Bjorn’s Corner: Electric aircraft, Part 3

By Bjorn Fehrm

July 14, 2017, ©. Leeham Co: In the last Corner, we developed a hybrid aircraft propulsion system and looked at system efficiencies. Today, we look a bit deeper at how hybrid propulsion can be implemented on an aircraft.

What are the advantages on an aircraft level, that such a chain can have? Can aircraft level efficiencies compensate for more parts and lower propulsion efficiency of a hybrid systems?

Figure 1. Zunum aero regional airliner. Source: Zunum aero.

Hybrid propulsion

In last week’s Corner, we developed the hybrid propulsion system in Figure 2.

Figure 2. A turbofan compared with a hybrid propulsion chain. Source: Leeham Co.

The chain contains more components than for a classical turbofan. We examined the hybrid chain by looking at the efficiency differences such a chain has compared to a Turbofan.

On a chain of the mid-next decade (with superconducting generators and motors) we would lose around 6% in overall efficiency compared with a turbofan with the same core and fan components.

“The same core and fan components” is a limitation that an electric aircraft shall not be constrained by. By clever partitioning and sizing of the components, the hybrid can gain back efficiency. Here is why.

Propulsive gains

The typical twin-engine airliner has its engines sized by the extremes of its flight envelop. The continued take-off with one engine inoperative is such an extreme, as is the one engine inoperative go-around after a missed landing.

The last part of the climb to cruise altitude (top of climb) is also stressing for the engines. Minimum altitude limits (over mountain ridges etc.) when an engine is lost can also be a sizing case for the engines.

Today’s turbofan propulsion on the twin-engined airliner lose half of its power when an engine breaks. This forces the engines to be larger than needed for normal operation. With a battery as energy store and a more flexible configuration of the propulsion units (motor+fan units), one can avoid having half the propulsive power disappear from a fault in a propulsor.

With three or four propulsors, a third or a quarter of the propulsive power disappear if one of these have a fault, instead of half.

Another area that can be sized more efficiently would be the gas turbine core. The smaller the core, the more difficult it is to get the core efficient. With two turbofans in the 15,000lbf thrust class, the size for the typical regional airliner, we are close to the size of the core where the last axial compressor stages have to be replaced with a radial stage.

The axial compressor blade dimensions get too small and we get high tip clearance losses for the last stages. A radial stage must replace the axial ones. It’s less sensitive to small dimensions.

But a radial compressor at the end of the chain lowers the compressor’s overall efficiency and pressure ratio.  With a hybrid setup with a battery to offer energy backup, one could replace the two smaller cores of the turbofan airliner with a single larger core, driving the generator. This is done by Zunum aero in Figure 1.

The efficiency of the core is improved as its larger size makes it suitable for an all axial compressor design.


With a single larger core, an alternative propulsive redundancy scheme must be found. If the core stops, the airliner must continue its flight. The airliner’s batteries can work as a redundant energy source to the core and generator. Batteries and inverters would be duplicated to provide redundancy.

It remains to size the aircraft’s aerodynamics (induced drag) so batteries as an emergency energy source have sufficient power level and endurance to cover the take-off and go-around case.The minimum altitude and emergency landing from cruise cases must also be covered. These will be tougher nuts to crack, as the energy endurance requirement will be higher.

Aerodynamic improvements

The turbofans of a regional airliner are typically placed on the wings. With their high weight (mass really) the propulsion units (nacelle, turbofan and pylon) are placed where lift is generated and close to the main landing gear forces.

But the wing placement of turbofans is not ideal from an aerodynamic perspective. The engines disturb the flow around the wing and the high-speed fan flow does nothing to improve the aerodynamics of the aircraft. If anything it creates interference drag.

The dominant drag component of an aircraft is the air friction against the airliner’s surface. Laminar flow air generates half the friction of turbulent flow. On a normal airliner, the transition from laminar flow to turbulent boundary layer flow is happening after less than 10% of the surface of the wing/empennage.

With a suitable designed wing profile and propulsors sucking a tired boundary layer off the aft wing/empennage, an electric propulsion system can be designed to keep laminar flow longer and thereby lower the aircraft’s drag. A similar effect is achieved with sucking a tired boundary layer off the last part of the fuselage.

These effects will need new research into the best placements of propulsors. Research of more laminar flow wings and other aircraft parts is ongoing, as is research into boundary layer ingestion propulsion systems. Figure 3.

Figure 3. NASA concept for a boundary layer ingestion aircraft. Source: NASA.

Sizing of an electric regional airliner

Our next step is to look at size and weight for the components in a typical regional airliner application. How much heavier is a hybrid setup? Can it be motivated with aerodynamic gains? This will be the subject of the next Corners.

22 Comments on “Bjorn’s Corner: Electric aircraft, Part 3

  1. Do you have an analysis of trade off between wing-mounted engine and tail-mounted engine when it comes to structural efficiency? I read somewhere that when Boeing designed the 737, they did a comparison and concluded that putting the engines under the wing make the airframe lighter as there is no need to transfer the thrush to the wing when the engine is on wing. The rear fuselage will only be under tension in this case. When the engine pushing the fuselage from behind, the rear fuselage also have to resist compression.
    What change have advanced materials since then that changed the equations?

  2. for a proposed lighter than air “zeppelin” in the 70ties the propulsion intake port was placed into the nose center as frontal dynamic pressure buildup is dominated by the central part of the nose.
    Sucking there does thus not suck 🙂

  3. Hi,
    What about fuel cells? They are evolving very quickly (I didn’t know that there are already commercially available fuel cell electric cars and they solve the battery weight problem.
    The hydrogen takes lots of space (plenty available inside the wings, tail and stabilizer) but weights very little

    • Hydrogen is hard to store.
      For fuel cells the future will be Methanol.
      Either as Direct Methanol Fuel Cells (DMFC) or with reformer producing Hydrogen right when needed.

    • Hydrogen can be stored in anhydrous ammonia more densely than compressing pure hydrogen itself. An energy density less than but not very much less than kerosene can be obtained.

      Ammonia can be used in a fuel cell or combusted conventionally – its an ideal fuel for a hybrid vehicle.

      This article concentrates on compressors as if they would be similar to those in a conventional gas turbine engine. I’m not sure that they would be. The mass flow down a gas turbine core is far in excess of the requirements to burn the fuel – most of the gas is nitrogen, which doesn’t figure except to provide a working fluid for the engine, and – I recall – only about 2% of the oxygen is used in combustion.

      In a fuel cell, you’d expect as much of the oxygen as possible to be consumed in a reaction with hydrogen. So for a given energy output, you’d only need about 2% of the core mass flow of a gas turbine.

      Technologies are starting to exist where sheets of graphene are given holes that let one molecule through, but not another. Already they can separate fresh water from seawater this way.

      So here’s an idea – use that technology to separate oxygen from nitrogen, compress just the oxygen (2-3% of what would be required in a gas turbine). Use the increase in temperature of the oxygen to heat fuel (ammonia) so that it can be reformed to produce nitrogen and hydrogen. Or to put it another way, use the fuel to cool the oxygen while it is being compressed, reducing the energy required to do the compression. Separate the nitrogen from the hydrogen (decomposition products of ammonia) and pump high pressure hydrogen into a fuel cell with high pressure pure oxygen.

      This will create a lot of electricity.

      Pump the exhaust products (which will still contain oxygen and hydrogen in small quantities) to a conventional burner and gas turbine that is also fed with ammonia.

      Result – a hybrid powerplant with no CO2 emissions.

      Admittedly, a lot of technological development remains to be done.

      • Thank you, Chris! I found your comment highly inspiring, which to me is an unusual feat in today’s evolutionary aeronautics.

        I also found that you write about some other cool stuff…

      • Spilling a bunch of kerosene or methane or hydrogen is one thing in an accident, but ammonia?!? We’re starting to talk about railroad tank car disaster there.

        • Rick – Ammonia is already manufactured and transported in large quantities for the fertilizer industry – it is stored at far less pressure than hydrogen (150 psi rather than thousands of psi) and though it does burn (it was used as the fuel for the X15 hypersonic aircraft flown by Neil Armstrong in the sixties) it is much less flammable than kerosene.
          It is toxic, and has an unpleasant smell – true – but essentially its as safe or safer than other fuels.

    • At today’s efficiencies you need a huge fuel cell to create enough electricity to push a aircraft through the air; wing size and inches thick. Depending on type they also operate at elevated temperatures, which makes locating them in the wing problematic from an incendiary point of view. If that’s not enough, the most efficient, lowest temperature ones need bulky hydrogen to operate, for which there is zero aviation infrastructure. All this to enable use of unproven electrical propulsion in the most safety critical equipment on the planet.

      How many problems are too many?

      • I agree – the idea of fuel cell powerplant is a futuristic prospect – the arrangement would have to be small enough and light enough to fit in an aircraft and I would expect to see the technology power ships and trains before it was ready for aircraft.

        I don’t think compressed hydrogen is a good fuel – I wouldn’t want to fly in a passenger aircraft or even drive (or crash) a car that used it. Ammonia is much, much safer, its already in mass production, it can be easily transported by road and it packs more hydrogen in than an equivalent volume of compressed H2. Ammonia at high temperature can be broken into hydrogen and nitrogen using the appropriate catalyst. So although the fuel cell would be hydrogen – oxygen, the hydrogen would only exist as hydrogen at the last moment, and never in a large quantity.

        Ammonia does have to be stored under pressure (150psi I believe) and that would make storage in the wing box problematic – it would probably have to be held in cylindrical tanks hanging from under the wing.

        I imagine the fuel cell itself to be held in the centre wing box – where the centre tank of most airliners is currently – and oxygen and hydrogen separators in the wing and perhaps the dead spaces of the fuselage.

  4. @MHalblaub

    “Hydrogen is hard to store. For fuel cells the future will be Methanol. Either as Direct Methanol Fuel Cells (DMFC) or with reformer producing Hydrogen right when needed.”

    Liquid hydrogen would be an excellent fuel and coolant for a subsonic derived version of the Scimitar* engine concept from Reaction Engines. In a distributed hybrid LH2/electric propulsion system, a single large subsonic gas turbine — derived from the Scimitar engine — would generate electricity to power electric fans by producing thrust from both the E-fans and the gas turbine engine. Because the peak power of an aircraft engine, which is presently only used at takeoff, is generated by both the single gas turbine and E-fans running on power from either batteries or fuel cells, the single gas turbine can be significantly smaller than it is today. Hence, the gas turbine engine integration into the aft fuselage would seem to be eminently suitable.

    Furthermore, the list of reciprocal positive influences can be continued. This includes superconductivity that causes certain materials to lose their electrical resistance when they are cooled to temperatures well below minus 100 degrees Celsius. Motors and cables that transport the power of the turbines and batteries/fuel-cells to the fans could be designed to be much lighter, smaller and more efficient. Cooling the components using LH2 which is already onboard as a fuel is, therefore, a no-brainer.


    The Scimitar precooled Mach 5 cruise engine has been derived from the Reaction Engines SABRE engine designed to propel the SKYLON SSTO spaceplane. These engines are able to achieve high-speed air-breathing flight by using hydrogen fuel as a heat sink to lower the temperature of the decelerated inlet air so that it can be compressed and managed through combustion by relatively conventional turbo-machinery. The fluid flow is entirely subsonic while passing through the cycle, being supersonic only in the intake capture and nozzle acceleration phases.

    Through clever thermodynamic design, it is also able to cruise effectively at Mach 0.9, with a range close to that when in supersonic mode. This not only allows the vehicle to come out of supersonic flight and continue at subsonic speeds during for example equipment failures; but more importantly it allows the vehicle to fly overland subsonically, eliminating the noise issues due to sonic booms. A subsonic exhaust at take-off has also been incorporated even though it is operating with reheat at the time, it therefore fully meets all current civil aviation legislation, hence overcoming a major operational obstacle which hindered Concorde.

    As for the fuel, liquid hydrogen is used for two main reasons. Firstly, it has a large calorific value (120MJ/kg) which allows for an antipodal range if the engine can be realised with near ideal theoretical performance for cruise at Mach 5. For an airframe with an L/D of about 6, this would give a nominal flight time of 4 hours, or less, to anywhere in the world. Secondly, although liquid hydrogen is a hard cryogen (having a low density of 68kg/m3 at a boiling point of only 21K), it has a very high thermal capacity, almost 3.5 times that of water. If stored at low enough temperatures to maintain its state, it can therefore be used to effect the precooling of the air entering the compressor up to Mach 5, while maintaining an equivalence ratio close to that for optimum performance. Hence, the liquid hydrogen fuel allows the engine to be designed to operate relatively unaffected by the increasing Mach number across the entire flight operating range.

    • Liquid hydrogen is forever futuristic. It’s problematic even for space rockets, let alone airplanes. How can an airplane left fueled on the apron for several hours with it? And its volumetric energy density is lacklustre.

      • By subcooling Liquid Hydrogen (LH) there will be no loss of the fuel due to venting.

        Now, LH2 is only “problematic” when used as a fuel in a SSTO or the first stage of a conventional multi-stage launch vehicle (LV), due to the fact that the first stage will have to be much more voluminous than a first stage using kerosene as fuel. LH2 remains superior, though, when used as a fuel in the upper stages of a LV. In future horizontal the take-off space launch vehicles, though, LH2 would be an asset throughout the subsonic part of the flight regime since the voluminous LH2 tanks should help to increase the lift of the vehicle — in contrast to a traditional vertical lift-off LV, where the voluminous LH2 tanks will only add to the drag.

        However, what we’re talking about here is a hybrid electric airliner, and not a LH2-powered conventional aircraft or rocket. A hybrid electric airliner would dramatically reduce the amount of fuel required, and the LH2 could conceivable be stored in a large spherical tank located on the aft part of the fuselage — between the wings and the single gas turbine located at the very aft of the aircraft.

    • Another issue with liquid hydrogen is infrastructure. Producing hydrogen requires lot of energy, or still emit lots of CO2 if produced from methane. It need to be cooled to liquid form which consume more energy. I don’t think I will live long enough to see it.

  5. I do note that two of the tghree aircarf showin in the first picture have two engines.

    Getting 4 means putting back on the wing.

    And oddly, the Hybrids such as Prius and Fords have good records reliability wise (as does the more electric 787) .

    I would not have guessed that, my experience was electronics are bad enough in a good environment. Far Tougher one? No way and they still did it.

    Maybe my electronics are just wimpy and poor design cause they can get away with it.

    It seems if they need to buck up they know they can.

    For our motor inverters, we go 50 to 100% sizes larger to get them to live a reasonably long life.

    • Electronics engineering for a harsh environment is comparatively easy if the components exist with the right temperature and shock/vibe specs;. Where it gets difficult is when you’re operating temperature is outside the spec of every component available…

      So a car isn’t too bad. It can get chilly, say -30C, and quite warm, say +50C, and handles comparatively little power (compared to the 30,00ohp of a big jet). Better still the electronics could be preheated if the low end spec of the components didn’t go down to -30C.

      Aircraft though? -70C to +50C is the target air temperature, which is really tough. Plus getting the waste heat out of it will be hard; several hundred kilowatts have to be lost somehow (that’s the inefficiency in the electronics soaking small percentage of 30,000hp). That’ll need slip stream cooling, and if it ever gets switched off then it’ll chill to -70 (or whatever you get at 35,000ft on the day) very quickly.

      Personally speaking I think a less ambitious architecture is more realistic, ie using a motor/generator on the shaft of a GTF as a top up booster. More easily built, everything is smaller and all of the electrics is non-critical.

      Even more currently, with the very electric 787 they’re not so far away; they have a big generator, which is also the starter? OK, just run the starter during takeoff, and that’s a hybrid right there.

      Not a very good one admittedly, but a lot closer than a system which has no mechanical coupling between core and fan.

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