Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 10.

February 21, 2020, ©. Leeham News: After spending several Fridays looking at different possibilities and technologies that can lower air transport’s environmental footprint we now turn the discussion to what path forward makes sense and how shall we set our priorities.

Figure 1. The CO2 concentration in the atmosphere and what causes it. Source: Wikipedia. Click to see better.

Firt priority, reset impressions and expectations

There is no question the world is facing a serious environmental challenge caused by the increase in emissions of greenhouse gases, Figure 1. The raise of CO2 emissions over the last 50 years is exponential.

Those who choose to ignore this trend are saying “I live a comforting life that I don’t want to change, any problems I leave to my kids to handle”. Or put another way: our parents worked hard to give us a better life, it’s questionable if we do the same.

Figure 1 also shows the sources of the problem. Air transport is not one of those. Yet the public opinion thinks it is. Air transport is very visible and not part of everyday life.

It’s easy to point to this pseudo-problem when changes in the daily commute (the bulk of the transport emissions) or our jobs (the largest emissions problem, our inefficient industrial processes) are closer and would affect us directly.

So “Flyshame” spreads without rebuttal and the unrealistic claims of the over 200 electric/hybrid projects that these will fix our environmental problems reinforce the notion air transport is the problem area “but hey, we have a solution”.

Nothing could be further from the truth. Air transport is NOT the problem and no, we don’t have a solution with those 200 projects.

Here is what we need to do:

First, we need to reset impressions:

  • Air transport, as is, is highly efficient. Our latest aircraft are as efficient as car transport despite transporting us at much higher speeds (the required energy for movement increases with speed).
  • As air transport has already achieved very high efficiency, any further gains are hard-won. It requires a major investment and will give limited environmental returns. Investments bring higher gains in other areas.
  • The natural driver for research and development in air transport is aligned with the environmental issue. Fossil fuel is the largest cost factor for an airline and there is substantial pressure to bring the fuel consumption per passenger-mile down further. This is 100% aligned with our key environmental problem, CO2 emissions.

We shall also bring expectations in check:

  • The ongoing development for new airliner generations has a clear path to a lower CO2 footprint. Complemented with alternatives to fossil fuel it will bring air transport’s part of an overall solution for the near future.
  • Electric airplanes will be confined to pilot training and Urban Mobility for decades to come. The weight and safety issues with major size batteries in airplanes have no solution in sight (I have no issue flying on the MAX but will not set my foot on battery airplanes. Most people have no idea of the risk levels these batteries entail. You can drive a burning Tesla to the curbside, an aircraft crashes).
  • The type of hybrids presented makes no sense. They bring weight, losses, and complexity for no real gains in fuel consumption and hence CO2 emission levels.

The above means we need to stop hyping hopeless projects presented by entrepreneurs who want to be the next Elon Musk. The investments shall be directed to stuff that really brings us forward, not inflate a hype bubble.

There are sensible developments in the electrification of flight. It’s called “more electric” and we will discuss these technologies and others in subsequent Corners. These technologies have been part of aeronautical research for decades, however, so it’s nothing new.

The resetting expectations is all about setting targets and directions for what we can and shall do that are challenging but bring real progress. A good example is an update of our ATC procedures, gaining us what a full generational shift of airframes and engines would bring to our 20,000 airliners flying every day.

So there are urgent things we need to do but it’s not about further inflating the electric/hybrid bubble. In addition to the above, there is more we can do and we will discuss this in subsequent Corners.

47 Comments on “Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 10.

  1. Bjorn, thanks for the read. With the reveal of the Maveric prototype at the recent Singapore Airshow, I was wondering if you could cover BWBs in this series (yes, I am aware of the 2018 article featuring Boeing’s Mike Sinnett). There has been a lot of talk about the drawbacks of BWBs like the Maveric and the X48, namely the need for compatibility with existing infrastructure, evacuation requirements, and passenger comfort when the aircraft is rolling.

    To that end, what are your thoughts about concepts like TU Delft’s flying V? It seems to at least partially address the compatibility and evacuation requirements. I suppose the uncomfortable rolling tendencies can be addressed with advanced control laws and pilot training (turbulence notwithstanding – although turbulence presents an interesting case, since shifting mass outwards should significantly increase the aircraft’s moment of inertia too).

  2. You are extremely right…
    Electric-powered airliners are many years away , if ever .
    Steady improvement in turbine-engines will continue to improve apace . New architectures , materials , and engineering , will continue to increase the performance/efficiency of tomorrow’s jet-engines , endlessly .
    *We’re off to see the Wizard !
    D.H.

  3. An interesting series Bjorn. In the context of your conclusions above, how do you see Wright Electric’s proposal for 186 seat 200 nm all-electric aircraft with service entry in 2030? What “secret source” have they identified?

    • In my opinion, and I seem to be one of the few who has analyzed the problem in dept, none. It’s one of the 200 projects.

  4. The above graph suffers from the all too common economics problem of aggregation. As a Westerner and at a personal level, the only level most of us can act at, restricting air travel is by far the most important change I could make most years.

    I’ve used many CO2 calculators dating back to the 1990s and this has always been the result. Basically, excluding new construction (eg vehicle, property) embedded emissions my significant CO2 effects (domestic energy, vehicle mileage, and food) are all ballpark, just 10s of % apart from each other, and a single transatlantic economy flight would be likewise. Drastically reducing food consumption or indoors temperature are really non-starters, reduced vehicle mileage is somewhat more possible, but by far the biggest impact I can have on CO2 (in fact, couple with vehicle mileage the only significant impact) is any ability to restrict my annual air mileage.

    • Woody,

      you must have missed my Corner where I showed our daily car commute has an efficiency below 10% if we use a smallest engine option mid-sized car. Use an SUV and this halves.

      We can start by selecting our daily cars with efficiency in mind instead of having a 200+hp 2 tonnes neighbor impression tool to accelerate with to the next stoplight.

      • Cars are the low hanging fruit,30% reduction without too much trouble, but I am not able to drive 10000 miles in a single day.As noted aviation is incredibly sensitive to efficiency, its easy increase that pressure and deter unnecessary journeys. Ryanair has stated that its planned new routes are unviable without the efficiency of the the MAX.
        When I first learned to drive, I had 34 bhp available in my lightweight mini and I was delighted with it. In the UK, new car mpg has actually got worse in every one of the last 3 years. This is the most irritating thing about electrification, huge powerful barges that have to lug huge batteries around with them.

        • Or in passenger vehicles in general. Check the l/km / mpg of European market post oil shock, pre safety shock vehicles and, due to lighter (admittedly less safe) structures, narrow rubber, lower A (of CdA) and a whole lot less comfort mass, they are often better than available today.

          My guess is that recent new car mpg decline is more to general SUV/crossover bargeification (increased mass and CdA) than to huge batteries. Fortunately Bjorn’s “impression tool” appears likely now to have some reversing effect, near term, as the still limited range of EVs leads to these fashionable vehicles being smaller and more car like than SUV barges.

      • No, I read that.

        I also have a good understanding of how to reduce my pollution, CO2 or otherwise, and have done so all of my adult life, including in the vehicles I choose to drive (I have never and would never use a vehicle like you describe and have never and would never use one as a status symbol, SUV, Prius or anything) and whether to drive at all. And every time I have checked my footprint, since the 1990s, has shown the same thing. Just a single long haul round trip is ballpark the same as each of the other 3 contributors.

        If I did drive a “200+ hp 2 tonnes neighbor impression tool” and flew 1 transatlantic type economy journey per year I could reduce my overall CO2 emissions by something like 10% if I swapped to a more efficient, smaller engine, lighter vehicle. But I could reduce my overall CO2 emissions by something like 20% if I chose not to take that return flight. Obviously each person’s circumstances are different and I’m also not going below the order of 10s of % here. But these are the facts, facts I have checked repeatedly over decades. If I was a metro resident (eg Paris, NYC, London) and like many there had chosen not to have vehicle access, as many do, the CO2 cost of flying becomes even more of a focus. The 2% in the graph above strongly misrepresents the reality of this personal level, the only level that almost all of us can act on, and thus misunderstands why many people choose to reduce or eliminate air miles.

        • Actually the best thing you could do is cut back on your meat consumption (if you haven’t already) – especially beef. The amount of energy required to turn 8 lbs of grain into 1 lb of meat is huge. Probably could save enough for several trips a year.

          • Cattle eat grass, as they are meant to, where I come from. As a result they are healthy to eat. Much of the land of the world is not suitable for tillage being too rocky or hilly or unable to sustain cropping or even fruit trees due to the nature of the soil and climate. Cattle if properly managed trample grass underfoot thereby ensuring that the grass can’t burn and that the water from rainfall is easily absorbed into the soil. Take the statements of vegan animal liberationists and extinction rebellion with skepticism. Counting the rainfall on grazing land that is often dessert and has low density as “water consumption” is absurd. So is talking of the vast amounts of land used. Soy and cabbage won’t grow there. We can make some incredible hyped statistics and choose the worst ones. To a certain extent the practice of feeding cattle grains is a result of subsidised grain that came out of the imperative of keeping food prices low. Grain, turnips and fodder have a place especially during draught. Were it not for the practice of using food as a weapon via embargo, sanft yin’s or trade or an arm of Tarif policy perhaps grain fed animals would be few.

    • There are a certainly choices we can make as individuals and as local communities.

      We can choose to live closer to city centres in apartments and townhomes rather than in suburban areas. This will allow us make more trips using transit perhaps even forgoing one vehicle. We can also choose to buy smaller vehicles or electric vehicles.

      I understand the this will not work for everyone. But even those who need to continue to live suburban areas can vote for parties that support denser cities, better transit, congestion charges, fuel taxes, toll and other ways to reduce traffic.

    • Woody, there is actually a lot more we can do to reduce CO2 emissions on a personal level.
      1) Commuting: For many years I am now commuting by bicycle instead of car. It was a 35 minute ride for several years and is now a 25 minute ride. Not only are the CO2 emissions reduced by 99%, but I’m also much fitter now than I was 2 years ago.
      2) Traveling: I have just sold my Audi diesel and pre-ordered the new ID.3 from Volkswagen. If the offer will not be good enough I will get me an Tesla 3 instead or maybe a Kia Niro or Hyundai Kona. They all offer a range that is sufficient for most of my business trips and I’ll be happy to spend some break here or there on the way for charging, knowing that’s the best we can do. The CO2 effect depends on the production of the juice, of course, but in Germany we are now heading towards 50% this or next year, and with the next government (that should include the Greens) should keep rising fast.
      3) Besides that we use the high speed trains all over Europe. All around the world more high speed trains are being built. (The US is quite a bit behind in that regard.)
      4) Heating: Our next project is both for our home and our company building to switch from gas heating to a heat pump. Again the CO2 effect depends on the electricity production, but that is improving year by year.
      5) Solar: The last project is that both the company building and our home will get solar panels, both for hot water and electricity.
      6) Flying: I completely stopped flying for fun/holidays/recreation. I only fly when I have to, mostly to international trade shows. I would still fly the same occasions if it was much more expensive. So taxation of fuel should be the same as with cars or buses if you ask me.

      Global warming is not a question if you believe in it or not. We know it’s happening for more than 20 years now. And we need to bring CO2 emissions down to ZERO in about 10 years to avoid the tipping point from where we will not be able to stop climate warming spiraling up and up. Anything contrary to that is either stupidity, negligence or, most times, propaganda from the biggest, richest and most influential companies of today – the oil companies. I have no doubt that they all have troll armies enrolled to work the public day and night. They also “buy” unscrupulous scientists. (Luckily most scientists can’t be “bought”).

      To keep flying we therefor NEED a CO2 neutral solution. Green Methane or Kerosine might be the only short term solution. It could be produced in the deserts. What you need is lots of solar panels, space, water and air. Of that we have plenty. Think of the Sahara, South-USA/Mexico, Gobi, central Australia…

    • The biggest single impact would be proper traffic signalling systems. There is no reason any vehicle should ever waste energy by braking or stop on the way to a destination or sit idling at traffic lights. Vehicles should bunch into “trains” that coast to half speed rather than stop at traffic lights. Without vehicles stopping fuel consumption would halve in cities, journey times decrease and road capacity increase. There is nothing to stop such a system being implemented through cellphones right now, you only need to know 20 seconds in advance when lights will go red and what speed to drive to avoid stopping . This is low hanging fruit. Should have 5 times more impact than decarbonised aviation.

      • Cities are too congested now , the queue for the lights is what stops you not the lights themselves and that will vary enormously.
        Pouring more cars more quickly will just create instant grid lock in a CBD or major employment centre like a large University or Hospital.
        Then there are the roads with signals like freeways/motorways, mostly hopelessly congested in rush hour to be stop- start movement.

        • I have thought of these issues. The main problem is that developers and their council lackeys and social housing folks keep approving High Rise Buildings: 4, 8, 10 even 20 stories without funding underground roads and providing underground car parking for residents, their casual guests or delivery couriers etc thereby cluttering the roads. High rise means underground roads and rail is critically essential. It’s a no brainier. I don’t just mean underground motorways but roads. Must be funded by the developers. Secondary problem is that too many cars are just too big and physically take up too much space. If the majority of cars were hatch backs much of the problems would go away, the little Mercedes Smart car is mocked by idiot motoring journalists for laughs but would dramatically improve traffic by allowing more cars to queue at lights and slip through. Cars also lack compulsory autonomous emergency braking which would allow separation distances to be safely reduced.

    • The psychological impacts are by far more devastating than anything else. The west has fallend into a suicide cult, one of nihilism, demoralisation and anxiety. The scare mongering about “climate change” is now being targeted to children rather than adults because they lack appropriate defense mechanisms and are developing eco anxiety and no longer care to have children. It has become a literal death cult. CO2 increase does not mean climate apocalypse. I have studied the matter. It’s not that bad. Recent data even suggests the Earth has been cooling in the last year. I am one for prudent action but there is no emergency.

      • William, you have obviously not been studying the climate catastrophe in a way that we might call scientific. I guess you have simply watched too much FoxNews. You are only repeating what the oil companies want you to think. The young people that you discard are obviously much clearer in their thinking than you. They are also a lot younger and will have to endure the dramatic consequences that may not fit in your head right now. If I was in your shoes I would now start to think of some very good excuses that you will tell your grand-children some day like “oh you know, but everybody was telling me we are fine” or “but it was too uncomfortable to even think of the consequences” or maybe “but it would have cost some money and I would have to make an effort”.

        I have been fighting all my life against the ever growing use of fossil fuels, though I have to admit that for many years not very actively. But I’m now back on the barricades, as you can tell.

        Go back to studying climate change. This time do it properly. Stay away from all those sources that might be influenced by oil companies, and then come back and tell us what you have learned.

  5. Gasoline has bout 100 times greater energy density that a lithium battery.Yes electric motors are more efficient but the weight of an aircraft ( critical of course for flying!) drops over a journey whereas electrons are massless.So the answer is not replacement but management of flying.
    There are only two ways of managing consumer behaviour.
    1. Legislation
    2.Taxation
    The former would be political suicide for any national government so that won’t happen.
    So it’s taxation.Business people don’t ‘have’ to travel in uneconomical 6+ foot business beds,indeed many meetings can be conducted electronically particularly with 5G and soon to be released ‘Starlink and others’.Consumers? Well perhaps they can fly ‘less’.
    But both can be taxed with that additional taxation being 100% spent on preventing further global warming. (100% carbon offset via tree planting?)
    I believe the 80:20 rule exists for flying.IE 80% of flights are done via 20% of the population.It should be possible to target these people.Perhaps one or two ( carbon tax) free return flights a year after which heavy taxation applies.
    Question.With this immediate climate emergency can flying be considered a right any more or more likely a ( taxable) privilege?
    Airlines themselves could also be taxed on the efficiency of their aircraft.Should a 25 year old 747 still be flying?

    • Fuel taxes are used to pay for roads, bridges & such infrastructure. They are not there for the largesse. Airport fees already pay for runways and terminals. Consumption Taxes (GST, VAT) on the ticket price already suck out an incredible 20% on the ticket price in Europe. It’s a important principle of taxation and economics to not tax the inputs of a business. Lufthansa already pays for 100% offsets and can rigorously prove to remove as much carbon as they put in. This can’t be said of fuel or carbon taxes which no government can prove had done anything. Taking money out of the airlines removes their ability to buy fuel efficient aircraft. The consumption taxes on a ticket are very much dependant on the amount of fuel burned so airlines are already motivated to reduce fuel burn. Airlines spend 3 times more on fuel than aircraft so the “signals” to motivate them are already there. I believe decarbonisation offsets can eventually be phased into the fuel price. CO2 can be sequestered, offset or it can be a carbon neutral synthetic hydrocarbon produced by PtL technology.

  6. Bjorn, you have given the problem statements very clearly here, so thank you. I wish we could get people on both sides to see these things. Instead we have the climate deniers on one side and the green advocates on the other. Engineering analyses like yours actually bridge them together, as some of their goals are already aligned, but they are not always able to see the big picture.

    In previous Corners, commenters have highlighted dozens of approaches to greener technology for aviation. That interest is a great thing and we need to keep researching & learning. I certainly have learned a lot by reading here, both columns and comments.

    But maybe recognize most of those ideas are not yet at a point of broad implementation, while more conventional areas of improvement are much closer. So being tied to reality, and real progress, is also a good thing.

    • Ever read “The Sand Pebbles” ( McKenna afair ) ?

      it is a “rice bowl” thing. ( and taking that away. Some people really can’t cope with just imagining it. )

  7. Taxes don’t work. Carbon taxes don’t really work except maybe as a threat. There are two problems 1 inelasticity of demand. People have limited alternatives and must pay and 2 the tendency of governments to absorb taxes into consolidated revenue and misspend on stupid things. I have nothing against “feminist dance theory” but my pasty chef shouldn’t pay for it through taxes on his 3 am commute to work. Offsets can rigorously prove that they’ve reduced emissions. Lufthansa offsets 100% of its emissions and that is certified.

    Batteries are dramatically improving and soon an electric car will be affordable but not right now.

    The first Graphene battery just went commercial. Graphene is 100s of times more conductive than copper so solves a lot of problems with electrode size and heating in batteries. It seems to increase discharge and charge rates 6 fold without effecting energy density yet still improving battery life. At the moment Tesla Model 3 batteries have 248WHr/kg with a 3C (20 minute discharge). Imagine them with a 20C (3 minute discharge) ie 1kg battery providing 5kw power for 3 minutes.

    In 5 years battery’s will drop 70% in price and I suspect will have nearly twice the energy density. Only around that time will 50% of people be able to afford electric cars. Wasting money on an expensive technology not yet ready actually means we won’t have resources when we do have cost effective tech.

    • High taxes on fuel ( petrol, diesel ) have always worked here in Germany. Just compare the average mpg of vehicles used in Germany and the US of A. :-))

      • Yes, and new EU regualtions on cars forced “everbody” to start selling charging hybrid cars this year in the EU. For those buying a new car many will choose the hybrids, plug it in at home and at work and reduce fuel consumptions in the EU a bit. Friends that drive the VW Passat hybrid fills up the tank maybe once a month.

        • But hybrids or full electric vehicles don’t actually reduce CO2 emissions overall, the only real benefit is stopping smog in cities.This is also where braking regeneration and the efficiency of an electric motor is most worthwhile.Taxing fuel is proven to be effective, just study what happens to road traffic when the oil price rises sharply.

          • Of course hybrids and full electric vehicles reduce overall CO2 emissions, and not only from the tech you recognized, regenerative braking and electric motor efficiency.

            Even if your electricity supplier is mostly using fossil fuel, combined-cycle power plants are easily more than twice as efficient as car engines.

            But more to the point, wind and solar power are far cheaper than fossil fuel, even with the current bargain natural gas prices in the US. Power companies that hold on to fossil fuel plants are dragging their feet to write off more of the existing plant’s capitol cost, or following a government dictate that they continue using coal despite the savings with wind (US EIA study in 2019 of full US that showed in fine detail where tower hub heights gt 80 miters would pay off also noted that land-based wind farms cost about $200/peak-kilowatt generating capacity, and with proper siting and hub heights their annual equivalent of peak-power is 3,500 to 4,000 hours, ie $200 capital buys 3.5 to 4.0 mWh annually for the life of the turbine.

        • Which definitely was not a VW thing.
          All manufacturers gamed their numbers.
          The bigger the car the more gaming.
          Mercedes Benz numbers where really hilarious.
          With Merc GL* overweight barn doors having “officially” nigh better mpg than our Hyundai I10

  8. Aircrafts are visible, expensive and Aerospace if often the technology leader, so it makes many point to Airlines to fix the problem Technology wise so the rest of the industry have an easy path to follow.
    For many CO2 emitters there are already solutions available and in use in some countries. Like replacing recidental oil heating with geothermal systems with drill holes 100m deep has been a success, but the political will to enforce it and the politicians dependence on energy taxes makes it move very slow in many contries.

  9. Interesting stuff. Yeah, very high energy density batteries are arguably small bombs, compared to todays fuel, unless some new innovation is discovered.
    Which are the major parameters to increase airplane efficiency?
    Reduce weight=less induced drag
    Less parastitic drag=thinner air/, higher altitude, less exposed front area (prismatic coff?)
    Ability to trade potential energy to kinetic energy better=minimized drag at decent.

    Which is the equation to this?

    Maybe a long tube with extreme high aspect thin wings and inboard engines are close to optimal. Some rocket scienstist should know.

    • Look at the Boeing Sugar “Trussed braced wing” projects, good pictures in last Aw-Week.

      • Yes. And they, if any, are “rocket scientists”

        The are also some bipane type sketches. The span csn be reduced by this and aspectt ratio still good as the wings can be even thinner still structural fine. Renold numbers limits?

  10. I think we need to be pursuing jet fuel that’s derived from non-fossil sources. I agree that electric aircraft are, for the foreseeable future, unlikely to be solution. The power density advantages of jet fuel seem overwhelming. But there’s no reason why jet fuel needs to be derived from fossil sources.

    In other words, given that the turbine technology is so efficient, how can we make the fuel for those turbines green.

  11. Good summary Björn. Please note though that transports including Air Transport only contributes with around 15-17 % (Depending on Source) of the CO2 emissions. This is mainly due to that Electrical Generation with Coal is contributing with much more CO2 per energy unit produced. Coal is used to produce around 40% of the Electrical power in the world, but emits 70% of the CO2 from Electrical power generation. Best to do in the short term would be to replace the Coal with Natural Gas , would reduce the world CO emissions with around 20%. This is more than 10 times the emissions from Aviation
    Last figures I saw on Air Transport CO 2 Emissions was around 1,5 % of Total.

  12. “Bottom line: the climate activists are decisively winning. The science no longer matters in the public policy debate. Activists have moved beyond it and the major science institutions no longer defend it against the activists’ exaggerations and misrepresentations.”

    Here is an interesting article explaining what happened and why and how it happened:

    https://fabiusmaximus.com/2020/02/12/climate-policy-debate-is-dead/

  13. Bjorn,

    Well said. Why people beat up on air travel that has connected the world together, when we aeronautical engineers have done an excellent job of squeezing enormous efficiencies out of our airframe and engines, is hard for me to understand. As you said, air travel is NOT the problem. Period.

    That brings me to ask the following question: How much can a nation reduce the carbon footprint of its people by building high-speed rail transports between busy hubs and convenient spokes of efficient trains to take people wherever they want to go, for personal or business reasons? Remember individual choices are nice but it is the collective contribution that matters in the end. I am spending my sabbatical in Japan and it is amazing how convenient, cheap and efficient its rail-based transport system is. Shinkasens can take you from one major city to another efficiently and comfortably, and connecting spokes of trains can then take you anywhere in another hour or so. No train station in Kyoto for example is more than 15 minutes by walk. With such efficient network, people don’t have to drive carbon-inefficient cars like we do in USA. Anyway, has anybody done an analysis of the per capita carbon footprint of a rail-based transport system as in Japan and automobile-based system as in USA? Are you planning to do so? That would be useful.

    • Japan is a small country , think the size of Washington and Oregon but with 125 mill people. Then take away 70% of the land area because its too mountainous.
      The Kanto region around Tokyo has about the same population of California but in 12,500 sq mi.
      Think of the USA Boston -Washington DC urban corridor , its got 52 mill people but its 50,000 sq mi.
      When you visit Japan , you perhaps arent seeing the wood for the trees.

  14. OVERALL I agree social expectations that aviation can produce all electric airplanes are unrealistic, however, there may be some potential for a hybrid electric gas turbine trijet and if Boeing are serious about regaining their pre-emminance building airliners theyneed to look at such a concept.

    I am alluding to NASA’s AURORA D-8 concept, which I raised here: https://qr.ae/TqVAE9 on QUORA’s website
    one variation on the Aurora has a B727 layout, but with an electric driven, ducted fan in the #2 engine location designed to suck air from the fuselage boundary layer,. Such a design could reduce carbon emission for a 757 sized airliner by 50%

    • More like 7% reduction in fuel burn according to NASA. ‘concept studies’, which means computer simulations not real world.

    • Bjorn looked at the BLI concept in an earlier Corner from this series. The benefit of BLI is that it can reduce or eliminate the form drag from the fuselage. However for a typical tube & wing design with streamlined fuselage, the form drag is only a small percentage of the total drag. So the benefit is not large relative to the penalties.

      That could change with blended wing or multiple-bubble designs, where form drag becomes a much larger component of total drag. In that case, as Bjorn pointed out, using an efficient APU for shaft power makes more sense than hybrid, since you don’t have the weight or the conversion losses of electric drive.

      Hybrid only makes sense where the weight penalty is offset by repetitive energy capture and reuse, combined with an otherwise inefficient engine, as occurs with ground vehicles. Aircraft are much more sensitive to the weight penalty, have much more efficient engines, and don’t have the mechanism of repetitive energy capture.

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