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Old 12-07-2019, 06:06 PM
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Electric airplanes


NASA is experimenting with an electric airplane, the X-57 Maxwell. It has several changes that should make it more efficient than just replacing the engines and fuel tanks with electric motors and batteries. They're taking their time on studying it, making only one change at a time, so it's going to be a while before they get done.

Assuming these experiments all work out the way they expect and adding in other ways to make airplanes lighter (e.g. carbon fiber fuselage and wings), how good of an electric airliner is likely? That is, how many passengers and what kind of range are we looking at? What speed are they likely to have during routine flight?
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Old 12-07-2019, 06:36 PM
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Nantucket based Cape Air actually already has electric planes on order, the Eviation Alice. Although that isn't an airliner along the lines of the 737, it will be an electric plane flying for an actual commercial airline.

Fun Fact: The airline portrayed in the sitcom Wings was loosely based on Cape Air.
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Old 12-07-2019, 06:47 PM
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What NASA is working on goes beyond the concept plane you linked to. I couldn't generate any links to the stuff they exhibit to the public but it's interesting in how diverse the research is.

As to the one you posted that would fall under the commuter airline. Using electric motors allows all kinds of engineering designs beyond what a traditional jet engine can proved. It would be really easy to have a series of engines/props that can tilt to change the air flow characteristics of a wing.

And the current Lithium battery technology probably isn't going to be the power source. However, they can use a traditional liquid fuel tank as a battery source with liquids that can be charged with electrons. Instead of just adding fuel like a traditional jet they would download the liquid for recharging and upload fresh fuel.

As for making them lighter with carbon fiber and other materials that's already being done by the airlines. You'll note that the winglets that were so popular on new aircraft have already been replaced by a tapered wingtip design that does the same thing more efficiently.

Probably the neatest thing I've seen was a ducted electric "coreless" engine. instead of putting the motor in the center the motor is in the rim with the traditional blades in the center. I wish I could find a link to it but I don't know what to call it. They're definitely working on propulsion for larger aircraft.
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Old 12-07-2019, 07:13 PM
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... they can use a traditional liquid fuel tank as a battery source with liquids that can be charged with electrons. Instead of just adding fuel like a traditional jet they would download the liquid for recharging and upload fresh fuel.
This is a new one to me. Any links?
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Old 12-07-2019, 07:18 PM
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Batteries don't yet have the energy density for long-haul flight. But shorter runs, say <1000 miles, have some potential in the relatively near future.

One advantage electric planes will have is that the engines are not dependent on oxygen. They can fly at an altitude limited by the wings and props/fans, not by the engines. Electric motors can put out maximum power at any altitude. There's no reason they can't fly at 70k feet or so.

It's also easy to get a very high power density out of motors without much additional weight, since it's really the batteries that are the limiting factor. Hence we may see VTOL or SVTOL flight out of electric planes.

Size is only an issue with regards to recharging the planes. Like other aircraft, they get more efficient the bigger they are. So electrics only get more practical as the size scales up. But they'll need many megawatts of ground power to recharge in a reasonable time and that will be a challenge. Like cars, the fastest charge time will be somewhere in the 30-60 minute range assuming they aren't limited by ground power.

Here's my mini-optimization for electric planes: sodium power conductors. Normally, sodium isn't thought of as a great conductor, and per cubic centimeter it's not. But per unit weight it is better than aluminum or copper. And its low melting point can be used as an advantage: pump it through the conductors and motor housing as coolant.
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Old 12-07-2019, 07:20 PM
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This is a new one to me. Any links?
Flow batteries. They don't yet have the energy or power density of lithium ion. I would say they're more interesting in a fixed storage situation.
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Old 12-07-2019, 08:45 PM
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Batteries don't yet have the energy density for long-haul flight. But shorter runs, say <1000 miles, have some potential in the relatively near future.
Yes, that's better than what's being done so far. Just replacing ICE engines+fuel tanks with electric motors+batteries and no other changes usually only allows about 100 miles range. If we could get a small to medium sized electric airliner with 500 miles range, it'd be a significant step up. A large one would be even better. There's lots of cities less than that far apart that have heavy air traffic between them. LA-SF and DC-NY-Boston, for examples.

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It's also easy to get a very high power density out of motors without much additional weight, since it's really the batteries that are the limiting factor. Hence we may see VTOL or SVTOL flight out of electric planes.
Air taxis seem to be going with that. But to get VTOL, don't you have to have rotating motor mounts or wings so that the propellors can blow air downward and then blow backward for level cruising? Won't that add a lot of weight? The range would be longer without it. Also without using up lots of power to do VTOL. It's not like we don't have runways for traditional take offs.

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Here's my mini-optimization for electric planes: sodium power conductors. Normally, sodium isn't thought of as a great conductor, and per cubic centimeter it's not. But per unit weight it is better than aluminum or copper. And its low melting point can be used as an advantage: pump it through the conductors and motor housing as coolant.
The idea of liquid sodium on an airplane does not appeal to me for some reason.
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Old 12-07-2019, 09:30 PM
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There's lots of cities less than that far apart that have heavy air traffic between them. LA-SF and DC-NY-Boston, for examples.
Yep, and I expect routes like these to be the first to be electrified. It'll need more work than a simple conversion to get there--just as long-range electric cars took more redesign than stuffing a bunch of batteries in the trunk and putting a motor in the engine bay. But there's no "magic" required here. They'll need to dedicate a large fraction of the internal volume to batteries and design the plane around that.

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Air taxis seem to be going with that. But to get VTOL, don't you have to have rotating motor mounts or wings so that the propellors can blow air downward and then blow backward for level cruising?
Right--I'm not expecting VTOL for the longish range routes. I'm thinking more puddle-jumper routes, like say the turboprops that run all day from Portland to Seattle. Airport fees are a higher relative cost the shorter the route, so there's an advantage in making use of shorter airstrips.

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The idea of liquid sodium on an airplane does not appeal to me for some reason.
Like anything else, it's safe if handled properly. Normal jets fill their wings with highly energetic hydrocarbons. Liquid sodium is pretty benign in comparison.
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Old 12-08-2019, 01:12 AM
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This is a new one to me. Any links?
here's an article talking about it. The rim driven propulsion is what I saw in the display only on a much larger scale.
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Old 12-08-2019, 01:17 AM
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Flow batteries. They don't yet have the energy or power density of lithium ion. I would say they're more interesting in a fixed storage situation.
Yes but the fire hazard of Li batteries can't be mitigated at altitude. they would be fine for low flying micro taxis but not airliners.
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Old 12-08-2019, 01:25 AM
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Yep, and I expect routes like these to be the first to be electrified. It'll need more simple conversion to get there--just as long-range electric cars took more redesign than stuffing a bunch of batteries in the trunk and putting a motor in the engine bay. But there's no "magic" required here. They'll need to dedicate a large fraction of the internal volume to batteries and design the plane around that.
for that to work financially that have to leverage the savings EV's should produce against the loss of freight revenue built into long distance flights. I don't know if electric propulsion is capable of generating the thrust of jet engines so it might involve electric/ICE hybrid motors.
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Old 12-08-2019, 01:40 AM
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But to get VTOL, don't you have to have rotating motor mounts or wings so that the propellors can blow air downward and then blow backward for level cruising? Won't that add a lot of weight? The range would be longer without it. Also without using up lots of power to do VTOL. It's not like we don't have runways for traditional take offs.
Lightweight jump-start gyroplanes aren't true VTOL - more like STO-VL. To jump-start, the engine drives rotors to lift the craft a few feet; then power shifts to the (usually pusher) propeller for forward motion and fast ascent. IANAP but vertical landing seems simple and autorotation is reportedly safe. But they're a bit short on passenger capacity and speed; electro gyroplanes would probably be best-suited as urban aero taxis as well as their surveillance roles.

Last edited by RioRico; 12-08-2019 at 01:44 AM.
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Old 12-08-2019, 01:46 AM
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Oops, missed the edit window. That last part should read,

But they're a bit short on passenger capacity and speed. Electro-gyroplanes would probably be best-suited as urban aero taxis as well as their surveillance roles.
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Old 12-08-2019, 04:14 AM
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for that to work financially that have to leverage the savings EV's should produce against the loss of freight revenue built into long distance flights.
Long distance, sure. But we're talking short-medium distance here. I suspect that anything less than a day's drive in a truck tends to go by truck in the first place.

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I don't know if electric propulsion is capable of generating the thrust of jet engines so it might involve electric/ICE hybrid motors.
Thrust is no problem in principle. The lithium-polymer batteries used with RC aircraft have absurd power density, and the cells can lift many times their own weight. However, this comes at the expense of energy density. One approach might be to dedicate some smaller fraction of the total to high-power-density chemistries, and the rest to high-energy-density chemistries. You only need a short burst to take off, and then when cruising you can draw from the energy dense cells.

Or maybe we'll just see chemistries with the best of both worlds. Cars aren't really limited by their power density, so there's not huge a motivation to optimize for that.
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Old 12-08-2019, 06:31 AM
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Right--I'm not expecting VTOL for the longish range routes. I'm thinking more puddle-jumper routes, like say the turboprops that run all day from Portland to Seattle. Airport fees are a higher relative cost the shorter the route, so there's an advantage in making use of shorter airstrips.
Yes, but existing smaller airports are not currently built to handle large volumes of passengers. At least not in the Portland area and for most in the Seattle area, although Boeing Field does handle small volumes of passengers. So there'd be a lot of expenses upgrading those airports. Any savings is going to be rather long term. Diitto if they create new airports for this.


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Like anything else, it's safe if handled properly. Normal jets fill their wings with highly energetic hydrocarbons. Liquid sodium is pretty benign in comparison.
I had in mind the standard HS chem demo where the teacher puts a small bit of sodium in some water. Not really applicable unless the plane crashes, but I'm not the only one who will have that reaction.

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here's an article talking about it. The rim driven propulsion is what I saw in the display only on a much larger scale.
What kind of advantages do rim-driven props have? Do they just make it easier to rotate the propellor mount or can they be made lighter and thus extend range?

Last edited by dtilque; 12-08-2019 at 06:32 AM.
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Old 12-08-2019, 07:54 AM
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What kind of advantages do rim-driven props have? Do they just make it easier to rotate the propellor mount or can they be made lighter and thus extend range?
Answering my own question here. After posting, I realized that rim-driven props have less obstruction of the air flow. Of course, electric motors already have significantly less obstruction than ICE engines. And a disadvantage is additional bearings needed to support the rim-driven prop. Not that we can't do bearings, but they're another thing that can go bad.

However, I can see that rim-driven could also be safer. The X-57 will have its main props way out at the end of the wings to reduce vortex drag. But that creates a hazard to ground crew servicing the plane. Enclose them and that hazard is much reduced.
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Old 12-08-2019, 08:13 AM
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One point to keep in mind is that, while lower vehicle weight would undoubtedly help an electric aircraft, it'd also help an internal-combustion aircraft, and so the internal-combustion aircraft with those same weight reductions would probably still be much more economical. If you're really serious about carbon footprint, probably the best way to make an airplane is to keep using hydrocarbon fuels, and either manufacture the fuels greenly, or offset the carbon in some other way.
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Old 12-08-2019, 08:18 AM
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Yes but the fire hazard of Li batteries can't be mitigated at altitude. they would be fine for low flying micro taxis but not airliners.
They can't? What is the issue here?
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Old 12-08-2019, 08:47 AM
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One point to keep in mind is that, while lower vehicle weight would undoubtedly help an electric aircraft, it'd also help an internal-combustion aircraft, and so the internal-combustion aircraft with those same weight reductions would probably still be much more economical. If you're really serious about carbon footprint, probably the best way to make an airplane is to keep using hydrocarbon fuels, and either manufacture the fuels greenly, or offset the carbon in some other way.
I suspect that hydrocabon-burning aircraft are mainly more efficient because they're allowed to externalize their environmental costs. At any rate, continuing to use them, even with mitigation of their carbon footprint, still leaves the problem of contrails, which also cause warming of the planet.
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Old 12-08-2019, 09:20 AM
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Long distance, sure. But we're talking short-medium distance here. I suspect that anything less than a day's drive in a truck tends to go by truck in the first place.
NASA is looking at all of air travel. Clearly electric power is only suitable for shorter flights NOW but the goal is for full range flights.

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Thrust is no problem in principle. The lithium-polymer batteries used with RC aircraft have absurd power density, and the cells can lift many times their own weight.
I don't think it's a hp power issue. You can only generate so much thrust with propellers. High bypass engines get a lot of their thrust from fan blades but there's a cutoff point. Jet airliners are 200 mph faster than prop airliners.

It will be interesting to see what the TBO (time before overhaul) is with electric motors along with the energy cost savings.
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Old 12-08-2019, 09:34 AM
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They can't? What is the issue here?
if a jet engine catches fire at 30,000 feet you turn off the fuel. jet fuel in the tank is inherently stable. Although there is a case where a fuel tank exploded and destroyed the plane the fuel is almost inert in the tank. you can throw lit matches on it and it won't ignite. It has to be injected at high pressure to get it atomized enough to burn or heated enough to do the same thing.

Lithium batteries have a different set of problems. If they go into thermal overload the resulting fire is not something that can be contained. The NASA site I linked to actually lists this as a reason for research into the liquid batteries I described up-thread.

Last edited by Magiver; 12-08-2019 at 09:35 AM.
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Old 12-08-2019, 09:49 AM
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I suspect that hydrocabon-burning aircraft are mainly more efficient because they're allowed to externalize their environmental costs. At any rate, continuing to use them, even with mitigation of their carbon footprint, still leaves the problem of contrails, which also cause warming of the planet.
There was a real world test of that theory on 9/11. According to that article the contrails reflected sunlight back and dropped the temperature during the day. It also reflected heat back at night. So it's not a cut-and-dried issue but at least there was test data from 9/11 to compare against modeling.

Last edited by Magiver; 12-08-2019 at 09:50 AM.
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Old 12-08-2019, 10:18 AM
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Thrust is no problem in principle. The lithium-polymer batteries used with RC aircraft have absurd power density, and the cells can lift many times their own weight. However, this comes at the expense of energy density.
that's also at the expense of lifespan. I'm deeply involved in R/C vehicles, and I've seen (and backed slowly away from) more puffy, swollen LiPo packs than I care to mention.
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Old 12-08-2019, 11:26 AM
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that's also at the expense of lifespan. I'm deeply involved in R/C vehicles, and I've seen (and backed slowly away from) more puffy, swollen LiPo packs than I care to mention.
yeah, I had a motorcycle battery do that. Scary stuff. the motorcycle sits in a detached garage so I don't worry about it burning the house down with me in it. Sadly the battery box is too small (IMO) for a proper sized lead acid battery. I really like the power the Li battery can deliver.
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Old 12-08-2019, 12:49 PM
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The main reason that fuel-burning engines are more efficient is that they get to keep the most reactive of their reactants outside of the fuel tank, in the atmosphere as a whole. The important ingredient isn't the hydrocarbons per se; we could replace those with something else. The important ingredient is the oxygen, but we take it for granted because it's so easily available.
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Old 12-08-2019, 01:31 PM
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The main reason that fuel-burning engines are more efficient is that they get to keep the most reactive of their reactants outside of the fuel tank, in the atmosphere as a whole. The important ingredient isn't the hydrocarbons per se; we could replace those with something else. The important ingredient is the oxygen, but we take it for granted because it's so easily available.
What if hydrocarbon burning planes were required to capture all the CO2 instead of emitting it to the atmosphere. Wouldn't be quite so efficient anymore, would it?
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Old 12-08-2019, 01:42 PM
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yeah, I had a motorcycle battery do that. Scary stuff. the motorcycle sits in a detached garage so I don't worry about it burning the house down with me in it. Sadly the battery box is too small (IMO) for a proper sized lead acid battery. I really like the power the Li battery can deliver.
um, a vehicle 12 volt charging system is totally inappropriate for use with lithium-ion or LiPo cells.
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Old 12-08-2019, 03:37 PM
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The important ingredient is the oxygen, but we take it for granted because it's so easily available.
Hence the lithium-air battery. Use ambient oxygen at the cathode instead of dense metal. Promising, but needs a great deal of development before they're appropriate for vehicle use.
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Old 12-08-2019, 03:46 PM
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that's also at the expense of lifespan. I'm deeply involved in R/C vehicles, and I've seen (and backed slowly away from) more puffy, swollen LiPo packs than I care to mention.
They're really pushing the extremes, though. A commercial aircraft doesn't need to zip up at 3 gees like a racing quadcopter can. Back off from that just a bit and you can have a more reliable system.

Also, I suspect the biggest problem with LiPo isn't with using a high discharge rate but rather squeezing it to the highest possible voltage. I've had a couple of phone LiPos go puffy on me and they weren't discharging at a high rate. Instead I suspect they were charging to a higher level than was safe because the phone maker decided to make the phone 0.1mm thinner by sacrificing battery volume. That's not something one would do in a more safety critical situation.
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Old 12-08-2019, 03:57 PM
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NASA is looking at all of air travel. Clearly electric power is only suitable for shorter flights NOW but the goal is for full range flights.
Transcontinental and intercontinental flight will require major, unknown developments in battery tech, though. I'm glad NASA is researching this, but it also shouldn't stop us from electrifying regional flights.

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I don't think it's a hp power issue. You can only generate so much thrust with propellers. High bypass engines get a lot of their thrust from fan blades but there's a cutoff point. Jet airliners are 200 mph faster than prop airliners.
Electric aircraft can use ducted fans with no problem. Propellers are limited by the prop tip speed--you really don't want it to be supersonic (otherwise you end up with a plane loud enough to cause seizures). Put it in a duct and it's fine because the shockwaves can't propagate outside.

Props may still make sense at lower speeds, and tend to be more efficient than ducted fans, so maybe we'll see those first. Turboprops are more popular on short routes anyway.
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Old 12-08-2019, 04:04 PM
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Quoth dtilque:

What if hydrocarbon burning planes were required to capture all the CO2 instead of emitting it to the atmosphere. Wouldn't be quite so efficient anymore, would it?
Only if, for some reason, you require the plane to carry the carbon-capture equipment with it. Just leave that on the ground: As long as it's still capturing as much as the plane is producing, it's fine.
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Old 12-08-2019, 05:55 PM
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Transcontinental and intercontinental flight will require major, unknown developments in battery tech, though. I'm glad NASA is researching this, but it also shouldn't stop us from electrifying regional flights...
Most aviation energy is not consumed by general aviation or business aviation -- it is by commercial air transport. I don't know the exact numbers but I'd estimate over 90%, maybe over 95% is by commercial air transport, ie airline and cargo operations. Therefore any meaningful electric aviation solution must include that or at least articulate a plausible path using attainable technology to achieve electrically-powered commercial air transport.

"Short haul routes" are under 3 hr, although some authorities use 3,200 km (1,727 nm).

What would be the energy or battery requirement for airline transport operations over (say) 1,500 km? Ideally this should provide roughly similar payload and travel time performance, else all of society is plunged back to the pre-jet era. Electric-driven jets are not really possible but electric-driven propellers are. Propeller-driven airliners with near jet performance have existed: the Tu-114 (based on the Tu-95). Fortunately its engines are rated in shaft horsepower which avoids the tricky conversion of pounds thrust to horsepower. https://en.wikipedia.org/wiki/Tupolev_Tu-114

The Tu-114 had 4 x 14,800 hp engines. If we assume cruise power at 77% or 45,584 hp, that is 33.9 megawatts. A short-haul flight of two hours would therefore require roughly 67.8 megawatt-hours. How much battery power would be required to fulfill that?

33.9 MW is engine output, not including gearbox and propeller efficiency. To provide this electrically we must consider battery efficiency (say 90%) and electric motor efficiency (say 90%) for overall efficiency of 81%. We won't consider efficiency losses from motor controllers or other sources. This means the batteries must hold about 83.66 megawatt hours (not including reserves) for a two hr "short haul" flight of about 1,500 km.

The battery pack on an 85 KWhr Tesla Model S weighs about 540 kg (1,200 lb). We would need about 984 of these which would weigh about 1.18 million pounds. The Tu-114 had a payload of about 55,000 lb, but 130,000 lb of fuel for a total payload of 185,000 lb.

So very roughly, we'd need an improvement in battery energy/weight ratio of about 10x to make this feasible. To recharge 10 of these after landing would require an on-site dedicated two gigawatt power plant, and the assumption each aircraft can sustain an approx 100-200 megawatt charge rate.

The maximum possible improvement for battery energy density improvement using currently-known physics is about 2x for lithium ion, about 3x for lithium sulfur, and about 4x for lithium oxygen. Lithium sulfur and lithium oxygen are mostly research items, not commercial products.

This doesn't mean the above improvements will be achieved, but those are limits beyond which improvement is unlikely using those chemistries: https://youtu.be/AdPqWv-eVIc

If you combine, say, 2x battery improvement, 2x energy efficiency improvement from lower speed, better aerodynamics, lighter materials, then it should be possible to eventually have (essentially) a battery-powered 72-passenger ATR-72 with an approx. 500 mi. range. This could serve routes like Hawaii inter-island service, etc. https://en.wikipedia.org/wiki/ATR_72#ATR_72100

There is currently no realistic envisioned technology that could produce a battery-powered 737-class plane, much less a larger plane. It appears that all airliners from "short haul" category on up must continue to burn liquid fuels, either hydrocarbon or hydrogen.

This fascinating chart shows the dilemma: Li-ion batteries are at the lower-left of the energy volume/density curve, whereas hydrocarbons or hydrogen is at the right or center: https://www.wikizero.com/en/File:Energy_density.svg
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Old 12-08-2019, 06:00 PM
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um, a vehicle 12 volt charging system is totally inappropriate for use with lithium-ion or LiPo cells.
and yet they sell Lithium batteries for motorcycles. go figure.
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Old 12-08-2019, 06:31 PM
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Electric airplanes as anything other than a novelty or experimental craft are not going to happen until we have a breakthrough in battery technology, and until we solve about a million certification issues. It's going to be extremely hard to get one of these certified for civilian private use, let alone for commercial transportation.

For example, these quadcopter-based 'personal flying craft' that you're seeing in development are death traps. An engine failure on any engine will result in a crash. They fly too low to use ballistic parachutes. The range is ridiculous - half an hour to an hour aloft is not nearly enough for a serious flying machine. In fact, if I'm flying a plane and I discover that I'm half an hour to dry tanks, I'm looking for a place to land NOW.

IFR flight requires that you carry enough fuel to make it to your destination, plus be able to divert to an alternate, plus be able to land with a 30 minute reserve. No electric plane can meet that requirement, and none will be able to until we have much better batteries.

Lithium-ion batteries aren't even approved for airplane transport, other than personal-sized batteries for laptops and such, which must be in your carry-on. There has already been a fatal crash of a jet when the lithium-ion batteries in cargo caught fire. I cannot see the FAA certifying an airplane carrying tens of thousands of pounds of the things and actively using them for power. Certainly not without huge amounts of certification trials and testing and explosion-proof or fireproof designs.

I could go on. We need to have certification trials for any electric motors used, new procedures would have to be developed for monitoring and testing them regularly, TBO's established, yada yada. The aircraft itself will need type certification, which is extremely hard to get. One of the reasons you don't see new aircraft designs very much is because the certification costs are outrageous. That's why 2019 Cessna 172's look nearly identical to versions made 40 years ago.

All-new aircraft designs are extremely difficult to certify. Cessna wound up with the Cirrus SR22 because Cirrus nearly went bankrupt trying to certify it. And those costs mean that these small aircraft, much simpler than a modern car, can cost a million bucks. Hell, just trying to get a conventional Porsche engine certified for private aviation just about broke Porsche, and they abandoned the effort.

Here we are talking about certifying entirely new airplane concepts with entirely new engines and power systems that have never been used in aviation before. There would also need to be extensive software, and that too has to be certified. Ask Boeing how that's working out for them. Now imagine trying to stand up a complete new flight control system for a radical aircraft with a new power system. Good luck.

Most of these planes are gimmicks anyway. If you gave me a plane with a half-hour range until it plummets out of the sky, I wouldn't leave the traffic pattern in it. It's entire range wouldn't even meet the requirements for VFR reserves for private pilots.

These things would also have to fly at lower altitudes, as props aren't going to fly in the jet levels. That means they'll be less efficient and the passengers will get to stay in the weather the whole flight. These planes also wouldn't have the endurance to be able to climb to a reasonable altitude and then descend again.

One of the Google guys has a company that is supposedly building an electric sport plane. Or rather, it started out as a potential plane until the reality of battery energy density and the inability to certify such a plane was discovered. Then they tried turning it into a sport vehicle for use on the water, with software limits preventing it from rising more than 15 feet and 25 mph. But even with those severe restrictions they can't get the thing safe enough, so they've scrapped it and are working on something else. Aviation is hard.

You're going to continue to see wacky experimental planes, and the companies that make them will be looking for funding so they will promise all sorts of bullshit to keep the bucks rolling in. Save your money until you see one of these things fly 500 miles with several people in it, and a proper track to reasonable certification can be shown.
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Old 12-08-2019, 06:37 PM
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For example, these quadcopter-based 'personal flying craft' that you're seeing in development are death traps. An engine failure on any engine will result in a crash. They fly too low to use ballistic parachutes. The range is ridiculous - half an hour to an hour aloft is not nearly enough for a serious flying machine. In fact, if I'm flying a plane and I discover that I'm half an hour to dry tanks, I'm looking for a place to land NOW.
To be fair, finding a place to land is a lot easier for a quadcopter than for an airplane.
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Old 12-08-2019, 06:44 PM
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and yet they sell Lithium batteries for motorcycles. go figure.
I believe modern ones are lithium iron phosphate (LiFePO4) with integrated battery management electronics. LiFe cells are known to be less temperamental than LiIon or LiPo cells.
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Old 12-08-2019, 07:01 PM
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To be fair, finding a place to land is a lot easier for a quadcopter than for an airplane.
Not if an engine is out. My guess is that a quadcopter that can be certified for land use will have to be at least a hexacopter, or maybe an octocopter, before the loss of an engine is survivable anywhere but on the ground. That will increase weight and complexity and maintenance costs and all the rest. But quadcopter-based designs are not passively stable either, and have other issues that may prevent them from being anything other than a novelty. For example, wait until you see how much crap on the ground is kicked up when one of these things tries to land. And some of that crap is going to go up and back through the blades and wear them down. A helicopter rotor is way higher off the ground than quadcopter blades.

We'll also need to develop the ability to inspect these electric motors and be able to tell when they are near failure. And you would need redundant electrical systems. Hell, light aircraft still use magnetos because electronic ignition is a point of failure and hard to certify. And we use dual magnetos for redundancy. Perhaps an electic plane would need multiple battery packs with the abiliy to shut down and isolate a failed one while still powering the plane with the others. But that too would need extensive testing and certification.

If you want to register on of these as experimental, you can certainly do so. Maybe someone will offer a kit. But don't expect to see one certified for commercial use any time soon. I'd say that even if we had all the requisite technologies available right now (better batteries, etc), it'd still be a decade or more before you'd see a plane certified to carry passengers commercially.

Consider the Beech Starship, a plane that used standard engines, control systems, etc. But it used composite construction and a pusher configuration with a canard. Designed in 1979, the first proof-of-concept plane flew in 1983, but the final certified production plane didn't leave the line until 1989, ten years after the design had been done. And most of that airplane used off-the-shelf certified components.
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Old 12-08-2019, 08:00 PM
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Transcontinental and intercontinental flight will require major, unknown developments in battery tech, though. I'm glad NASA is researching this, but it also shouldn't stop us from electrifying regional flights.
Current batteries are not nearly capable of reasonable duration, even for regional flights.

A Boeing 737, a common regional jet, burns on average 750 gallons per hour. There are about 39 kWh of energy in a gallon of jet fuel. The best lithium ion batteries have an energy density of around 250 Wh per kilo, or about 159 kilos of batteries to replace a gallon of jet fuel So one hour of flying a 737, all else being equal, would require 119,000 lbs of batteries, while the fuel weight would only be 5100.

The lithium ion batteries for one hour of 737 flight would weigh more than the maximum takeoff weight of an entire fully fueled and loaded 737. And one hour of fuel would only give you a 15 minute range with a reserve. That gives an idea of how much efficiency you would have to gain to create a viable commercial electric aircraft.

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Props may still make sense at lower speeds, and tend to be more efficient than ducted fans, so maybe we'll see those first. Turboprops are more popular on short routes anyway.
Until and if batteries have much, much more energy density, these planes aren't going to be in the air long enough to be able to climb to and descend from the flight levels. A 737 uses as much fuel to climb to altitude as it does in an hour of cruising. If our plane only has an hour of endurance, it isn't climbing far.
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Old 12-08-2019, 08:09 PM
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Right, I was just addressing the limited range, not the robustness against failure. An airplane needs a half-hour reserve of fuel, because if you can't land at your intended destination, then you might need to fly a half hour to find some other suitable place to land. That's not an issue when your suitable landing spots consist of "how about right there?".

But yes, having four independent mechanical systems, the failure of any one of which is catastrophic, is poor engineering, especially for something that's going to carry humans.
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Old 12-08-2019, 08:22 PM
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The Tu-114 had 4 x 14,800 hp engines. If we assume cruise power at 77% or 45,584 hp, that is 33.9 megawatts. A short-haul flight of two hours would therefore require roughly 67.8 megawatt-hours. How much battery power would be required to fulfill that?
You've definitely demonstrated that retrofitting an electric system onto a 50s-era Soviet craft is a non-starter. I would dispute the relevance, though.

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If you combine, say, 2x battery improvement, 2x energy efficiency improvement from lower speed, better aerodynamics, lighter materials, then it should be possible to eventually have (essentially) a battery-powered 72-passenger ATR-72 with an approx. 500 mi. range. This could serve routes like Hawaii inter-island service, etc. https://en.wikipedia.org/wiki/ATR_72#ATR_72100
Agreed, more or less. These are the kinds of electric planes I expect to see in the first round (aside from air taxis, etc.). They require some advancements, but no "magic".

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There is currently no realistic envisioned technology that could produce a battery-powered 737-class plane, much less a larger plane.
Size works in favor of electric planes. Electric planes will live or die on their efficiency, and larger planes are more efficient. If you can build a 72-passenger electric plane, you can build a 500 passenger plane.

What is difficult is range and speed. There isn't much demand for an A380 with a 500-mile range (or a 9000-mile range, for that matter). And the lower speeds we can expect matter more for long routes. So initially, I think we'll see replacements for the kind of smaller turboprops you mentioned, where an extra 15% flight time is not a big deal. It's not because large passenger counts are impossible, but just because the smaller craft work better in that market.

If nothing else, since many of the technologies are a step change, keeping the R&D budget down by starting with smaller craft will probably be necessary.
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Old 12-08-2019, 08:39 PM
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There are about 39 kWh of energy in a gallon of jet fuel.
So what? The battery in my Model 3 has the equivalent of 2 gallons of gas by that metric. And yet it goes 325 miles on a charge. My previous car would have only gone 60 miles on that. And it's not nearly as range-optimized as it could be, if the price were right--it could be doubled, trivially, with only small changes and an increase in price.

I'll see if I can whip up some numbers later to see how far off things are. However, I can start with this one:

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Until and if batteries have much, much more energy density, these planes aren't going to be in the air long enough to be able to climb to and descend from the flight levels. A 737 uses as much fuel to climb to altitude as it does in an hour of cruising. If our plane only has an hour of endurance, it isn't climbing far.
A good lithium ion battery has a specific energy of around 250 Wh/kg, or 900 kJ/kg. It takes 9.8*10000 = 98 kJ to lift a kg to 10 km.

If the plane is 1/3 battery, then there's a factor of 3 remaining. Planes are generally pretty efficient at producing lift, because they push a tremendous amount of air downward at a relatively low velocity. I'm not sure how it works out in practice, but if they're 66% efficient, we've used half the capacity to get to 10 km.

That doesn't sound half-bad as a ballpark. And the early planes on regional hops aren't going to fly to 10 km anyway.
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Old 12-08-2019, 08:48 PM
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Canada's Greg McDougall hopes to fly a Magnix retrofitted electric Beaver this Wednesday.

https://crosscut.com/2019/12/worlds-...could-take-pnw
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Old 12-08-2019, 08:54 PM
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I think we all hope to ride an electric Beaver one day.

That's pretty neat, at any rate. I expect there are lots of these little regional airlines like Harbour Air that serve only very short flights (30 min in their case).
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Old 12-08-2019, 09:01 PM
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Everyone Otter.

(If the Beaver retrofit works, he hopes to also retrofit Otters)
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Old 12-08-2019, 09:14 PM
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I'm hopeful that the Beaver is the first electric in the air, for with floats and skis De Havilland's Beavers and Otters opened up much of Canada. Bridging from one epoch to the next would be pretty nifty.
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Old 12-08-2019, 09:20 PM
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I believe modern ones are lithium iron phosphate (LiFePO4) with integrated battery management electronics. LiFe cells are known to be less temperamental than LiIon or LiPo cells.
Yes, I probably have an Iron battery. I have to take it out and store it in the winter so it doesn't freeze. It may be more temperamental than other Lithium batteries but there is no way to deal with a failure at altitude. A cascade failure would impossible to control. The article I linked to talked about mitigating the fire risks associated with lithium-ion batteries.
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Old 12-08-2019, 11:03 PM
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A little napkin math as a demonstration:

joema mentioned the ATR 72 as a good starting point, so I'll use that as a baseline.

Although I can't find a drag coefficient for the ATR, efficient aircraft can be in the ballpark of Cd=0.022. This Cd uses wing area as the reference area and includes both parasitic and induced drag (i.e., lift drag). The ATR has a wing area of 61 m^2.

The ATR has a cruise speed of 510 km/h, or 140 m/s, but I'll reduce this to 125 m/s for efficiency reasons. We'll also assume it flies at 5 km, which has an air density of 0.53 atm.

So, the drag force comes to:
F = 0.5*p*u^2*Cd*A = 0.5*1.225 kg/m^3*0.53*(125 m/s)^2*0.022*61 m^2 = 6807 N. At 125 m/s, that's 851 kW.

Propellers are efficient; around 90% is achievable, but we'll go with 85% as more typical. That puts us at 1001 kW.

If we want to sustain that for an hour (traveling 450 km), we need 1000 kWh. For the ATR at max takeoff weight, we need an extra 23000 kg*9.81 m/s^2*5000 m / 3600000 kWh/J = 313 kWh for the potential energy in getting to altitude.

So we need 1313 kWh of cells, and with current cell tech (250 Wh/kg) that means 5252 kg of them. The dry weight of the ATR is 13,300 kg, with the max takeoff weight 23,000 kg. So there's enough capacity. But it's better than that, because if you're doing a clean-sheet design then you will use the cells as part of the structure. Also, the electric motors should weigh less than the jet turbines, and you can get rid of a fair amount of other stuff.

Exactly how this balances out in the end depends on the design, but it's not totally crazy to think that the net weight increase might only be, say, 3000 kg instead of 5300. Especially since there would be a strong motivation to reduce the weight in other ways, like using composite materials. Whatever the case, the cells have not completely killed the payload.

Note also that efficient commercial craft have a glide ratio of around 20. So, once at altitude, you get another ~100 km of range due to trading the 5 km altitude for horizontal distance.

Of course I've left out a bunch of stuff, such as safety factors in the charge level--this is just napkin math after all. But I made conservative assumptions in some areas and unconservative ones in other ones. Some of those will balance out.

I also didn't go for somewhat more extreme approaches, like immediately flying to an extremely high altitude (say, 30 km) and then gliding for long distances. There's a huge design space to explore here that's completely new due to the differing characteristics of electric power. Some of these are bound to be improvements.
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Old 12-08-2019, 11:47 PM
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One thing I've always wondered is the effect on aircraft performance of hauling all the dead battery weight around. One big advantage, I would think, of jet fuel is that as you burn it off, the aircraft is getting lighter. As you get lighter, you can manage the power to generally give you greater range, a higher altitude, or more speed. With batteries, once they run out of energy, they are just extra weight. (There may be a more elegant way to describe this, but essentially, if I have a fuel tank with 1000lbs of Jet-A and burn off half of the fuel, I am now only hauling 500lbs of fuel and getting lighter by the minute. With 1000lbs of batteries and just half a charge left, the batteries still weigh 1000lbs and will weigh the same amount even at zero charge.)


Plus, you can't lighten the aircraft by defueling a battery. So if you need to remove weight for performance (maybe to get in and out of a little bit shorter runway) it would have to be passengers or freight, which no airline is going to appreciate. That leads to not being able to dump fuel and the landing gear structure has to be even stronger because it always has to be able to land at max takeoff weight. (The Embraer ERJ140, a typical 44 seat jetliner, has a max takeoff weight of about 46000lbs, but a landing weight of only 41000lbs. So the gear would have to be stronger to handle the excess weight.)

Am I overstating that, or would batteries actually have to be some significant percentage more efficient at storing power then hydrocarbon fuels just to haul the extra weight around?
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Old 12-08-2019, 11:54 PM
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Am I overstating that, or would batteries actually have to be some significant percentage more efficient at storing power then hydrocarbon fuels just to haul the extra weight around?
It's a fair point. However, for the short ranges viable in the near term, the fuel weight wasn't a significant factor anyway. The fuel burnoff matters a lot more after thousands of miles as compared to several hundred.

This is neither here nor there, but the Electron rocket by Rocket Lab uses electric turbopumps. As it depletes its batteries, it jettisons them so that they don't have to be carried any longer than necessary (on a more conventional rocket, the pumps would be powered by rocket propellant, and thus be burning through the mass as it flies). Of course, the whole rocket is expendable, so there's no real cost to doing this. Maybe we can have the batteries fly back to an airport on a little glider .
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Old 12-09-2019, 10:22 AM
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Also, the electric motors should weigh less than the jet turbines ...
I'm not too sure of this.

Googling suggests that modern jet engines can achieve specific power around 5 to 6 hp per pound - about what state-of-the-art brushless electric motors can manage. But the jet engine is complete, whereas you must add a propeller (or ducted fan) to the electric motor.

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Note also that efficient commercial craft have a glide ratio of around 20. So, once at altitude, you get another ~100 km of range due to trading the 5 km altitude for horizontal distance.
Yes - same with all aircraft. You consume extra energy to climb, and "get it back" when you descend in a glide.

Last edited by Xema; 12-09-2019 at 10:25 AM.
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