Bjorn’s Corner: Sustainable Air Transport. Part 10. Where Hybrids work.

By Bjorn Fehrm

March 11, 2022, ©. Leeham News: After our articles about Serial Hybrids and Parallel Hybrids showed they were unsuitable for airliners, where do these make sense?

The obvious answer is for our stop-and-go cars (as we then can recover the brake waste energy). Still, there are aeronautical special cases where hybrids can bring advantages. Let’s look into these.

Figure 1. The variable angle of rotor blades. Source: FAA Helicopter Flying Handbook.

Where the Hybrid can bring advantages

If a vehicle can gain major architectural benefits from the power output being electric rather than shaft power, a hybrid powertrain can be motivated. An example is to replace the complex and expensive mechanical rotor gearbox and rotor head of a helicopter and go for an electric-driven multirotor concept instead.

As explained in Figure 1, a helicopter needs to vary the rotor blades’ angle continuously during flight. It comes from the helicopter’s forward speed adding to the forward-going blade’s speed and decreasing the speed of the reward-going blade. To have an even lift, the angle of attack of the blades are continuously varied during the rotation.

A multirotor design like a drone or VTOL simplifies this by having at least four rotors, one in each corner, half rotating clockwise and half anti-clockwise. Now you can control the lift and pitch/roll/yaw by varying the RPM of the electric motors driving the rotors, Figure 2.

Figure 2. Multirotor control principles. Source: Wikipedia.

So a complex gearbox plus rotor head has morphed into several electric motors with fixed rotors. Control is now by shaping the alternating current from the motor control electronics to the motors, a much simpler design.

Here a Serial Hybrid makes sense, as it produces electric power that can be converted to the alternating current needed to control the motors.

The Parallel Hybrid can solve some other problems. One is as a supplementary safety system to a gas turbine propulsion system. Airbus is exploring a parallel hybrid configuration for its helicopters, where an electric motor working in parallel with the gas turbine can improve the helicopter’s autorotation emergency mode.

As it’s a short-term power injection to the rotor gearbox, the energy store weight problem we’ve seen, doesn’t become acute (we talk power for minutes only). Still, the electric motor and its energy store, the battery, is dead weight for all non-emergency situations. We will, therefore, see configurations where the motor is used as the main electric generator for normal flight and the battery used as the standard vehicle battery. In an emergency these switch to a short-term power boost for the helicopter rotor.

Finally, we have the enlarged starter-generator that helps a gas turbine be optimized closer to its limits, assisting with speed up and down transients. But this has always been called a “more electric “ technology, and we will continue to call it such.


A hybrid works in some instances, our everyday car being the prime example. For the car, it’s because of the stoplights; for air vehicles where we have no stoplights, the exceptions are different, as described above.

For the general aircraft case, where a hybrid delivers shaft power to a propeller or fan, the hybrid’s added complexity only increases manufacturing and operating costs, with no tangible reduction of greenhouse emissions.

32 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 10. Where Hybrids work.

  1. The catch, with quadcopter designs (if they really are QUADcopters, i.e. have just four rotors), is that if you lose one rotor, you’re spiralling out of control (literally).

    Most UAM concepts have at least eight rotors, adding to the complexity/weight. Some make do with just six rotors. These also have a fixed wing for cruise, so I suppose they could still land conventionally IF the failure comes during horizontal/cruise flight.

    Still, I’d expect all such multi-rotor designs to feature ballistic chutes.

    • Some redundancy can be obtained by having two motors on a common shaft or two motors integrated into one housing or a double wound motor with each winding fed by a separate inverter. There are also contra rotating coaxial props powered by a hollow shaft motor with a through shaft motor. That would in some ways be an octocopter with the rotors simply coaxial pairs. Since these are fixed pitch the reliability will be high.
      Such arrangements don’t cruise well but they hover well. The test pilot for the volocopter C200 (18 rotors and 9 batteries) reported extreme ease of control, excellent precision and response and piloting.

      • Not disputing any of this, but it’s showing that the system isn’t quite as simple as one might assume, hearing of a quad-copter with fixed rotors.

        I’m not familiar with the volocopter. Are these 18 rotors fixed-pitch?

        There are some practical limitations with fixed pitch rotors, as they get bigger. Since control relies on them varying their RPM, the bigger they get, the slower their response in speed changes becomes… which limits their controllability. I suppose that’s less of a problem when there are 18 of them, i.e. they don’t need to be very big. But a simpler (?) system with just 8, bigger rotors, might not work that well.

        • Volocopter 2X uses 18 fixed pitch rotor with 9 rotating clockwise and 9 counter clockwise. This gives a high degree of redundancy. Yaw control is by varying relative rotor speeds. I think variable pitch would introduce a great deal of weight and complexity as well as failure modes though might overcome some of the inefficiency in forward flight of such aircraft. It has a ballistics recovery parachute.
          -Volocopter VoloConnect reduces to 6 vertical lift motors and 2 horizontal for much greater range. Thinking about failure modes leads me to be sure that it can easily handle one motor failure and maybe two depending on which ones. It might even handle 3 or 4 and retain some control, at least enough to remain upright when ‘falling’ at a controlled rate. In theory even complete battery depletion might allow the aircraft to fall with the rotors wind milling and generating at an individually controlled rate so that the battery is charged during the fall and the aircraft retains pitch, yaw and roll control. Not sure, just conjecturing. I think ballistics parachutes will be the norm for these aircraft.

  2. Can multiple smaller diameter propulsors, relative to one large diameter ditto, lessen the phenomenon of blade tips that go supersonic and generate heavy drag? Or is this a non-issue?

    • Supersonic blade tips are very rare and abandoned decades ago. The NK14 of the Tu 95 has a slight tendency to go supersonic in some regimes but that would be the only such engine. The Republic XF-84H “Thunderscreatch” is a 1950s design and on one occasions rendered maintenance crews unconscious and vomiting from high sound levels.

  3. @Bjorn – for quadcopter how does the serial hybrid vs hydrogen fuel cell comparison work out? Certainly on the maintenance side is seems the fuel cell would have a bit advantage.

    • Helicopters are small payload as it is compared to small winged aircraft
      A fuel cell is going to be even worse than having one on a plane, lost all your payload for its weight and a fraction of the range. You want the turbine to charge the ( small) battery

    • We will come to fuel cells. From a greenhouse gas perspective, they are ideal but they have major heat and mass problems. The heat is the worst as a fuel cell produces more heat than electric power and the heat management system ends up weighing more than the fuel cell. It will take time to master this all. Hybrids can be made now.

  4. There is the Airbus Flightlab Helicopter testbed which uses a 100kW motor and a 30 second battery to provide safety in case of engine failure. The Leonardo Koptor AW09 hybrid will also be converted into a similar test bed. It’s fairly obvious that both designs could progress to an electric tail rotor.
    I do wonder whether some of the eVTOL will end up as a hybrid with for instance the batteries optimised for hover and something like a fuel cell or turbine providing cruise. Lilum uses 10x more power for hover than cruise so this might work.
    Distributed electric propulsion is interesting from the point of view of thrust vector control and maybe boundary layer ingestion propulsion. In addition it could be used for wing boundary layer control by suction and blowing. Though likely powered of generators or fuel cells batteries might figure somewhere perhaps to provide a surge of power at critical times.

    • Its based on a single turbine engine helicopter (H 130) anyway. I dont see adding a battery is really necessary for safety ( single engine piston and turbine helicopters are quite common- autorotation is the safety part)

      It seems that other on board autonomous features is the real aim and the battery is useful there
      ‘The autonomous technology being integrated with Flightlab includes vision-based sensors and algorithms to enhance situational awareness and obstacle detection, fly-by-wire for auto-pilot enhancement and an advanced human-machine interface via a touchscreen and head-worn display for in-flight monitoring and control.
      Integration of these technologies will allow in managing navigation and route preparation, automatic take-off and landing, and sticking to a predefined flight path.”

      Kaman helicopters always used a mid blade ‘flap’ to adjust the blade angle without complicated rotor head mechanics and is a more flight stable system. They have had their completely autonomous single engine intermeshing rotors K-max running for years now – was in service with Marines in Afghanistan and have recently added more features

      • -“I don’t see adding a battery is really necessary for safety”. Single engine & piston helicopters are highly restricted in operating over densely populated cities in most western countries. Thirty seconds of 100kW of power it is hoped will greatly ease safe transition to auto rotation and retain directional control & may allow a $1 million dollar single engine helicopter to do the job of a $2.4 million twin. The pilot will still need to find a place to land but it should completely eliminate the problem of spinning or corkscrewing into the ground at a high descent rate while the surprised pilot tries to figure out how to recover rotor speed in time. It may not be suitable for winching people up but it could be suitable for medivac or transporting across a city.
        -As you say the HH130 Flightlab is about testing a range of technology not just a hybrid with a 100kW electric safety motor. That 100kW “safety motor” may pay for its weight through multiple use: Starter motor, generator-alternator, rotor brake and will facilitate the development of an electric tail rotor.
        -An electric tail rotor might look like a single fan with a double motor, contra rotating double fan, two separate fans or a honeycomb of 7 small fans. Maybe even a mechanical fan with an electric. It would eliminate loss of tail control problems even on twins.
        -The Koptor AH09 is remarkable. A cheap single engine helicopter with its CFRP body, single 1050shp engine it can hover in any hot and high conditions yet still fly 800km. Sydney to Melbourne with 33% reserve.

        • Its a complicated area which I know nothing about but the single engine helicopter restrictions ( and only for some designated airways tracks not a blanket ‘no urban areas’) were mostly about other systems redundancy…hydraulics , electrics and minor systems etc to get IFR. It doesnt seem that the engine failure alone was the sticking point to become IFR approved. Now of course there are single engine models available . AW109 and Bell 407 which meets FAA requirements, while MD say the very light twin IFR MD902 is still affordable option

  5. Finally you’re touching on the areas where electric propulsion is not only beneficial, but in some cases necessary. The operational capabilities of multi-rotors are not going to be ignored, particularly by the military. That means whatever energy source is required will be developed, possibly including a new modern and efficient turbine to generate electricity. Or a fuel cell suitable for aviation…

    • There are existing ‘new modern efficient ‘ turbines for rotor craft existing right now. However the small size means their isnt really any in-efficiencies left to squeeze out. The costs of the engine have to be related to what fuel savings they deliver. Its like improved turbo props, but even harder to do. The military market helps but they often dont care about production cost so much.

      • small turbines are no match by far for (Diesel) IC engines.
        PT6 ~360g/kW/h smaller ones and/or reduced power settings are even worse
        AE300 ~185..230g/kW/h ( best to full power )

        reason why a single diesel + hybrid e-Motor booster
        can be competitive to a twin turbine setup while providing similar savety.

        • -Interestingly only slightly modified diesels can be made to run of a blend of hydrogen and diesel. At Idle they can run of 97% hydrogen and a full power 70% hydrogen. Furthermore the engine becomes more efficient and 80% cleaner.
          -Hence we could power an aircraft of a SAF diesel with fuel in the wings (free of Centre of /Gravity issues) and hydrogen in the tail.
          -Time to dust of the plans for the Jumo 223. German engineers envisaged aircraft plying a passenger trade to the Americas in the 1930s with aircraft like the Junkers EF100. Perhaps a little slow at say 250 knots but good for cargo.

          • The Junkers opposing piston design is attractive but
            the two stroke aspect makes it a PITA to get clean. ( lubricating oil in the gas path.)

    • Turkish Air, Korean Air/Asiana ,Air India and China Southern and the other state owned majors are still using their normal direct routes to US or Europe.

      • Given that at least two Korean Jet Airliners have been shot down by the Russian Airforce in peacetime I find that cruising for a bruising.

        • And the US Navy 1 ,Iran Air over Gulf and Ukraine Air defences 1, over Black Sea Flight 1812 from Tel Aviv
          or the Israeli AF shootdown of Libyan Air 114 727 over Sinai

          Its never bad when negligence leads to the ‘right people’ shooting down civilian airliners
          Military spy flights over Russian or Warsaw pact territory was quite common in those times . 200 lives lost in spy planes.

          • Your link is to information on overflights that happened in the late 1940s, 50s and early 60s. Everything changed after 1960 when Francis Gary Powers U2 was shot down. Overflights essentially stopped after that because the Soviets forced the US in order to release powers and because it was clear the SU was developing capable SAM defences. SR-71 flew on the edge of borders using side looking cameras and scanners, the faster single seat YF-12 recon variant Mach 3.5+ which was meant for over flights never was used.
            Hence the Soviets shooting down stray B707 and B747 with common navigation errors is simply typical of their cavalier and reckless attitude of the Soviet military in the 80s.
            In defence of the USN Aegis shoot down of an Iranian airliner it has to be noted that
            1 There was a hot war going in. It wasn’t peace.
            2 The Iranian Airliner had its transponder out of action.
            3 The USN certainly screwed up seriously but it all did actually come out in the open and they owned up in days not decades.

    • Wow! Thanks for the details on the extra flight times. This is just what we needed immediately after the COVID hit on international air travel?! Will we ever get back to “normal” so aviation can flourish again?

  6. Back on topic, on bullseye.
    ‘The all-electric CityAirbus NextGen is designed to carry up to four passengers with a 80km (50 miles) range and a cruise speed of 120 km/h (75mph), for use in cities. The aircraft is being optimized for hover and cruise efficiency, while not requiring moving surfaces or tilting parts during transition.’
    I wonder why our colleague William hasnt salivated over this before ?

    • -I am aware of it and have posted. Basically the uBer elevate white paper suggested that the majority of UAM revenue is going to come from super commuters travelling distances of 100 miles/160km. Airbus Citybus will be well short of this. Competitors are Joby Aerospace claiming 150 miles/240km, Vertical Aerospaces VX4 claiming 100miles+/160km+ and Lilim claiming 160 miles/250km and EmbraerX eVTOL is a little undefined.
      -However Airbus Citybus is taking a conservative approach. There are no tilt rotors (as all he others have apart from lilium) but there are a pair of pusher propellers to improve efficiency. Citybus’s focus on hover is probably needed assuming piloted flight rather than a rapid transition.
      -I can see Airbus’s strategy is to get to market cautiously, perhaps not making any money, but building up on that.
      80km is not very far and although one can imagine this increasing to 100km perhaps we can only expect 60km at most. I can see it used in Sydney to transfer passengers between the curfew free Badgeries Creek airport to the Sydney (if ever built), island hoping.
      Lilim is interesting as they have a stretch very of their 6 passenger aircraft that looks like 10 passengers. If that happens I’ll be forced to support VFT trains because it will be able to transport passengers into high speed rail.

  7. Not particularly connected with the insanity of “Net Zero” (i.e. net zero civilisation) is the “slowed rotor” concept of the high speed helicopter where the electric motor is the prime mover of the rotor (two rotors actually) and their speed easily modulated throughout the flight regime.

    • -Would probably work well with hybrid synergy.
      -One of the leading concept for uBer elevate was an electric autogyro with a high inertia slowed rotor. Rotor was pre spun electrically to allow a vertical take-off using the high inertia. As propulsion was by stub wings with outrigger electric motors the outrigger motors could produce an anti torque effect so I imagine a small amount of power still going into the rotor. In forward flight the rotor was slowed.

      • Conceptually, a high speed slowed rotor compound aircraft would have contra-rotating rotors with one electric drive and one magnetic gearbox turning the second rotor with a single ducted fan pushing.

        The prime mover would be a turbine with ultra capacitors providing redundant power reserves.

  8. -There is another area I can see hybrid working very well that is VTOL. Recally the 1960s German supersonic program the VAK191, VJ101, Dornier Do 235. These used a vectored thrust engine (similar to Pegasus) and some vertical thrust engines. One could also imagine the F35.
    -The vertical lift jets could be replaced by vertically lift electric ducted fans. The electric fans being far more maintainable and cheap and devoid of ground erosion problems and even ingestion. They could be powered partially by a generator and partially by a battery optimised for vertical flight only.
    The Electric Ducted Vertical Lift Fans would provide pitch and roll control and supplement when there was a main engine failure.

  9. Bjorn,

    Off topic, sorry, but any comments on Eviation Alice claim of 815 km range on a 820 kWh battery (specific energy 220 Wh/kg), if you have not done so already? Including 30 min for an alternative airport, my back-of-the-envelope calculations suggest a L/D ratio of over 25. No information on wing area but graphics suggest an aspect ratio of 15, Cl of 0.58, Cdi of 0.01 and total Cd of 0.023 during 407 kph cruise at 3,000 m. Does this sound right? What does your sophisticated code say? Is 220 Wh/kg realistic at present for a battery pack (not individual cell)?

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