Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 4.

January 10, 2020, ©. Leeham News: We continue our series why e in ePlane shall stand for environment and not electric.

Before we continue the discussion about low CO2 footprint propulsion opportunities we look into some of the distributed propulsion concepts proposed for electric/hybrid airliners.

In this and the next Corner, we examine the substance in claimed aerodynamic gains and increased efficiency from such concepts.

Figure 1. The Ampere distributed propulsion concept as presented by ONERA. Source: ONERA.

The gains from distributed electric propulsors

The proponents for electric/hybrid propulsion solutions claim one of the advantages is it enables distributed propulsion like in Figure 1. This is an ONERA concept called Ampere, and it will represent the ideas of spreading the propulsion over the wing, by it increasing the aerodynamic and propulsive efficiency of the aircraft.

The other idea which is often presented as enabled with electric/hybrid is the aft Boundary Layer Ingestion (BLI) fan concept shown in Figure 2.

Figure 2. Boundary-Layer Ingestion (BLI) aft fans driven by electric motors fed by turbofan placed generators. Source: JADC with Leeham annotations.

The graph is from a recent JADC (Japan Development Corporation) study which promised a 3% gain in efficiency with this technology.

We will now examine the realism in these ideas. We can do this in two ways:

  • We can have an involved aerodynamic/propulsive discussion where we go deep into the fundamental physics of the concepts. I feel comfortable in such a discussion but it’s very difficult to keep it simple, which is the idea of these Corners.
  • Alternatively, we can examine if these concepts are possible with existing technology and why in such case they haven’t been realized.

We will use the second method as this makes for an easier and clearer discussion. We will start this week with the BLI concept and look into wing distributed propulsion next week.

Boundary-Layer Ingestion (BLI) concepts

Figure 2 shows the concept. The normal wing-mounted turbofans of an airliner are fitted with generators that can deliver an additional 1MW of power per side, which drives a 1MW electric motor/fan combination on each side in the tail of the aircraft.

The suction of the thick boundary layer at the tail of the fuselage into the fans increases the aerodynamic efficiency of the aircraft by reducing the thickness of the turbulent (and thus energy consuming) boundary layer at the aft end of the fuselage.

Now to the acid test of the idea: Could this have been done with today’s technology and if so, why wasn’t it done?

The sobering fact is the answer is yes. And more sobering: it’s more efficiently and conveniently done with today’s technology. Even worse, every airliner for 50 years has a powerful gas turbine where these motors/fans are placed, sitting idle during cruise, the APU, Figure 3.

Figure 3. The APU installation of our typical airliner. Source: Boeing’s Aero magazine.

This APU is perfectly capable to power any BLI fans sitting where the exhaust of the APU is depicted (the exhaust can be placed in the middle of the fans or any other convenient place). These APUs are generic power generators, generating pneumatic/electric or hydraulic power to the aircraft. They are used on the ground to provide the aircraft with these powers but they also work as standby power if needed all the way up to maximum flight level for the aircraft.

During cruise climb, cruise and descent they are not used, so they would be free to drive BLI fans via a gearbox/clutch and short driveshaft during these flight phases. With the present use spectrum, they are not designed to be highly efficient but this is just a requirement/design choice as they use standard gas turbine technology. The weight/volume and cost consequences of making them efficiently drive BLI fans during cruise would be marginal. No new technology or knowledge is needed.

Despite being almost for free and sitting there for 50 years on every airliner in the world, the aft BLI fan idea has not been realized. It hasn’t even been tested to my knowledge. If the gains are so obvious as the electric/hybrid studies show, why is this the case? The volume, weight and system complexity of the fan power source is already there on our airliners?

And it’s not because the benefits of boundary layer suction/ingestion are recent knowledge. This is known since the 1950s and its use was more popular then than now. It was implemented on both military and civil aircraft. But the gains are not that large. It’s been better to spend the money, weight and system complexity elsewhere on the aircraft.

And if we adopt the JADC and other BLI electric/hybrid aft fan ideas we have to find a (less ideal) place for the APU. With two 1MW electric motors, it gets crowded in the tail.

So instead of adding a gearbox/clutch and short shaft to the APU we now shall add 1 MW to each engine generator. Then a 3000V electrical distribution system to the rear, pumping 330 Amps per side to motors driving the fans. The propulsion chain loss of this is between seven and 13% dependent if we use cryogenic generators/motors or not and there would be additional weight, system complexity, and cooling requirements.  But hey, it’s a good idea, it’s electric!

This shall be compared with less than 0.5% gearbox losses for the drive from the APU and marginal weight/cost consequences.


The conclusion from this lacmus test is clear. This concept, if it had the large gains that are now put forward would have found its way onto our airliners 30 to 40 years ago. It hasn’t.

And it’s not because we didn’t have a suitable distributed propulsion system available. The perfect power source was there since the 1970s. Time to think instead of just buying into what is presented as enabled with electric technology for airliners.

Next week we will look at wing distributed propulsion.

32 Comments on “Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 4.

  1. You should also consider the reverse, a S-Duct configuration, with a single jet engine.

    The singe engine will provide thrust, power to 2 or 4 electric motors on the wings, and generates a and stores electricity on an in plane battery pack which is good for say 30 minutes powering the electric motors.

    How would this look for say an A220-300?

  2. Thx for article and I look forward to the distributed thrust area.One or two electric aircraft that are test flying use this technique so will be interesting to know the logic.
    As for boundary layer ingesting.There have been various illustrations of this over the years and it’s a little sobering to read it’s true ( small) value. As stated it’s a good answer as to why it’s not been used when clearly it could have been.
    I also read a while back that there is a further problem.The article was looking at the upper surface of a BWB.
    They noted that the air pealing of the boundary layer was very ‘lumpy’ and was v hard for the ingesting Fan to cope with ( uneven thrust on blades).
    So yup a non starter as you say.

  3. What are typical power capabilities of an APU? The issue may be that the APU is too unpowered to warrant the weight/complexity of doing BLI with it alone.

  4. Actually on the B717 the fairly big aft mounted RR engines achived this effect, the effect was better than expected. The 717 suffered from an old wing design, had it got a Gulftream GV type of wing its fate might have been different.

    • The B717 was an underrated aircraft. It’s dispatch reliability and maintainability in both engine and airframe is outstanding.

      • Yes, the 717 was the DC-9/MD-80/MD-90 done right except for reusing an old wing design. Most airlines buying Douglas Aircraft instead of Boeing back then are now Airbus customers. Had Boeing quickly decided to boost Douglas wing designs starting with the 717 and made a fly by wire/modern systems 717-300 the stream of customers jumping to Airbus could have slowed down (Boeing calculated that 3 aircraft families fighting for the same market would depress prices and probably was the 737NG cheaper to make than the 717-200), still having a Fly by wire 717-300 in production in Long Beach today would have helped Boeing keeping cuotmers happy under the 737MAX groundings.

  5. Bjorn, thanks again for another good article. The main argument for BLI is the reduction of drag in the area behind the tail, as the air has been re-accelerated behind the aircraft, removing the drag-inducing depression in the velocity profile. That notion is similar to bleed air in projectiles, which greatly extend their range. Are you saying that effect is not sufficiently large or justified, for a highly streamlined airliner?

    • I’m saying the aeronautical world has had 50 years to ponder if it’s worth doing and it hasn’t been done. Then comes full electric or hybrid. Their cases are not very convincing when doing the sums (I’ve done it, it was sobering, go back to my electric airplane Corners from 2017, nothing has changed since). So the proponents need further benefits and suddenly schemes that have been dormant for decades magically bring big benefits and electric/hybrid holds lots of promise. Go figure.

      • Ok, thank you Bjorn, for responding. I guess all old ideas become new again, and the lessons have to be re-learned. It’s good to have a long-term perspective.

      • I looked into this a bit more. For a modern tube & wing airliner, the after-body form drag is 3% to 4% of the total aircraft drag. This is the component that a BLI propulsor would reduce. So the benefit is not large for the required cost.

        The benefit could be doubled if podded engines are instead blended into the after-body section of the fuselage. Again the potential benefit is not enough to justify the issues associated with such a design change.

        For future blended-wing or multiple-bubble designs, the potential becomes much larger because the form drag becomes a larger percentage of the total drag for those cases. So possibly BLI will need to wait for that.

        In the meantime research continues on distortion-tolerant turbofans as those will be required for BLI with a turbulent boundary layer.

      • I still expect some kind of hybrid propulsion to make sense.
        I see a good case on helicopters. IC engine plus e-storage
        provides for single engine out and the energy boost demanded for hover ( limited time though).

        for airplanes the power boost provided by add n electrics
        reduces design thrust requirements for takeoff.
        This then works towards higher efficiency in cruise.

        • I agree in principle. It’s only when you put in the volume and kg for the battery system (you have to include all components, not just the battery cells) you see the consequences. Your top-power assisting battery system is now eating into your payload BIG time.

          Do the sums. An airworthy battery system has 0.24kWh/kg by 2025 according to Airbus UAM chief, Puerta. You need at least 30min assist or the assist system becomes an unacceptable hazard for the Pilots.

          The typical single-aisle turbofan develops 2*23MW from the LPT to the fan at a takeoff power setting. Say you want to shave off 20% (otherwise you don’t gain anything in cruise efficiency for the turbofan) This equals 9.2MW for 0.5 hour = 4600kWh/0.24=19.2t. Your typical payload is 13t, your battery on top of the normal empty weight is +19t.

          The problem with electric airliners only surfaces when you do the sums. I initially thought it had some merit, then I did the back of the envelope check. HORROR!! Those who peddle it hasn’t even done this check.

          • I presume this calculation is for the case of a turbofan being replaced with an EDF (Electric Ducted Fan). If we reduce wing loading we could reduce power loading and thereby lower the power requirements greatly, couldn’t we? Also a propeller is more efficient at take-off and further reduces power requirements.
            I did a simple calculation for conversion of a DC3 to battery power and found that through substation of fuel for an equivalent weight in batteries and consideration of the lighter motor we end up with a 4 ton battery with capacity of 1MWHr to supply 2 x 890KW engines. This is good for about 34 minutes of full powered flight. I appreciate I haven’t considered conductors, battery packaging, firewalling and cooling but then nor have I subtracted the weight of the no longer needed fuel tanks and lines nor the lighter structure and propeller needed due to less vibration and weight. Scaling up by a factor of 7 to get a 160 passenger narrow body weighing 50 tons we would need 5.4MW for each engine. Obviously using composites saves much weight, drag and allows higher aspect ratios. Some kind of ‘niche’ market with a commercial range of less than 400NM would seem viable eg for Sydney-Melbourne, Dublin-London, Stockholm Western Europe.
            There are maybe some other possibilities such as Q-STOL very quiet operation that might allow a new type of airport with a small footprint.

  6. There is a possibility of an aircraft with EGTS (electric ground taxiing system) that might be combined with an significantly upsized battery to completely replace the APU. So long as the aircraft is “more electric” (electric pressurisation, air conditioning, deicing, electohydraulic actuators) it may work out with future high energy batteries of around 500-800WHr/kg in the labs now. The engines could recharge the batteries but some recovery should be possible when the fans/props windmill during descent idling as some electric aircraft do now. The idea would however be to recharge on the ground when connected to the grid.
    It seems to me that there is a potential to use electric ducted fans for local laminar flow control for purposes of STOL rather than just minuscule fuel savings. For instance the ShinMaywa US-2 Flying Boat uses an APU to provide boundary layer control which gives it the ability to land on 20ft swells. This is mission critical and transcends economics and perhaps the laminar flow control APU can be replaced by electric motors. Electric flight will find its place in eVTOL and eSTOL for short range urban mobility where runway size, safety and noise are paramount.
    Slightly off topic but still relevant. The airline industry is approaching the PR aspects of emissions wrong. Air transport is 12% of transport emissions but 2% of total emissions (5.5% for sea shipping). It’s insignificant. The elephant in the living room is industrial processes. Production of ammonia, cement, calcination, iron and steel, aluminium, smelting, agricultural traction, industrial drying will all easily yield vastly greater reductions in emission at far greater efficiency and lower cost. What tackling these technically easier issues will expose is that the costs of wind and solar makes it somewhat a farce and nuclear needs to be restarted.
    I’m convinced by Bjorn’s articles which to me are saying is that the various electric and hybrid electric aircraft don’t work out with foreseeable battery tech because the increased weight negates any emission gains.

  7. I think you will find Shin Maywa themselves don’t really agree with you there. It is BLC over control surfaces for low speed handling, effectively STOL. I don’t think electric fans can match piped high pressure air forced out of narrow slits, can be turned on and off and be placed in front of flaps and other control surfaces. Indeed ‘the fans’ could increase drag

    • If you reread my post you’ll see I gave the ShinMaywa US-2 as an example of boundary layer control used for purposes of STOL. In the US-2 a 1017kw LHTEC T800 gas turbine weighing 145kg provides the power for the boundary layer control compressor. How much weight would it take to replace this with an electrically powered compressor? The Siemens SP260D Aviation Electric Motor generates 260kw with a mass of 50kg which is a ratio of 5.22kw/kg. It would thus require 2 x 195k = 390kg of generator motor combo to replace the 145kg LHTEC T800 gas turbine. The system cost of boundary layer ducting, exhaust ducting, gearbox not counted. There is some system weight saving from distributing boundary layer compressor.

      Specification for the Schübeler DS-215-DIA HST® EDF (electric ducted fan) with DSM10066-290:
      Inner shroud diameter: 195 mm
      Fan swept area: 215 cm²
      Weight incl. motor, wiring,
      Plug and Secure Fan Fix: 3400 g
      Thrust range: 215-250 N
      Exhaust Speed Range: 84-98 m/s
      RPM range: 12.000-14.000 U/min
      Input power: 9,8-15,6 kW
      Allowed battery: 12-14S 20000 mAh
      Overall efficiency: 78 %

      NB Flying boats have the problem that their rotation angle is limited. Hence they tended to use Göttingen Go Airfoils even after the NACA 4 and 5 digit airfoils became available. Rotation angle probably explains use of boundary layer control.

  8. One of the 2019 Nobel Prize winner who is behind Li-Ion batteries, think it was Akira Yoshino, said on TV that within 2-3 years Li-ion batteries without liquid electrolyte, half the mass and double capacity will start getting into production. As they can be aircraft certified they will help the economics of electrical powered short range aircrafts and various electrical quadcopters.
    He made no mention of cost and life though….

      • I know, still the exact number of future capacity with the new design is yet unknown but it will be positive…

    • Professor John Goodenough invented the Lithium Ion Battery. He wasn’t happy with his invention and eventually he found what he was looking for in the work of Maria Helena Braga in conductive glass. New batteries based on conductive glass solid electrolyte promise to be fire proof (critical in aviation), have 2-3 the energy density, be able to substitute cheaper Na for Li and have 5 times greater life. There are things afoot. If electric flight can be viable for 500NM (long battery life) then it will cover city pairs such as Dublin-London or Warsaw-Stockholm. Even if it doesn’t reduce emissions maybe we’d do it to shut greenies up?

      • Professor John Goodenough was one of many scientists on the path to a working LiIon cell.

      • Looks very promising, we will see how much of its potential end up in certified Aircraft batteries. Just the combination of increased engergy content and reduced mass is really beneficial for Aircrafts. The Life of these new batteries are very important, can they do 20 000 recharges and only lose a few % capacity it would be great

  9. Ignoring boundary layer effects, there is a much simpler way to employ the APU during flight. One could simply use the generators in the main engines as electric motors, powered by the APU. If the APU fails, they would revert to working as generators.

    The main engines could thus be weaker and smaller, while the outer fans would remain about the same.

    Yet another idea would be to use the APU for full takeoff power, perhaps also during climb, then switch it off during cruise.

    The main engines would become cheaper because of reduced power requirements, while the APU would probably have to be spruced up a little.

    Another advantage would be that in certain cases of engine failure the fan of the failed engine could be driven electrically from the APU and from the other engine. After all we now have something like a three-engine plane.

    Efficiency advantages would be small, resulting mainly from the smaller main engine cores, but the other advantages may still be significant.

    Ideas, ideas …

  10. On the Glenn research center site they mentionned about BLI the following technical issue :
    “It sounds like a simple design change, but it’s actually quite challenging. Boundary layer air flow is highly distorted, and that distortion affects the way the fan performs and operates. These new designs require a stronger fan. These new propulsor designs require a specialized inlet to help straighten out the swirling flow before it gets to the fan, and a stronger, more durable fan to resist the constant pounding be applied by the flow distortion.” Why did you not mention such issue ?
    In your very interesting solution ( increase of engine generator power to 1MW to power BLI fans at the back through very high voltage circuit ) there might be some other “system redundancy” concerns of this Distributed propulsion system:
    – in case of Failure of above turbofan generator do we rely on the relocalized APU generator to run BLI fans?
    If not ,such BLI concept would not allow in my mind a significant downsizing of the wing- mounted turbofan.
    May I have misunderstand the definition of your solution ?

  11. Mike, the BLI propulsor is meant to recover lost energy from the form/pressure drag of the fuselage. It’s not meant to provide a substantial fraction of the aircraft thrust, or to provide redundancy.

    Roughly speaking, a 110 kn LEAP 1B engine generates about 20 MW of motive power. So twin engines are maybe 40 MW, while the BLI propulsor as described is 1 MW (or 2 MW if 2 propulsors are used).

    So no downsizing of the podded engines. The 787 generator systems can produce about 1 MW of electrical power from both engines, smaller aircraft like A320 and 737 much less, possibly 150 kw or so. So they might need to be enlarged.

    As Bjorn says, the APU could also be used, and possibly scaled up to provide all the BLI power.

    The turbulent air distortion research for the BLI propulsor is on-going, it’s one of the probelms to be solved.

  12. Rob, thks for all your explanations. Considering current 787 figures ( 2005 technology ) the proposal in this article is increasing from roughly 1.4 MW ( engins+ APU) to 3MW the global on board power in order to get 1or 2 MW from BLI.
    I do not know the weight increase of such 0.5 MW VSCG ( 2020 technology) but if kva/ kg ratio decrease by half it can lead to a neutral balance on this issue. After you have to consider the global BLI system installation weight ( including APU relocalisation -which is already very well optimized on the tailcone- plus 3000V feeder liner weight).
    To recover 2 MW lost energy , a difficult global weight balance to reach for me when you start from an existing classic 787 configuration even if technology improvements in 15 years brings weight saving. May be the intermediate conclusion of this Part 4.

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