Bjorn’s Corner: Electric aircraft, Part 10

By Bjorn Fehrm

September 1, 2017, ©. Leeham Co: in the nine previous Corners, we looked at 50-seat regional jets and turboprops with hybrid electric propulsion systems.

We have seen that at the state of technology until the mid-next decade, such aircraft have dubious efficiency. The hybrid propulsion chain weighs too much, and can at best match the propulsion efficiency of gas turbine based aircraft when one includes any airframe gains that can be made.

We will now finish the series by looking at a pure electric concept, designed for extremely short-haul routes. The Zunum Aero 10-seat commuter in Figure 1 will be our reference for such a design.

Figure 1. Zunum Aero’s short-haul turbofan commuter. Source: Zunum Aero.

A battery powered commuter

Our commuter aircraft, similar to the concept in Figure 1, will serve routes below 500nm. It’s a pressurized 10 seat commuter, used for short-haul routes of 100-450nm in its initial version.

The electric propulsion is now battery-based, Figure 2. This is simpler than a hybrid system. By using motors with 95% efficiency we can stay with air cooled motors and can therefore avoid a cryogenic cooling system in the aircraft.

Figure 2. The commuter’s battery based electric propulsion compared with a classical turbofan concept. Source: Leeham Co.

Power switches as discussed in last Corner will still be needed, as the battery will be split in two systems for redundancy, each capable of feeding each motor with power.

The commuter will use electric fan based propulsors, each of 500kW maximum power. The reason for using fan-based propulsors for an aircraft with a maximum cruise speed of ~300kts is the lower noise levels from shrouded fan blades.

A nacelle shrouds the fan blade tip noise in the radial direction and it contains sound damping sections in front of the fan and aft. A low noise level is important for a commuter, which shall use close-by urban airfields.

The 500kW of the propulsor motor is only used for five minutes during One Engine Inoperative take-off scenarios. Normal take-off power is 450kW per propulsor with cruise consuming 350kW each.

Weight of system

Normal empty weight for such 10 seat commuters lies around three tonnes (6,600lbm) with Max Take-Off Weights of five tonnes (11,000lbm). Fuel load is one tonne with full passenger load, which gives a range of 900nm.

If we assume a commute sector of 150nm, our fully electric aircraft would consume 45kWh during take-off, 160kWh during climb, 250kWh during cruise and 20kWh during descent and landing.

This totals 430kWh, which with 0.3kWh/kg specific power for our battery, gives a battery weight of 1435kg. This is the battery weight for our mission consumption. To this we shall add the safety reserves and a margin as not all routes are 150nm.

If we assume the routes shall stay below 450nm, our battery will weigh four tonnes. To this we shall add the propulsors of 150kg each. Our total propulsion system weighs 4,300kg.

This shall be compared to a turbofan system of 900kg and one tonne of fuel, giving us a range double that of our electric aircraft.

Our electric commuter would have an empty weight of around six tonnes with a Max Take-Off Weight of seven tonnes.

This shall be compared with the turbofan variants three tonnes empty weigh plus the one tonne of fuel = four tonnes weight before passengers are loaded.

Our electric aircraft weighs 50% more and has half the range.

Economics

The question is now: can an aircraft which is tanked from the Electric Grid compensate the higher weight with the cheap energy from our power stations? We will explore the operational economics of our 10-seat electric commuter in the next Corner and compare it with a turbofan equivalent.

23 Comments on “Bjorn’s Corner: Electric aircraft, Part 10

  1. If the energy for the batteries comes from the fossil energy networks currently dominating, we are fooling ourselves. If it’s wind energy probably too (bottom line energy balance to produce / operate a wind mill park). Biomass is non-sense, an d solar tiny tiny/ expensive). Nuclear seems possible if we can contain the waste but it scares the public.

    If the Zunum battery takes e.g. 2 hours to be fully loaded between flights, utilization /costs take a big hit. I think we as a society should focus on the big environmental issues and stop pumping billions in tech geek project that only do a tiny bit for a big problem.

    -> 75% Of airline passengers are for fun / holidays. Lets reduce that with 40%.. 6 fun holiday trips instead of 10 over a time period. Real action instead of hiding behind cool techutopia projects.

    Problem is, it has become a eco business / lobby in itself drifting on subsidies, fear and politicians that dropped physics when they were 15 yrs old.

    • Toyota put out an announcement a couple of weeks back. They’ve been working on solid state lithium-ion batteries, and the PR was about how they’re now building a factory. Coming to a Toyata near you in the early 2020s.

      The “so what” is that with a solid electrolyte, everything is held in place, it’s much more electrically robust. The result is that they can be charged very quickly. Suggestions of 5 minutes for a full charge on an electric car. Better lifetime too, and a touch more capacity.

      Ok, that’d need a phenomenal mains supply to deliver that much juice in such a short time, but the point is that batteries are beginning to head towards sensible re-fill times.

      Of course we’ve still got to solve the generation problem. With big enough batteries that could sink / source juice more easily than today’s slow charging Lithium ions, the role of renewables becomes saner as batteries can be more easily used for supply / load balancing on gusty days.

      50% heavier, half the range? Well, it sounds like high speed rail is a strong competitor.

      I’m deeply concerned that an awful lot is being mooted involving the use of hybrid this, electric that. Battery energy density simply hasn’t moved on that much at all in many years. That’s especially significant in weight-sensitive applications like aircraft propulsion.

      For cars it’s becoming harder to ignore electric propulsion (and Toyota’s solid state batteries are going make it harder still). Replacing steel built / petrol fuelled cars with CF or Aluminium built / battery driven cars isn’t a radical weight increase. In contrast, aircraft have always been light weight, so there’s very little room to recoup the battery weight by improving one’s design with better materials.

      It feels like we’d be better off making further optimisations to what we currently have (i.e. better gas turbines, lighter materials, limit the damage we’re doing), and wait for batteries to make a big improvement (so the switch becomes a no brainer).

      Making the switch to electric airliners too soon, before the batteries are really good enough, before grid generation has really cleaned itself up, feels to me like we’d be risking burning just as much (if not more) fuel now building / operating these things, only to throw them all away the moment someone does crack the battery problem. Sort of like building a wind turbine (which takes a lot of energy), but running it for only a few years (thus never recouping that energy) because a better design has come along.

      • The forgotten advantage of liquid fuel is the relatively short time it takes to refuel which supplies a very large amount of energy. I dont have the numbers at my finger tips, but the comparison with what you can do in less than 5 min holding the refueling nozzle for a car would takes many hours by battery recharger.

        • I think that for an airliner, with dedicated ground infrastructure, the way to go is to replace the batteries with a fully charged ones after every flight

    • Things have changed a lot in the last 5 years in the renewable energy field, specially on the solar power price.

      The wind stability is getting tackled with a mix of size (one region stabilizes another), smart batteries (electric companies will a % of connected batteries for stabilize the system) and Electrolysis (https://goo.gl/1tmSMp)

      And the bigger interest in the field will add more R&D to the field.

      PS: I still think that a mix of batteries for take off and landing and a power cell for cruising will help in the range issue

  2. The analysis is made with 2x500kW Engines assuming a modest L/D number and a cruising speed of 300kn. I suspect that a brand new design will have the best aero/structures available today and hence a +40 L/D number and reduced Power requirement at T-O. For such short range flights the cruising speed could be reduced a bit to save battery Power. If it could get ground assisted T-O (like sailplane wire T-O’s) it helps the range.
    Most likely the pax load must be approx 19 pax to cover the cost for a pilot, the 10pax design will maybe be popular and cheaper if unmanned short range all electrical passenger flights becomes allowed.

  3. Why push for a range of 450nm in v1 of the plane? I’m guessing there are sufficient opportunities for 150-200nm max pairings, where road networks aren’t up to scratch and where the total time of a business oriented travel via rail is inferior to relatively low traffic airfield to relatively low traffic airfield. Get v1 up and running, learn from it and upgrade the batteries sufficient for 300nm, 400nm, whatever, in the v2, as and when it is possible.

    • Certainly her in New Zealand there are a growing number of twelve seat aircraft covering routes to small towns. Mostly less than 300nm

    • Hi Woody,

      the idea of going for a first version with modest performance is the right one. But I’m afraid you can’t reduce the range requirement much. You need a certain minimum range/endurance to have weather security. Weather forecasts are inherently insecure so when you arrive at a say 200nm target and the area is non landable your alternate could be more than 200nm away.

      400-450nm range is probably the minimum for safety reasons. On top you have 5% route reserves for bad wind forecasts and 30min circling time minimum, this is included in my calculations.

      • Wouldn’t a procedural change mitigate this? For eg a 200nm route, flying at say 300mph equivalent, mid point is (I’d guess) 30 minutes or so from landing time. The very near time weather forecasts I use are pretty accurate 30 minutes ahead and my weather is quite variable. Might get light shower when dry forecast, gusts might be 10-15mph higher than forecast but that is as big as the delta seems to get.

        So, 200nm route, low traffic field to low traffic field (ie no queue issues) decision point is at mid point, 30 minutes from the 2 (or more) available fields. Might not suit fields prone to twisters etc but probably pretty workable for large parts of the planet for large parts of the year.

        May be pick a single jurisdiction (eg Paul’s NZ) that is comfortable with the new tech/operational packages. Trial v1, and take it from there.

  4. Airbus has a system for trim that moves the flaps rather than the conventional method of adding downforce at the horizontal stabilizers. Seems like a more sophicticated way of matching center of lift with center of gravity. Question: With the fan/electric motor setup, can trim be achieved through swiveling the propulsors a couple degrees?

    • “Down force on tail” in normal flight attitude is a core element in horizontal flight stability.
      Moving COG into COL reduces drag but also reduces intrinsic stability ( to zero ) though you can then use the FBW wrapping to show a stable airframe to the pilots.

    • Something similar was proposed for the eurofighter as a way of increasing the range. Could swiveling propulsors be used to improve stol performance?

      • Eurofighter was about 2 magnitudes different in performance. Its parameters were to use instability for low speed maneuverability and reduce supersonic drag
        The plane most like a short range commuter is the Piaggio Avanti which has forward lifting surface like a canard to unload the tail surfaces. 3 lifting services help increase its range and cruise speed as its capability is more like a small jet such as the Cessna M2

        • Grubbie is referring to a proposed TV upgrade to the design, one of the modeled benefits of which was that by moving them off axis aerodynamic efficiency improved.

  5. Doesn’t a large part of e-flight’s economic viability come down to not the in flight operating costs, but rather the much reduced cost of maintenance (with direct increase in TBO and lifetime operating times) and insurance, both driven by the drastic increase in simplicity/fewer moving parts, and thus far fewer points of failure?

    • This would be OK for the electric motor and control, but the battery is only lasting around 1000 charge cycles, and its the most expensive component in an electric aircraft IMO.

      • I agree that simplifcation, together with economy of scale, is the real strength of electrification, if combined with simplified structures maufacturing and designs. The economies of scale advantage of using very similar technology and perhaps even some identical components with road vehicles could be enormous for costs, reliability and development momentum.

        Re charge cycles, what I think would be of perhaps more concern at the moment is that with currently deployed energy densities the capacity loss after repeated charge cycles becomes significant. As energy densities rise I’d worry more about the maximum number of cycles, although an order of magnitude greater has already been demonstrated, IIRC, with slightly modified LiIon chemistry.

  6. Over the last 25 years, billions have been invested to improve battery density, loading time, safety etc. by the biggest corporations, top R&D institutes. Evolutionairy , small improvements have the results. Not even keeping pace with the booming demand from portable devices.

    We might all hope for breakthroughs, but not count them in to save our environment, and keep consuming meanwhile.

  7. My conclusion: we are not right there, but we know which points to tackle. Fossil fuel will get more and more expensive and carbon fuel, therefore, have to generated differently (e.g. from algae). And batteries will get better (e.g. Lithium-Air type with factor 10-20 higher energy density. Who knows what comes next). So, good to see there are attempts to get on track. Before Carl Benz build his car, there where a couple of other tries for individual driving that failed. I look forward for fascinating development! And, again, thanks Bjorn and to all others in this forum, to accompany this with an interesting, although critical discussion.

    • “And batteries will get better (e.g. Lithium-Air type with factor 10-20 higher energy density.”

      Nope, as far as I can see not. Sorry. We have to start planning with realistic expectations, not what we love to believe.

    • Fossil fuel will be with us for a very long time. Now there is a glut. When demand catches up w/supply prices rise, demand drops and new development becomes economical. In 1973 we were “running out of oil”; not then, not now. As Keesge says we really need behavior changes especially by the world’s better off people.

      Solution: a carbon tax (mostly refunded on a per-capita or similar basis) that makes the monetary cost of fossil fuels better reflect their environmental cost. The world (we people that is) is certainly not ready to do this globally; it would have to be by nations or groups of nations like the EU.

      If enacted at an adequate level (ramped up over maybe 15 – 20 years) this would, mostly, eliminate thermal coal use (this will be more difficult in China/India, etc), make petroleum much more expensive and nat. gas significantly more so. (Methane emissions must be tightly regulated.)

      If we (the rich world) could do this the market really would mostly take care of the rest. Short term (how much to fly or drive, etc.) and long term (what vehicle to buy or what aircraft to develop, etc.) behavior will change with the right incentives.

      I doubt we (especially U.S.A where I live) will do this anywhere near enough, but without real behavior change by us all, technology will not save the planet from radical warming, suffering, extinctions, large scale death of poor people, etc.

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