The true cost of Electric Aircraft. Part 2.

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By Bjorn Fehrm


July 8, 2021, © Leeham News: Last week, we looked at the cost of running an electric 19 seat airliner based on energy stored in batteries. We found the energy costs were lower than for the equivalent turboprop aircraft, but when we add the maintenance costs for the batteries, the operating costs were higher than today’s 19 seat commuter.

This was under the assumption that the battery aircraft had the same energy consumption as today’s aircraft. We now run this check. The result is eye-opening.

Figure 1. Heart Aerospace ES-19 battery-based airliner. Source: Heart Aerospace.

  • Battery based aircraft weigh significantly more than jet fuel based ones. It increases their energy consumption.
  • Last week’s findings were conditioned on the same energy consumption. This week’s analysis proofs this is not a valid assumption.

The energy consumption of battery-based regional airliners

Last week’s article assumed our battery-based 19 seater had the same energy consumption as a 19 seat commuter like the Beech 1900 (Figure 2) or Jetstream 31 (Figure 3). The block fuel for the 200nm trip was 320kg with 308kg reserve fuel (IFR reserves of 5% route contingency, 100nm alternate, and 30 min circling) for the turboprop.

Figure 2. The 19 seat Beech 1900D. Source: Wikipedia.

It equates to 1,037kWh block energy and 998kWh reserves when we convert with 12kWh energy per kilo for fuel and take 27% of that as the shaft energy transferred to the propellers (the gas turbines for the turboprop have 27% fuel to shaft energy efficiency).

Transposed to battery energy, this equates to a 4,508kg block energy battery with 4,339kg reserves. To reach these values, we used a 92% efficiency for the electric propulsion chain, battery-to-propeller-shaft, with 0.250 kWh energy per kg of battery (the energy density on a system level). Densities are today at typically 0.160 kWh per kilo but we upped this with 56% to cater for development in batteries during this decade.

Figure 3. The 19 seat Jetstream 31. Source: Wikipedia.

So instead of 628kg jet fuel, where 320kg gets consumed during the flight, we have a constant 8,847kg battery system. The electric energy is 14.09 times heavier for the maximum range flight for the electric airliner.

It’s a devastating difference for an aircraft, making it practically impossible to design an aircraft with any reasonable performance around this excess weight.

But the bad news doesn’t stop there. The heavier turboprop, the Beech 1900, has an empty weight of 4,900kg. Laden with 19 passengers at 100kg we are at 6,800kg. The rest up to takeoff weight is fuel, in our example, 628kg. So we takeoff with 7,428kg and land with 7,108kg.

If we want to fly longer, we can fill an additional 338kg of fuel, doubling our range, before we hit Maximum TakeOff Weight (MTOW). If we want to fly longer still, we load only 16 passengers, and we can fill an additional 300kg of fuel. And so on, until the tank volume is the limit.

The battery aircraft is different. If we assume we can store the batteries in the wings (distributed along the wing to put the weight where the lift is), we could best case get away with an additional tonne of empty weight according to our performance model’s weight estimation part (this includes lighter electric motors instead of turboprop engines).

To this, we shall add the batteries. We land on an operational empty weight of 14,757kg. The only variable component is the payload, 1,900kg of passengers with bags. The takeoff weight for the 200nm trip is then 16,657kg. It stays constant during the whole flight and is also our landing weight.

Our wings must lift 16.6t of aircraft whether we fly 50nm or 200nm, and we can’t extend our range by offloading passengers as our energy doesn’t increase (the range increases with a few nm as the whole aircraft is now 300kg lighter, but this is only 1.8% of the total weight).

An aircraft that can handle the weight

How do we design a 19 seater that weighs 214% more than a typical 19 seater? With difficulty! In fact, it’s virtually impossible to make something sensible around this weight.

Heart Aerospace says it’s possible. It’s even a good idea, as it’s a greener aircraft than the Beech 1900 or Jetstream 31. Let’s examine their proposed aircraft and apply the learning we have so far.

Figure 4 shows the ES-19 airliner that transports 19 persons 200nm according to the company.

Figure 4. The ES-19 battery-based airliner compared with the Beech 1900D airliner. Source: Leeham Company.

The aircraft looks pretty normal compared with the Beech 1900 except for two aspects: the very large wing and wingspan and four engines instead of two. The four engines have little to do with any efficiency gains by having more propellers. Together with the large wing, with a wide span, it’s there to try and handle the hurdles this aircraft faces in performing a flight.

The hurdles start at takeoff field length and how the aircraft handles an engine out situation at takeoff. The field length for the ES-19 should be shorter than normal, according to Heart Aerospace. I can’t understand how this is possible with an aircraft that takes off with 2.4 times the weight of the Beech 1900.

Using our airliner performance model, the ES-19 has twice the runway requirements of the Beech 1900 at 7,600ft when we dimension the takeoff thrust of the engines to keep the V2 climb at over the required 3.0%.

The 3.0% are connected with the V2 safety speed directly after rotation, where we shall climb with a minimum angle of 3.0% with One Engine Inoperative (OEI). Four engines make us lose only 25% of the thrust instead of 50% for a two-engined aircraft. This certification requirement explains why we have four engines instead of two.

The next problem for the aircraft is that the climb drag is 55% higher than the Beech 1900, despite optimizing the climb speed to a lower value to decrease the drag.

It continues in the cruise where the drag is 91% higher, once again after cruising at an optimum 180kts instead of 230kts to keep the drag down. The 91% additional drag comes from the larger wing (parasitic drag) and the extra weight (induced drag).

This problem of higher drag continues through the trip’s descent and landing.

The energy consumption for the trip

The increased weight of the aircraft makes it accelerate slower to takeoff speed. It doubles the runway distance needed for takeoff. Once in the air, the increased drag from the weight and the larger wing increases the aircraft’s drag. To compensate, we need to run the engines harder to generate more thrust. It increases energy consumption.

Despite optimizing the aircraft speed in all flight regimes to lower drag and thus thrust, the aircraft consumes more energy than the Beech 1900. As it flies slower than the Beech 1900 to minimize drag, it keeps this higher energy consumption for longer.

The result is the aircraft consumes three times more energy for the 200nm trip, at over 3,000kWh. As we have 1,000kWh available for the trip (the rest has to remain reserves), our range of the aircraft is less than 100nm.

There is no point in increasing the battery size to increase the possible range, as when we increase the battery size, and by it the aircraft weight, the range decreases. The aircraft drag rises faster than our available energy increases.

The problem with available energy means we can’t size the battery for optimal time on aircraft before changes. We need to load the battery to 100% for each flight to get somewhere (not the 200nm we should though). The operating costs skyrocket as we need to change batteries after 1,000 flights, which with six flights per day is every six months.

The finding is the battery energy density has to climb above 1kWh per kg to change this, and we are today some 600% from this point.


Electric aircraft came in vogue when electric cars worked, with Tesla as a good example. The point all missed was that our petrol cars are miserable energy hogs. They use about 5%-7% of the energy in the gasoline as they coast from stoplight to stoplight.

Battery or hybrid cars recover energy as you stop for the light. But there are no stoplights in the sky and our airliners use between 25% to 50% of the energy in the jet fuel. It means electric aircraft, whether with batteries or hybrid, doesn’t improve state of the art; they degrade it.

It’s typical that non-aeronautical people run companies like Heart Aerospace.

Aeronautical companies, like Airbus, Embraer, Britten-Norman, and Pipistrel, started by propagating for battery aircraft. Then they swung to hybrids and are now at hydrogen. With SAF (Sustainable Aviation Fuel), it’s the only technology that can bring us greener air transport.

80 Comments on “The true cost of Electric Aircraft. Part 2.

  1. Hi Bjorn, as old school aeronautical bloke, I highly sympathize with your efforts to bring some reason into electrical flight. There’s only so much joules you can put into a kg. And kg’s are essential for aircraft.

    But all our future dreams & ambition are build around electrical vehicles and clean wind energy (equally sucking efficiencies if you look honestly, completely). And technology shifts around the corner. Unfortunately. Hard to accept realities are washed away with awesome graphics and techno optimism on an enormous scale. Tesla & my phone got 10x more powerful in 10 years, as a reference..

    What amazes me is all the skilled engineers working on all these taxi, electrical flight, flying car projects. When they have a coffee break & a piece of paper, they see reality.. And you didn’t touch safety/ATC. But they probably love their jobs, colleagues now & aren’t responsible / accountable for the total endeavor / R&D budgets / “investments”..

    Saying it’s not possible for a long time makes you old, retarded and blocking progress, so better sing along.. tragic.

    If you google images on my username & aircraft you can see I’m hobbying around with designs for many yrs, but I try to stay on the safe site of old school aeronautics 😉

    • In the current investor environment — full of retail investors, millennials, Robin Hood users, and hordes of others looking for some sort of capital growth in a zero-interest-rate world — there are lots of opportunists who hang out an upbeat (but outrageously unrealistic) story in order to attract rookies to invest…and it works! Plenty of examples in the ground-based EV world, so why not in the aviation world also? As you point out, naysayers are tarred as “out of touch”.

      What’s frightening is that the same attitude of “just believe, and anything becomes possible” is also widely present among totally unrealistic environment activists.

      • All well said. And Thanks Bjorn for the realistic and coherent analysis, as always.

        “What’s frightening is that the same attitude of “just believe, and anything becomes possible” is also widely present among totally unrealistic environment activists.”

        Its not just the environmentalists. The same attitudes also currently prevail in esteemed aerospace firms like Boeing. Seems to me that too many people are expanding the popular “post-truth” philosophy to subjects with little forgiveness for sloppy work or over-optimism (such as aircraft design). The inevitable wake-up calls for these investors will be painful. Sadly these would cause an over-cautious investment attitude going into the next couple of decades and beyond: as a result, when real breakthroughs finally arrive, they will not be believed and will not be funded by the same investors that got burned on these aeronautical smoke and mirror shows.

        • There was similar completely;y unrealistic attitudes at the start of the civilian nuclear age….trains, ships and even planes were going to be nuclear powered. There is a place for nuclear power in that picture , but not to meet mass market needs.
          The nuclear plane did get to a more advanced stage, its hard to believe the experts thought it could be viable but they tried as there was government money for doing so. Thats what I see the major aerospace companies involvement, they want to be seen as ‘part of the conversation’ but the battery dream is almost over .
          In an ironic twist to what many car can be aware of, a dead battery- its the summer heat that kills the performance of a battery who has reached their cycle limit, but its the cold hard reality of winter that buries it. The aviation battery dream has some cycles to go yet but reality isnt far away from killing it dead.

          • There is even a term, a field of study called “The Gartner Hype Cycle”

            “The hype cycle is a branded graphical presentation developed and used by the American research, advisory and information technology firm Gartner to represent the maturity, adoption, and social application of specific technologies.”

            Having said that much of the speculation about nuclear powered locomotives, cars and aeroplanes came from magazines such as “Popular Science” rather than scientists and engineers in the know.

            At the inception of nuclear power US nuclear scientists and engineers wanted to develop molten salt breeder reactors based around plentiful Thorium. Molten salt was used as
            1 it would allow easy reprocessing of the material including transmutation of long term waste to medium term waste.
            2 No highly stressed reactor vessel as it was unpressurised
            3 A containment failure would lead to the liquid draining into a holding vessel underneath the reactor which was not critical and which was designed to conduct heat away.
            4 Thorium is plentiful, Breeding it into U233 makes it almost infinite.

            Unfortunately the technology was neither suitable for submarine propulsion or an adjunct to nuclear explosives and was not funded by the military.

            If the world ever does end from CO2 driven global warning I will blame it on anti nuclear protestors for crushing nuclear development.

            At the moment the only country seriously developing this technology is China. They intend to control and dominate it in the future.

          • @ William
            The Netherlands is doing a pilot study on the construction of 4-5 thorium-reactor nuclear power plants. As you might expect, ill-informed environmental groups are up in arms. Realists, on the other hand, recognize that wind and solar just aren’t going to cut it in a small, densely-populated country with a huge industrial and agricultural sector. Who’ll ultimately win this debate?

          • @ Bryce,
            An interesting article in nature: “Emergency deployment of direct air capture as a response to the climate crisis” basically suggests that reducing emissions in order to limit atmospheric CO2 buildup has become hopeless and that best solution is a massive program of DAC “direct air capture” of CO2 with geological sequestration. They don’t come out and say it directly but using nuclear power is the only practical option. For aviation of course DAC means vast amounts of CO2 to turn into carbon neutral PtL jet fuel.

            Chinese engineers are publishing articles on producing plastics and carbon fibers from Direct Air Capture using power molten salt reactors will produce.

            The authors only look at using SMR “Small Modular Reactors” not something efficient such as a molten salt or other Gen IV or Gen V.


            Its kind of a silly in that the 50%-100% increase in electricity production would in effect make carbon neutral iron and steal and concrete viable as well as carbon neutral fuels so long as they are subsidized to be cheaper than fossil fuels.

            Unfortunately our nuclear engineers are getting old. The solution is probably SMR followed by Molten Salt

          • @ William
            Thank goodness DAC is starting to get some attention…and thank goodness there’s finally some realization that reducing emissions does NOT address the PRESENT climate problem. The DAC concept has been resisted for years by environmentalists, because they thought that it was just a ploy by multinationals to avoid having to reduce emissions. Another example of ill-informed activists doing more harm than good.

    • Just a point about your analogy.

      Neither your Tesla nor your phone has added 10x the battery power.

      Tesla S has I believe always offered an 85 kWhr version, and now is at about 100 kWhr max. That’s not a ten-fold increase….

      As for your phone, maybe the processor is ten times more powerful, but the battery in it too is surprisingly little changed over the last decade. And while processor efficiency had (has?) the opportunity for great boosts, the existing efficiency levels of converting electricity to thrust don’t allow for anything like ten-times boosts.

      Until a revolutionary (not evolutionary) battery tech appears from nowhere, Bjorn’s analysis, which already gives the “benefit of the doubt” and factors in some improvements, will remain closely correct.

  2. We need to always be sure to take into account the true environmental cost of power stations for charging, harmful impacts of hydro dams on migrating fish like salmon, wind farm impacts on birds, and the amount of land lost to and heat generated by solar farms in order to generate the power to recharge the batteries. All of this is before we get into a discussion about the impact of rare earths mining on wilderness areas.

    If all of this stuff is truly accounted for in a realistic fashion the actual cost of all of the battery powered air and surface vehicles may well be a lot higher than thought when the above social costs are considered.
    Maybe not such a great deal after all, particularly when you consider the degradation in battery performance over time.

  3. Spot on. I work in aviation and these start ups come to us all the time wanting to be “disrupters” and the next Elon Musk. Most come from Silicon Valley. They have a mindset of “if it isn’t written, then nothing says I can’t” but the reality of the FAA is “if it isn’t written, then you can’t.” Viable certified electric planes for commercial operations are at least a decade away. Not only does technology have to catch up, but the regulations do too. There is no section in Part 33 for an electric motor as the primary power source. If the large OEM’s aren’t doing it then that says something there. Hybrid and hydrogen propulsion are the next logically steps. Don’t believe the articles that try to keep the hype alive about electric passenger aircraft.

  4. The figure 4 is not displaying. Can you please update its link, would love to see what it shows.

    Thanks for making this excellent article open access.

    (No need to release this message, it is just an easy way of advising you about the broken image link.)

    • Thanks, the labeling needs to change from paywall to freewall. Both this figure 4 and the one in the previous article are now freewall.

      • Glad you caught the issue in the earlier article, too. I saw that subsequently, but hoped you’d realize and catch it without me pestering you a second time. 🙂

  5. Yes, some developers exaggerate, but I think Björn and some correspondents here will be surprised by events in the marketplace. Some overlooked points:

    – Several makers’ batteries storing 400–500 Wh/kg are moving from lab to pilot production, with higher densities in view (think e.g. safe lithium-air or Li-S with a nonflammable solid-polymer electrolyte or nanostructured ceramic). Start with C4V, Sion Power, Solid Energy, then there are more.
    – Electric planes actually do have an analogue to hybrid autos’ regenerative braking: they can recover energy in descent for significant recharging that should be creditable toward reserve range.
    – Otto Aviation’s remarkable Celera 500L suggests major further potential for laminarization and other aerodynamic advances. Its claimed 8x fuel efficiency and 6x lower opex (vs standard business jets) look plausible.
    – Advanced polymer composite manufacturing was shown in DARPA’s affordable-composites program in 1994-96 to offer e.g. a tactical fighter airframe ⅓ lighter but ⅔ cheaper (at T100) than its 72%-metal predecessor. (Veitch, L., “Assessment of the DARPA Affordable
    Polymer Matrix Composites Program,” Institute
    for Defense Analyses D-2068, 1997, II-4 and III-3,
    – Cellular lattice structures covered with tough polymer membranes can weigh ~98–99% less than corresponding metal airframe structures, e.g. wings, yet have comparable mechanical properties. Indeed, some can passively morph real-time to optimize their shape for best aerodynamic efficiency, like a bird’s wing. (N. Cramer et al., Smart Materials and
    Structures 28:055006, 1 Apr 2019, https://doi.
    org/10.1088/1361-665X/ab0ea2 (https://cba.
    – Such structures configured as a spherical shell can form a vacuum balloon, buoyant in but not crushable by the atmosphere. Its lifting power will equal the mass of air evacuated from it minus its own tiny structural weight. A big version could have ~24x the net lifting power of a 747 ( Perhaps small versions in parts of a plane not otherwise useful could help offset battery weight.

    There is more in heaven and earth….

    • When creating a new product, you seldom innovate multiple things at once, because every single innovation infers additional risk.
      Electric flying is the basic innovation, a large one, with new aircraft configurations and systems architectures involved. (The hope for) better batteries comes on top of that, and if that doesn’t suffice, you bring in new structures, and then, laminar aerodynamics. Finally, multiply your chances for realization (each < 1), and you will arrive at nearly nil.

      By the way, the energy you claim to recuperate in descent is actually needed for gliding. There's absolutely no surplus energy to be harvested. Actual aircraft engineers know that.

      • Yes there is. I’ve flown the Pipestrel Alpha Electro…it regenerates as you descend. You can clearly see the current going back into the battery and the battery charge increasing.

        • Yeah sure, it certainly recuperates, in a dive… But not if you gently and efficiently glide towards your destination.

          The topic is not about leisure flying, but about commercial aircraft. The most efficient mission trajectory is one where you switch your engines off at end of cruise (top of descent) and use all of the potential energy to reach your destination at maximum L/D. If energy can be recuperated, you cruised too far in the first place.

      • We evidently have different design philosophies. From a half-century of designing superefficient buildings, vehicles (land/sea), factories, and equipment, I’ve learned the value of rigorously minimizing demand first, then simultaneously innovating and shrinking supply. This can reduce overall project risk as well as cost. In short, optimize the airplane as a whole system, for multiple benefits—not its components for single benefits. If you can redesign only one thing at a time, your innovation will be too slow to compete.

        This approach is summarized in a half-hour talk at and in the foundational paper at, applied to autos at, and sketched for airplanes at

        I therefore think your proposed design sequence is backwards. And by the way, very light laminarized planes already exist. The most impressive powered one I know is Otto Aviation’s Celera 500L, currently diesel-powered but a candidate for electrification.

        It’s interesting how relatively little commentary my long post yesterday has elicited. I hope this means that some knowledgeable readers are checking its references. Conservatively, I didn’t mention the obvious opportunity to put photovoltaics on exterior surfaces (a plane aloft can get about as much sunlight from below as above). Today’s best lightweight multi-bandgap PVs are 47% efficient (at a high price); 80s may become feasible with optical-scale lithographed antennas. PVs can be a major range extender.

        Your comment below about “the most efficient mission trajectory” is interesting, but what’s “most efficient” in commercial flight is not always the same with batteries as with fueled engines.

        • Mr Lovins, I highly respect your lifelong experience of building (and envisioning) super-efficient stuff, but I’m afraid that doesn’t qualify you as an aircraft engineer. Your comments strike me as a bit naive in this respect; no offense.
          Firstly, useful aircraft are not designed for sunny days, but for the worst weather, where the sun doesn’t shine. Range extension cannot be relied on in a robust operational environment, and thus, it is pointless.
          Second, the physical principle of energy conversion doesn’t discriminate between fossil-fueled and electric aircraft.
          Also, I was talking about parallel innovation, not about parallel optimization. Of course you optimize in parallel; in aircraft design, it is called Multi-disciplinary Design and Optimization (MDO). But do have a look at how much innovation established aircraft OEMs are putting into their aircraft. You will be surprised how incrementally progress happens there, for the reason that the technical environment is extremely challenging.
          You might have interesting ideas and theories; but your laudable optimism most probably fails in this special circumstance, imho.

          • Thank you, Bernardo. I’m indeed not an aircraft engineer. But my many millions of miles of air travel included zero longhauls that didn’t fly above the weather. My comment on energy recovery in descent was not about physics (my original discipline) but about optimal operation, which may change if you’re carrying batteries but not burning fuel. I’m familiar with OEMs’ published innovations but dismayed (not surprised) by how slowly it’s adopted, as much for cultural as technical reasons. That’s why my keynote of ATAG’s 2019 sustainability conference—suggesting a potential solution to those lags—was so well received, especially by major OEMs.

          • Mr Lovins, the first instance of commercial electric aircraft will most probably have to stick to lower altitudes than those seen on international flights, very much for efficiency reasons. You might know that, because you explicitly said “long-haul”. Electric flights will encounter clouds on the regular. There’s not only no use for PV cells, they are even harmful ballast.
            Please describe a commercial flight operation where electric recuperation makes sense. I cannot think of one.
            Lastly, don’t look at what OEMs say, but at what they do. They employ very smart people, physicists amongst them. They might know a thing or two.

        • Thanks Mr Lovins, your posts are enjoyable to read. IHere is a thought experiment. Consider the case of an aircraft using some of the technology your refer to. Say an aircraft with microcellular structure achieving 15% airframe & propulsion system weight, 15% payload and 70% weight of batteries of 600WHr/kg. This aircraft should have a range of about 1500 nautical miles extrapolating from Eviation Alice figures. Assume the alternative is a RCCI/Diesel of 55% efficiency extracting 6kW.Hr/kg of PtL carbon neutral aviation fuel. The alternative would thus have a relative weight of 15% payload, 15% structure and 7% fuel. The structural weight can be reduced by a factor of 5 to about 3% so our new weight is 15+3%+7% = 25%. The fuel required can now be reduced by a factor of 4 because our aircraft is 4x lighter. We now end up with an aircraft of 15%+3%+1.75%. And it goes on. Our petrochemical aircraft would be less than 1/5th to 1/6th of the weight due to compounding effects and the weight reduction during flight of the fuel burn.

          Assuming the production of PtL is 65% efficient and the engine 55% efficient we have an efficiency of 35% which is reduced compared to a electrical efficiency of the battery charge-discharge and variable speed drive of about 90%. However the PtL vehicle needs 1/6th the energy so is actually 2.5 times as efficiency.

          Obviously as desired range is reduced the Electric Aircraft gains relative efficiency and in this case I suspect the cross over point is about 600 nautical miles. Our electric aircraft is still at least 2-3 times heavier and larger.

          On the other hand PtL fuel can be made in the desserts of Ethiopia or Australia and the wind swept coasts of Norway, Canada and NW Australia and shipped anywhere.

          • Microcellular structure is just ‘foam plastic ‘..they have very low tensile strength.

  6. Let’s see what happens when you fly in cold climates and at higher levels with freezing conditions.
    Beautiful battery will be even more useless and world polluting.
    Not enough people are interested in how they mine lithium. Have a look at Chile on YouTube.

    • Similarly, not many people seem to be interested in how oil is extracted, refined, and transported around the world. Somehow, Exxon Valdez, Deepwater Horizon and the destruction caused in oil extraction regions such as near the Alberta Tar Sand region are forgotten or ignored by most people who use those fossil fuels.

      These totally eclipse lithium mining, which is actually mostly done in Australia these days using conventional mining methods. No, mining for battery materials (especially Cobalt in the DRC) is not perfect, but certainly better than extracting oil and then burning the stuff and dumping it in our atmosphere, especially when taking into consideration that those fossil fuels cannot be reused, but batteries can be reused for many cycles, and those battery materials can be recycled and turned into new batteries afterwards.

  7. RE ‘ miracle ‘ propulsion schemes for an air vehicles- look up a program my college roommate spent a lot of time on. The nuclear ramjet missile program called Pluto. It had essentially unlimited range at low level and high speed using heat generated by a nuke core. But there was this little drawback- a radiation trail which would not only injure the target along with explosives, but almost anything along its flight path. And the orion rocket program which used multiple low yield a bombs against a massive pusher- shock adsorber plate, etc.

    Now using high capacity batteries for small drones to carry small packages cfan be a winner, but carrying more than one or two passengers more than say 50 miles may well be the practical limit.

    Waiting for the equivalent perpetual motion version engine to fix all the polution problems. The closest to date is al gores carbon credits.

  8. Aviation can’t be done in an environmentally friendly manner, huh? Guess there’s only one alternative, stop aviation. If you can’t do it without screwing up the environment, then you need to damn well stop.

    • A similar statement could be made about human procreation.
      Focusing on a single “scapegoat” sector of human activity isn’t going to make the environment problem go away.

      Incidentally, SAF offers a way for aviation to continue in an environmentally responsible manner, without the various penalties associated with batteries and/or LH2.

    • The SNAP type nuclear isotope batteries used on the Pioneer space probes were rated 150 Watts. In a 24 hour period they would accumulate 3.6kWHr which is enough for 36km/22 miles of driving in a 1 tonne vehicle. (EV consumption tends to be 10kW.Hr/tonne per/100km.)

  9. It seems to me that so long as the mass fraction of battery weight to MTOW is kept to about 25% electric flight is effective over very short ranges (100 nautical miles) with respect to piston powered aircraft in limiting emissions in part because although electric motors are not lighter than gas turbines they are much lighter than piston engines. At the moment therefore self launching sail planes and flight trainers are the only legitimate winged electric aircraft.

    Two things need to be considered. One is Elon Musk claim that when aviation batteries exceed 400W.Hr/Kg electric flight will be viable. We have about 55% of that available for automobiles but lab scale sample production of 400WHr/kg automotive batteries (Sion) are now available and may become viable for aviation in a few years. Certainly 400WHr/kg changes things for short range flight since a good range with a mass fraction of 25% becomes possible.

    Against this will be improvements in piston engines. In the 1930’s I.G.Farben (BASF) was awarded patents for HCCI engines (homogenous charge compression ignition engines) that Daimler Benz and BMW built into prototypes. These engines replaced the spark plug ignition with an injection of a small amount of DME (dimethylether also a widely used refrigerant).

    The HCCI engine has been perfected in Formula 1 racing as the RCCI engine (reactivity controlled compression ignition engine) which use a combination of DME injection or spark plug. (you don’t need the DME its just better).

    These engines are more efficient than diesels yet have non of their NOX or particulate issues. They can operate at stoichiometric ratios of lambda 2.5! This means they will be extremely efficient at low partial loads.

    Mazda are developing a SkyActive version of the RCCI engine they claim will be 56% efficient. These engines will compete with PEM fuel cells for efficiency. (petrol heads rejoice)

    If a Direct Air CO2 capture PtL (Power to Liquids) fuel plant were build right now it would easily be 45% efficient at turning electricity into fuel. However 60% is possible with direct air capture and 65% from concentrated sources.

    The field of eVTOL over short ranges does however have a future I feel because electric flight adds something helicopters don’t have. Electrical flight in this instance has the advantage of being extremely silent and also safe in an urban environment due to massive redundancy possible with multi-motor multi battery systems. eVTOL craft may also be extremely simple to learn to fly and maintain. Blade strike and landing footprint are also better.

    So we will see eVTOL both in the form of Urban Air Mobility and drones of increasing size performing such tasks as urgent parcel delivery, precision crop dusting, fire fighting, emergency services. The large Volodrone (being used by John Dear for crop dusting) suggest to me suggests that we may yet see LD-3 containers shipped by drone between airports to logistics centres 20-40km away.

    Finally there is the Hydrogen PEM fuel cell. Advances has reduced the platinum requirement to less than that required of a automotive catalytic converter. Polymer tanks can store at 700Bar. Electrolysis-Compression is approaching 80% efficiency. Toyota’s MRAI car will be one answer.

    However for short range flights it may also be an answer better than batteries.

    • William,

      you should read the first article from last week (link is in the ingress). An aviation battery adds 50% of mass to the cells for all the cell protection measures (a temp sensor for each of the 300,000-500,000 cells, connection leads to a battery management system with Voltage and Current sensing for each cell, thermal runaway protection material (Cert require that several cells can go off without the battery running away), for some systems liquid cooling to keep the temps in control, etc).

      You need 600Wh/kg cells for a 400Wh/kg battery system consequently, this is always missed when discussing promising cells run in labs.

      • The point I’m making is that 400WHr/kg cells doubles or trebles performance against current cells and batteries. In fact doubling cell power density should improve battery power density disproportionately since containment and packaging should stay about the same?

        The Pipistrel Alpha Electro trainer achieves around 60 minutes endurance at 85 KIAS cruise with 30 minutes VFR reserves.

        It’s hard to find this information but it has a 126kg battery pack which has 21kW.Hr. One battery is behind the engine and the other behind the cockpit.

        It means it has a mass fraction of 23% at MTOW and a battery power density of about 160WHr/Kg.

        I assume this pack is based around about 200WHr/kg cells.

        So if 400Whr/kg cells came along the Alpha Electro more than doubles its range. The aircraft designers were concerned with endurance (for training) not range and so with some aerodynamic and propeller refinements a range of about 200NM looks comfortably achievable. Not entirely useless.

        • It’s not to surmise that the infrastructure stays the same. The certification requirement is for several cells in the battery system to have simultaneous runaway and for it not to propagate to adjacent cells (if it does the aircraft is lost). This is dependent on the energy content of the cells that go into thermal runaway. Cells with higher energy density probably demand stricter cooling and larger/different padding around the cells.
          Certifiable battery systems is rocket science and the cert requirements will skyrocket the moment we have a battery accident in the air on a UAM/airplane.

          • I’m on record as asserting that winged electric manned flight is of little value. So I agree with your analysis.

            It is in eVTOL that battery flight will likely create new markets. I suspect that ballistics parachutes will probably be part of electric flight, certainly the larger eVTOL aircraft and drones, because of the battery fire issue. Perhaps rocket extracted and instantly inflated by an explosive like a zero zero ejection seat. That’s where I’d be buying shares.

            Lilium is relying (hoping they get certified) battery cells with an energy density of over 300WHr/kg and a short term power density of the 2700W/kg to allow hover to achieve its goals. They will have 9 battery packs. The nightmare scenario for Lilium is it finds its batteries at a SOC ‘state of charge’ of less than 15% perhaps with a partially shutdown battery which will be enough to fly for quite while but won’t be enough to provide hover power for landing. Without an ability to land on wheels it will need to save the passengers with a ballistics parachute.

            There are some anticipated solid electrolyte batteries that promise freedom from thermal runaway and potential battery fires.

            Looking at the Heart Aviation ES19 animations seems to suggest the batteries are going in the rear like Airbus’s hydrogen tank?

            I think they should go in the engine pods suspended of pylons so that a battery fire is well away from the wing spar and passengers.

          • Björn, Tesla has long prevented thermal runaway in its automotive lithium packs by spacing the cells far enough apart that runaway is impossible. This naturally and very manageably raises volume but immaterially affects mass, cost, and complexity. Boeing’s 787 battery unfortunately ignored this solution (though that debacle had multiple causes: wrong vendor, wrong chemistry, wrong geometry, wrong informants, etc).

          • Tesla makes thermal runaway impossible?

            Tesla’s advice:
            ‘Battery fires can take up to 24 hours to extinguish,” Tesla says in an emergency response guide for the Model S on its website. “Consider allowing the battery to burn while protecting exposures.”
            Thats nice to know for something thats claimed to be impossible

      • Whilst I can agree with the general thrust of the article and certainly with respect to the larger aircraft over longer ranges – in these cases electric only flight is currently not feasible. When Tesla started in 2003 they weren’t feasible and were the same parameters you state would also not be viable now. However this specific comment and a number of the assumptions you make in the article are not true. Not even automotive have a 50% mass cost and they are not optimised for mass. Each sensor for voltage is for each parallel string. No one puts a single cell in parallel. No one advocates the use of a temperature sensor per cell either. Thermal runaway protection doesn’t require the mass of automotive and problems that you are referring to in lots of cases can be negated by changing materials. If you are going to make a critique of a system that may be warranted I would suggest actually making sure your points are accurate and researched completely otherwise it can negate your entire argument. You make no reference to novel materials to reduce the overall mass, the fact that EASA have already stated they think electric vehicles for category enhanced will enter service by 2025. You translate directly an electric powertrain into a structure and an aircraft that is not optimised for it, there aren’t many engineered systems that could pass that type of translation.

        • And you have no idea what you are talking about.. aviation is different to road vehicles as weight plays an enormous part. And petrol driven cars are extremely inefficient , a change to electric makes big jumps for that. Turbine powered by flight with propellers is streets ahead in efficiency already so there is hardly any free lunch to gain from electric motors and penalties induced drag from their heavy batteries. (Drag is hardly an issue for modern cars)
          Certification rules cant be wished away by ‘magic materials’

  10. Electric flight will reduce CO2 emissions. PtL fuel and SAF will reduce it much more. Oddly there is no hype around SAF or PtL.

    • Silent flight?
      Doesn’t exist unless it’s gliders, propellors create the same noise no matter the power source. For a 19 seater plane the airframe will be noiser than the very efficient glider shape.
      As for the rest of the ‘Musk says so’ , take it with grain of salt, as he has his own category of hype

      • “propellors create the same noise no matter the power source.” this is simply wrong or out of context. Read the articles on noise on Volocopter and Liliums web sites. Lilium uses electric ducted fans lined with the same sound absorbing material as the RR Trent. Volocopter cancels out noise with destructive interference from its 18 rotors. Both will produce about 60dbA at 100m about the same as a good dishwasher.

        From Volocopter
        “Wonderfully quiet
        When you experience a VoloCity for the first time, you won’t believe your ears. Yes, flying can be this quiet. All 18 rotors acoustically operate within a narrow frequency range, thus canceling each other out to a high degree. By comparison, VoloCity air taxis are four times quieter than a small helicopter. A true treat for urban ears.”

        From Lilium

        “Figure 6 shows how our engineering efforts translate into the predicted absolute perceived noise level across the given flight envelope. Except for the initial hover phase with the corresponding 60dBA at 100 meter distance, the aircraft will be virtually inaudible during cruise flight. A good reference: normal conversational speech is circa 65dBA at 1 meter distance whilst a dishwasher is circa 60dBA.”

        “Reassuringly, even at this early stage of optimisation, the measured noise levels not only correlated well with pre-test predictions but also gave maximum peak values in the 60-65dB range, at 100m distance, without the addition of acoustic liners.”

        • “All 18 rotors acoustically operate within a narrow frequency range, thus canceling each other out…”
          Its rubbish , just you cant see it and instead believe any sort of quasi sciency – cartoon world.
          I suppose you think that what happens in headphones is transferred to 18 rotors ?

          • It’s just school science, the rotors have the same frequency but because there are 18 of them in different phases the pressure waves of nodes and anti nodes cancel or smooth each other out.

          • The technical abalysis of the Lilium proposal from here:

            But is well above my knowledge level but I checked the noise attenuation you talk about and it only has a passing reference and no methodology to back it up. They seem to rely on ‘acoustic linings’ in the ducts to do the job.
            It may be ‘school science’ but they dont mention it at all in the way you suggest so its another fantasy claim
            But like all the electric design models they assume a power supplied per unit weight from batteries is 3x times that available now . Even looking ahead a decade leaves them well short . As usual the range is a paper one with no allowance to reserves etc that the real world requires.

          • @Dukeofurl. Lilium have themselves stated they need a battery energy density of 300 Watt Hours /kg and a battery power density of 2700W. In other words they need to dump the energy from those batteries in 6.5 minutes in hover. Of course they plan on hovering only 10 seconds during take-off and landing before the load reduces during transition. 1 minute is allowed for as a reserve.

            For Comparison current Tesla Batteries are about 210 Watt Hours/Kg and can discharge in 20 minutes (C20 rate). By preheating the battery and putting the battery in ludicrous mode (which compromises its life) the battery can discharge in the C15 rate. Of course Lilim needs the C7 rate.

            If you played with electric remote controlled model aircraft you would know this many of which operate on the C3 rate. Batteries can trade of energy density for power density by increasing the size of the battery conductors and electrode area. I think to double power density one would loose about 25% energy density.

            There are pre production batteries, lab scale products that could fulfil Ilium requirements. Of course not certified and not in mass production.

            If they fail to find 300WHr 2700W/kg batteries they may end up with an aircraft that uses batteries that have compromised energy for power and therefore loose range. If lilium has a range of 125km instead of 250km they may still have a product that is marketable to the extend they can start limited operations while the battery issues are solved.

            They may just end up putting an APU in the boot.

  11. A minor quibble. In the section on takeoff climb requirements you state that “Four engines make us lose only 25% of the thrust instead of 50% for a two-engined aircraft”. The oversizing penalty for the use of two engines isn’t quite as bad as the section implies. The aircraft has a takeoff weight that is well over 12,500lb, so it will have to be designed to FAR Part 25 requirements. FAR 25.121(b)(1) states that the OEI climb gradient requirement at V2 is 3% for a four-engined aircraft, but only 2.4% for a two-engined aircraft.

    • Hi Tony,

      yes, I omitted this as to not bog the article down with too much detail (we have at least another 15 areas that I could dwell on, like needed tail volume due to the large wing…, etc, but it would make the article boring). The difference for a two propeller aircraft going from 3.0% to 2.4% is 4.6% lower thrust need in this case. In the scheme of this troubled aircraft, it changes nothing.

    • I accept thats less climb gradient for 2 engine, but I would have thought it was the other way round. Many years ago in my engineering job, field work would take us to rural areas and one place we could see an international airport runway over the water. The big twins took off and climbed fairly steeply compared to the 4 engine ones who climbed comparatively slowly…not many 4 engines these days. I considered the twins needed the height more quickly if one engine was lost, while losing 1 of 4 engines was less of an issue.

  12. I think you need to redo your maths… flawed assumptions. The ES-19 will kill turboprops.

    • It will never get into production, they may get to a prototype which can takeoff, do a circuit and then land and they will claim success as there’s dreamers who want to believe.

        • Time has already been allowed for
          ” 0.250 kWh energy per kg of battery (the energy density on a system level). Densities are today at typically 0.160 kWh per kilo but we upped this with 56% to cater for development in batteries during this decade.”
          The numbers still dont work in the assumed 56% power/system weight over the next decade. ie from 160W per kg to 250W ( when you account for all the packaging equipment etc)
          You cant ever get around ‘5% route contingency, 100nm alternate, and 30 min circling’ on top of the actual distance required.
          The promoters arent interested in a manufacturing business that makes planes with electric motors , they are going for a disruption- technology business that packages dreams and wont be aiming for passengers but for investors with money to burn ( as well happen in development)

  13. The interesting thing is that electric aircaft must be almost complexly immune to bird strike a leading cause of engine failure.

    • Small turbo props generally are anyway, the PT6 is a reverse flow engine with compressor inlet at back and the turbine at front nearest the propeller gearing.

  14. Some of the commenters on this article, especially those who wrongly think they are stating laws of physics or chemistry, will probably be embarrassed by what the more capable developers do in this decade.

    Developers who understand those laws very well—and apply them with greater imagination and integration than traditional designers did—love to have competitors who think like many of the skeptics writing here. So let’s review, say in 2030 and 2040, and see who was right.

  15. First, I wanna refer to the conclusion “ it’s typical that non-aeronautical people run companies like Heart Aerospace ”
    Saying that is not really wise and has no place in an analysis. This ruins the consideration of your analysis.

    In second place, some points of the analysis based on the comparison, of battery-based electric planes, seem to be correct. And I can add the problem of recharging batteries in the future where all planes would have to recharge their batteries at an airport.

    But this is where the challenge lies, if we take a look at the amount of energy that a battery can store per kg only 10 years ago we can notice an exponential improvement. If it is possible to see Antonov An-225 flying, then there is no place to believe that electric planes will not be the future of aviation.

    Lighter models are under development, and even ”liquidless” wafer batteries are under study. Yes I mean wafer batteries.

    Finally, the problem of recharging batteries that I added, in fact some companies are developing devices that will be able to meet these needs. As an example of the ”KI GENERATOR” device which solve all the problems, Weight, Endurance and Recharging of the batteries, at once.

    An efficient ” ENERGY HARVESTING” model. The device will be able to recharge the batteries while the aircraft flights.

    • “The device will be able to recharge the batteries while the aircraft flights.”
      Says it all for a complete farrago of nonsense, but cant be callenged as its ‘high school science’
      Theres the magic devices, the special materials- I loved the one where the plane is built from plastic foam- but the major requirement is always a ‘crystal ball’

      • I don’t understand the objection. Photovoltaic aircraft surfaces work fine when exposed to light, whether the plane is on the ground or in the air. Their weight can be small and their other properties suitable. That’s why specialized military and intel aircraft are PV-powered. Some can stay aloft indefinitely. The concept is proven; the application to civilian commercial flight awaits the right technology, application, and economics.

        If “plastic foam” was a reference to plastic lattice (covered with a tough elastomeric membrane), please check the reference I provided; the lattice’s cells are of ~cm scale and made of engineering plastics like ABS, though they could be made of carbon fiber, carbon nanotube, etc if necessary.

    • Aircraft do often need to apply air brakes or spoilers as they maneuver to descend rapidly and line up with the runway. They are often forced into this by air traffic controls. Putting the propellers into wind mull mode this way is merely an efficient air brake that comes almost for free (apart from some electronics)

      Any use of regeneration is however truly a waste of energy. The aircraft is much better gliding down to its landing.

      I think Robert Maxwell proved that only 57% of energy could be extracted from an air stream so you can only recover a small amount of potential kinetic energy.

      Nevertheless regeneration has its use. It will be an highly effective air brake.
      Also Putting the propellers into ‘wind mill mode’ after landing may save some brake pad wear as the drag may be quite high as well as recover small amounts of energy.

      If cars had 10 seconds of warning before a traffic light turned red they would simply coast to a stop and never waste fuel. If cars had a 35 second warning they would simply coast to half speed, bunch up and never stop at traffic light again. It would probably render the need for regenerative braking in cars moot except for hills.

      I suspect better air traffic management will end up saving larger amounts of energy.

      • Does the logic count the potential benefit of carrying less battery weight for reserve range to the extent one can count on some degree of energy recovery in descent (particularly under emergency conditions) to recharge?

        • I Pipistrel who make the alpha electric trainer variously refer to a 6% to 13% recovery in charge. I don’t know how that is measured but it might give them an additional take off and landing given that as a trainer they might be doing that say 4 times every hour. It might also give them an additional ‘go around’ charge and save 5-10 minutes on recharge.

          Regeneration will be a feature of electric aircraft simply because 1/ aircraft benefit greatly from having air brakes to manoeuvre (eg runway line up) and 2/”regenerative wind turbine” mode comes at zero hardware cost. 12 transistors are needed irrespective and the software is already written by the motor supplier (Siemens now Rolls Royce)

          It can be hard to loose altitude in a high performance sail plane without exceeding maximum speed or gaining so much speed a landing is impossible so these aircraft must have air brakes. Since electric aircraft will all have high L/D ratios they will need air brakes.

          There are self launching electric sail planes (good for 3-4 launches per charge) and at least one ultra high performance sailplane on the market a 2 seat side by side seating model which has conformal photo voltaic cells integrated into the wing that can recharge the battery in flight. It must be quite some engineering feat to keep wing smoothness.

          Sailplane flight is a glorious thing to experience but the wealthy men that own them will pay up to $600,000 for one. They do seem to have success in gliding clubs where you share the costs on slightly less expensive models and in gliding mad Germany and Switzerland you can launch an aircraft without earning the ire of the neighbours noise objections. Else you’ve got to winch it up.

  16. One area not adequately being discussed here is the certification of the battery system, and the re-certification requirements if/when battery technology improve after the initial type certification. The second has to do with residual values and aircraft financing.
    We all know the failure modes of a modern fuel system, but there is a great deal of uncharted waters on battery systems when they become the primary source on a large Part 23 design. How well do we understand the failure modes, and are the current FARs adequate for battery aircraft? Then, the failure mode analysis and the certification would have to be repeated when battery technology improves, so it is not a plug and play each cycle when a new battery arrives – making a 20 year program level investment a challenging business case. In fact, continuous breakthroughs in battery technologies can be the single largest threat for electric aircraft financing as the risks in residual values become more uncertain. Most Part 25 aircraft can structure a 20 to 25 year amortization at LIBOR +1% to 2% at 3 to 5 year terms. This makes aircraft lease or financing payments workable from an operator’s cashflow needs. The 20 to 25 years is attained not just because of the design specs, but low true depreciation as aerospace technology evolves slowly and there haven’t been true game changers since we went from piston props to gas turbine. Electric aircraft can be self-disruptive with rapid changes to battery technology (e.g. doubling range with lower gross weight every 5 years, etc.) making long term 20-30 years amortization unlikely.

    • Battery leasing and maintenance might become a business. The lessor will charge a fixed rate to account for the capital of the battery plus an additional amount for each KW.Hr charged and discharged since this effects cycle life. The rate may increase for that component of charge that exceeds 50% . Some airlines doing only 80 nautical mile hops between islands twice per day may not need premium or new batteries.

  17. One of the downsides of electric flight is that recharging will be high value energy delivered at high power levels on demand. We may have the situation that electric aircraft are recharged by other stationary batteries (because the wind farm is becalmed) or at night by hydrogen powered fuel cells because their is no sun. The only way around this is exchangeable battery packs or the use of PtL fuel.

  18. Is the assumption that the Heart ES-19 and Beechcraft 1900D use the same amount of energy a justified one?

    The Beechcraft is a clever little small aircraft (I’ve enjoyed a few flights in the passenger seat) but it is almost 40 years old, and many things have advanced since then.

    It is reasonable to assume that the ES-19 airframe and wing will have reduced drag and greater efficiency. It is also reasonable to assume that the ES-19 will have greater efficiency in descent with the ability to fully feather props, and that it will use less energy on the ramp as there are no turbines that must be kept hot until takeoff. Improved avionics and control systems, alongside lighter airframe and wing construction will also reduce weight and improve efficiency. Quite apart

    None of this negates the penalties imposed by weight of batteries, but these might not be as severe as described in this article. United Airlines seems to have decided that there’s a point at which the tradeoffs become worth it.

    • However if we built a modern Beech 19 seater we might expect it to also use technology as advanced as the Heart ES19. I would expect a lightweight composite wing and wing box, slippery laminar flow surfaces, lithium alloy fuselage, more efficient engines, lighter props and likely the aircraft battery would be lighter.

      There is also a German proposal for a electric 19 seater. They propose to optimise the twin engine aircaft for flights less than 200 NM but have gas turbines that can be attached to the rear of the electric motors to supplement them during longer flights and allow ferrying. Possibly a workable idea.

      To me it seems much simpler and more feint to produce a PtL fuel.

  19. The comparison seems slightly unfair, because it seems to assume the fuel system is weightless? Pumps, pipes etc. Also not clear if there is a weight difference between the PW engines and the electric motors. I would also suspect the electric airplane to dispense with hydraulics in favour of an all-electric architecture, which should save further weight? Plus new materials etc as pointed out already. It won’t overcome the weight penalty but will mitigate it.

  20. I love analyses like this, thank you for sharing. Almost everyone accepts the benefit of “sustainable” technologies as a given without questioning the premise. I’d like to see a similar analysis applied to SAFs. I simply cannot ignore the devastating impact on our planet, let alone the direct and opportunity costs (e.g., plants for fuel not for food), that SAFs present. Watch Michael Moore’s documentary “Planet of the Humans” and you’ll see how the claimed benefits of biomass/biofuels is all a massive lie. The SAF house of cards is build on a claim that the plants absorb the CO2 but in fact requires
    massive deforestation if any scale is to be achieved. The cost and enviromental impact of accumulating the feedstock, which needs to be converted (energy, pollution and water consumption), refined (just like petroleum, more energy, pollution and water consumption) and then burned – just like petroleum actually makes SAFs WORSE than petroleum. Heck, just leave the trees as they are and do as we always have – very EFFICIENTLY extract hydrocarbons from the ground with comparatively minimal environmental impact, just work to do it more efficiently and cleanly. I challenge anyone to find a similar critical analysis of SAFs. I’ve tried, unsuccessfully. The media and policiticans and the ignorant just accept it on faith, in fact it is their faith. We engineers needs to stand up and speak out.

  21. Bjorn, any plans for an analysis of United’s 100 plane “order” from Heart Aerospace?

  22. Thankyou Bjorn.

    So the Heart or such is unworkable, perhaps a fantasy, but employs some people out of someone’s pocket including taxpayers.

    Remember the ‘dotcom bubble’ – investors are not necessarily smart.

    In this case has the cachet of a fad and politics.

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