Hybrid Aircraft: Nuts or Realistic?

Discussion in 'Architecture & Engineering' started by exchemist, Sep 25, 2018.

  1. exchemist Valued Senior Member

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    I read an article in the Financial Times last week claiming that we may be on the verge of a revolution in aircraft, whereby electric power will become the future. The obvious objection that battery energy energy/weight ratio is terrible for aviation seemed to be dismissed and there was a lot about something described as a gas turbine/electric hybrid. However the basis on which this would provide an advantage over a conventional gas turbine only was not explained.

    I don't get it. The principal advantage of a hybrid car, bus or commuter train, surely, is to recapture kinetic energy during braking for re-use. So, in stop-start operation, a lot of energy can be saved. But one cannot do this with an aircraft, given that air resistance alone provides the braking during descent and it is about as far from stop-start operation as it is possible to imagine. I can see that during take-off one uses more power than in the cruise, so maybe one can fit a smaller turbine and use a battery to boost during take-off, but then the battery will have to be recharged by the turbine, because descent won't give any energy back to the battery. (Not to mention the absence of any safety reserves of power, once the battery is depleted.).

    I struggle to see how, given the inherent weight penalty of a battery, this can possibly be worth doing. But evidently I must be missing something. Does anyone know what the logic is behind this idea?
     
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  3. Xelasnave.1947 Valued Senior Member

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    I dont know if you are upto date with rc planes but electric works for them.
    There are folk working on solar powered flight but it is difficult to imagine electric would be of much use.
    Maybe the passengers could all pedal little generatorsâ˜ș
    Maybe once up electric could assist...recover energy by dropping ten thousand feet.
    Maybe it just an idea to attract investorsâ˜ș
    Alex
     
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  5. Gawdzilla Sama Valued Senior Member

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    Figure the power-to-weight ratio for a passenger liner to fly, then figure how long any reasonable load of batteries would last under that demand.
     
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  7. el es Registered Senior Member

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  8. Gawdzilla Sama Valued Senior Member

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  9. Michael 345 Bali in Nov closer Valued Senior Member

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    How do you jump start a flat battery in mid air?

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  10. exchemist Valued Senior Member

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    Thanks for this, quite informative. Again it is not specific about how the energy demand is handled, but it suggests that electricity is used for the cruise phase, supplemented by a gas turbine for takeoff, rather than what I has assumed which was the turbine for steady state boosted by the battery for take-off. What is interesting is the relatively large power needed for the climb, compared to the cruise. I had not realised the difference was so marked.
     
  11. billvon Valued Senior Member

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    Nope. That only makes up a few percent improvement in efficiency. The biggest advantages for a car like the Prius is:
    1) It allows an Atkinson-cycle engine to be used and avoids the problems that engine has with idling.
    2) It allows the engine to operate at its most efficient RPM
    3) It allows the engine to be sized for average, not peak, power.

    Right.

    Consider a DC-8 (old 3 engine aircraft.) Replace the two outboard engine with electric turbines. Keep the center engine.

    On takeoff all three power the aircraft. For cruise only the center engine runs. For descent, the outer engines provide drag and recharge the batteries a little. This small recharge allows go-around capability. When the plane lands the battery (sized only for takeoff plus a margin) is recharged.

    Two fewer engines to maintain, less fuel used overall - and a single engine that is always run at its most efficient power setting.
     
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  12. sculptor Valued Senior Member

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    a solar powered airplane has already flown around the world

    (probably not much good at night?)
    or

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    Solar Impulse 2, piloted by Andre Borschberg, flies over Manhattan on June 11, 2016
     
  13. Gawdzilla Sama Valued Senior Member

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    Get out and push, of course.
     
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  14. exchemist Valued Senior Member

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    Aha, thanks, I was hoping you would show up.

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    So the hybrid concept is really mostly about peak-shaving rather than energy recovery. I had no idea. Now it starts to make sense.

    Also I did not know about the Atkinson cycle, which is very ingenious: power stroke longer than compression stroke, I see.

    Good. I've learnt something.
     
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  15. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Yep, this is bolts of it... aircraft engine are highly inefficient when taxiing and powering up to their optimum. Taxi around on electric and you'd save literally bucket loads of fuel. Recharge the batteries during descent and /or through regenerative braking on the tarmac should give enough to taxi to the nearest power socket.

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    At the moment there just isn't the necessary fuel density in batteries to make them viable for all-electric replacement of your 737s et al, but there are much smaller all-electric aircraft around. So in the meantime, using electric to cut out the most inefficient usages of the current engines is the obvious way forward.
     
  16. river

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    Perhaps it is about less fuel . Fuel is an enormous weight , tonage .
     
  17. DaveC426913 Valued Senior Member

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    Every time I read the title of this thread, I think you're going to be talking about ekranoplans.

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  18. exchemist Valued Senior Member

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    Yeah, they're nuts all right. Perfectly useless. Just the sort of thing a communist regime would waste its money on.

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  19. Seattle Valued Senior Member

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    I would assume that the concept behind hybrid cars is much like rowing a kayak or getting a powered boat on plane. The work load is less at that point and requires less energy. If you never had to stop in a car and could simply travel on an interstate the energy required would be much less.

    If you didn't have to taxi and take-off in an airplane the energy requirement would be much less. If you have a rotating blade, like a helicopter in auto-rotation, you could build up kinetic energy as well.

    Airplanes require less energy (fuel) to cruise at altitude as well just due to a less dense atmosphere. The work is getting up to altitude. You can become a glider for landing (in theory). Gravity is a bitch.
     
  20. billvon Valued Senior Member

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    Yep. Unfortunately, batteries weigh even more.
     

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