Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 5. Distributed propulsion.

January 17, 2020, ©. Leeham News: We continue our series why e in ePlane shall stand for environment and not electric, where we now examine the gains with electric/hybrid distributed propulsion systems.

We started last week with the type of boundary layer ingesting aft fans shown in Figure 1. Now we continue with wing mounted distributed propulsors.

Figure 1. Boundary-Layer Ingestion aft fans, driven by electric motors. Source: JADC.

The gains from distributed electric propulsors on the wings

The proponents for electric/hybrid propulsion solutions say there are advantages of using distributed propulsors mounted on top of the wing as in Figure 2.

We are here showing an ONERA concept called Ampere but there are other similar concepts which are researched at present.

Figure 2. Distributed electric propulsion creating a higher lift wing and a higher bypass ratio (really lower specific thrust) propulsion. Source: ONERA.

The concept uses distributed electric propulsors to energize the aft wing boundary layer and by it creating a higher maximum lift for the wing. The propulsors also create a higher total bypass ratio which leads to a lower specific thrust. This increases the propulsive efficiency.

The concept is similar to the Coanda effect STOL transporter that was produced by Antonov during the 1980s, with 50 still in service (Figure 3). Boeing also made a similar prototype aircraft, the YC-14, when competing for what became the US Air Force C-17 transport aircraft (developed from the conventional McDonnel Douglas YC-15 competitor).

Figure 3. Antonov AN-72 STOL transporter from the 1980s using the Coanda effect. Source: Wikipedia.

There are advantages to the concept but also disadvantages. Instead of arguing these factors we will use the same principle as in the last Corner to test the realism in the gains the idea shall bring. We will test if the idea can be realized with classical mechanical means, and if so, why didn’t it go beyond research and the single production aircraft using the idea of an energized boundary layer, the AN-72 transporter.

The realization of wing distributed propulsion with electrical and mechanical means

The concept in Figure 2 can be realized with a gas turbine which drives a generator. This then feeds the electric motors of the distributed propulsors through an electric power distribution network.

To do this we need an electrical distribution concept like last week’s aircraft, containing high voltage cabling (to keep the current down and by it losses) and inverters to condition the right electrical waveforms for the motors.

Alternatively, the propulsor fans could be driven over a mechanical power distribution system. Such systems are active on all airliners flying today. They are used to drive the slats and flaps from a central motor over shafts and gearboxes, Figure 4.

Figure 4. Slat and Flap actuation systems for a 747. Source: Shimadzu Corp.

The system shown uses centrally placed hydraulic motors. In our case, it would be a gas turbine that would drive the torque shafts over a gearbox like it’s done for Boeing’s CH-47 Chinook (Figure 5) and Bell Boeing’s Osprey dual-rotor helicopters (Figure 6).

Figure 5. The Boeing CH-47 helicopter with rear engines driving the rotors over a mechanical torque shaft distribution system. Source: Wikipedia.

When one examines the weight of such systems they come out at about even, with the power to weight ratio of the generator/motors similar to aircraft gearboxes of the same power rating. The electrical distribution network and inverters weigh about the same as the torque shaft and angle gearboxes needed in the mechanical system.

Where they differ is in the efficiency losses. If we use a cryogenic central generator and high-efficiency motors on the wings we have a total chain loss to the fans of about 9% for the electric system (the gas turbine and fans are the same in both cases). The losses for the mechanical system would be around 5% end to end.

There are some troubling issues in the electrical system which are not present in the mechanical system, however.

The most troublesome is the safety of the systems. One of the major issues with a distributed electrical system like in Figure 2, is the safety aspect according to the NASA report “A Review of Distributed Electric Propulsion Concepts for Air Vehicle Technology”. The sudden loss of part or all of the propulsors from a fault in the distribution network, inverter/s or the generator could have fatal consequences for the aircraft.

The mechanical system has been used for the last 70 years with good reliability and risk profiles.

A good thought experiment is if someone would fly on a Bell Boeing Osprey (Figure 6) that would use an electric link between the propellers should one engine stop while in hover flight. I wouldn’t volunteer for such a pilot job.

Figure 6. The Bell Boeing Osprey V-22 helicopter with a torque shaft joining the two engines/rotors in case of a one-side engine problem. Source: Wikipedia.

We can see distributed propulsion concepts are realized already today, with a mechanical distribution of the power. This is safer and more efficient than if we use an electrical system.

Yet, no project exists which has realized a distributed propulsor concept like in Figure 2. It has stopped at the STOL transport AN-72, a concept with two propulsors. When a power distribution system is needed for the safety of a twin-rotor helicopter a mechanical system is used.


It’s clear electric and hybrid/electric airliner concepts have problems with weight, volume, and efficiency. To fix these problems and argue for their advantages the proponents mix in new propulsion concepts in the proposals.

As we have seen these “new” concepts could have and would have been realized decades ago if they had convincing characteristics. Realizing them with existing mechanical power distribution schemes would be cheaper (no research and development needed, it’s all existing technology), more efficient and in the case of the BLI fans with lower weight consequences. For distributed propulsor concepts, there would be a major difference in safety.

The feeling that grows when checking one concept after another is: It’s first decided the airliner has to be electric or electric/hybrid and then the justification quest begins. It’s a matter to find a propulsion concept that is exciting and opaque enough so it hides the fundamental problems of electric/hybrid so one can get investor/research money. The ever-present mantra is: it worked for cars, therefore, it will work for airliners.

Let me state again: the fact it works for cars doesn’t mean it works for airliners. The use cases and sensitivities for installation effects are dramatically different.

Now, this is for the record: I’m not negative about electric propulsion. In fact, I studied electrics and electronics during extra years at University as it has so many technological possibilities. I have kept a keen interest since and I spent 11 of my working years in the World’s largest electrical company, Siemens before I returned to aeronautics.

It makes all sorts of sense to go more electric for our airliners. But the keyword is more electric, and in a process where it has to earn its place each step of the way.

It shall not be in some after-the-fact justification process why electric/hybrid aircraft will be with us soon and fix all our efficiency and environmental problems. No-one gains from such over-inflated expectations. Our CO2 problems are grave enough to merit the best and most serious solutions. And for the foreseeable future, the solution for our airliners is not electric/hybrid.

In next week’s Corner, we will look at concepts that have more promise than electric/hybrid.

50 Comments on “Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 5. Distributed propulsion.

  1. Dear Bjorn, with your knowledge in aeronautics and electrical engineering, could you please discuss concepts for improving the Pipistrel Electro light-sport aircraft for longer range and greater payload? For example longer and slender wings, retractable landing gear, changes in propulsive configuration, and so forth? – Thanks!

  2. “A good thought experiment is if someone would fly on a Bell Boeing Osprey (Figure 6) that would use an electric link between the propellers should one engine stop while in hover flight. I wouldn’t volunteer for such a pilot job.”

    But isn’t that exactly what virtually all the forthcoming rash of urban air mobility electric/hybrid vehicles will use – an electric-only link between battery/hybrid power source and the propulsors? A power supply interruption on one side only would be catastrophic. How do their manufacturers propose to certify them?

    • They will likely use a ring or mesh or grid power distribution network for redundancy. As Bjorn said, this will result in weight and efficiency penalties, but for those craft the power levels are much smaller, and the main attraction is mobility, at whatever cost.

      Much of that technology can’t be readily scaled up, they are already pushing the limits of what is essentially smaller drone technology. So for now they will likely remain more of an enthusiast or personal technology, maybe eventually branching out to short-hop air taxi.

      • It is also about cost. Lower volume model specific and so more expensive to certify and produce mechanicals vs higher volme more widely used electricals. The electrical design in itself may, and the more widespread use of similar components will, also lead to lower maintenance costs and more access to suitably qualified technicians.

    • Lilium’s EVTOL has 36 EDF and 6 battery packs all with appropriate redundant and fault tolerant electrical distribution. Volocopter with 18 rotors has almost the same level. While Lilium and Volocopter are all electric wheras SureFly is a quadcopter (actually 8 electric motors and rotors paired into 4 coaxial pairs) which is powered by a single piston engine driven generator that nevertheless has batteries to provide about 5 minutes of controlled flight in case of failure in the generator system. All of these will have ballistics recovery parachutes. I don’t think anyone would make an electric tiltrotor modelled on the V22 Osprey as it doesn’t consider the weaknesses and strengths of electrical propulsion. At the minimum each rotor would be replaced by a pair of individually driven coaxial rotors. The reality is that a sort of 4 wing canard quadcopter tiltrotor is more appropriate.
      My own view is that should the climate alarmists win aviation will turn to synthetic hydrocarbons made from CO2 and hydrogen. The CO2 is efficiently extracted directly from the air or from concentrated sources such as cement or steal works. The process is now being referred to as “Power to Liquids” abbreviated as PtL. Also referred to as electricity generated green kerosene or PtL kerosene. It’s already happening. Here is a reference from the German Government but its happening in many countries particularly the Netherlands and Switzerland. Such a fuel will not be as cheap as mineral oil but it is likely to be affordable for aviation though fuel efficiency will be paramount. Personally I believe we will need to turn to fission for most of our energy needs.

      • Hot burning of PtL kerosene would only solve the CO2 issue, but not the environmental impact of the NOX exhaust.
        To eliminate the NOX exhaust, you need cold burning in a fuel cell.

        What Bjorn skipped in his analysis, is that electric propulsion allows different redundacy concepts. Electric wiring allows it to group the redundant systems symetric on both wings. Furthermore, electric motors can be regulated within much shorter response time. Two unique characteristics of distributed electric propulsion, which can’t be done with mechanic power distribution.

        Together this means, that there is no longer a need for a rudder and the elimination of the rudder will increase aerodynamic efficiency!

        • Much shorter response times ?
          What would that be used for, so they can stop and start again. The reason why existing turbine powered aviation has efficiency wheeled transport can only dream about even when all electric is the constant running at most efficient speed.
          A flying wing doesn’t have a tail or rudder, I don’t think you have thought about what rudders or the tail they are attached to are used for.
          But your comments are typical of the barely informed hot air of the very hype which is clouding any discussions of these matters.

        • I do not believe there is a serious gas turbine environmental NOX problem. A recent paper strongly concluded that California’s NOX photochemical smog problem came from nitrate fertilisers not combustion engines. The NOX panic is reminiscent of the ‘acid rain’ panic where it was claimed that sulphur pollution, primarily from coal combustion, was leading to deforestation of pine trees. It turned out the problem was due to changes in agricultural land use. To this day many organisations, including a shamefully, a history of the EPA, make themselves heroes for solving a problem that didn’t exist. NOX is often presented as a potent greenhouse gas but what is seldom mentioned is that its half life persistence in the atmosphere is short. Of course we should strive to reduce NOX and Sulphur emissions anyway so that the industry can go and because one day maybe in 2050 the population will be 14-21 billion instead of 7 billion. The only countries that have controlled their birth rate are still all increasing in population due to immigration from countries that haven’t, a pointless tragedy for the people of the receiving counties.

          Technically I agree with you, there is the possibility of fuel cells, perhaps turbocharged ceramic units capable of 85% efficiency but closed cycle brayton cycle turbines have a much lower NOX than traditional open cycle brayton (gas turbines)

          Power vectoring is also a great idea from the point of view of being able to generate a sidewise crab motion against cross winds while landing. A Handley Page H.P.42 Horsa is the perfect configuration! It is easy to see how a yaw force can be generated and a pitch force is only a little harder but I suspect roll control may best be handled by flight control surfaces though even here drones manipulate the speed of their rotors to generate a yawing torque it should be possible to use coaxial props to generate a roll.

          • The US is aware of highly efficient EU designed car diesel engines reducing fuel burn by 15-25% and part of their high efficiency is combustion at high pressures helped by the fast and precise piezo electric fuel injection and variable vane turbocharging, like on the latest Mercedes diesel engines. The drawback is high NOX generation at many operating points, by holding NOX emissions rules very strict the US can stop the flood into the US of these engines and helps its domenstic car industry. The EU used to help its industry by a leanient test cycle and not including high power Autobahn operations into the NOX measuring drive cycle. So NOX regulations are highly political.

          • I would say the opposite is also true. NOx regulations in the EU have to recognize the large number of diesel vehicles operating, so have not advanced as quickly as they could in the US, with the dominant reliance on gasoline engines.

            Also with technology (Volkswagen not withstanding), NOx can be controlled from diesel, so it shouldn’t be an issue to import those cars if the technology works well.

            An issue in the US is the constantly changing cost of diesel vs gasoline. At times diesel is significantly more expensive, so the cost benefit becomes reduced or erased. In the EU, fuel cost is controlled to favor diesel.

            Also with hybrid tech, gasoline engines can come close to the performance of diesel, gaining torque and fuel economy, but the benefits are not as great for hybrid diesel, since they’re not as complimentary.

            Diesel still makes the most sense for large engines running at steady speeds, so it will always be valuable. The case for smaller car engines at variable speeds is not as strong.

          • The VW emissions scandal is highly political. When portable emissions testing equipment was applied to a range of 25 relatively new diesel cars in the UK around the same time as the US it was found that VW was not even in the worst 50% and that only two (German) manufacturers fully complied. I suspect everyone was ‘cheating’ the test a little or that the VW system was actually more robust in long term use. Happily VW is now exceeding the US requirement by 60%. Roughly these cars were exceeding overall California emissions of NOX by a factor of 4 in real world. But as stated nitrous oxide pollution from fertilises use/misuse is actually bigger problem. The ex VW CEO got in to Jail Time not because he ordered the cheat (that was done by a few engineers in a sub contractor on their own initiative) but because he rolled out a solution to the market place that he announced to the stock market but that actually didn’t work enough. He’s up for securities fraud type thing. Some of the overall optimistic things Mr Muilenburg stated about MAX return to service seemed to skate close to this. The B737 MAX grounding may end up costing as much as the VW emissions scandal and the latter didn’t kill anyone. Using the UREA system is very effective at reducing NOX.

          • Jet Engines have made great progress in reducing NOX emissions and the latest Engines are just at a fraction of allowed limits. Modern car diesel Engines depend on Adblue injection to split NOX into N2 and O2 and I am not sure how effective it is in all driving conditions. Trails are beginning on small effective coolers develloped by British Reaction Engines on jet Engines to cool the air thru the compressor and hence have cooler air for cooling, more efficient compressor work and higher temperature rise in the burner, all this increase efficiency, power density, max speed and reduce emissions.

          • William, the VW and other manufacturers emissions likely killed tens of thousands. From The Guardian (

            The European Environment Agency said the UK had 11,940 premature deaths in 2013 from nitrogen dioxide (NO2), a toxic gas mostly caused by diesel vehicles and linked to lung problems. The number is down from 14,100 in 2012, but still the second worst in Europe.

        • Just wanted say that I agree from many points of view electric propulsion via a fuel cell makes much sense. Solid Oxide Fuel Cells that are turbo compounded to take advantage of the 1000C operating temperature to raise efficiency from 60% to 85%. PtL synthetic hydrocarbons fuel with fuel cell electric propulsion represent an ideal.

  3. The electrical driven Aircrafts and quadcopter will start with short range. One key is the capacity, cost , life and mass of the battery packs. The other is a automatic flight control system where you can klick on the screen where you want to go and the Aircraft connects with ATC computers to take you there automatically, you can just press a panic button on route and it will land you at the nearest alternative landing spot. I bet the first applications will be from San Fransisco Airport down to Silicon Valley and London City Airport to helipads in the city. Maybe also DXB to helipads in the city.
    For Commercial jets the new Boeing SUGAR truss/high wing design will probably set the pattern for the rest to follow. Would be interesting to read a Bjorn analysis of it for 220pax and UDF/Geared fan Engines A321neo competitor.

  4. “When one examines the weight of such systems they come out at about even, with the power to weight ratio of the generator/motors similar to aircraft gearboxes of the same power rating. The electrical distribution network and inverters weigh about the same as the torque shaft and angle gearboxes needed in the mechanical system.”

    This is the key argument made here, but it isn’t universally true for all aircraft, and especially not for smaller aircraft. The use case matters.

  5. Hello Bjorn,

    The appeal of electric propulsion is not in the efficiency gain (as it is in cars). Rather, it is an opportunity to phase out the use of fossil fuel. Whether or not aircraft can be made electric, I’m fairly certain that the aviation sector will be forced to stop using fossil fuels by the latter half of this century.

    Given that petroleum-based jet fuel will simply become unavailable by the latter part of this century, what is the viable alternative to electric propulsion? Hydrogen? Biofuel?

    • “Simply unavailable” Based on what ?
      Do you also claim that plastics or concrete and steel will become unavailable too as they are ‘forced to stop using fossil fuels’. Just because an 17 yr old claims stuff doesnt mean its true.

      in a slightly related matter, when the Royal Air Force was created in 1917, near the end of WW1, one of the reasons given was that this new military service would replace the the existing services which would ‘become obsolete’. Some even claimed later in the 50s that ballistic missiles would make ‘airforces obsolete’.

    • The point however that these articles have been making is that the massive weight increases more than eliminate any gains from battery power and its efficiency as a storage medium. For instance using PtL kerosene derived from nuclear or renewables is costly in the sense that it is perhaps 60-65%% efficient to generate and will then be combusted in a gas turbine of 40-45%% efficiency giving a 25% however the aircraft is enormously smaller and lighter than a battery aircraft and overall will use less energy.

  6. Hybrid power.
    A commercial helicopter friend of mine was extolling the potential advantages of an electric driven main ( and tail) rotor system such are the complexities/weight of the present mechanical systems.
    Furthermore if the electrical generation is fed via a smallish battery then in the case of an engine turbine failure you would /could have ( say) 5-10 mins of power in which to land the machine.
    The total package may well be simpler/cheaper/safer than the present engineering solutions.
    Not sure if any OEM is looking to go down this route with helicopter.

    • Doesnt have any idea of the weight of batteries required and those that are certified for aviation …his quest to get rid of ‘weight and complexity’ means it wouldnt get off the ground. Even a ’10 min battery’ would take all the current payload.
      Not sure ‘your friend’ has much idea on how helicopter blades work either as complexity is part of what gives them lift and horizontal thrust. A main gearbox is a simple and reliable unit and maintenance work is always much greater in aviation and that wont change.
      Its another instance of the Tesla effect and mistakenly thinking cars in traffic are like aircraft operations.

  7. Hi Bjorn,

    Your articles on the topic of electric/hybrid propulsion and advanced propulsion concepts are music to my ears. I agree with your conclusions. Finally, someone who thinks similarly!

    Many ideas look great in concept but when you dig into details, they do not appear to be any more feasible, either due to cost effectiveness or in the aviation field, safety, efficiency and redundancy issues. That does not mean one should not look at exotic concepts like distributed propulsion or “all-electric” planes. One day one of these ideas may pay off big but … Not every idea becomes reality, probably a few percent ever reach practical application status. Things like boundary layer suction to increase max CL of the wing have been researched since 1930’s but have gone nowhere.

    Now for an example. The BLI concept of “cancelling the wake of the fuselage” and hence reducing frictional drag. Wonderful in theory but can we build distortion-tolerant turbofans? We take extreme care in current turbofans to make sure we feed them as clean and distortion-free flow as possible. Compressors don’t like non-axisymmetric flow, because they can cause rotating stall, which would severely stress the blades. And we are deliberately ingesting distorted flow! Perhaps a remedy could be found/perhaps not. And the BLI gain is not that substantial to begin with, to go through all that trouble.

    Electric/hybrid propulsion for passenger airlines is very nearly impossible, unless we make an order of magnitude improvement in the energy density of batteries, and I don’t see any proposed battery technology coming anywhere close!

    Also all this so-called “adverse environmental impact” of aviation is largely overblown. Aviation contributes, I believe, less than 5% currently to CO2 pollution and even in the next 2 decades, it can grow to perhaps less than 10%. There are much worse polluters like cars and trucks and power plants, especially those burning coal. Yet the public is focused on aviation as contributing to global warming! Ignorance id dangerous! We need to tackle those worst polluters before blaming aviation. Don’t get me wrong. I am all for reducing carbon footprint, but aren’t we looking at the wrong perpetrator? We have done a wonderful job increasing both airframe and engine efficiencies (that is the only way to cut down on CO2 emissions) over the past few decades. What has power plants and other polluters done in those same decades? Talk about barking up the wrong tree!

    Anyway, thanks for a wonderful series of articles on teh topic.

    • Kant, typically concepts will need a good amount of development work when implemented into prototypes. And that’s a tremendous blessing, because if there weren’t any development work to be done, all the skilled engineers of the world would be awefully depressed.

      To put it slightly differently, we haven’t reached the end of history yet. We do have a future to live!

    • Aviation contributes at most 2% of global emissions and 12% of transport emissions. Sea transport is close to 5.5% of global emissions. It is indeed overblown but I argue counter productive to ppur massive resources into aviation when the same money will reduce emissions elsewhere far more successfully. Recently Dutch (Netherlandish?) researchers were looking for a source of CO2 for the production or PtL kerosene (electrical Power to Liquids) and found that the nearby Tata steelworks was generating enough CO2 for production of half of Amsterdam Schiphol airports fuel requirements. All sorts of industrial processes such as iron smelting (can be reduced with hydrogen instead of coke or natural gas), aluminium production (electrodes are graphite and emit CO2, ammonia production through haber bosch, cement calcination can far more easily be dealt with. There is a certain plebeian insanity in regards to reducing emissions in high profile targets such as aviation or even automobiles while the low hanging fruit is ignored. I despair sometimes.

      • “Aviation contributes at most 2% of global emissions and 12% of transport emissions.”

        Look at specific pollution ( i.e. per transported volume/mass )

        12% of transport emissions to do 1..2% of the overall transport work?

        • Even if you completely banned air travel or taxed it so that only the ultra wealthy could travel one will only reduce global emissions by 2%, not only would that be an austere world it would impact productivity greatly as communication collapsed. Moreover the alternatives, such as high speed rail, is not viable except in rare circumstances of large city pairs with wealthy populations about 300km apart. It has some very bad environmental impacts that are downplayed. Put it this way taxing aviation will acchieve little but cost a lot in both money and quality of life. The same effort elsewhere will have several times more effect. Electric cars, trucks and buses, introducing the smelting of Iron by smelting it with hydrogen will have a far more dramatic effect. Collecting the CO2 from cement calcination or aluminium electrolysis and using it for PtL will have far more effect than doing something difficult in aviation. Have you heard of Pareto’s rule named after Wilfredo Pareto?
          The Aspergery 17 year old scold was transported from Europe to America on the family yacht of a infamous trillionaire banking family whose banks own most of the world certified carbon credits. They stand to benefit from cap and trade and hysteria is their friend but our enemy.
          At the moment we cant even provide renewable electricity to replace more than 60%, there are a lot of more pressing things to focus on. If the climate alarmists were at all serious in their belief and not just levers of crisis we would be restarting nuclear to save the planet. As Bill Gates says: one look at Tokyo should let everyone know that running that on renewables is impossible.

  8. Cryogenic Generators: ?

    Huge plants yes, aircraft? Phew

    The Russian copied or stole the the YC-14 design !

      • Bjorn clearly knows his stuff having worked with Siemens. There is, of course, the case of cryogenic superconductivity. However lowering the temperature of a conductor lowers its resistance considerably without superconductivity. If one used cryogenic hydrogen fuel to cool a conventional motor or generator prior to its combustion or use in a fuel cell the resistance of the windings of any motor or generator goes down by a factor of over 100 and one can build much smaller motors and generators. Cryogenic Nitrogen would be very effective as well. I suspect about a 5:1 reduction in resistance. Such low temperatures are also beneficial to insulators and electronics and I suspect improve magnetic permeability. Cryogenic nitrogen is probably not to hard to generate.

      • Aidan, by cryogenics I believe Bjorn is referring to superconducting generators, that would have near-zero losses but require cooling to stay below the superconducting transition temperature. They are wound with High Temperature Superconductor (HTS) materials.

        The newest second-generation HTS materials have a transition temperature around 75 kelvin, which can be maintained by liquid nitrogen. Since the resistive losses of superconductors are effectively zero, an insulated liquid nitrogen reservoir can be sufficient, with either a small vapor capture and reliquification system, or a small chiller. Or if the reservoir is large enough, it might sustain the flight with evaporative cooling alone. Nitrogen vapor could be used for fuel tank inerting as well.

        This is new technology and the first commercial systems are still being built. There is interest for aviation because superconduction allows devices to be much smaller and lighter for the same power, but then they also have the cooling issue. I think Bjorn was assuming they could become viable eventually, so as to make the best case for electric propulsion.

        But to answer your question, the goal is to lower the generator winding resistance to zero, so there will be no heat produced, and thus none to reuse or convert to useful work. The idea is to keep heat out, so as not to exceed the transition temperature. The device will suffer a destructive magnetic quench event if any superconducting component shifts into normal conduction. So a lot of development is still needed.

        • Thanks Rob. This is what I described with cryogenic generators. This is the term used by the people in the trade it seems and it lowers the typical losses of a generator/motor from 5% to 2%, but is more complicated and is still to be perfected.

          And yes, I applied it to the central generator to put the electric hybrid in the best possible light, otherwise, we have losses of 12% versus 5%.

          • Here is a good overview of the cryogenic motor – generator technology as of 2017. There is a discussion of aircraft applications, aswell as wind turbines. A lot of development still needed to meet the goals.


            The typical tube & wing design of airliners doesn’t lend itself well to superconducting electrification. Blended wing-body designs are more feasible as size/pax are increased, to accommodate the internal components.

    • “The Russian copied or stole the the YC-14 design !”

      Always interesting to see select persons projecting their own way of life onto others.

      • Coanda was Romanian but worked in Germany ( before WW1), Britain ( chief designer at Bristol) and France.
        Not unusual for Americans to think they invented everything. Even the Smithsonian Aerospace neatly avoids saying carbon fibre was an invention of a British research institute by jumping from Thomas Edisons carbon filaments to a Nasa spacecraft, leaving the reader to make the connection they want.

        One of his principles is used for the inflation of aeroplane slides.
        “A small stream of a high-velocity fluid could be used to generate a greater mass flow, at lower velocity.”
        The small quantity from a cylinder of very high velocity gas released through a small orifice draws in a much larger quantity of air to inflate the slide quickly and makes it rigid enough for passengers to use

        But Henri Coandas 1910 plane , an experimental aircraft driven by a ducted fan brings us right back to the current day- along with saying it could be something ( a jet engine) which it couldnt. Now thats the core of this topic too!

  9. I think this particular corner is interesting and a little different than previous offerings. Previous corners have been a tutelage of physics and engineering principles relating to aeronautics and the mathematics used to prove their validity.
    This corner has all of that woven into it, with the addition of some exasperation by Bjorn as to why electric propulsion aircraft are not following a rational development process. It’s frustrating for a competent engineer.
    There are many reasons why a company/corporation would spend resources on a project with little chance of commercial success. It could be for the visibility it gives the company in the press, sometimes cheaper than buying advertising. It could be incompetent management. It could be wanting to hit a home run and and beat all the competition, viewing incremental product development as a loser’s philosophy.
    The structured approach is supposed to be pure research, product research, and then product development. The risk is supposed to decrease with each stage. This process is not always followed. I’ve seen a lot of money wasted over the course of my career because of these issues.

  10. I think Bjorn is pointing out that alternate electric technologies for aircraft are not feasible at present. They are still far too early in the development cycle, and have many hurdles ahead before they could become feasible.

    So the focus for now should be on improving existing fossil fuel technologies. There are still many unexplored avenues available for that.

    That doesn’t mean research shouldn’t continue on electric technologies for aviation. But we have to realize it’s still research, at this point.

    Also the issues in aviation are unique, so this conclusion doesn’t means electric technologies are not mature and ready for other land-based applications. Obviously they are, or we wouldn’t have electric cars. But the current technologies for those things don’t translate well to aviation.

    • IATA (International Air Transport Association, the Industry Body) has set industry targets of a 50% reduction of emissions by 2050. This challenge is raised by what must be close to a doubling of air travel by then. The IATA 50% reduction conforms reasonably closely to Paris Agreement and ICAO (International Civil Aviation Organization, UN body).

      My estimate in improvements in airliner efficiency are about 20% from conversion to turboprop right now, maybe 15% reduction in SFC per ehp by improved hot cores incorporating heat recuperation, intercooling and higher temperatures, maybe 10% lighter airframes and 20% from better aerodynamics such as true laminar flow. That compounds to about 50%. This is about half of what is needed because of future growth of air transport.

      The remainder will need to come from PtL Power to Liquids which uses CO2 DAC (Direct Air Capture) and hydrogen to generate synthetic hydrocarbons.
      (I see biofuels as a band aid)

      Nordic Blue AS is building a PtL plant for commissioning in 2020 for the production of 8000 tons per year of carbon neutral oil using the technology of Sunfire GmBH and Climeworks AG. It will consume 20MW of hydro electricity (about what 10 of the largest wind mills will produce on average).

      Efficiency should be around 45-50% and the production costs seem to be about Euro 1.7 (maybe Euro 1.1 in Norway due to cheap hydro).

      At current consumption of 2 Litres of jet fuel per passenger per 100km for a B787-9 or A321neo at optimal range we would need 120 Litres costing about Euro 240 to get the 6000km across the Atlantic. Expensive but affordable. In consideration of a potential 50% reduction in fuel burn over the next 30 years the cost gets down to 120 Euro/Seat.

      The process of course can be dramatically improved, the real world improvements seems to be to 65% and direct thermochemical processes promise more. Thermochemically driven PtL using heat direct from a hit temperature reactor would have stunningly low cost. Else there are offshore wind resources and sunshine in Western Australia.

      I’m curious that EGTS “electric ground taxiing system” hasn’t made it into service.

  11. While this idea is rather Rube Goldberg-ish, electric-powered planes would not need to carry batteries capable of the entire flight if their batteries were recharged by drones mating with the planes as needed.

    Could such a concept yield a viable long-distance electric-propulsion plane in advance of better batteries/denser hydrogen gas storage media/better fuel cells?

    • A stratospheric catenary would be more appropriate.
      Suspended from balloons? 🙂

      • That system can be further simplified by replacing the plane with the balloons or sliding the passengers along the arc of the (very long) catenary between airports.

        My original question is perhaps better stated as ‘Would in-flight refueling of electrical energy change Bjorn’s analysis?’ He eliminates electrical propulsion systems based the weight of the systems required, but there are experimental hydrogen storage systems with higher energy density than jet fuel, which when combined with light fuel cells would greatly the weight of the power generation equipment.

    • Ken, what you’re suggesting is akin to aerial refueling. Although that is routine in the military world, it’s not been done for commercial aviation because it has elevated risks.

      If the transfer of energy is electrical rather than fuel, the flammability risk is reduced, but others still remain.

      Another issue would be charging time. Refueling generally takes under 10 minutes. So the recharge time would need to be on that order.

      • Rob,

        The 10-minute recharge capability has been developed (cite below). If drone-carrying batteries could safely mate with planes without adverse aerodynamic impact there wouldn’t have to be a limit.

        I agree there is a degree of risk, but as you note, with electricity transfer the risk is less than that conventional refueling.

        I would hope that recharge drones communicating with the plane could cut the risk to near zero, but I posted the idea as a thought experiment:

        Are electric planes carrying 500+miles of battery power practical now, achieving increased distance by in-air recharge?

        A paper published in the October 30, 2019 issue of Joule showed that a simple design change to Li-ion batteries allows them to recharge to 80% of full capacity in 10 minutes. After 2,500 recharging cycles the batteries had only lost 8.3% of their capacity.

        The Pennsylvania State University team responsible is working on reducing the time required to 5 minutes.

        Easier read:

        Full paper:
        Asymmetric Temperature Modulation for Extreme Fast Charging of Lithium-Ion Batteries

    • Too silly for words… beyond silly even.
      It easier to land and recharge, and for long range range planes land and refueling can actually save fuel if they do so aproximately along intented route.
      Thats what cargo planes do now on long haul routes , so they can carry more cargo they carry less fuel and have stopovers. Anchorage is still a popular airport for trans pacific cargo flights to refuel.

      • Dukeofurl,

        The landing/takeoff cycle would dramatically slow the flights at currently available battery energy density, making the exercise pointless.

        • Some batteries allow exchange of the electrolyte with the electrolyte replenished or recharged. The electrodes are either not consumed or replaceable after a dozen uses. Vanadium and Aluminium Air. Complicated in that several fluids are being moved in multiple directions.

          The interesting thing about electric flight is that it is anaerobic. Elon Musk made a comment about a supersonic transcontinental airliner he ‘invented’ he didnt’t let much on but with some advanced battery tech it is conceivable. This is how I think it would work.

          1 Use eVTOL or sSTOL to ensure there is a small wing and undercarriage.
          2 Climb to very high altitude where there will only be minimal parasitic drag. At 100,000ft parasitic drag is minimal but Mach 2 flight should be efficient.
          3 EDF (electric ducted fans) or Turbofans do work at up to Mach 2.8.
          They can either work supersonically or slow the incoming air down to subsonic, put it through the fan and then accelerate it again.

          I really don’t know of what power densities he was thinking of. 750W.Hr seems achievable but there is talk of fullerenes greatly increasing this.

  12. this review is so biased, nobody wants to use this with a kerosene fuelled 747

    you ignored onera because is probably out of your reach, a round ducted array mounted in the wing as a boundary layer suction is probably one of the most efficient aircraft propulsion systems to be used at low speed air transportation for urgent deliveries rather than for stuffed slave tourist planes.

    747s and concorde sub or hypersonic jet engines aircrafts are gone for good with that era of WASTE.

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