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Are there any hybrid planes (planned), or would this be impossible because we don't have big enough batteries?

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  • $\begingroup$ You might find this YouTube video by Real Engineering interesting: Are Electric Planes Possible? $\endgroup$ – Alexandre Aubrey Jul 21 '18 at 19:36
  • $\begingroup$ the main problem with hybrid/electric aircraft is that the batteries and dual propulsion systems add considerable mass while taking up considerably internal space, leaving an aircraft with very limited carrying capacity, performance, and range for the same cost. $\endgroup$ – jwenting Jul 23 '18 at 5:09
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There are experimental electric planes. Some are promising enough that they might end up useful as trainers and private GA flown for the love of flying.

Long-range electric will not work. The electric equivalent of the Breguet equation is R=L/D*E/W*n/g, where E is stored energy and n is propulsive efficiency. For a 100% efficient L/D=20 aircraft that is 100% battery with 250 Wh/kg cycle depth, that's 20*250*3600*1/9.8=1.8*10^6 meters, or 990 nmi. This is the hard limit. With 80% efficient props and real world factors, you can expect 5-8 nmi for each % takeoff weight dedicated to batteries.

Hybrid aircraft are, even on paper, only moderately interesting. Aircraft throttle in cruise at 45-75% for GA, 70-90% for small jets, and 80%-95% for long-haul widebodies. This is due to the engines "self-throttling" in a way - power and fuel consumption reduce due to lower air density as you climb.

So major airliners have nothing to gain from going hybrid, they can only shrink their engines by 5-10% with a hybrid system. This is very different from cars, which run duty cycles as low as 10%.

And there are no start-stops or regenerative braking to recover energy in flight. More precisely, aircraft already use regenerative braking - by pitching up or deploying flaps, speed gets traded for lift. So that braking energy is already being harvested, and stored as altitude (potential energy).

Smaller propeller aircraft could replace up to 30% of their power, so it's less cut and dry. However, when you want full throttle, you often want it for a while, like in climb. The only way for a non-plug-in hybrid to save energy is to use a lighter/smaller engine, and it's difficult to shrink enough to offset the hybrid system's weight.

A plug-in hybrid could benefit from the lower cost of electricity vs avgas, but fuel averages to less than a third of private flying expenses. Pure electrics have a shot at making flying more accessible through simplicity. With no fuel, very simple and reliable engines, it makes sense to relax the rules, like is done for ultralights. Hybrids have all the complexity of a piston-engined plane and then some.

The hybrid route makes sense for cars. The only safety-critical parts of a car are steering and brakes. Engine power is almost arbitrary, most hardware is just nice to have, and there's the universal failsafe of pulling over. In a plane, things need to work. Two systems that both have to work to fly safely is a complication and will likely increase costs.

Alternately, either system needs to supply takeoff thrust on its own. That could work in the 19-seater class, if the authorities accept a hybrid "twin"-engine single prop as sufficient redundancy. But there's a lot of "ifs" in this.

In a zero-weight scenario, some military aircraft (fighters, naval) could benefit from a hybrid system, as they have excess power like cars do. That's where reality hits: for given output, electric motors are lighter than car engines, but much heavier than aircraft jet engines. Since they're not lighter, there's nothing to be gained.

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    $\begingroup$ May be worth noting that most hybrid cars recharge their batteries via regenerative braking and that's a big part of their efficiency; I don't know whether a similar method could be applied to planes. $\endgroup$ – Walt Jul 20 '18 at 21:33
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    $\begingroup$ @Squareoot Unless you're making very short flights, a hybrid engine primarily charges its battery from a gasoline engine. So there's little CO2 savings without an increase in efficiency. The answer explains why that's the case for cars, but not for aircraft. A purely electric plane would help, but that's a huge engineering challenge. Or running a plane with a biofuel as the USAF is experimenting with, a rare case where national security and environmentalism coincide. scientificamerican.com/gallery/… $\endgroup$ – Schwern Jul 20 '18 at 22:09
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    $\begingroup$ You say that the electric engines of a hybrid only add complexity and thus reduce reliability. But what if they work independently and are able to power the aircraft alone over short distances? Then it would be additional redundancy, which would be especially useful in small, single engine planes. $\endgroup$ – Michael Jul 21 '18 at 6:45
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    $\begingroup$ @Michael As I've mentioned, if either system can do takeoff thrust OEI, it adds redundancy. But engine redundancy is only required with 10+ pax in the US or even more elsewhere. Aircraft engines are reliable enough that engine failure in singles accounts for very few incidents. Small planes can glide to a landing, except in takeoff or over-water failures. Whether a hybrid-twin single-prop will be accepted for 19-pax operation deserves a separate question. $\endgroup$ – Therac Jul 21 '18 at 7:14
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    $\begingroup$ Re, "A plug-in hybrid could benefit from the lower cost of electricity vs avgas." I drive a plug-in hybrid (Cheverolet Volt). The battery weighs about 450 lbs. It can power the car for approximately the same distance as a gallon and a half (12 lbs.) of gasoline. $\endgroup$ – Solomon Slow Jul 21 '18 at 17:27
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There are a number of small (ultralight, Light Sport categories) aircraft that run on pure battery power for anywhere from a few minutes to (as I recall) nearly an hour of powered flight -- a couple of these are essentially motor gliders, so they can extend the total flight time to multiple hours with soaring flight.

Larger hybrid aircraft have been proposed, even in to the regional airliner class -- they'd use small, efficient jets or turboprops, just adequate for cruise, augmented by electric ducted fans or propellers for takeoff thrust, or alternately shaft turbine generators driving pure-electric propulsion fans, augmented with batteries for takeoff power; the result would be, as with a hybrid car, the ability to install a "too small' engine that burns much less fuel, and still get acceptable performance.

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    $\begingroup$ Zunum Aero has a great article [here][1] on their entry into the hybrid market for a 12 passenger regional plane. Goes into pretty good depth on how the plane will work. [1]:spectrum.ieee.org/aerospace/aviation/… $\endgroup$ – CrossRoads Jul 20 '18 at 18:04
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    $\begingroup$ "Smaller engine to burn less fuel" - that's the idea. However, modern jets could lose less than 20% in engine size before they have to cruise lower and face more drag. And such a reduction is not 20% less fuel burn, but just 4-5% less. It's quite questionable if hybrids ever get a niche. Full electrics will. $\endgroup$ – Therac Jul 20 '18 at 18:30
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There is a company call Bye Aerospace here in Colorado that recently celebrated a successful first flight of their "Sun Flyer" prototype, an all-electric 2 seat trainer. I believe they are planning a 4 seat version as well. Check 'em out! enter image description here

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    $\begingroup$ This is interesting, but since it's all-electric it doesn't seem to answer the question about hybrid aircraft. $\endgroup$ – Pondlife Jul 20 '18 at 22:34
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Actually, NASA just announced a new X-plane program. While they haven't selected a design yet, it looks like a hybrid-electric system is one of the designs they will try to build see here. The point of the hybrid system in this case isn't so much about providing additional thrust from an efficient system, but rather to use electricity to transfer power to where it is needed for drag reduction.

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    $\begingroup$ That program does not use a hybrid system as used in cars, it rather uses a hybrid system as used in submarines with a turbofan charging batteries which in turn power an electric engine. I wonder how they think to save fuel that way, all it does it increase the mass of the aircraft significantly $\endgroup$ – jwenting Jul 23 '18 at 5:06
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    $\begingroup$ Not sure there are any batteries in there at all. The electric system there acts as more of a means to transfer power across the airframe than to preform any regeneration or the like. Drive shafts would be difficult to use there and electric motors are fairly light. You have to remember that the rear nacelle only houses a low pressure fan, there is no need for complex cooling systems and high pressure containment that happens at the back of a turbine, therefor lightweight materials can do the trick easily. Wouldn’t be surprised to hear that it only takes one person to lift that fan. $\endgroup$ – XRF Jul 24 '18 at 6:53
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    $\begingroup$ Even without batteries, there is the extra mass induced by the additional equipment, the drag induced by the extra external fittings, etc. etc. Nice concept in theory, but the practicality seems dubious. $\endgroup$ – jwenting Jul 24 '18 at 7:00

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