Bjorn’s Corner: Sustainable Air Transport. Part 7. Hybrids.

By Bjorn Fehrm

February 18, 2022, ©. Leeham News: Having covered batterie-based electric aircraft in the last three articles, we now move to the next ideas to improve our aircraft’s efficiency and burn less carbon-based fuels, hybrids.

A word of caution first. The words Sustainable, Electric, and Hybrid are, because of their success in ground transportation, virtually a must in any news release from anyone involved in aviation these days. We will unveil what is behind all this and what is real and what’s Greenwashing.

Figure 1. The successful ground hybrid, Toyota Prius. Source; Toyota.

What does Hybrid mean?

It used to be when a major portion of the propulsive power of a vehicle comes from an electric motor, like for the Toyota Prius, Figure 1 (for the Prius it’s 100%). While this used to be the case for aeronautical vehicles, it’s no more.

Today, with the Sustainability value embodied in the word Hybrid, anything that has the tiniest electric motor involved somewhere in the aircraft’s propulsion, even if it contributes perhaps a percent or two, is labeled as “ using Hybrid Technology.”

We have explained why the concept works for cars. These energy hogs (they use 5-7% of the energy in the fuel) waste all the energy used for acceleration when braking for the next stoplight. The Hybrid instead brakes with the generator and regenerates the energy to the battery. That this is the case shows by hybrids being super efficient as town taxis, whereas, on the highway, they show the same mpg as a regular car.

As we have no stoplights in the sky the regeneration of energy to a battery doesn’t work (and no, you can’t regenerate energy in the descent, we come to that).

Back to Civil Air Transport and the Greenwashing going on today.

When Boeing launched the 787 as “more electric’ in 2003, it would today be “using hybrid technology.” By this, I’m not accusing Boeing of doing it (it hasn’t done it); I’m just giving examples of what things used to be called and what’s the marketing hype of today.

So what’s a Hybrid in the aeronautical sense

It’s where an electric motor is substantially involved in powering the thrust-producing part, the propeller or fan.

A Hybrid is not when the engine’s starter generator is used to generate power to an electric system. Starter generators have been around for 100 years or more, have always worked as a motor to start the aircraft from a battery or ground electric source and then, once the engine runs, change role to a generator for the aircraft. It then supplies the aircraft’s system with electricity (an electric motor can serve as a generator, you change some electric fields and done). This is nothing new, it’s been done this way on small aircraft since the beginning of aviation.

This hasn’t been called “hybrid technology” during these 100 years, but it is today. We won’t call this a hybrid, and others shouldn’t. What we call a hybrid is the following:

A propulsive concept where the electric motor supplies an important part of the power to a fan our propeller as in Figure 1.

Figure 2. Different propulsion alternatives. Source: Leeham Co.

Figure 2 shows the principle differences between turbofan/turboprop propulsion, battery-based, and hybrid propulsion concepts. It also shows the typical path projects take as they work through the plus and minuses of these concepts as a replacement for today’s turbofan/turboprops.

Serial Hybrid

A serial hybrid often results when a project starts as a battery aircraft and the design team realizes that the range/endurance is not practical. Most projects start as battery-based, then migrate to battery plus “range extender” to fix the problems we saw with the Alice and ES-19.

The range for such aircraft (pilots always think in flying time/endurance, weather/wind then convert this to range) is not enough for everyday use. The risks for pilots and passengers are too high that a flight runs out of Alternate or Go Around energy. What battery entrepreneurs don’t understand is that experienced pilots NEVER fly with only regulatory reserves; they always add on top, and often they add a lot. Real weather is too unpredictable for you to rely on a forecast. Risking passengers’ life because “we fly Sustainably” is not on.

A range extender is harder to make work than it seems. Project after project abandons the idea because when all the safety precautions (failure mode and effects analysis, etc) are worked through, the project now has two propulsion systems. The result is, the Hybrid is heavier, more complex, and most times not better in fuel economy than the turbofan/turboprop propulsion it shall replace.

Parallel Hybrid

Often hybrid projects start with a serial hybrid, then discover it doesn’t bring enough improvement over the turbofans/turboprops it shall replace. The next stop is a parallel hybrid.

A downsized gas turbine is working in parallel with the electric motor feed by a battery system for takeoff and climb. During cruise, the gas turbine generates the power for the fan/propeller.

This concept assumes a downsized gas turbine, that doesn’t have to be sized for takeoff and climb, will be more economical than one sized for a mission’s varying loads. We will check if this is the case.

The final case

There is a final case, the embedded starter/generator, that we will look at, Figure 3.

Figure 3. The more electric engine. Source: Rolls Royce and Leeham Co.

By an enlarged embedded starter motor cutting in and helping the gas turbine during transients (increasing/decreasing power), the gas turbine can be optimized harder. While this is valid technology and has been studied and tested for decades, it’s questionable if it is to be called hybrid technology. But it is today.

We will look at all these concepts in the following Corners and analyze if and how these contribute to lower emissions.

32 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 7. Hybrids.

  1. I think Airbus (maybe others too) uses the term “hybridized” for what the article describes, i.e. simply using electricity instead of hydraulics for some systems.

    Marketing at its best. Or worst, depending on your viewpoint.

    • Definitely for the worst.

      Aviation is not allowed to come up with its own definitions.

      So, we don’t use those terms and they fail as no traction.

      A Hybrid uses a significant % of a motor for propulsion.

      A Prius was not intended as a long distance steady traveling highway speeds vehicle, it was oriented to around city driving where its benefits pay off.

      In an interesting twist (pun intended) is the Ford F-150 that is dropping its diesel because its Hybrid variant has better mpg and torque than the diesel does (our of a small V-6 though its high hp and twin turbo)

      • -> “A Prius was not intended as a long distance steady traveling highway speeds vehicle, it was oriented to around city driving where its benefits pay off.”


        EPA fuel economy (city/hwy):
        2003 Toyota Prius (the first one available for the U.S., IIRC) 42/41
        2003 Toyota Corolla 25/34

        The Corolla would still burn around 20% more gas than the Prius.

  2. Very nice, Bjorn.

    One of the benefits of electrification, regardless of application, is that one typically can reduce the parts count; seals, gaskets, airfoils, injectors, valves, hoses, pumps, regulators, particle filters, flange connectors, sensors and so forth. Typically with a hybrid solution, one will end up with an increased parts count, which is kind of a retrograde step.

    Some argue that hybrid systems are still valid as a practical bridge solution, and I suppose that’s a fair point in some ways.

    I think the 787 constitutes a relatively significant milestone in the development path toward electrically powered auxilliary subsystems as opposed to ones powered by pneumatics and hydralics. Kudos to Boeing for moving the boundaries forward!

    • John B:

      I agree on the 787. Its more electric was the one area I was concerned with on it.

      While fewer moving parts the electronics end and in minimum weight needed on an aircraft was a complete stretch (or so I thought)

      One of the areas I worked in was Building Pneumatic Controls. In that case, they tried to go cheap for the elecric/electronic actuator replacements and lots of failures and total throw away items (early 80s, Electronic Drive for Fans speed control also kicked in during that time).

      By the 2005 or so they had it down pretty good though for fan controls we found you needed to up-size pretty much double the Variable Speed (VFD) units size, it it was a 10 hp motor, 20 hp VFD to drive it. Otherwise they failed in a few years (I blame the Europeans with the IEEC rating garbage vs us NEMA that was seriously robust and good)

      On the 787 they acualy got more complex over bleed air (aka Pneumatic) but the benefit was routing, duct size and much easier and flexible with wires.

      All my building controls from 2005 (roughly) on were electronic.

      I converted a lot of fan/boiler and room controls to Electronic control using the old Pneumatic actuators via a EP (Electronic to Pneumatic ) Transducer. That was a wonderful mix as the actuators end was really simple and reliable.

      That said it took an air compressor to supply the air (20 lb systems) and of course a distribution of tubes (easy as wire due to the size and low volume).

      A Pneumatic actuator is nothign more than a can with a diaphragm in it and a shaft. The Electronic actuators had to have gear systems. The heating valve type were the worst.

      Sadly, like the 787, power is easier to grab off a local panel than a compressor plant (you need at least a dual compressor so you can work on one as needed).

      One of the electronic solutions was to have a 100 VA transformer bank in the panel room and off one circuit and run the 24 volt wires to the Actuators (one for a damper and one for a valve) in a typical room control.

      Others would put the transformer at the room unit and daisy chain 120 to them from the nearest panel.

      Cost wise the all electric worked better, function wise I liked the Pneumatics but new building went with cost and it was impossible to argue with both the lower install costs and the lower costs overall as no compressor was needed.

      Ironically for the flight Simulator we had 3 air compressors as two were needed for the Fake Oxygen supply and one to keep their life raft blown up!

      • IEC ratings are generally type 1 and type 2. In type 1 (lighter duty) circuit breakers, contactors may become damaged during a severe fault. In Type 2 they do not sustain damage. Generally its a matter of oversizing. NEMA equipment is very tough but simply too expensive hence its limited popularity outside of North America. NEMA have come up with a lighter specifications.

        To be reliable complex electrical systems need good, professional termination technology. The plating on the wires and terminals has to be with good alloys that don’t interract galvanically. Terminals need to be torqued properly or better still gas tight insulation tech used. What works for 100 terminations in a house won’t work for 10.000 in a complex aircraft or plant.

        Cars need to have air tighter ferrules embeded in epoxy so they are absolutely corrosion and moisture free for high reliability.

        Terminations need to be studied and understood carefully.

  3. -If an electric motor powered aircraft (either propeller or EDF) could be powered by a combination of batteries and fuel cells it would still be an hybrid wouldn’t it. This would eliminate at least the drive train complexity. SOFC cells burning hydrocarbons would be ideal.
    -Also an aircraft which could use hydrogen stored in the tail and ‘kerosene’ in the wings thereby allowing enough fuel to be carried for say transatlantic service. It might use the hydrogen in fuel cells and the kerosene in gas turbines or some kind of integrated gas turbine-electric system. Possibly an interesting way to obtain reverse thrust.

    • Mr. William ,
      That’s a pleasing thought , but fuel-cells are not competitive with gas-turbines on a power/weight basis . They also require much more space , this necessarily being with the fuselage .
      As to the battery-power ; this is definitely far to inefficient on an energy-density basis to be useful for most applications . The only practical version would be a “Roll-on , Roll-off” architecture , whereby the plane made a series of small trips , having a Ro-Ro battery quickly switched-out at every stop .
      The above is a challenging paradigm , but would avoid passenger impatience and wastage of the aircraft’s valuable time .

      • We are seeing improvements in PEM fuel cell volumetric and weight density such that we are now seeing mass produced cars as well as several conversions of aircraft such as ATR, DASH/Q series and Deutsche aircraft D328 being offered. They’re not there yet but they are flying with real test pilots.
        I’m thinking the PEM fuel cells should be sized as small as possible but sufficient to allow the aircraft to ‘hold’ in case of a runway problem and battery depletion and extend divert range. and compensate for head winds. Things that will occur rarely. During flight they will opperate at a low level so as to increase battery life (worsened by having to reduced battery size to allow the weight of the fuel cells.)
        SOFC cells are seeing progress as well but not as much nor is their 5000 hours satisfactory. Attractive because of their higher efficiency and ability to use hydrocarbons.

  4. Well , “Hybrid” says you mix/merge two or more “things”.
    The rest is ideologic freight 🙂

    Best application of “hybrid” is mixing orthogonal technologies.

    an IC sustainer engine or fuel cell is good at delivering a constant medium amount of power. Batteries and electric motors are good at delivering lost of energy in a short timeframe.

    In this vein: The Airbus Hybrid Helicopter project is a perfect application of what I said.

    • -Do you mean the Eurocopter X3?
      -Or the Airbus Flightlab helicopter’s whose main gearbox will have a 100 kW electric motor connected to it to provide electrical power for 30 seconds in the event of engine failure, according to Airbus. The aim is to give pilots extra time to react to a possible failure?
      -There are clearly going to be opportunities to use electric motors in the tail rotor or on stub wings in helicopters and autogyros. Batteries can be made to have extremely high power outputs for very shot periods.

        • Every rocket that reaches ‘inner or outer space’ has some sort of pumps to combine fuel and oxidiser.

          • There were three traditional ways to power these turbopumps each with its own advantages and disadvantages.
            1 A separate turbopump system with its own fuel and oxidiser. This is what the German V2 (EMW Aggregate A4) used with hydrogen peroxide and a type of hydrazine methanol mix (T Stoff).
            2 Burning a portion of the fuel and oxidiser to drive the turbopump and venting the exhaust or putting it through the engine.
            3 Using the heat of the nozzle and chamber to expand the fuel to drive the turbopump and injecting the fuel into the camber. This system was used with LH and LOX on the upper stages of Saturn V. It is simple, reliable and efficient but is inefficient if there is atmospheric backpressure.
            4 A hybrid which uses regenerated cooling energy boosted by burning, turbo pump and injection to the chamber. This was used on the space shuttle SSME and their impulse of 455 seconds.
            These SSME were incredibly maintenance hogs due to the pressures and temperatures required. Very difficult.

            So the electric turbopumps really simply things and are easy to control. The battery system probably also does duty powering the hydraulics for gimbals and electronics.

            Interestingly the German V2 was the first use of a ‘molten salt battery’. Heat from the turbopump exhaust melted a salt battery that powered the onboard electronics and invertors and I think the hydraulic system. I think also destined for a proximity fuse.

            Today such batteries with a pyrotechnical charge to mobilise them are the main power source for missiles, proximity fuses. In event of duel engine loss on a Panavia Tornado (interdictor variant) a molten salt battery activates to keep the hydraulics going (when the NiCd is exhausted)

            In the ADV variant a RAT deploys as it is blocked when the wheels extend you have to have balls for a wheels up landing or eject.

          • Topic is Electric, isn’t it?

            Had you followed the link you’d have seen
            that it is about electtrically driven pumps running from LiIon Batteries.

        • Electron Rocket: Both stages use the Rutherford rocket engine, the first electric-pump-fed engine to power an orbital rocket. The electric pumps are powered by lithium-polymer batteries. The second stage uses three batteries which are “hot swapped”, two of the batteries are jettisoned once depleted to shed mass.[25] There are nine Rutherford engines on the first stage and one vacuum-optimized version on the second stage. The first stage engines deliver 162 kN (36,000 lbf) of thrust and the second stage delivers 22 kN (4,900 lbf) of thrust. Almost all of the engines’ parts are 3D printed to save time and money in the manufacturing process.

          They also have a booster called “Neutron” under development which is a fly back system. The electric pumps would greatly simplify fly back and landing. Perhaps a RAT during fly back can charge the batteries for relight for landing.

          I imagine the batteries no only operate the turbo pumps but the gimbal actuators and power the electronics.

  5. Mr. William ,
    That’s a pleasing thought , but fuel-cells are not competitive with gas-turbines on a power/weight basis . They also require much more space , this necessarily being with the fuselage .
    As to the battery-power ; this is definitely far to inefficient on an energy-density basis to be useful for most applications . The only practical version would be a “Roll-on , Roll-off” architecture , whereby the plane made a series of small trips , having a Ro-Ro battery quickly switched-out at every stop .
    The above is a challenging paradigm , but would avoid passenger impatience and wastage of the aircraft’s valuable time .

  6. Car hogs @ 5-7% ? Hmmm. A 737 on the runway from zero to Rotate. I make the fuel “efficiency”, from JETA1 energy to aircraft kinetic energy 8 %.

      • Yes. All thats hes shown is the accelerate from zero to take off speed with the tyre rolling resistance and inertia of a 90 tonne plane with 175 passengers isnt the best place to look at its overall fuel efficiency. And never compare to a care with 4 passengers which might accelerate to highway cruising speed which is far less than takeoff speed.

  7. -I suppose the ultimate eco friendly aircraft will be hydrogen fuel cell powered, about 60% efficient and emitting only water in a form that does not condense. The only sound will be that of the Electric Ducted Fans, A giant whisper quiet flying wing which makes almost no sound.
    -I can imagine a mix of fuel cells and a gas turbine plant perhaps with a downstream supercritical carbon dioxide engine. One or two large gas turbines optimally placed where aerodynamically and structurally convenient with fuel cells distributed in the various spaces of the airframe to allow flight with the turbine plant shutdown or faulted.
    -The only battery that could supply power at the levels to almost compete with hydrocarbons is the Aluminium Air battery. It must be replenished by anode replacement as a cartridge but this seems to be a practical task. This form of propulsion won’t even emit water vapour. We may see aircraft that are propelled by electric fans or propellers use a mix of battery, fuel cell and turbine plant if technologies achieve that level

  8. -> ” … whereas, on the highway, [hybrids] show the same mpg as a regular car.”


    In U.S., for example:

    EPA rating (city/hwy)
    Toyota Prius 54/50
    Toyota Corolla 30/38

    Honda Insight 55/49
    Honda Civic 1.5 turbo 4 dr 33/42

    • The best comparison would be the Toyota Corolla Hybrid versus the standard Corolla. EPA figures city/hwy/combined:
      Corolla 1.8L Hybrid 53/52/52
      Corolla 1.8L Auto 30/38/33
      (Note manual, automatic, 1.8L and 2.0L models all show the same economy)

      It’s clear that there is a great deal of manoeuvring (accelerating to overtake then slowing/braking again) and hill climbing and hill descent in the EPA highway cycle. They hybrid has a double effect: recovering braking energy and hill descent energy and keeping the ICE running at optimal.

      The Hybrid might struggle on a German Autobahn

      On an aircraft its better to glide but occasionally a steep descent is called for and energy can be recovered by using the propellers/fans.It’s about 6% on the Piperstral Alph Teainer.

      • 1) ” … hill climbing and hill descent in the EPA highway cycle.”

        Lol. Such fiction writing.

        2) “Note manual, automatic, 1.8L and 2.0L models all show the same economy”


        EPA rating (city/hwy)
        Toyota Corolla 1.8 M6 29/39
        Toyota Corolla 2.0 M6 29/36

        Toyota Corolla 1.8 CVT 30/38
        Toyota Corolla 2.0 CVT 31/40

        Toyota Corolla XLE 1.8 CVT 29/37
        Toyota Corolla XSE 2.0 CVT 31/38

        3) What evidence do you have to claim that: “The Hybrid might struggle on a German Autobahn”??

        • 1 The EPA cycle since 2008 included the high speed cycle (as well as a cold and air conditioning cycle) which is used to adjust the highway cycle. The high speed cycle includes aggressive acceleration and stops. These are equivalent to climbing/accelerating and descending/braking. See the link you provided to see the curves and I think a regime the hybrid performs well in.

          2 The difference in fuel consumption between various non hybrid corolla models are small.

          3 Reviews of the Prius in motoring journals 15-20 years ago sometimes noted that the fuel consumption increased at high speed. In one case I read that a BMW 3 series could outperform the early Prius on a high speed highway run (Sydney to Canberra).

          The Prius works in part by using the electric motor to ensure the petrol motor is always running at its optimal point. Past a certain point of car speed the petrol engine must generate power by higher RPM which reduces efficiency due to intake suction losses, exhaust losses etc. Larger engines can be more efficient than small in these situations where the small engine is running in an inefficient part of its map. The modern Prius is a far more refined product and the ICE has been refined.

          The “rightful speed” on a German Autobahn is 130kmh/(80mph) however if the autobahn is 3 lanes and there are no intersections (merging traffic) one has no speed limit on the inside lane except that governed by responsible driving. Speeds of 160km-170km are pretty common. I’m suggesting that this is a speed a Prius would start loose its advantage. It apparently performs well on autobahn in part because there are so much congestions that that slow but don’t stop traffic and construction zones. One reason the German makers were slow to adopt the Dutch developed van Dorne CVT is because they don’t work well on autobahn. The German developed Scaefler Luk chain drive (used in Subarus) works at a wide gear ratio.

          However the hybrid now works very well on realisic autobahn. The battery charges at cruise to be available for accelerations.

          Either way given the enormous savings the technology could have been widely distributed by now. I find it bizzare the tech wasn’t regulated into the market.

    • It’s worth noting that assuming a 37% saving in fuel burn that were all automobiles and light trucks hybrids that CO2 emissions would be down by over 10% globabaly more than 3-4 times what zero emissions aviation will save.
      The Prius came to market in 1987, 35 years ago.

        • -Prius production started in 1997 according to Wikipedia. That is 25 years ago.
          -Sheesh, you could have just correct my minor mistake which is not substantial to the point that the technology has been available long enough for it to be installed on almost all cars by now?

          • In U.S., the Prius sales has sunk from a 2012 high of almost 240,000 to less than 60,000 last year!

            Total hybrid sales only reached 5% of the U.S. market after almost twenty years!!

          • Look at US SUV and Small Trucks sales.
            Environmental considerations are not a going concern
            ( except you can suck money from abroad.)

            Squandering resources is a US nation sport, apparently.

        • 1997

          Today’s Prius is not the ’97 Prius anymore.
          ( 4th Gen now. things improve. It isn’t a US designed small Truck)

          Major gains in Batteries and VFD Electronics.
          ( Why I think Boeing went into “High Power more electric” too early. Water cooling could have been avoided.)

          • Power Electronics and Permanent Magnet motors were fairly well developed by the year 1997 and certainly by 2012. I worked in a cable plant installing Variable Speed Drives as a graduate. No doubt cheaper, smaller and lighter now but they worked quite well. The 1.0-1.3kW.Hr NiMH battery was also a reliable device.
            Some of the recent gains in hybrids have been due to the surprising improvements in the ICE.
            American engineers developed quite a few hybrids in the years that followed Prius’s release. I think Pickup Trucks featured quite a lot and some provided generator power for use by tradesman and farmers. They never took sold much and I can only assume fuel was too cheap and the love of these powerful trucks too great. That may be changing. (They are easy to drive, handle well and I notice that there performance is such they don’t hold up traffic at lights causing jams like I see in some countries)
            So with the benefit of hindsight I have to say regulations of some kind are needed to push the hybrid.

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