Half a croissant, on a plate, with a sign in front of it saying '50c'
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I think this would be a great thing to not do.

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Hybrid Plane

  [vote for,

I suspect this isn't a great idea, so I'll keep it short to offer a smaller target.

Man-carrying electric aeroplanes exist, but only as research tools or for very short hops. An all-electric A380 is out of the question with current or foreseen battery technology.

Howevertheless, a hybrid turbofan aircraft might be feasible. Replace the existing electrical engine-starting thingy with a decent motor; a turbofan gets most of its thrust from the bypass, so it doesn't matter much whether you spin it by burning fuel or with an electric motor. Manageable batteries will only power the engine for a couple of minutes, but that's enough to get off the ground and a few thousand feet in the air, at which point you open the taps and light the burners. During the flight, the batteries are recharged by the on- board generators, allowing you to do your approach and landing electrically as well.

Energy density of good batteries is about 1/30th that of jet fuel, per kg. So, all other things being etc, batteries equivalent to 1/5th of your fuel-weight would allow you to fly electrically for 1/150th of the flight, or 4 minutes of a long-haul flight.

The point? Well, you won't save any fuel and you have to lug around a few hundred kilos of batteries. But it should allow quieter take-offs and landings. This, in turn, might make earlier (or later) operation from airports near houses, which is most of them.

MaxwellBuchanan, Jun 21 2019

Wickety Pink https://www.ft.com/...9-bf5c-6eeb837566c5
The "hybrid" reference is some way down the article. [pertinax, Jun 22 2019]


       The space below is reserved for retrospective comments to pre-empt the scathing critique which will doubtless come from [8th].   





MaxwellBuchanan, Jun 21 2019

       To begin with:   

       // it should allow quieter take-offs and landings. //   

       No. You still have to accelerate the same amount of reaction mass, by whatever means. High-bypass turbofans emit less noise because they move a bigger mass of air at a lower velocity. If you get rid of the high pitched noise from the fast-rotating core, it will be a bit quieter, but you'll still have the low frequency noise from the fan.
8th of 7, Jun 21 2019

       //accellerate// ha.... cat purrs contentedly as 8th skulks into a corner wearing the dunce's cap.
xenzag, Jun 21 2019

       I suspect that the //high pitched noise from the fast-rotating core// has disproportionately high annoyance value.   

       Also please return that cap - it's the Intercalary's.
MaxwellBuchanan, Jun 21 2019

       I'm pretty sure I saw hybrid planes described in the Financial Times last week. WKTE?
pertinax, Jun 22 2019

       //Energy density of good batteries is about 1/30th that of jet fuel, per kg.//   

       We don't get all the energy of the fuel out as useful work. Put motor efficiency at 90% Vs. Jet engine at 40% and the sums look a little kinder, although getting motor output to thrust is going to have losses that are included in the jet figure. What might help things further is powering the aircraft as a serial hybrid. A jet turbine generator tuned for efficiency might outperform conventional jet propulsion, only a small battery bank would be needed to smooth the supply-demand mismatches in take-off, cruise and descent.   

       //a turbofan gets most of its thrust from the bypass, so it doesn't matter much whether you spin it by burning fuel or with an electric motor.//   

       There is a slight snag. As far as I can work out, 1 747 engine at cruise is about 18MW of propulsive power. 25,000 horsepower or so. Even a 90% efficient electric motor is going to generate 10% of that as heat. With the best will in the world, it's going to be impossible to reject that heat from something as dense as a motor at the low temperatures the magnets can handle. Jet engines get around it by being inherently high-temperature tolerant.   

       This is OK though. We don't need 2 huge engines. You could easily have 20 smaller units, and take advantage of propeller/prop-fan efficiency gains. Large numbers of smaller units keeps diameter, and therefore tip speed down. Supersonic tips are noisy and inefficient. Small units could be built into the wing and used in creative ways to increase lift through whole-wing blown flaps and the like. Control surfaces could be eliminated and functionally replaced by differential thrust.   

       Some thrust units could be closed off, eliminating drag during cruise. Better yet, some energy harvesting could be done during the descent phase when losing energy quickly is a real advantage (see the Hawker Siddeley Trident/C17).
bs0u0155, Jul 03 2019

       // Jet engines get around it by being inherently high- temperature tolerant.// That is a good point. On the other hand, modern jet engines are the result of decades of metallurgical tweaking. With a similar effort, we ought to be able to make magnets that will take a bit of heat. Plus not all electric motors rely on permanent magnets.
MaxwellBuchanan, Jul 03 2019

       //magnets that will take a bit of heat. Plus not all electric motors rely on permanent magnets.//   

       Depending on how hot you want to get, the Curie point stops permanent magnets at ~1000K, ruling those. I had to learn quite a lot about magnetism to understand how it operates with temperature. I'm still largely a moron on the subject, but it hit me pretty hard that the magnetic field of the Earth isn't anything to do with it being mostly Iron, it's above the Curie point by a lot, so it's really about the movement of charge. Iron's got bugger all to do with it, it could be copper or gold. I need to have a word with whoever it was that taught me physical geography.   

       Anyhow, you'd think that any charge would disipate in a liquid metal, to the point where it moving material wouldn't have any real bearing. As it happens, the resistivity of stuff goes up a lot with temperature, usually linearly, but it's all over the place. That has problems with induction motors at high temperatures. You only need a few hundred K to double the resistance in your aircraft motor.   

       That's actually a serious issue, we know battery density can't approach kerosene by a long shot, but I'm not sure people realize that there's a power density limit, cooling a rotor spinning at 50-200,000 rpm isn't trivial. You can make the motor superconducting, perhaps, but then there's a magnetic field density limit, I'm not sure how far away that is from practical applications though.
bs0u0155, Jul 17 2019


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