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2MW Electric Aircraft Engine

weflywright.com

99 pointsnixass118 comments

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The most effective technology for hard to decarbonize sectors like flight is a carbon tax.

It simultaneusly:

- reduces usage by pricing in the externality and changing consumer preferences

- makes existing alternatives (trains, boats, zoom calls) more competitive

- funds efficiency improvements in current tech

- gives a clear business case for developing new alternative approaches

- can be used to fund fundamental research

- can be used to accelerate uptake of low hanging fruit in other areas, postponing the time that the new solution is required in the hard to decarbonize sector.

Having said that, electric flight looks like it'll generate a lot of interesting stuff, but like in EVs it'll be the short, regular flights that'll switch first.

Uber has a conference with talks online on youtube that has lots of interesting information about this in the context of urban air taxis, e.g. working around noise limitations on city airports, new design possibilities etc.

I never really thought the engine the limiting factor in electric flight, but rather the heavy batteries.

Found this somewhere, is it still up to date?

> Jet fuel has an energy density of 9.6 kWh/L. What this means is that jet fuel, pound for pound, is nearly 50x more energy-dense than batteries. However, the inefficiency of internal combustion means that 1,000 pounds of jet fuel yields only about 14x more power than 1000 lbs of batteries.

Why are they using volumetric energy density when speaking of weight? Anyway, jet fuel has a specific energy of 46MJ/kg (HHV) compared to 1MJ/kg for good lithium ion (conventional chemistry). Drop that 46 by a third for HHV efficiency of 33%, and you’ve got 15MJ/kg.

But better chemistry lithium ion (lithium sulfur, lithium metal anode, etc) gets you to 2MJ/kg and the difference is only about a factor of 7.

A factor of 7 sounds like a lot until you realize jet fuel is better than it needs to be and conventional aircraft still leave a huge amount of efficiency on the table. (This is the same reason why gasoline is ~15 times better than lithium ion, and typical gas car ranges are 400 miles but you can get a Model S with the same 400 miles range). The Virgin Globalflyer jet went over 40,000km on a single fueling of jet fuel at Mach ~0.5, and there have been aerodynamic and structural/material improvements since then. One seventh of that is 5700km, enough for long haul battery electric flight.

How does change in energy density over time affect the viability of the energy source?

Going by your numbers, jet fuel maintains 46MJ/kg as it is depleted—the last kg has the same energy density as the first kg. I would exclude the weight of the enclosing tank because I think that’s usually the airframe itself.

Batteries start at 1MJ/kg for the first MJ and fall to 0MJ/kg when depleted (I don’t know what the curve looks like, but I’d expect it to be worse than linear).

What is the practical impact of this difference, if any?

If energy is consumed uniformly over time, and all fuel is consumed during a flight, it would mean a factor of 2. Using a lot of fuel during takeoff would mean a slightly higher factor, but keeping a fuel reserve would decrease the factor, so it would probably come out even?

An important characteristic of liquid fuels are that as you extract the energy from them you reduce the total weight. In particular for flight this helps increase overall range and efficiency.

Batteries weigh the same full or depleted. So after you deplete 50% of your battery power you’re still carrying 100% of the weight.

In cars this is less of an issue. But for planes, where you are defying gravity, it can make a large difference.

Interestingly, if you ignore air drag for cars, the equation is basically the same. Lift to drag is just the reciprocal of rolling resistance. But yeah, that does change the answer a bit. One possible solution is to stage off some of the battery mass.

The wheels on my Tesla hold up the batteries from gravity easily but in a plane you have to continuously lift them, does that change the dynamic?

It certainly does. To fly, an airplane has to push air down, it creates wing tip vortices, these vortices are wasted energy. The heavier the plane, the more air it has to push, the stronger the vortices and the more energy is wasted.

Cars push on the ground, and the ground gives back all the force applied to it, so no losses here, planes that fly very close to the ground can also take advantage of the ground effect (that's the name) for more efficient lift.

To how much extent, I don't know, but one thing for sure, manufacturers strive to make their planes as light as possible.

As a side note, heavier gliders actually fly a bit better (faster) but unlike powered airplanes, they don't lift themselves.

Edit: added ground effect

Not as much as you might think. A huge amount of energy is required to gain the altitude for long distance flight. Cruising much less so.

The way that airplane wings work allows planes to sort of float on a cushion of air, so modern planes, for example a 747 at cruising speed, have a lift to drag ratio of 17. So one pound of thrust gives you 17 pounds of lift.

>conventional aircraft still leave a huge amount of efficiency on the table

Go on. I’m listening and I’ll bet Airbus and Boeing are too

Not the GP, but I'll take a stab.

Roughly speaking, you can increase the efficiency of an airplane in 2 ways: increase the thermodynamic efficiency of the engine, and make the engine push more air at a slower speed.

For the first one you need to make the engine burn hotter. Since metals melt at a certain temperature, you need to use ceramics if you want to exceed that temperature. Such as Silicon Carbide [1] which "melts" (or decomposes) at 2830 degrees Celsius. The efficiency of an ideal Carnot engine is 1-T_cold/T_hot, so if T_cold = 300K and T_hot = 3000K, you end up with 90% efficiency. Of course the jet engine is not quite the Carnot engine, but the idea stands that higher burning temperature leads to higher efficiency. Now, if manufacturers decide to use Hafnium Carbide one day, that one melts at 1000 degrees Celsius higher.

The second method is due to the relationship between momentum and energy. Energy is m v^2/2, which means that for a given energy (think quantity of jet fuel burned in the engine), you can either push a certain amount of air at a given speed, or 4 times as much air at half the speed. But momentum is m v, so the second choice gives you twice the momentum. In other words, twice the thrust. To push more air, you want to grab more air, so you want turbines with higher diameter, and a higher bypass ratio.

General Electric and Pratt&Whitney do exactly that with their engines. Engines with higher bypass ratios and using ceramics (more precisely, ceramic matrix composites). In fact GE's engine is so fantastic, that Boeing decided to put it on the 730-Max, and the rest is history ...

So, yes, Boeing and Airbus know what needs to be done, but it's just not that trivial.

[1] https://en.wikipedia.org/wiki/Silicon_carbide

You missed the big one: improve the lift to drag ratio. A Cessna does like 10. State of the art airliner does like 20. But the Virgin Globalflyer did 35-40 and modern sailplanes do 50-70+, and it’s possible to do up to 100.

I did skip that one, as the topic was engines (gas fueled or electric), not overall efficiency. But since you mentioned this, here's yet another big one (known by very few people): just go faster. Much faster. The lift-to-drag ratio goes down, but the time spent in the air goes down more, so you might end up burning less fuel overall. In cruise mode, a Concorde quite efficient, but it was horrible at takeoff and climbing; because you need fuel to carry fuel, the problem was compounding and overall Concorde was not economical. If someone can solve the takeoff problem (e.g. by using catapults) and climbing (maybe using detachable and reusable boosters?) then a Mach 5 jet could be very competitive. I think this is what the Boom guys had in mind when they say they aim to get to where anyone can travel to any destination on Earth in 4 hours for only $100 [1].

[1] http://edition.cnn.com/travel/article/boom-supersonic-four-h...

Cost is the missing element there. They know how to make more efficient planes (e.g. Dreamliner) but the industry can't afford to apply that efficiency to the entire fleet.

> This is the same reason why gasoline is ~15 times better than lithium ion, and typical gas car ranges are 400 miles but you can get a Model S with the same 400 miles range

I think the real reason is there is that no need for bigger gas tanks.

But I'd need >1100 km range to make my road trip between family cities without stops, and each stop substantially increases travel time, even if refueling and paying are all I do.

Still waiting for a ground vehicle that I can pilot* on ordinary roads and highways, go a bazillian range units without stopping, and thereby usually avoid the dramatic'ly worse flying aggravations compared to when I was grammar school. (^_~)

* not technic'ly "driving" when you're not behind the thing that you're causing to move. Although really, any linguist would acknowledge that in reference to ground vehicles, the verb "drive" has acquired the additional meaning of the verb "pilot", so I'm just being silly.

Particularly when refueling is so fast and fuel stations so widespread.

It's the synergy of everything that matters. This same factoid is and was used by people doubting the feasability of battery electric cars and trucks because, especially in it's very misleading pre-combustion version, it's seems like an insurmountable difference.

But the total impact is related to the full system, which in cars includes:

- The useful power at the interface of road and wheel.

- The weight of the engine you get rid of (including the safety precautions to stop this killing passengers).

- The location of the weight.

- high torque at low speeds feeling "peppy"

- The type of journeys (stop'n'go traffic vs long distance trucking)

- The ability to use regen to save brake weight and pollution.

And so on.

Some of these have unexpected impacts, in EV the low center of gravity, the ability to put the wheels closer to the corners, the quiter operation, the ease of home refueling, ease of building on a shared platform are among the things that emerged.

For a while it seemed like hybrid cars might make the most sense, but a thousand small changes added up to basically make fully BEV the logical next step.

In aircraft, it's all still a bit up in the air (pun totally intended) but things like operating in thinner air that give combustion engines trouble, the ability to have lots of propellers and rethink a bunch of design assumptions, quieter operation in urban areas are already being noted and explored.

Jet fuel has the advantage that it is consumed as you fly. Some planes can start with full tanks, but not land. If they need to do an emergency landing, they have to dump the fuel. I imagine that would be difficult with batteries.

Thats an excellent point. I wonder if flight batteries could be developed in small distributed cells with a weight / surface area that would allow them to hit a non-dangerous terminal velocity. And I wonder what the financial and environmental impact would be as compared to dumping jet fuel.

We are making the assumption that dropping the batteries would actually be useful in an emergency situation. Considering that the plane needs to be designed to land fully loaded, since you're not dumping the batteries every flight, I think you would need to be way outside the operations planned safety envelopes to ever need to do that.

I think building battery cells designed to fail safely in an accident is probably a more reasonable option than trying to dump them.

Planes are safe to land fully loaded, but not designed for doing it repeatedly. The undercarriage, IIRC, needs a separate inspection after such landing.

This is why, time permitting, planes will first burn off or drop fuel before doing an unplanned landing.

While it‘s technically true that they can land fully loaded, using the word „safe“ is not ideal [1].

They _can_ land fully loaded but you are exceeding the maximum weight for landing which is neither advised by the manufacturer nor is it guaranteed that nothing will break. So if you it‘s not an emergency that requires immediate landing, you won‘t go for it.

[1] https://aviation.stackexchange.com/a/33253

Yeah that’s better said.

Dumping solids along flight routes doesn't really sound like a good idea. People don't like getting hit by small pellets, so matter how non-dangerous their terminal velocity is. Just imagine all the broken windshields for example.

Easy, the battery bank is a separate drone ship that can be detached at any time. In fact, you could have booster electric aircraft to help the planes reach cruising altitude and then return to the same airport it left from to do it again.

That doesn't sound easy at all.

You're right, I should have said "solvable" instead. Maybe some people wouldn't react so poorly to the comment. The point was that dumping the combustion products into the atmosphere and as a result reducing the total mass of the plane as it flies isn't some unsolvable advantage of jet fueled planes compared to the electric ones.

1) Detaching heavy objects while in-flight is a tried and tested technology. We've been doing it ever since flight was invented. 2) UAVs are not making the news anymore and fully autonomous landing has been commercially available since the mid-60s, granted, for very different type of aircraft, but it's becoming more common for light aircraft too.

Dropping UAVs from airplanes is of course a whole different ball game, but wouldn't you say it's.. doable? Ultimately whether the added complexity is worth it depends on the proportion of energy used during taxi, takeoff and climbing compared to the total which varies wildly depending on distance and other factors, but tends to be highest for very short (but also very long) flights which is where electric aircraft currently stand a chance.

Do planes use most energy to get airborne?

If so catapults like aircraft carriers maybe a good idea.

Or extension cords that pop out when it gets airborne?

You can power an electric plane with jet fuel, just like diesel electric trains convert diesel to electricity to turn the wheels.

I think for electric planes hydrogen fuel cells may be a real option since liquid hydrogen is so much lighter than pretty much any fuel or battery out there. The main problem is the density though.

Liquid hydrogen or some other type of fuel cell is definitely a solution for flying big heavy jets. Another potential solution is simply burning the hydrogen in jet engines. Or methane. Both have been used as rocket fuel. Both could be sourced in a sustainable way. The technology to build jet engines that burn that type of fuel already exists. The reason that fuel cells are potentially interesting is getting better efficiencies in combination with light yet powerful electrical engines such as described in the article. The reason we still use kerosene is simply that the infrastructure to distribute that is there and it's a cheap fuel to create.

Changing that industry is about ramping up the manufacturing and fuel infrastructure to the point where it is profitable. It's a very slow and expensive process.

The right question to ask though is why big heavy jets are interesting to begin with. They are big, heavy, expensive, and complicated machines. Why do we go through the trouble of building those at a cost of around 100M per vehicle for a small one?

This has to do with a lot of things but the two main ones that stand out are that fuel is expensive and pilots are expensive (and scarce). So the optimal conventional plane is one that moves around a lot of stuff with a minimum amount of fuel per kg of useful payload and the requisite two pilots. Hence the A380 briefly existed until it got to expensive to operate it.

An autonomous small battery operated plane would fix both those pain points. Electricity is comparatively cheap and autonomous vehicles means no expensive pilots are needed anymore. People that are talking about making electrical airbuses or boeings are missing the point: these form factors become a lot less relevant when we can fly dozens/hundreds of autonomous vehicles for orders of magnitude less money. Mostly small planes are pretty cheap to manufacture. So, are electrical engines and batteries. And the electronics needed to run them. You could feasibly build hundreds of them for the price of a single A320. And they'd be comparatively dirt cheap to maintain and operate. And a lot more flexible in where they could go. You are not going to cross ocean in one any time soon. But short haul flight is a different matter. Neither of the best selling jets (737 & A320) is commonly used for intercontinental flights. They are used for short haul flights.

> these form factors become a lot less relevant when we can fly dozens/hundreds of autonomous vehicles for orders of magnitude less money

A big and multi-faceted factor not considered here is airport capacity, in terms of terminal building footprint, aircraft ground handling, flight control and airspace.

Many large airports are already at capacity, so it's difficult to see how higher numbers of aircraft could work.

The counter point here is that small electrical and autonomous planes don't need large airports or long runways. They are very efficient in terms of space. All of the long suffering smaller airports that can't even handle jets would potentially be of use for this. And given the lower noise and pollution levels, building more airports would not be as controversial as building new airports for jets currently is. Possibly even within city limits.

And that is of course before you consider VTOL.

You make a good point; Now I'm curious how efficiently capacity is used with flight schedules exchanging large numbers of passengers with each flight, and whether smaller but more frequent flights would have any real impact on that equilibrium.

Trains solve all of those issues: short haul, high capacity, electric

I certainly agree that many countries (US & UK included) should be investing much more in rail, but there is the pesky problem of rails and land to put them on - it's very expensive, and takes a lot of time.

Liquid hydrogen sounds rather impractical, with its deep cryogenic requirements, compared to regular compressed hydrogen. Compressed hydrogen already has high energy density by volume to easily compete with hydrocarbon based fuels.

Yes, the 15x factor (14x in your quote) is what is often cited in discussions, given all else equal. What complicates is that fossil fuels can drive propellers, turbofans, and jets, where as electric motors can only drive propellers -- each technology with its own efficiency envelop.

That’s not true. You can drive a turbofan or jet with electric motors as well. For high bypass turbofans, a prop fan is functionally identical. For jets, the heat increase can be produced by multistage compression (although there’s little reason to do that versus a single stage turbofan/prop fan except for maybe supersonic, and even then single-stage makes a lot of sense).

Technically that would be a different thermodynamic cycle. For a true jet engine, you need compression, heating, and then expansion. Using the heat of compression doesn't count; it's thermodynamically equivalent to a propeller (just less efficient because not all of the work is being converted directly into kinetic energy, but rather a bunch is converted into heat).

One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor.

I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades).

Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.

That's funny, I just wrote about the exact same idea! I literally had not heard about it until now, but clearly many people have had the thought. Do you know of anyone actually attempting to build one, toy scale or otherwise?

Someone get Colin Furze in here

That would be interesting! The only way I know to ionize air en masse is a strong electric arc. These planes would sound like science fair Tesla towers!

Also there’s nothing that says a jet engine needs to use jet fuel. You could use electric heaters to heat the air, because ultimately it is a heat engine and it doesn’t care where the heat comes from.

Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets).

The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.

The issue is that jets are super inefficient for thrust. That’s why all modern passenger jets use the jet core just as a power source pretty much and rely on the bypass fan for almost all of the thrust. There’s no good reason to replicate the jet core when you can just use an electric motor to drive the bypass fan directly.

People seem to be forgetting that the fuel is also a heat sink, and is often used to dump heat from the air conditioning systems. Planes can and do take off in +100 degF weather, I'm not entirely sure how you're going to manage all that heat on an electrical plane with no readily available sink like a wing full of fuel.

The web page claims that the electric motor can output 2000 kW at 10 kW/kg.

For comparison, Europrop TP400, a modern turboprop engine, has

8200 kW 1938 kg (= 4.1 kW/kg)

So, although the electric motor is smaller by a factor of 4, its power density is about 2.5 times higher. The web page claims that the motor design is scalable up to 4000 kW, which is great.

The fastest turboprop airplane was TU-114, a Soviet airliner in the 50's that flew almost as fast as modern jet airliners. TU-114 uses four engines (Kuznetsov NK-12) that are the most powerful turboprop engines ever in production (11000 kW at 2900 kg). We need more powerful electric motors to emulate this performance..

Big claims for sure, but it looks like they're tied in with ARPA-E and a major airline. So, it's not some backwoods mechanics making bold claims about their infinite energy machine that runs on their special brew of moonshine.

https://mediacentre.easyjet.com/story/14628/easyjet-partner-...

Not surprising, the electric motor has some fairly high quality energy flowing in but the gas motor has to act as a heat engine as well, converting localized heat into higher quality energy as it goes.

That means a comparison of power densities between electric and internal combustion motors aren't entirely fair. But it also means that electrical motors have a big advantage in that it's a lot easier to add more of them. It looks like aircraft with many propellers along the length of the wing might have real advantages over single propellor aircraft and that's only something that's practical with electric motors.

> The web page claims that the electric motor can output 2000 kW at 10 kW/kg. > For comparison, Europrop TP400... > 8200 kW 1938 kg (= 4.1 kW/kg)

FROM SITE:

>2 MW - 2x improvement in []size[] over megawatt scale motors being demonstrated in the aerospace industry >10kW/kg specific power - a 2x improvement compared to available aircraft propulsion motors.

To make a fare comparison, you need to also compare the volumetric power densities (i.e. power delivered within a volume envelope vice just under a given mass threshold).

This is true for both energy and power densities - and it drives me bananas that these metrics aren't clearly specified for every single energy storage and conversion device.

Why does this matter? Because increasing a vehicle engine volume demands more supporting structure (frame), which generally drives up overall vehicle mass (hence cost of construction) and if the vehicle is operating in some fluid (air or water) the resistance to motion increases which drives up your installed energy and power for the same speed.

Imagine if you increase your car's mass pwr density by 2X, but doing so grows your engine compartment volume by 2X. Was that a wash? You can't tell until you eveluate the construction cost, energy storage and mpg impacts of having a larger car frame. Same is true with any vehicle.

Mass (specific) density is important, but volumetric density is too. Both need to be compared for a more thorough apples:apples comparison.

Why not make a ducted engine? Just replace the fuel burning part of a jet engine with this motor?

The compression effect of the jet engine blades is what ignites the fuel. I don't think that part is replaceable, it's what makes a jet engine a jet engine.

The hot gasses from the fuel burn produce thrust, and also sends rotational force back into the turbine blades to keep the compression effect going.

> The compression effect of the jet engine blades is what ignites the fuel

Not exactly, the igniter ignites the fuel initially, and after a short while the combustion becomes self-sustaining (i.e. new fuel is evaporated and heated above its autoignition temperature by all the residual heat around it). The igniters are turned off after a while and are only turned on again under certain conditions (high risk of flameout for a variety of reasons).

Also the "hot gasses from the fuel burn produce thrust" part has slowly become more and more incorrect, because nowadays in airliners we use what's called high-bypass turbofans where the primary purpose of the hot engine core is not to generate thrust, but to drive a turbine connected to the huge fan in the front, which sends most of the air AROUND the core ("high-bypass") in order to create thrust. The core does produce some thrust, but the air coming out of it is pretty fast and creates more noise, so we try not to have too much of that.

And the core itself doesn't really need to burn fuel, you could rip the combustion chamber out and put a nuclear-reactor-fed heat exchanger there and it would work the same.

Or a beefy electric motor that would drive the fan and also utilize its waste heat to imitate a very-high-bypass turbofan.

Thanks for the detailed explanation. I am not connected to the aerospace industry.

In the context of retrofitting an electric motor where the fossil fuel combustion happens today, it still doesn't seem possible.

> Or a beefy electric motor that would drive the fan

Isn't that what Weight is doing with their motor? Motor attached to fan blades to produce thrust?

The compression is the easiest part to emulate. The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid. Combustion is great on both accounts for that, but you could do it electrically as well given a sufficient power supply.

> The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid.

Without a combustible fuel in the picture, isn't it more efficient for the kinetic energy of an electric motor to directly spin fan blades or turboprop propellers, vs converting that energy to heat?

Maybe! There are certainly some operational regimes where it would be less efficient to spin props than it would be to shoot out hot gas, but they might be places where an electric aircraft wouldn't ever operate.

Greater parasitic drag from the duct?

Why do you need a duct?

Jet engines achieve higher speeds and efficiencies.

No, not efficiencies. Jet engines have lower efficiencies than propellers up to near 0.8 speeds of sound, and terrible ones at low speeds, and low altitudes.

Last generation airliners have designed cruise speeds below 0.8 speeds of sound.

So, a propeller can still be a viable option at that speed, it will just have to be enormous.

If propellers are more efficient, then why do airlines use jet engines? Noise? Reliability?

There's a reason jet engines completely dominate the market for commercial airliners. Would making the propellers electric change that?

> then why do airlines use jet engines? Noise? Reliability?

- Noise, yes. EU effectively bans turboprops by noise limits

- Reliability, yes. Gearboxes needs frequent, and expensive overhauls. That's acceptable on smaller regional planes, but not airliners costing tens of megabucks.

- Size. There is an upper limit on propeller blade size after which aeroelastic effects cannot be overcome by any known material.

I would argue then if you can't practically build a propeller to replace a jet engine on an airliner, that being more efficient in theory is meaningless.

I'll refine my initial claim to say jet engines are more efficient in many practical uses and configurations. Would you agree with that?

Would agree partially.

5-6 seat abreast planes can be switched to powerful turboprops for practical use.

Widebody airliners, not at the current level of technology.

Because jet engines have better power to weight ratio than cylinder engines.

The comparison talked about here is not between jet and piston engines, it’s about turbofans and turboprops.

> although the electric motor is smaller by a factor of 4, its power density is about 2.5 times higher

Is there any standard way of calculating this with the battery / full tank?

I think it’s most appropriate to compare vs “tanked for a given/defined range (including reserves)” rather than fully tanked. Airliners are rarely flown with full tanks, because that’s rarely needed. (Tanks are often sized for maximum ferry missions rather than maximum cabin load missions.)

See my comment to answer this question. Volumetric power density impacts are neglected in the OP's 'back of the napkin' estimate comparison. As you suspect, you need to account for the motor size and the specific application to see if this motor is a benefit to you.

Complicating matters is the fact that the OEM does a poor job of presenting these metrics up front on the landing page.

>TU-114 uses four engines (Kuznetsov NK-12) that are the most powerful turboprop engines ever in production (11000 kW at 2900 kg).

Interesting detail about the NK-12 (according to Wikipedia):

The design that eventually became the NK-12 turboprop was developed after World War II by a team of Soviet scientists and deported German engineers under Ferdinand Brandner, who had worked for Junkers previously; ... Thus, the NK-12 design evolved from late-war German turboprop studies.

Do longer/larger prop blades translate to better efficacy?

I wonder if we would be better off with array of smaller electric motors rather than pursuing 1:1 replacement of ICE engines.

Depends on flight speeds and prop rotation (tip speed specifically), but the propellers have for a long time had a 90+% efficiency already. Ideally, a long and narrow one-bladed propeller would have best performance, somewhat similar to sailplane's wing. For practical reasons (ease of balancing and limited diameter) we use two or more shorter blades, even if it causes minor efficiency loss.

A compounding concern is that of a propeller tip speed - when it approaches speed of sound, the physics of airflow become complex and it's very hard to maintain efficiency - and near impossible to maintain reasonable noise levels. This necessitates keeping blades of high speed propellers rather short. The Tu-114 was exceptionally loud for a plane, both due to propeller speed concerns, and also due to contra-rotating propellers inherently causing increased turbulence.

There is a good four-part series on propeller theory and practice:

https://www.youtube.com/watch?v=SgoPPg8oVt8&list=PLD2EcpzcvT...

If all else is equal (in reality it isn’t), you always get more efficiency by increasing blade length.

You want to maximise the momentum of the air your moving in any unit time for a fixed energy cost. If you double blade area you double both momentum and energy, if you halve the blade speed you halve the momentum and quarter the energy, so if you quadrupled blade length while halving the speed you get double the force for the same power.

This is of course a gross oversimplification, but it is why the human-powered helicopter a while back had such massive blades: https://en.wikipedia.org/wiki/AeroVelo_Atlas

all of those options have been studied for years; how many motors, what they weigh, where they are mounted, blades on the prop, rpm on the prop, any gear reduction. there is a room of aircraft designers that will show you why the current design is the "sweet spot", and the technical reasons that drove them there.

my point being this was all analyzed ahead of time, in modeling. no-one just said, "lets go with 4 foot props"..

Electric motors are more compact, need less cooling, work well at all altitudes, have likely more mounting options, can spin fast, offer different power curve, probably easier to service and diagnose.

I think there's enough of differentiating factors that could move the "sweet spot" when compared to ICE motors.

This NASA project seems to be pursuing that line if thinking

https://www.nasa.gov/specials/X57/

I think lots of little motors are the approach lilium is taking. https://lilium.com/ I’m a firmware engineer not an aviation engineer so I have no idea what the efficiency would be.

the original article shows ten electric engines on their planned plane.

A question to ask... is it the engine weight only, or engine + its gearbox?

It's very easy to make very high power to ratio electric engine, if RPMs, and voltages can be arbitrarily high.

They are actually a YC funded company.

https://www.ycombinator.com/companies/wright-electric

Anyone knows how much more efficiency is gained (due to reduced weight) once a jetliner consumes 50% of its fuel?

Curious if for flying it’ll be more practical to use electricity for some kind of direct or indirect carbon sequestration (to be carbon neutral) fuel generation and continue to fly with some type of combustion based fuel.

I know that stopping to refuel in the middle of a flight vs carrying all the fuel you need to go nonstop is about 20% more efficient because of not having to carry fuel to carry fuel.

So at the end...they are saying that soon we might be seeing commercial airplanes with 10 props? How would that be configured do you think?

Like 5 under each wing seems like a stretch...could you put maybe 3 under each wing and 2 on the back tail somehow? Or is 5 under each wing the only way to go?

Well, the aircraft mockup on their website[1] has what looks like 4 ducted fans in the tail and 6 on each wing. Might be an older mockup from when they thought they would need 18 motors.

Though, I suspect very little engineering thought has gone into that mockup. Looks more like something marketing threw together in an afternoon.

[1] https://www.weflywright.com/wright-1

Why is 5 on each wings a stretch? With electric motors you have a lot more flexibility, and distributed propulsion has some significant benefits.

Check out maxwell X-57 for research on this: https://en.m.wikipedia.org/wiki/NASA_X-57_Maxwell

Electric planes will probably look very different from modern turboprop/turbofan planes by the time they’re fully optimized for electric propulsion.

Could electric planes not benefit from having the initial energy heavy and take-off process boosted through some external source? Some sort of tow/tug, some sort of push, or the use of a supercapacitor? This way battery need not be depleted for the initial take-off?

Unless you literally tow it for many minutes to cruising altitude, whatever push you can give it in the ground won’t really amount to much, compared to the energy use for the rest of the trip.

Imagine you’re embarking on a cross country road trip, and someone offers to initially tow you to get you to 60mph, but then leave you on your own. Will that save you a lot fuel? Obviously not.

The way specifications are given makes it very hard to compare this to fossil fuel engines.

How so? 10kW/kg is a pretty relevant metric.

Seems they're ignoring the weight of the fuel/batteries.

Should it be so easy for anyone to market their products with the Wright name ?

The Wright brothers’ company is still around, albeit as a successor entity through many mergers and reorganizations. See https://en.wikipedia.org/wiki/Curtiss-Wright

We could say the same about Tesla.

The inverter is more interesting here imo. Actually, that's only part of the problem. Delivering 2MW+ from batteries to the engine and other systems seems pretty hard. Delivering it without failure, even more so...

It’s probably easier, fundamentally speaking, than delivering jet fuel to turbine engines without failure over a huge range of environmental conditions. It’s just that over many years we’ve perfected the later and already figured out and solved pretty much everything that can go wrong.

> 800 mile range

Screams of a scam

It's an impressive piece of mechanical engineering. I don't see it as being able to fly without some really aggressive advances in technology. If we stay on our current track then maybe in 200 years we could have a direct conversion compact fusion reactor as the power source of a high-power electric plane. Until then I don't think this could reasonably fly.

200 years is a little less than twice as long humans have been flying powered aircraft for. It's more than 3 times longer than humans have been putting things in to space for. The idea that you can predict that far in to the future is just silly.

... and what exactly are you basing any of this on?

Battery-powered airplanes are perfectly capable of flight. They are just inefficient for long trips.

None of them have 20 MW to feed.

The largest machines humanity has built (typically mining machines) are electric. I don’t see why people think total power is the real limiting factor vs, say, specific power.

Specific power is also not a problem. Specific energy is.

Indeed, but the article is about a motor.

An aircraft motor*.

At the upper end, the largest aircraft engines such as the GE CF6 (used in 747) produce about 40MW [1] and there are 4 of these.

[1] https://en.wikipedia.org/wiki/General_Electric_LM6000

The GE CF6 does not use motors for propulsion.

The LM6000 is not used in aircraft because it can't be with current technology.

The CF6 is a gas turbine engine, not an aircraft.

Virtually all aircraft engines outside of very small personal planes are gas turbines. This includes turboprops, helicopter applications, all commercial aircraft (turbofan) as well as fighter jets.

The primary difference is how the thrust is used. If mechanical energy is required a power turbine is used - as is done in the LM6000.

The LM6000 is the industrial version of the CF6 (the cold section is largely the same). The thrust is converted to mechanical energy using a power turbine. I included this link instead of the CF6 since it provides a direct reference to power output in MW (i.e. not thrust).

And if you just look at the state of battery tech, that does not actually look unreasonable to provide.

There are commercially available high-energy cells that provide ~900W/kg. That's ~22t for the cells, round up to 30t for the other stuff. Coincidentally, that's roughly similar to the fuel capacity of an A320, to which they compared their engine power.

You could push the specific power up quite a lot, but you'd have to start sacrificing specific energy, which is the sticking point of electric airplanes. With those batteries, you only get 5MWh, or you could only provide that 20MW for ~15 minutes.

This is not as bad as it sounds, because airplanes only need to provide near their maximum power for relatively briefly. There are many short hop routes where that airplane can operate profitably.

Still, it should make clear the point that the deficiency of electric airplanes is not their power, but their range. When you fit a plane with batteries that contain sufficient energy to go anywhere useful, you also provide it with frankly excessive power to run it's engines.

>There are commercially available high-energy cells that provide ~900W/kg.

Can you please provide more information? My understanding is the best lithium is under 400wh/kg.

Edit: Sorry looks like you are talking about power density not energy density (watts vs watt/hours).

My understanding is range (energy density) is the problem not power output. Cruise takes about 1/3 the power of takeoff so maybe an hour of flight with 5 Mwh?

Of course it will be able to fly. With 20MW of engine power at your disposal you’ll be able to lift an awful lot of batteries.

The real question is, how long and how far will it be able to fly? And how much payload?

Where's the 20MW from?

The 5th and final line:

>Wright will use 10 2MW motors on its Wright 1 aircraft - that's a total of 20 MW, which is as powerful as an A320 Airbus aircraft

The blog also links to a "deep dive" with a few more details.

https://medium.com/@jeff_60994/wright-has-begun-testing-our-...

> ”Where's the 20MW from?”

Quite a few tonnes of batteries, I’d say. But potentially there are other options like fuel cell stacks.

vs. quite a few tonnes of jet fuel.

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