Bjorn’s Corner: Sustainable Air Transport. Part 6. Energy consumption.

By Bjorn Fehrm

February 11, 2022, ©. Leeham News: In a sister article, Part 6P. Energy consumption, the deeper discussion we use Leeham’s Aircraft Performance Model from our consulting practice to generate the aeronautical energy consumption for aircraft like Eviation’s Alice and Heart Aerospace’s ES-19.

This is the energy needed to combat the drag of the airframe during flight (Figure 1). We then add the losses in the chosen propulsion system to arrive at the energy drawn from the energy source.


Figure 1. Simplified force model for an aircraft. Source; Leeham Co.

Energy consumption for an aircraft

The energy needed for the aircraft is found through the force required to combat the drag on the airframe as it flies through the air, Figure 1. When we multiply the force with the traveled distance, we get the energy consumed.

When we climb, the force combines drag and the force needed to get the aircraft to a higher altitude. The potential energy we gain can be used to neutralize the drag on the way down.

A passenger aircraft must descend at an acceptable rate for the passenger’s ears, therefore, we cannot descend at a rate where we gain energy back in the descent, but the energy consumption is low in this phase.

The results for Eviation’s Alice and Heart Aerospace’s ES-19

The claimed range for the Alice is 440nm with reserves, for the ES-19 216nm including reserves. The results from the model doesn’t agree. Neither aircraft can fly the distances promised, not even with a 30 minutes VRF (fair weather) reserve.

The Alice, with its large 3720kg 820kWh battery, can fly a still air 200nm route in Europe and land with a 30 minutes regulatory VFR reserve. For the 45 minutes US reserve, there is not enough energy.

If an IFR alternate of 100nm is required with 30 minutes circling (EU rules), the range falls below 100nm.

The ES-19 can’t fly 200nm and have any reserves; in fact, the battery is empty before we land. It can’t even fly a 100nm flight and land with European VFR reserves. This assumes the 3,000kg (four times 750kg in the nacelles) batteries give us 660kWh of energy (the same energy density as the Alice batteries).

How can this be?

The problem for these aircraft is not that they consume a lot of energy, it’s the amount of energy they carry. The ES-19 takes off with 1/12 the energy of the 19 seat Beech 1900. As a result, everything is done to mimize energy consumed with a low cruise speed and wide wings. But it’s not enough when the energy is 8% of what a normal Commuter has.

Eight percent is not enough to fly to the destination and have anything left for reserves, not for bad weather conditions nor for good weather conditions if we have a headwind.

Didn’t the organizations behind the aircraft realize this?

The described companies are upstarts, started by enthusiastic entrepreneurs wishing to change how air transport is done. Unfortunately, architecture choices and presentations on what’s possible were made before aeronautically competent people arrived. When experienced engineers work through the problem, it’s too late. Media have been briefed on the “fantastic news,” and investors promised groundbreaking new aircraft.

The reality was all the time what we see in the Tecnam slide below. Realistic data from an experienced manufacturer. Solid, though not very exciting information.

Figure 2. The TECNAM P2012 in fueled and battery versions. Source: Tecnam.

What happens?

If we go by the more than handful of battery aircraft projects that have preceded these two, the next step is to look at “range-extending” by adding a gas turbine driving a generator to get more energy. As you calculate this through, the battery gradually shrinks to gain weight (the gas turbine with generator and fuel systems is not for free), and the fuel burned from the gas turbine increases.

In the end, you have a more complex and expensive aircraft than the one you replace without gaining anything in fuel burn. Then you start looking at hydrogen as an alternative.

27 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 6. Energy consumption.

  1. What a cold shower!
    I suspect that the slick CEOs of these hot-air upstarts would counter that “huge advances in battery tech are coming”. However, where that’s concerned, “seeing is believing” — lots of battery tech looks good on paper but is hampered by significant practical impediments (chemical stability, toxicity, flammability,…), making it difficult to apply in aviation. Just look at the “high-temperature superconductor” craze from 30 years ago, and the wet firecracker that that turned out to be.

    • Or the Hype over fusion generated electricity…and 25 years later we are still have 20 years to go….but it will run my fridge now

  2. Yes, one solution is pods with LH2 and fuel cells. Like expensive JATO tubes for bad weather and flying into known icing conditions. Those are not cheap and will be needed most days of the year once certified and allowed to be hoisted and plugged into the engine nacelles.

  3. -Ignoring the relatively unrefined E-19 and focusing on the Alice with its 820KW.Hr/3720 = 0.22KW.Hr/kg battery pack. My rough calculations suggest that a 15% improvement in battery capacity will increase the 30 minute VFR hold reserve to 45 minute VFR hold reserve. A 30% increase in capacity over the original battery would probably get a 100nmi increase in range. So a 45% improved battery would allow the Alice to fly 200nmi, hold for 45 minutes and divert 100nmi. This would seem to be within the capacity anticipated performance of batteries expected in the next 5 at most 10 years. This does mean only 50% of the battery will be used in typical operation which is good from the point of view of battery life.

    -I do not hold that there is much point to these aircraft which would have small niche markets island hoping (and perhaps green washing) when gaseous compressed hydrogen can fly further without the weight problems of battery and the infrastructure problems of cryogenic.
    -Incidentally there is a 10,000 ton Australian ship being built that will export 2000 tons of gaseous compressed hydrogen at 250bar. Basically just 2 side by side cylinders in the hold of the ship. It will join a ship already exporting cryogenic hydrogen. Gaseous hydrogen may beat cryogenic. The 700 bar vessels possible in cars and aircraft should allow a very usable range.

    -I see eVTOL a far more useful and practical development than fixed wing electric flight.
    1 Won’t need extensive hold and divert reserves since multiple nearby alternate landing zones are easy to provide.
    2 Since vertiports are easy to provide they can be placed so as to shorten the journey. It can be useful in journey’s from 30km to 150km perhaps to 300km whereas there are unlikely to be runways that close together. eVTOL will eat up most of the fixed wing electric market as well as create a new market.
    3 eVTOL, while placing demands on propulsion also needs smaller wings and lighter undercarriage.

    The Aluminium Air Battery may achieve an energy density of 2kW.Hr/Kg and that is going to be useful, ranges of 1500nmi I think.

    • Batteries on their own aren’t used , like home vacuum cleaner ( great things by the way)
      Packaging and support structure is needed too adding more weight. Cooling spaces and or liquids to cool the batteries especially during charging has to be considered. Im sure the electric motor needs cooling as well but as that highest requirement occurs at takeoff and climb it’s less an issue but the small diameter of the motor means a liquid cooling probably needed

      • -The cooling requirements and battery firewall packaging certainly add and overhead but there are ways around it. The “custom cells” to be used on the Ilium Jet have a chemistry that can operate at 150C which would allow air cooling and the heat pump to be dispensed with.
        -The improvements that increase charge and discharge rate also reduce thermal load.
        -At the end of the day the cells and batteries need to achieve higher densities. Elon Musk said that electric flight becomes compelling at 400Watt Hours per kg. Nothing Bjorn has said contradicts that at least for shorter flights of practical length. We are a long way from that, about half of that, though there are 400WHr preproduction cells in the lab.
        -Money from the automotive and consumer electronic industry will finance battery development and solve these problems (including recycling). It will take 15 years for automobiles to become substantially electric or fuel cell.
        -However road transport will also use a lot of fuel cells and I suspect general aviation as well.

        • William,

          we talk about the mass of a battery SYSTEM, the cell mass is 2/3 of this. So always do this calculation to understand what the system density is. And re non-cooled batteries, Lillium has to prove it can get these cells through certification. Pipistrel, who is the only OEM with a certified battery today, had to add liquid cooling to pass certification (the non-certified Velis has an air-cooled battery).

          • -Thanks, Musk when he quoted “400Watt Hours per kg” as the threshold for practical electric flight he was just talking & didn’t make any caveats but the numbers seems to work. Assuming your 66.6% battery packing rule of thumb it does get the 200nmi range Eviation Alice gets a little beyond the 45 minute VFR reserve.
            -I’m of the opinion that fixed wing electric aircraft (range 100nmi-200nmi) will be a fizzer in the market given the ease of making SAF but that eVTOL with half that range (50nmi-100nmi, 92.6km-185.2km) will be useful and successful. They’ll need to operate under different flight rules.
            -Fascinating is the Aluminium Air Battery supposedly capable of 2000W.Hr/Kg. Even if it only achieves 1200W.Hr/Kg it should produce a range of 1000nmi. I don’t see the refuelling by replacing electrodes as cartridges or whole batteries in LD-3/45 style containers as problematic. I imagine there would be say 4 batteries with 3 in use and the 4th kept as an emergency reserve that is seldom ‘activated’.

  4. Thank you Björn for an excellent analysis, and also to the commenters for adding a lot to the discussion.

    While aerospace is not my area of expertise, I do wonder if the operating economics of electric aircraft may make the case in some situations. I think we all agree that the range is less than ideal, seems with today’s technology the Alice manages 200 nm in perfect conditions (that could be London to Paris, so not entirely useless).

    But the operating economics might be fantastic. For example the Taurus-Electro claims in the advertisements that you can “fly-for-free” (see in particular the section on the solar trailer).

    Any thoughts on how the economics look like for the Alice?

    • Batteries are expensive and have a finite useful aircraft life. It add “maintenence cost reserves” per flight, similar to the life limited parts in jet engines.

    • I agree with that Elephant, but I think one that is just as big is the production of batteries and the recycling of batteries, plus the ecological issues associated with how the electricity used to charge them is created.

  5. The “range extender” can be a turbocharged ICE.
    Look at the performance of a modern F1 power unit. It mix amazing efficiency with low mass, good reliability and high power.
    And for a range extender you do not need the gearshift nor the fast response to the throttle.

    • -Hybrid automobiles work well because the electric motor is used to keep the ICE engine operating at the optimal point and by allowing the ICE engine to be optimised (Prius engines have light components because they don’t rev fast). Engines are switched of when the vehicle is stationary or coasting. If the vehicle needs to climb a hill or accelerate the ICE remains at optimum efficiency as the electric motors provides the power. The effective of braking regeneration is perhaps the secondary advantage.
      -These advantages are not as pronounced in hybrid gas turbine electric as aircraft engines are often running at 70% near their optimum anyway.
      -Nevertheless both Pratt & Whitney and GE are working on hybrid turboprops and they seem to have some hope. They allow the compressor to be controlled well and provide good handling. The electric motor can effectively replace the starter and alternator etc. Tight integration is needed to avoid the weight growth hybrid systems bring.

      • William, braking regeneration is the primary advantage of hybrid cars. Urban cycle mileage is the proof.

        • -The energy flows in hybrid drives systems is complex, subtle & interrelated. We can’t isolate the flows. Inter related to the point we can argue for anything. For the Toyota Hybrid Synergy system the larger part of the savings come from the ICE engine operating always at its optimal point nearly 44% thermal efficiency. Further savings accrue from the engine being optimised with the foreknowledge that it shall never be reeved at high speed, the engine is thus larger and lightly constructed saving weight (a big fuel saver) but providing good low RPM power and torque at efficient speeds.
          -In the Prius applying pressure to the brake pedal causes the motor to regenerate while concurrently making a request to the ABS system to mechanically brake the car (past a certain pressure).
          -You are indeed correct in saying the regenerative braking is the primary source of energy (for charging the battery) but the system would still work very well without regenerative braking by charging during driving and coasting as well as switching the engine of at stops.
          -A Tesla and most BEV are different in that the regenerative braking function is applied through the speed setting function of the accelerator pedal rather than brake. Some Tesla Model S owners have reported that 32% of a drives energy came from regenerative braking in hilly terrain. It’s more likely to be 20%. This is different from the regenerative braking that occurs when braking to zero speed in traffic as opposed to undulating hills.
          -The situation may be different for Formula 1.

          • Regenerative braking is VERY limited on most cars.
            very simply because its power is limited!
            The motor is used as a generator, therefore the braking power is identical to the motor power.
            Suppose you have an average car (not a TESLA!) you have a 200KW braking power.
            Good to slow down in a hilly terrain, but not good at all in urban conditions.
            A serious brake on a 1500Kg car needs 1500KW!
            hard braking requires a lot more: these who watch racing cars at night (for example Le Mans) will see the discs red or yellow…
            Regenerative braking is useful in some conditions but far from the miracle solution that most people believe.

          • But you don’t normally brake at the limit of your braking power.
            It would be a hell of a ride for your passengers.
            Fact: urban mileage is much better for a hybrid car compared with a similar ICE car, highway mileage stay same level.

    • Worth noting that Formula 1 is now using the HCCI and RCCI engine which can achieve an efficiency of 58% and is probably doing 50% in the race. They are as efficient as fuel cells when inviter losses are considered. The piston engine should not be discounted given that eFuel/PtL like SAF can be made at 60-65%

  6. One problem is that the people who are true aviation experts tend to be less vocal than the CEOs that are hoping for the best. That is why they are difficult to locate and engage. Plus there may be a tendency to keep them out of the public eye in case they reveal too much.

    • -There certainly is a naiveté out there. Some of it from people who don’t have any technical background who truly believe there are going to be transatlantic electric aircraft (believe me I chat to them) but as Daniel Wiegand (A fully educated aeronautical engineer) and the man who started Lilium Jet with friends said “Without Naïveté you don’t enter into these things” . By that he meant you don’t see all of the problems you need to overcome. The company now has nearly 500 million, Tom Enders as CEO, the guy that managed the Trent XWB as technical officer, an new test pilot with decades of experience in the RAF and at Leonardo/Westland, a flying testbed and is about to move to full transition flight in Spain.

      -The reality is batteries are improving all of the time. Apart from energy density there is charging rate (down to 10-15 minutes with energy density) and increased battery life. When Lilum first started batteries were such that when they got to 35% depletion they lacked the power to transition to vertical flight. Batteries now manage 10% for that.

      -While we might be sceptical of electric flight in 5 years what about 8-12? Some serious private money is now going into this from professional investors who know how to bet on tech.

      -Will the Lilium 6 seater eVTOL be in service end of 2014 flying 6 passengers with reserves 250km? Maybe not but I’m pretty sure it will be flying and probably able to take 4-6 passengers 150km and that it will probably achieve its full objectives a few years latter.

      -I see the Eviation Alice as being properly designed using the ‘extreme’ engineering needed on an airframe to make electric flight work over a useful distance.
      -The Tecnam P-Volt will be a test bed of such low performance range it will be without a market. The Hart ES-19 is a bit of a joke which unlike the ES19 doesn’t take the radical steps needed, it will attract subsidies. Maybe something will come of it, probably with fuel cells.

      • That’s mixing up solvable problems with basic research done to the electric plane promoters who claimed to have solved the problems before the research even began

        • Serious money comes largely because of how the US tax system works.
          Make huge paper profits on capital gains on a ‘startup’ of dubious/minor value and to avoid paying tax its spread amoung a plethora of possible new technologies which are called Qualified New Business Stock -QSBS and pay zero %. If it tanks then that loss is offset against other gains.

    • @ AndePac
      “CEOs that are hoping for the best”

      I don’t know if the verb “hoping” is the most appropriate…perhaps “playing” is more suitable. As long as the illusion of a viable company is kept up, the corporate officers get to enjoy a nice salary, lots of travel, media coverage and networking. Ever notice how many of today’s young CEOs try to emulate Steve Jobs? Black casual clothes, headset, stage podium, special effects – the whole thing is more about “look at me” than about the actual content. Young start-ups wallow in a world of “Corporate Instagram-ization” — looks, showmanship, upbeat message with carefully chosen hip terms are all more important than reality. And as long as they’re able to continue hoodwinking super-wealthy venture capitalists, the party roars on.

  7. -Electric flight makes a little less sense when you consider that much of the renewable energies generated (around 50%) will need to be stored as hydrogen then converted back to electricity by turbine or fuel cell to allow for long term storage. In Both cases the efficiency will be less than 50% (80% x 60%) . It thus makes sense to utilise the hydrogen directly in an aircraft (or car) rather than statistically running 50% of it through a gas turbine to generate electricity. We’ve seen that in Germany renewables have not been generating for months. A pipeline could probably store 2 months.
    Furthermore much if not most of the hydrogen in the network will need to be blue hydrogen so it makes even more sense to use it directly in a fuel cell or engine.
    -It’s worth noting that Tesla, who wants called Fuel Cells “Fool Cells” have developed a fuel cell car. Not a big thing for them to achieve. Hybrid cars and Fuel cell cars usually have 1.3-1.7kW.Hr batteries but pluggable versions with 8kW.Hr will give perhaps 40km on Battery only. (perhaps 16,000 cycles) and will have the best of both worlds: a small amount of direct charging off peak with hydrogen for longer trips.

Leave a Reply

Your email address will not be published. Required fields are marked *