Question About Electric Aircraft Propulsion

[gmax137]

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.

Hmm what is the terminal velocity of a shipping container? Say 100 MPH or 45 meters/second. 100,000 tons (10^8 kg) at 45 m/sec gives 100 gigajoules (check my arithmetic). About 24 tons TNT equivalent. Better wear your hardhat when those things are floating overhead

 

[Anand Sivaram]

That could be used as an argument against many possible ways to reduce CO2. I can see the logic behind it but an awful lot of that CO2 is due to unnecessary travel. Why have foreign holidays, for instance?

Edit: Unnecessary trade is also a factor. Items are transported from A to B whilst identical items go from B to A; the only reason is the trade deals and the politics involved. As with many such issues, 'Freedom' to trade does not mean that it's justified. This is not Physics, of course but it's certainly a point worth noting.

Absolutely agree..

 

[Klystron]

I'd really like to see a calculation on the volume of lifting gas required to carry 100,000 tons, to replace a cargo ship.

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.

The reference to water-borne cargo was meant as analogy, perhaps inapt. I have seen an articulated truck loading three standard containers. Most ground vehicles carry only one or two containers. All air traffic entails strict weight and size limitations plus overflight restrictions. Though heavier than air, consider the C5a (https://amcmuseum.org/at-the-museum/aircraft/c-5a-galaxy/).

My point was to consider designing air frames around improved electrical motors. While interesting, the idea of replacing liquid fueled jet engine components with "electric" in an air frame designed around jet propulsion lacks merit.

 

[gmax137]

I was going to let this go, but I just can't.

That could be used as an argument against many possible ways to reduce CO2. I can see the logic behind it but an awful lot of that CO2 is due to unnecessary travel...

Who says what is necessary travel and what is not? Our system in the US and in most of the world is, if you can afford the tickets you can take the trip. Is that a perfect system? Of course not. But it is better than any other I can think of ("Sophie decides," "gmax137 decides," "the FAA decides," "nobody travels...").

Why have foreign holidays, for instance?

There are too many reasons to list. First & foremost might be, "better understanding between different cultures."

Edit: Unnecessary trade is also a factor. Items are transported from A to B whilst identical items go from B to A;

Again, who says what is "necessary trade" and what isn't?

the only reason is the trade deals and the politics involved. As with many such issues, 'Freedom' to trade does not mean that it's justified.

Besides "trade deals and politics," the people involved in the unnecessary trade get paychecks, allowing them to provide shelter and food for their kids. Almost anything can be viewed as "unnecessary." But people find a niche, and make their living filling it.

This is not Physics, of course but it's certainly a point worth noting.

Agree, it isn't physics.

This kind of commentary just feeds the denier trolls, giving more fuel to their belief that climate change is just another excuse for social re-engineering.

 

[sophiecentaur]

belief that climate change is just another excuse for social re-engineering.

I think a bit of global/social re-engineering is seriously needed. "Feeding your kids" is very laudable and we all do it when we can. There is a big difference, though, between enough and so much that other people suffer. When we feeding clothe our kids with food and clothes that are produced on subsistence wages I think we could ask whether that's justified. I find it amazing that 'good Christian people' often seem to limit their 'caring' to their nearest and dearest. "Who is my neighbour?" is a question which was answered in some detail by (according to Luke's Gospel) the founder of their religion.

Who decides what's justified? Well, we can all decide, on the basis of how it affects the less fortunate sections of the world and the environment (there's plenty of information available for good decision making). It isn't an easy one to solve and the last thing I would want would be to impose arbitrary limits. Ninety percent of foreign holidays are truly not 'necessary' and the old chestnut about holidays providing "better contact" with other cultures doesn't apply to most holidaymakers and most destinations. Hotels and swimming pools are indistinguishable the world over and so are theme parks. Staff in the entertainment industry will nearly always speak English and your average holidaymaker could get more information about true foreign cultures from TV or even books than they get on most package trips.

Clever marketing is what has made people desire particular lifestyles. People spend money on things that they are targeted with. (And the 'they' is a small wealthy sub-set of the population.) Individuals can always justify their own holidays and there are a number of extremely 'worthy' and enriching holiday activities available that don't involve a lot of 'consumption'. (Personal choices should involve thought, not reading adverts.)
I can only hope that 'thinking' people will make decisions about their lifestyles that are better informed than if they just the advice they get from the big corporations.

This kind of commentary just feeds the denier trolls, giving more fuel to their belief . . . .

I'm not sure where that argument takes us. The presence of climate change is a separate issue from the possible consequences. Denyers will use all sorts arguments but the basic reason (seldom stated) is that they are terrified that their lifestyles may have to change. Head in the sand is preferable to the logical consequence of actions they would have to take if they concede that it's happening. But denyers don't feed off evidence, as we well know.

 

[skystare]

A 100,000 ton airship would need somewhat over 3 billion cubic feet of gas and would be roughly a mile long. That's a couple of decades of global helium production, so H2 will have to do. Be careful, y'all.

 

[skystare]

Sophie, 'thinking' people are a smallish minority. Any scheme to save the world/build utopia that depends on most people behaving notably better will founder. Even getting the few hundred decision makers in China and India to abandon their plans for 1400 new coal fired power plants is only moderately likely, and even then entirely dependent on viable technical alternatives.

 

[russ_watters]

Hmm what is the terminal velocity of a shipping container? Say 100 MPH or 45 meters/second.

Doubt it - a human intentionally acting draggy has a higher terminal velocity than that. A densely packed shipping container is probably triple that. But in either case...

100,000 tons (10^8 kg) at 45 m/sec gives 100 gigajoules (check my arithmetic). About 24 tons TNT equivalent. Better wear your hardhat when those things are floating overhead

Yeah.

 

[russ_watters]

The reference to water-borne cargo was meant as analogy, perhaps inapt. I have seen an articulated truck loading three standard containers. Most ground vehicles carry only one or two containers.

Fair enough, but whether it's one container ship or 2,500 twin-container trucks or twin-container airships, the problem is pretty much the same. To put it another way, if every airship carried two containers (and big cargo aircraft carry more than that), replacing just four big container ships with airships would require doubling the number of aircraft in the air at anyone time. That's how big a scale we're talking about here. Replacing trucks or container ships with airships is just never going to happen. The capacity scales are orders of magnitude apart.

My point was to consider designing air frames around improved electrical motors. While interesting, the idea of replacing liquid fueled jet engine components with "electric" in an air frame designed around jet propulsion lacks merit.

On that we agree.

 

[sophiecentaur]

Sophie, 'thinking' people are a smallish minority. Any scheme to save the world/build utopia that depends on most people behaving notably better will founder. Even getting the few hundred decision makers in China and India to abandon their plans for 1400 new coal fired power plants is only moderately likely, and even then entirely dependent on viable technical alternatives.

You make good points there. However, that implies that there is no hope for the human race. So be it. We will be dead before the crunch really comes but our grandchildren etc. will not escape. We could possibly be rescued by an appropriate extremist, fundamentalist religion taking control. Many of the "thinking" people will end up against a wall but after a millennium, things could settle down again. (A Seldon Crisis, perhaps??)

 

[gmax137]

Doubt it - a human intentionally acting draggy has a higher terminal velocity than that.

Yes, that's why I picked that number -- everyone will agree, the real value is higher, so my result of 100 gigajoules is a lower limit. The actual kinetic energy on impact will be higher.

I just spent some time goofing around with the shipping container dimensions and the terminal velocity equations... And then there is the time required to reach a given speed... Anyway my velocity estimate is low by a factor of about four, so the energy would be 16 times higher. If the box is nose heavy (so it falls "head first") the result could be much higher.

 

[bob012345]

That's very unlikely in the near to moderate future in my opinion. Fossil fuels are extremely energy dense, which is one of the features that makes them useful over other energy sources, and it's very difficult for batteries to even approach their energy density. Jet fuel has an energy density of around 43 MJ/kg, while lithium ion batteries are about 0.8 at the top end, so we're talking roughly a fifty-fold difference between them.
The issue is that jet engines don't rely on a moving prop to generate their thrust (with the exception of turboprop engines), they rely on the heating and expansion of gas powered by the combustion of fuel. How would you do something similar using electric power?

I think fuel cells may be a bridge technology between fossil fuels and batteries. You still use liquid fuel and still get CO2 but the conversion may be more efficient so less CO2 per pound of fuel along with more energy available to power an electric turbofan or propeller. Then as batteries become a lot better you can swap out the fuel cells.

 

[skystare]

Sophie . . . I don't think that there's no hope for us, just that it mostly lies with the thinking people coming up with technical solutions.

 

[skystare]

Bob01234 . . . Aluminum/air fuel cells would be a marginally practical option for an electric aircraft in the prop commuter or lightplane market. Aluminum carries a bit over 30 MJ/kg energy differential over aluminum oxide. One drawback would be the increasing weight of the cell throughout a flight, as the "exhaust" is powdered Al2O3 accumulating inside the cell. One upside of e-lightplanes is that electric motors have lifetimes and reliability more comparable to turbines than piston engines.
Al/air could be a terrific system for cars, though; comparable range to an IC vehicle & replacing the fuel unit would be far quicker than charging a battery. Intermittent power sources like solar and wind would be ideal for "recharging" by powering the smelting of the oxide back into metal.
A consortium of an Israeli and a Canadian company are apparently making progress on such a system.

 

[bob012345]

Bob01234 . . . Aluminum/air fuel cells would be a marginally practical option for an electric aircraft in the prop commuter or lightplane market. Aluminum carries a bit over 30 MJ/kg energy differential over aluminum oxide. One drawback would be the increasing weight of the cell throughout a flight, as the "exhaust" is powdered Al2O3 accumulating inside the cell. One upside of e-lightplanes is that electric motors have lifetimes and reliability more comparable to turbines than piston engines.
Al/air could be a terrific system for cars, though; comparable range to an IC vehicle & replacing the fuel unit would be far quicker than charging a battery. Intermittent power sources like solar and wind would be ideal for "recharging" by powering the smelting of the oxide back into metal.
A consortium of an Israeli and a Canadian company are apparently making progress on such a system.

I once heard a lecture by a Purdue professor on such a system with Aluminum proposed to replace gasoline for cars. Even though it could be workable, it probably won't be adopted. Too much inertia for a marginal gain meaning the adoption of a whole system of regeneration of the fuel. I just meant a fuel cell using existing fossil fuels as a transition to electric aircraft with compact high energy density batteries.

 

[Anand Sivaram]

Recently I came across NASA Glenn Research Center website.
NASA Electric Aircraft Testbed (NEAT)
They are working on EAP - Electric Aircraft Propulsion

https://www1.grc.nasa.gov/aeronautics/electrified-aircraft-propulsion-eap/eap-for-larger-aircraft/

 

[FRVI]

The enthalpy of burning jet fuel is (approximately) 40 Mj/kg. But mr. Carnot puts an upper limit to conversion to "shaft energy"aka mechanical work(optimistically) to 0.20. The electric motor is 0.9 (this number measured) from e energy to shaft energy. LiIon is (available now) 0.9 Mj/Kg. Just for fun, Li air has 41 Mj/Kg. So, the ratio of energy from jet fuel to Li ion is not 40 : 1, it is 8:1,and there are much higher energy chemical reactions. Furthermore, electric power system can be fragmented, requiring completely different aircraft design, allowing VTOL or STOL. The slow conversion of small and slow aircraft to e energy is inevitable, driven by economics, not by the goodwill of the majority in advanced countries. Which, tongue in cheek,does not include the US,perhaps temporarily. Another point is the size of aircraft' fossil fuel usage,of the order of 1-2%. To fill a 747 ,it takes 1000 round trip from home to airport by car;this contribution is negligible (compared to the traffic around SFO).Conclusion: 1) electrifying aviation will not reduce fossil fuel consumption and 2)trucks and cars are first priority; and finally 3)we need an increasing number of dedicated, knowledgeable,curious, free of worries about tomorrow' bottom line scientists-that is, working in national labs and/or universities.

 

[FRVI]

Well, theoretically you could use a giant electric heater to heat the air in the "combustion chamber", but I'm not sure if that's really doable.

 

[FRVI]

The compressor /burner behind the duct ed fan spin at high rate. The fan must keep the tip of blades below sound speed. Latest jet engines have a reducing gear from the compressor shaft to the fan ( just like turboprops).An electric motor will spin the fan ,with practically 100% efficiency, without any gear.A Dreamliner burns 1.3 Kg/sec.That is 50 Mj/sec ( or Mw). My guess is that 4 Mw electric will be needed operating the fan from an electric motor from a battery. weighing 120 Tons (6 hours flight). The plane will burn 28 tons of fuel.280 passengers @60 Kg/passenger is 17 Tons. Conclusions: 1)I am not suggesting to convert a Dreamliner to e drive,2)batteries are no more than an order of magnitude away from intermediate range, subsonic aircraft for passenger flight.

 

[cjl]

The enthalpy of burning jet fuel is (approximately) 40 Mj/kg. But mr. Carnot puts an upper limit to conversion to "shaft energy"aka mechanical work(optimistically) to 0.20.

This is pretty inaccurate. Gas turbine engines regularly achieve efficiencies in the ~40% range, and modern turbofan engines can achieve over 50%. You also have to account for the fact that at the speeds jets fly, props are not 100% efficient either, so your overall efficiency for an electric drivetrain might be down in the 75% range. In addition, electric motors aren't anywhere near the power density of turbines, so you would require larger, heavier engines for the same performance. Finally, electric doesn't have the efficiency advantage of getting lighter throughout the flight. Combine all of these and I don't think you could match modern jet performance even with batteries in the ~30-40MJ/kg range.

 

[cjl]

The compressor /burner behind the duct ed fan spin at high rate. The fan must keep the tip of blades below sound speed.

Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.

Latest jet engines have a reducing gear from the compressor shaft to the fan ( just like turboprops).An electric motor will spin the fan ,with practically 100% efficiency, without any gear.A Dreamliner burns 1.3 Kg/sec.That is 50 Mj/sec ( or Mw). My guess is that 4 Mw electric will be needed operating the fan from an electric motor from a battery. weighing 120 Tons (6 hours flight). The plane will burn 28 tons of fuel.280 passengers @60 Kg/passenger is 17 Tons. Conclusions: 1)I am not suggesting to convert a Dreamliner to e drive,2)batteries are no more than an order of magnitude away from intermediate range, subsonic aircraft for passenger flight.

4MW isn't anywhere close to enough. Propulsive power at cruise for a modern jetliner the size of a 787 is more on the order of 40MW. At takeoff, each engine has to be making more like 50-60MW of shaft power just to run the front fan. Rerun the electric numbers knowing that and you'll see why running jetliners on electric power is a pipedream without a massive breakthrough in technology.

 

[gmax137]

Overall efficiency here refers to the efficiency with which the engine converts the power in the fuel flow to propulsive power. It is the product of thermodynamic efficiency of the process that converts fuel flow power to shaft power (herein called motor thermodynamic efficiency) and propulsive efficiency (the conversion of shaft power to propulsive power).

The most efficient commercial aircraft gas turbines in service or entering service in this decade have takeoff thrusts of 20,000 lb and above. These turbines operate at cruise, with motor thermodynamic efficiencies of up to 55 percent and propulsive efficiencies of well over 70 percent, yielding an overall efficiency (the product of the two) of about 40 percent

https://www.nap.edu/read/23490/chapter/6#36

 

[essenmein]

Just for reference we work with reasonably state of the art electric machines (automotive), so a stake in the ground:
55kW peak, 35kW continuous liquid cooled PM 6ph machine, 22k rpm max, 135mm stator OD, 120mm stator stack length, weight about 12-15kg (estimated weight, too much other stuff connected to it to measure machine on its own...). This is at a reasonable limit for air gap, air gap flux and demag on relatively cost effective PM material.

If a number mentioned earlier is correct (100k Hp for dream liner turbo fan), then this is about 75MW e machine, and if built using similar tech as the PM machine mentioned, then you're looking at about 22500kg machine. I don't know what a core of a turbofan weighs, but I somehow doubt its 22 tones... Thats using the 55kW peak number not the 35kw continuous.

In cars you can play games with big short term numbers, unlikely for example you'd sit at full throttle for more than 10-20sec continuously, compared to a plane or boat, the power is needed for much longer periods (eg climbing to altitude) so relying on thermal mass is not really going to work, at least the air is cold up there...

 

[cjl]

You could probably get away with more like 50MW, and you only really need about a minute of full throttle capability, so you might be able to pull that mass down a bit. That having been said, an entire GEnx engine (including the fan and nacelle, which you'd still need for the electric) only weighs about 6 tons, so you're still at a massive weight disadvantage there.

 

[essenmein]

Yeah and that's just the metal in the machine, stator steel, PM, wire, bearings etc, you'd still need to add the control electronics etc, not huge numbers there, but they are not zero weight.

 

[essenmein]

And when people say "the technology needs to catch up", a lot of the time that is just wishful thinking, the laws of physics just get in the way.

If we are talking electric machines, then the tech is pretty stable, they are basically determined by F=BIL, B is fundamentally limited to about 1T in the air gap due to the magnetic material limits (B sat of stator steel, demag of the PM material, bearing tolerance etc), I is limited by the capacity to cool a wire and if PM then demag also will limit current, then L is a physical dimension (length), so if B and I are limited by existing materials/physics then all you can do is change L, ie larger size. I think its highly unlikely we'll discover a magic bullet around the B sat limit and the demag issue with permanent magnets...

 

[merriam]

The key to getting a flyable aircraft is the power/weight ratio of the engine. For Jets, the power outlet depends a lot on the speed, but roughly speaking a Boeing 777 engine (GE 90) puts out about 6 hp/lb (10 Kw/Kg). Very good electric motors can manage power/weight in this neighborhood. Viability depends on having light batteries that hold a lot of energy.

Electric aircraft exist, generally as small, experimental, short range aircraft. However, the Solar Impulse 2 has circumnavigated the globe (powered by solar cells). A number of projects are exploring hybrid aircraft, using the electric motors to increase the takeoff thrust, but turning them off in flight.

The power required for supersonic flight more or less precludes electric supersonic aircraft irrespective of the jet/propeller question.

Regarding jets and flight speed, the key is to look at the diameter of the column of gas coming out. You can have a large diameter at low speed or a small diameter at high speed. For the same level of thrust, low speed/large diameter takes less power. Example: A Harrier jump jet (hovering) vs any helicopter (hovering). The former has (4) small diameter jets of gas and requires a lot of power to hover. The latter has a big blade circle (and a big jet of air going down) and requires a lot less power. Top speed of helicopters is under 200 mph though, while the Harrier is over 600 mph.

That's why commercial aircraft have high bypass engines. Effectively they increase the diameter of the blade circle. This improves the gas mileage and lowers the noise. However, the top speed in level flight is limited to the speed at which the air comes out the back - the higher the bypass, the lower the top speed. Since all commercial aircraft (since Concorde) are subsonic (M = 0.85), the bypass is optimized for this speed. The fans are powered by the middle of the jet (so called hot section) where all the fuel burning happens. This de-energizes the exhaust from that part and makes it a lot quieter.

In summary, there is a long way to go in electric batteries and motors, before they enter commercial service (let alone military service).

 

[OCR]

Top speed of helicopters is under 200 mph. . .

You are real close, and for all practical purposes I agree. . . .

But, to get every thing right on the money. . . lets say 249.09 mph . .
It's rather amazing. . . the record still stands as of this year!

.

 

[merriam]

I'd really like to see a calculation on the volume of lifting gas required to carry 100,000 tons, to replace a cargo ship.

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.

Lift is about 1 gram/liter. Let's call it 100,000 metric tons or 100 million Kg = 100 billion grams = 100 billion liters. Roughly speaking, a cube that is 1500 feet on a side.

 

[merriam]

The key to getting a flyable aircraft is the power/weight ratio of the engine. For Jets, the power outlet depends a lot on the speed, but roughly speaking a Boeing 777 engine (GE 90) puts out about 6 hp/lb (10 Kw/Kg). Very good electric motors can manage power/weight in this neighborhood. Viability depends on having light batteries that hold a lot of energy.

Electric aircraft exist, generally as small, experimental, short range aircraft. However, the Solar Impulse 2 has circumnavigated the globe (powered by solar cells). A number of projects are exploring hybrid aircraft, using the electric motors to increase the takeoff thrust, but turning them off in flight.

The power required for supersonic flight more or less precludes electric supersonic aircraft irrespective of the jet/propeller question.

Regarding jets and flight speed, the key is to look at the diameter of the column of gas coming out. You can have a large diameter at low speed or a small diameter at high speed. For the same level of thrust, low speed/large diameter takes less power. Example: A Harrier jump jet (hovering) vs any helicopter (hovering). The former has (4) small diameter jets of gas and requires a lot of power to hover. The latter has a big blade circle (and a big jet of air going down) and requires a lot less power. Top speed of helicopters is under 200 mph though, while the Harrier is over 600 mph.

That's why commercial aircraft have high bypass engines. Effectively they increase the diameter of the blade circle. This improves the gas mileage and lowers the noise. However, the top speed in level flight is limited to the speed at which the air comes out the back - the higher the bypass, the lower the top speed. Since all commercial aircraft (since Concorde) are subsonic (M = 0.85), the bypass is optimized for this speed. The fans are powered by the middle of the jet (so called hot section) where all the fuel burning happens. This de-energizes the exhaust from that part and makes it a lot quieter.

In summary, there is a long way to go in electric batteries and motors, before they enter commercial service (let alone military service).

I stand corrected. At the Paris Airshow (june 2019) an electric aircraft was offered for sale. 3 engines, propeller driven, 650 miles at 500 miles/hour. 9 passengers. $4 million each. Roughly a dozen orders.