because they use variable speed fans and compressors, compared to older model heat pump tech which didnt , it was either running or not at a single speed.
          Same idea about the APU on the 787, it has effiocency gains as its purely a generator and does supply bleed air at all. Clearly more efficient.

          I forgot the link to the Boeing article from their quarterly magazine.
          787 No-Bleed Systems:Saving Fuel and Enhancing Operational Efficiencies

        • Also, do not forget that using a higher voltage power distribution, you can cut down on the cable diameter, hence the weight of the cables. This at the same time helps improving the efficiency as the power distribution loss due to resistance also drops.

          You can use high voltage electric motors for compressors & pumps with fiber-optic control lines to isolate the control and power delivery systems for improved safety. It’s always more energy efficient running motors when needed than pumping a couple of tons of hydraulic fluid throughout the whole plane.

    • Mike Sinnett gave an interview where he talked about the 787 not achieving as much savings as predicted for the more-electric design. He said it performed very well but the final build weight got away from them. It was a new design and new aircraft often turn out heavier than anticipated. The power electronics and batteries played a role in that, but there were other unrelated weight factors as well.

      If that design was repeated today, it would have far fewer constraints. The cost/size/weight of the electric components is steadily decreasing. Pretty much every forward-looking study recommends the more-electric aircraft (MEA), while not yet endorsing electric propulsion. That more or less reflects what Bjorn is also telling us. It makes sense that this would be an area where significant improvement is possible

      • You can’t leverage PR talk as a replacement to
        an objective engineering approach.
        Additionally Boeing has a thing for attributing “effect X” to mechanism “A” where the real causal chain is set around mechanism “B”.
        Where “A” showcases some uber engineering not accessible to other manufacturers and “B” is what was hard forced from some other design deficiency or hard engineering requirement.

  4. Scraping for ‘e.’ .
    The push for this has become almost fanatical . Unfortunately , this tends to cloud judgement , and engender costly and ineffective actions . These often have undesirable consequences , such as expensive and unreliable systems being mandated/employed , or highly destructive/polluting mining and manufacturing operations being used to produce related materials that have marginal environmental benefit .
    A complete and global perspective must be had , when judging whether or not to pursue/utilize proposed technologies , in regards to protection of the environment .
    *Future tech. will continue to develop cleaner derivatives inherently , thus a patient and measured approach will yield the most beneficial pathways and strategies , without unnecessary reaching/grasping for straws .

    • I see it as Boeing got a start and learned a lot.

      Unfortunately you have to model safe (or should) so you can not take full advantage of the possibilities (reducing the wing join area was a case of cutting too much trying to shave weight)

      I believe they took the 787 wing to 155%, safe build but too heavy. Airbus failed at 147% on the A380, a bit too light but fixable as it broke where they predicted.

      But, you have to start someplace.

      You don’t just one day design a DC-3 and shake the world, the Wrights built an aircraft that was a menace and barely flew, and rapidly the design changed and got better.

      Tail out in front and wing warping were proven to be wrong tech paths but no one knew that until they got it going, then found out what worked better.

      Wrights were so busy defending their patents they forgot you needed to compete and got swept under the rug of history.

      Nothing says Boeing can’t upgrade the electronics but that is also a cost to certify vs staying with what they have.

      Its never perfect, engineering and production is all compromises and trade offs.

      If it was easy everyone would be doing it (yes, a couple are trying but that is not the same as acualy doing it)

      • I agree that Boeing knows more now and has a good starting point with the 787 systems for further improvement in mass, cost and performance. The electronic brakes was a problem initially but probably fully fixed now. Airbus also learned from the 787 systems and its suppliers and in some areas on the A350 moved to electronic power buyond the 787 systems.

  5. Read the news story and all the comments up to here, and I am not sure about one thing: Does the 787 have more advanced technology in her systems or are the A220 and the A350 which came later have more new technology in them? I understand the airframe and the engines, but I wondering about the auxiliary power, etc.,…

    • 787 has PR achievements.
      The other frames show real technological achievements. 🙂

      Most prominently the 787 is a PR guys wet dream.
      Noting more.

      • Sam: Its a bit of a coin flip. Technically A350 is less advanced.

        But an A350 is still more advanced than an A330.

        When you first introduce technology, there is a learning curve.

        My work involved a different form of pneumatics but pneumatics never the less.

        A pneumatic actuator is a simpler and easier to trouble shoot device than an electric /electronic one. I did a lot of converting pure pneumatics to processor control via a transducer and worked very well.

        I also worked on pure elecric diverter that were incredibly reliable and a hell of a lot faster than the pneumatics type divers were.

        So its kind of, a highly developed bleed air vs a less developed but high tech electrical system.

        But you can also design the electronics with feedback and fault codes where a pneumatic fault would be, its just not working.

        Go to the processor, ok, its telling it to work, signal levels are right so then onto the pneumatic device.

        737 is easier to trouble shoot than the latter more advanced A320.

        Over time the electronics/electrical tends to get very reliable.

        Next all new commercial aircraft will follow the 787.

        MC-21, C929 will be the bleed air, they can’t manage the cutting edge stuff so will go with what they can copy and know.

        • Only until very recently pneumatics was not capable of he required precision control due to the problem of airs compressibility and stiction in the actuators. Furthermore unless properly dried the lines could freeze up. Which is no fun at 37,000ft. It probably has some potential for primary controls. I suspect it was used on flaps and undercarriage on the F27.

        • I am not expecting many new aircraft from Airbus or Boeing. I think Airbus might extend to a A322. I think they will then tweek the a350 for Qantas project sunrise order and anyone else wanting a long range plane. Next I expect an a 220 500, then 700, then 900 then 1000. Which will eventually provide a family of modern efficient single aisle Airbus. By the end of the decade I expect newer engines with 25% less fuel burn. I think these can make an attractive a350 neo. After this I expect to see Airbus launching planes that don’t emmit emissions sometime in the 2030s.

          So what is Boeing plan? I think they need to match Airbus. Single aisle airplanes are the best sellers by far. Why not have a common family ranging from 150 seater (1 class), all the way up to 250 seater (3 class). They will still have the 777x flying and the 787 which can be upgraded with new engines and more efficient electronics as an when these features make financial sense. Surely Boeing is not going to try and get the 737 Max is to compete with newer A220 airplanes of the future. The middle of the market plane can be part of this family of single aisle airplanes. From a customer perspective I prefer twin aisle but the single aisle just seem to be so much cheaper to purchase and operate that it makes sense to make something that can out compete a a220 1000

      • “787 has PR achievements.
        The other frames show real technological achievements. 🙂

        Most prominently the 787 is a PR guys wet dream.
        Noting more.”

        “If you repeat a lie often enough, people will believe it, and you will even come to believe it yourself.” 😉

    • The point of Bjorn’s article was that the B787 was the first aircraft to go big on electrical generation, as well as electrical functions that had previously been non-electric. So it serves as an example of where the technology might go. It wasn’t meant as a comparison of Airbus vs Boeing or statement of superiority.

      The B787 has 1.45 MW of electrical generating capacity, with 4 engine starter/generators with 250KVA each, and two APUs with 225KVA each.

      Th A350 has 550 kW of electrical generating capacity, with 4 engine generators with 100KVA each, and one APU starter/generator with 150KVA.

      The A380 has 840 kW of electrical generating capacity, with 4 engine generators with 150KVA each, and one APU driving two starter/generators with 120KVA each.

      Aircraft generators are variable frequency due to the varying engine speeds. Some of the new electrical loads are frequency-insensitive and can be driven directly, others require power conditioning for constant frequency.

      The conditioning electronics used in the B787 are inefficient by current standards, as the technology has progressed significantly. So they require greater cooling and space allocation. Newer generations of aircraft will not have the same penalty. I would expect the A220 to be better in this regard.

      The idea of using the more powerful starter/generator to assist the engine in certain operational regimes, is really interesting. I hope that goes forward.

      • Look at how power distribution is handled in the Toyota Prius Hybrids ( with power take off ( and on ) from both spools you can transfer energy between spools. that is a control “lever” that currently is handled partly via bleed flow control .

        • Uwe, engine manufacturers such as Safran have said that starter/generators could be used to expand the re-light envelope and otherwise assist the engine. It’s a new possibility and so requires research. Moving to the More Electric Aircraft (MEA) will open up new ways of thinking and design.

          The Prius transmission uses a planetary drive which allows for multiple inputs/outputs of torque, depending on which gear train is used as the reference.

          Bleed air relies on the engine operating at a sufficient level for one section to assist the other. That’s fine, but using the starter/generator with an independent power source expands the available options.

          • you are regurgitating what I wrote. .. but missing the point: Toyota uses the 2 electric machines integrated into the gearbox to transfer energy ( in their use case they achieve CVT like behavior.)
            In the turbine use case variable bleed is used to “impenadnce match” airflow from one spool to the other beyond other effects to increase surge margins.

          • Uwe, I’m sure Safran and other engine manufacturers are aware of bleed air options, as am I. The point was that the starter/generator offers some new options. They think so, Bjorn thinks so, I think so. If you don’t, that’s fine, you’re welcome to your view.

          • Rob. You are not understanding what you are replying to.
            ( There is a limit to what can be achieved with leveraging the Schopenhauer’s “The Art of Being Right” ways. Schopenhauer wrote those not as a “How to” but as a “not conducive to discussion” enumeration.)

          • Don’t engage Rob, it’s not worth it! LOL

  6. Bjorn, I was wondering if you know of any efforts to use the starter/generator to rotate the engines after shutdown, to prevent rotor bow? That seems like it might be a good use, the power requirement is very low and might be met by the on-board battery on the 787, or ground power. Could be done with a low-hertz drive or by selectively energizing the individual motor poles, as a crude stepper.

    • Rob:

      Rotor bow is an issue with fast turn around single aisle, not a wide body.

      Obviously its a starter so you can turn the engine with it any time you want.

      Batteries won’t do it, you would start the APU then use its starter now Generator to turn the main engines.

      Normally at the gate you will use a GPU, APU are expensive things to run.

      • Large turbofans can also suffer from “rotor bow” often showing itself as a stuck LPT. It normally happens after shutdown with hot air raising in the engine making the top of some casings hotter and expanding and thus bending the engine casings giving an effect of a stuck rotor. Running on idle a good time or cranking the engine with starter helps getting the hot air out, some new engines are tricky as the LPT does hadly move at low idle and thus does not extract energy making the exhaust gases hot so you cannot reduce idle too much at engine design.

      • TW, to avoid rotor bow you only need a periodic half-revolution of the shaft. You don’t need to spin up the engine to any significant extent. Existing starters may not be designed for that, I’m just wondering if they could be, or if that would be a value.

        You could also spin it constantly but slowly to provide cooling from air circulation. That would involve more power but still not the same as attempting to start the engine.

        Its just an idea, that’s why I was asking Bjorn.

        • I think it’s a good idea. I haven’t heard any engine OEM mention it but I will ask next time I talk to an engine OEM’s advanced development guys.

          • Thank you Bjorn, and also thanks again for this series of articles. Really helps to inform our thinking about the future of avaiation.

  7. At a certain point SOFC solid oxide fuel cells will come into service likely with an efficiency between 60% and 80%. This will replace the APU and battery because it is very much more efficient than a gas turbine make electrical services far more attractive. B787 is already fuel cell ready.

  8. My understanding of the benefits of the Boeing ‘all electric’ 787 was in production efficiency and the skill levels required ( i.e. Putting in a plant with no previous aircraft manufacturing skills – read cheaper).

    Boeing used the phrase ‘clip together aircaft’. I.e. The wings and body came together ‘ready stuffed’ with ‘only’ ( over simplified) wires to join.
    In theory you could then put your final assembly anywhere ( no Unions) and build the aircaft quickly.

    Happy to be corrected but I always understood that this was the big gain -sort of making an aircraft factory more like an automobile plant.

    • Philby, that may have been an element of it. But I think the main driver was increased efficiency. All the studies of the more-electric aircraft have pointed to this.

      With bleed air, you extract energy from the engine in the form of compressor work. The result is compressed and heated air, which you can then route around the airframe as needed. In some cases you only need the pressure so the heat value is lost. In some cases you only need the heat so the pressure value is lost. Those losses are irreversible, so you have to extract more work than needed to overcome them.

      With electrics, you extract energy from the engine in the form of shaft work. The result is electricity which you can route around the airframe with wiring. It’s more readily convertible into the form of energy you need at each location, with fewer losses and irreversibility.

      Another aspect is that bleed air systems are mature technologies, there is not much more improvement to be gained. Electrics are still in the improvement phase and so will get better over time.

      I’m sure there are manufacturing considerations and efficiencies as well. Just not sure those would be the dominant reason.

      • One advantage with Power Electronics vs pneumatic/Hydraulic + Electronic Controls are that you can integrate the controls with the Power Electronics, like for a brake ballscrew actuator or Thrust reverser actuator, still with age Electronics have their problems if not perfectly designed and brazed as any buyer of an old BMW7-series or Mercedes S-klass can tell and reflected into their used car prices…
        You quickly get into massive parallell systems with special software to disconnect non functioning Electronics and risk of confusing the central maintenence computer software.

      • Rob, you are missing an important negative with bleed air systems. The air from the compressors (doesn’t matter from which port) is to hot to route around the aircraft. Therefore, there is a pylon placed pre-cooler in all bleed air systems that dumps a significant level of energy overboard (it’s cooled by fan air, sits behind the fan exit guide vanes). This is an energy loss that is not present in the more electric system.

        • Thanks Bjorn, I didn’t realize there was precooling, but it makes sense. That would be an additional large irreversibility.

  9. Thanks Rob very instructive reply.All seems totally logical and I am totally happy that it is indeed a double whammy both for efficiency in the air and in manufacture-indeed it should lower maintenance cost as well I imagine.

    A general question environmental efficiency.
    Recently their was hope that shipping would reduce av speeds by 10% as this would lower fuel consumption by >20% through drag reduction.
    Since the maths does not change for drag in gasses ( as opposed to liquids) .If all airlines agreed to reduce cruising speeds by 10% i.e. Roughly 550mph to 500 mph.Would that lower SFC thus CO2 emissions by 20%?
    Am sure it’s more complicated than that but thought I’d ask as it would be an instantaneous reduction for zero investment/time.

    • Yes, I think that would be true. Some of the efficiency improvements coming along, such as open rotor, might require a rethinking and speed decreases. It might also involve a remapping of engine performance to take full advantage. So the airlines and the traveling public would have to embrace that. It seem to me like a small loss to absorb relative to the benefit.

      In the US, we had 55 mph speed limits for awhile to conserve fuel, but that went away as vehicles became more efficient and fuel prices fell. Similarly there was a trend toward smaller vehicles for awhile, which now also has evaporated.

      It mainly depends on what people want and see as being important.

    • less speed, less lift, lower cruise levels : more fuel.
      Flying slower could quite well be counter productive.

  10. Yup Uwe that the $65k question.Perhaps Bjorn knows the answer.
    Which is ( I think)
    How much speed margin is there at cruising altitude? At present they fly at 550’ish mph.Is the wing totally designed around that speed or could the aircraft cruise happily at 500 mph at the optimal altitude ? I.e. 30-35,000 ft.
    Historically I believe the need for speed ended in the oil spike in the mid 70’s and wings were optimised slower after that ( leaving us the 747 as the speedster ).
    Anyway that is the key question.
    Perhaps a further question since this is a ‘forward looking’ set of articles.
    Just how much could fuel consumption be reduced if an all new aircraft wing was designed from the outset to cruise at 500mph?
    I guess the development work on fully laminar wings is part of this equation.

    • Philby, I looked into this a bit further. Airlines tend to fly at the LRC condition (Long Range Cruise). The MRC condition (Maximum Range Cruise) gives lowest fuel consumption. LRC gives up 1% to 2% of fuel economy over MRC, in exchange for greater speed. So that gives you an idea of the margins that are possible with optimization.

      Drag always increases with speed, but with turbofan engines the loss is offset by increasing engine efficiency with speed, up until the peak at MRC (around Mach 0.7 – 0.75). After that the drag begins to dominate, until LRC (around 0.8 – 0.85).

      So it may be more related to the type of engine than wing design. Open rotor would require lower speed than turbofan, turboprop lower still. These both lower the Mach number for MRC.

      Studies have shown that optimization of both speed and altitude can yield up to 5% improvement, but there are also real-world constraints due to ATC and congestion.

  11. Thx Rob,
    I guess there is ( unsurprisingly) very little low hanging fruit with existing short term remedies.
    Perhaps a next gen’ aircraft could ( not easy) go both slower -and-higher (say 40,000-45,000 ft) where drag is lower and the air is colder making the engines more efficient.
    But I guess 500mph at that altitude gets awfully close to ‘coffin corner’!

    • Yes, all true. There have been proposals for flying at 60,000 feet but they require either higher speed or more lift area. It’s interesting to learn how all those factors interact to yield fuel economy.

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