Cruise altitude for electric airplanes.

[Lord Lucan]

Electric airliners are presumably going to be a reality at some point in the future.

When engine efficiency considerations are no longer a factor, will there be any effect on the optimum cruise altitude for such aircraft? Would high Flight Levels still be the best? My aerodynamic knowledge has become somewhat rusty.

[B2N2]

Propulsion is a function of moving air.
Any sort of mechanical propulsion regardless of the fuel has an optimum altitude.
Under standard atmospheric conditions in an atmosphere with no wind it would depend on chosen system ( propeller, ducted fan) and wing design.
This would be an ideal cruise altitude but in reality many more factors need to be considered.
Winds aloft, sector length, GPS direct free-flight vs airways, regional and seasonal weather avoidance blah blah blah blah.
If it’s not fuel efficient ( energy efficient) to climb to FL380 now then it won’t be if we choose another fuel source.

[EEngr]

This would be an ideal cruise altitude but in reality many more factors need to be considered.

This is all true. What an electrically powered aircraft do is to remove the constraint of combustion performance on the selection of the most efficient altitude. Where a traditional jet engine core might exhibit some efficiency loss as air density (and partial pressure of oxygen) are reduced, I think other trade-offs dictate ideal cruise altitudes. Engines can be specified to cover the altitude envelope that the aerodynamics require without much problem. Given this, it doesn't matter whether the fan or prop is spun by a gas turbine or electric motor.

High altitude engines have been designed and built for planes like the U-2, Concorde and other special cases. The aerodynamics run into efficiency limitations before the powerplants do. One major input into the overall efficiency equation is fuel weight. Jet fuel burns off over the course of a flight, reducing the lift required and changing the most efficient altitude. Electric airplanes will be carrying the same load of batteries throughout a flight (aside from some heretofore unassumed fuel cell technology).

[Lord Lucan]

Yes, it was the lack of combustion efficiency considerations that I assumed would be the main difference.

Though it would have no effect on optimal cruise level, another difference is that perhaps there would be a possibility of recovering some of the energy used in the climb by using a windmilling fan to charge the battery during the descent

[FlightDetent]

perhaps there would be a possibility of recovering some of the energy used in the climb by using a windmilling fan to charge the battery during the descent A stimulating thought. What would be the optimal descent RPM of an electro-prop? Descent with the power on, recuperating, or even feathered?

[BleedingOn]

Also, if we’re talking some sort of lithium based battery in the mix, temperature would be a big factor. Certainly the range of electric cars is markedly reduced in cold temps. I can only imagine that would be a major consideration of high altitude operation

[nicolai]

Electric cars don't have a big (outside temperature related) range issue once they've warmed up the battery pack, and the operation of the motors and even of extracting power from the battery pack heats the pack up and keeps it warm. So if you pre warm the battery pack, or start off from a warm place (garage) then the range issues for cars are largely negated. The problem is with extracting energy, both in quantity and in rate, from a cold battery.

Cars have the obvious problem that you tend to park them in places where they have no external energy source so they get cold. Some electric cars improve the effectiveness of energy extraction from the battery by first using a bit of battery power to warm the battery, thereby getting more stored energy out of it later. In very cold places, it's common to plug in the car engine block heater to avoid the entire vehicle cooling down too much when you park the car, and you can imagine this continuing for electric vehicles.

A (transport) aircraft parked on a ramp with ground power could warm itself up from ground power before using onboard batteries, and in flight there will be heat from the motors (and air compressors, and so on) that can be used to keep batteries (and passengers) warm. Trivially, connect the heat-reject side of the air cycle machines to the batteries via a heat transfer loop and you'll warm the batteries while you keep the passengers supplied with air.

Transport aircraft in very remote places might need to self-heat their batteries, but the analogy these days is having to start and run the APU before boarding to provide passenger light and ventilation if you're at an airport too small to have ground power and conditioned air, and taking the fuel hit for it. The electric jet starting at the cold regional airport in the morning would need to heat itself up to get started as well.

Obviously, ground power to recharge the aircraft would have to be available - just like you need fuel to be available today.

[ShyTorque]

Also, if we’re talking some sort of lithium based battery in the mix, temperature would be a big factor. Certainly the range of electric cars is markedly reduced in cold temps. I can only imagine that would be a major consideration of high altitude operation

There are other considerations, too.

"Ladies and gentlemen, welcome on board this six hour flight to the Caribbean. I trust you are all wearing your thermal underwear - as soon as everyone has donned the arctic clothing provided for your safety and comfort we will be on our way. I regret to say that due to a headwind there will not be any hot food or drink served on our journey today".

;)

[speedrestriction]

perhaps there would be a possibility of recovering some of the energy used in the climb by using a windmilling fan to charge the battery during the descent

There would be loss of efficiency by doing this and in any case there would no need to keep the engines turning during descent. It might be more efficient to house the engines in the wing à la B2 and use conformal blocker doors to cover over the wing intake during descent and basically to glide from cruising altitude to somewhere between 5 an 10 thousand feet. I am sure there are a million and one different considerations to be taken into account regarding design not least fan diameter etc but personally I fail to see pure electric propulsion being used for anything other than an ATR sized aircraft on regional operations.

[Lord Lucan]

There would be loss of efficiency by doing this

I am not sure what sort of efficiency you are referring to. Air Nautical Miles per kWh? Could you elaborate a bit on this efficiency loss.

[pattern_is_full]

Bottom line - an electric aircraft is always going to need knife-edge efficiency. No power will be diverted for anything other than "go" and "payload" - so no pressurization. Thus the practical limit on altitude will be 12500 feet or so, regardless of the physics at higher altitudes. I'm sure there will be some "built-for-purpose" World-altitude-record-for-electric-aircraft contenders, with engineering devoted soley to lifting 1 person, 1 oxygen bottle, and x-many kilos of batteries to 40/50/60,000 feet. But in that case the optimum altitude will be "beats previous record before the O2 runs out" ;) . U2 aerodynamics would seem to rule in that case.

I'll go out on a limb and say that the optimum altitude for an electric aircraft will be "as low as possible" for the forseeable future. Think "Piper Cub." Better technology (materials, battery weight/amp ratio) may result in bigger and bigger "Piper Cubs" with larger and larger ranges and payloads. Except that the requirement to continue a takeoff with one engine failed will limit that also, as far as regulated commercial transport ("airliners") is concerned.

Net - somewhere around 2000 feet MSL or 1000 feet above terrain and obstructions, whichever is higher. I think having "air to push for reaction mass" will dominate or drive all other aerodynamic considerations - airfoils and such will be optimized for "enough speed" with "as little as possible" drag. Fine-tuned for function - transport, loitering, private, professional.

[speedrestriction]

I am not sure what sort of efficiency you are referring to. Air Nautical Miles per kWh? Could you elaborate a bit on this efficiency loss.

Every time you convert energy there is a wastage. Considering windmilling: you take off and climb to cruise - chemical energy to electrical to kinetic and the kinetic to electrical to chemical. At each of these changes energy is lost (friction, noise, heat, drag). By windmilling you are doubling the points at which energy can be lost - why not start your power off descent earlier (ie shorten your cruise phase) and glide to the runway rather than deplete your batteries longer in the cruise and then try to recapture it using windmilling in the descent. Recuperative technology makes sense in road vehicles due to the frequency with which deceleration is required. In an aircraft ideally there is no deceleration requirement nor is there a thrust requirement until you wish to go around on a missed approach/balked landing/reverse. In practice we keep gas turbine engines running to supply the various systems and due to the long start up times but in an aircraft powered electrically, these limitations would not apply. Glide the machine down to landing as much as is possible with the minimum drag possible (ie no big wind turbine hanging from the wing!)

[ATC Watcher]

To try to answer the original question : my guesses : Electric aircraft are going to be propeller driven *( at least for the foreseeable future ) = slow.
For unmanned or cargo aircraft ; I would guess 10.000 ft max. ( or above FL600 is solar rechargeable for long missions , like television relays for events, , observations, etc..)
For passengers carrying aircraft : electric pressurization cost energy, so why fly high? , but turbulence is an issue , so above weather , my guess will be between FL240 and 280 max.

[safetypee]

Viewing modern large fan engines as having a core ‘motive power unit’ driving a large ducted fan; then when replacing the core with a large electric ‘motor’ and driving a conceptually similar ducted fan, then why expect a reduction in speed.
My assumption is that a ducted fan is more efficient than an open rotor / propellor.

The characteristics of an electric power unit might be easier to match to a fan - still requires gearing, thus aircraft speed / altitude could be better matched without the limitations of requiring ‘stable’ core airflow which has to pass through a fan.
The primary driver for cruise altitude is more likely to be the balance of engine thrust / weight, battery weight, solar panels, additional fuel, and design range / payload of the aircraft.
A larger choice of altitudes for best wind speed - reduced block time, higer = faster ?

[Denti]

I'm sure there will be some "built-for-purpose" World-altitude-record-for-electric-aircraft contenders, with engineering devoted soley to lifting 1 person, 1 oxygen bottle, and x-many kilos of batteries to 40/50/60,000 feet. But in that case the optimum altitude will be "beats previous record before the O2 runs out" ;) . U2 aerodynamics would seem to rule in that case.

What point is there for something like that? A glider has already reached over 60.000ft (pressurized of course) and will eventually reach over 90.000ft. Not really impressive for a powered airplane to reach less than that.

[pattern_is_full]

@Denti - no, but "records" are set within categories: balloons, jets, gliders, props, helos, microloghts and paramotors, parachutes, etc. for various things: altitudes, un-refueled distances, endurance etc.

In just the same way that gliders don't have to compete against jets, electric-powered planes will almost certainly get their own category from the FAI (Fédération Aéronautique Internationale).

https://www.fai.org/records

[BluSdUp]

Quite possibly any altitude!
Once fuel burn at low altitude is not an issue a prop/fan driven electric aircraft will have much greater practical and economic flexibility.
The need for a jet and a turboprop for cold air at altitude to be efficient is gone, ie , a much greater range of efficient cruise altitudes!

My guess would be to make it efficient from 15 000 feet to 35 000 feet
And the level from FL 260 to say FL 320 is mainly unused for cruise,

[dogsridewith]

(Responding only to thread title)

For solar powered ones that can stay up indefinitely: Above the clouds...and above turbulence, since they're built light with enormous wings.

[tdracer]

Bottom line - an electric aircraft is always going to need knife-edge efficiency. No power will be diverted for anything other than "go" and "payload" - so no pressurization. Thus the practical limit on altitude will be 12500 feet or so, regardless of the physics at higher altitudes.
I'll go out on a limb and say that the optimum altitude for an electric aircraft will be "as low as possible" for the forseeable future. Think "Piper Cub."

Pattern, that's simply nonsense. It takes energy to pressurize the aircraft - regardless of the fuel source - but the lower the altitude the higher the drag and the more thrust required. The reason we normally cruise at 30k and above is simple - the reduction in drag means less fuel (energy) is required to fly a given distance, overwhelming the relatively small amount of extra fuel (energy) needed to warm and pressurize the aircraft. That is simple physics - going to electric propulsion doesn't change the physics.
Now, the early generation electrical aircraft will be relatively short range given the weight and energy density of current batteries. Shorter range means lower cruise altitudes simply because the additional energy needed to get to higher altitude is not justified by the smaller amount of energy saved due to short time at cruise.

[pattern_is_full]

td - I respect your oft-displayed engineering skill, and of course agree with your general point about altitude, drag and thrust.

I simply think the "early generation" limitations may last as long as the "kite" phase of powered flight - e.g. thirty years, roughly 1903-1933. Unless there is some "Black Swan" increase in the energy density of batteries and decrease in the general density of aircraft structures. Which is certainly possible, but not guaranteed. But pull out your slide rule and persuade me - I'd love to have an ETA for, say, a 19-seat electric transport.

And I say that as someone who welcomes electric flight and expects something generally viable and functional - on a small scale - within a handful of years (e.g. the electric Islander I've heard about).

[Sorry Dog]

Pattern, that's simply nonsense. It takes energy to pressurize the aircraft - regardless of the fuel source - but the lower the altitude the higher the drag and the more thrust required. The reason we normally cruise at 30k and above is simple - the reduction in drag means less fuel (energy) is required to fly a given distance, overwhelming the relatively small amount of extra fuel (energy) needed to warm and pressurize the aircraft. That is simple physics - going to electric propulsion doesn't change the physics.


Bingo! Reward yourself two Pesos from petty cash.

In fact, the plane may well carry high pressure air tanks that can be filled with ground power and serve as a backup air supply or even backup power source since it would already be there... but maybe not if its too hard to make the air compressors dual purpose: high volume low pressure for cabin air, high pressure low volume for tank air.

Anyway, with modern turbojet planes is it is much easier and fuel efficient to fly M .8 at FL 32+ than 12k. Drag being a drag with still be true no matter the propulsive force... in fact, electrics should have an advantage considering they can make their full power at altitude versus SL minus the any cooling capacity loss. Combustion engines lose much more power, but the reduction in drag more than offsets this.

[Sorry Dog]

Pattern, that's simply nonsense. It takes energy to pressurize the aircraft - regardless of the fuel source - but the lower the altitude the higher the drag and the more thrust required. The reason we normally cruise at 30k and above is simple - the reduction in drag means less fuel (energy) is required to fly a given distance, overwhelming the relatively small amount of extra fuel (energy) needed to warm and pressurize the aircraft. That is simple physics - going to electric propulsion doesn't change the physics.


Bingo! Reward yourself two Pesos from petty cash.

In fact, the plane may well carry high pressure air tanks that can be filled with ground power and serve as a backup air supply or even backup power source since it would already be there... but maybe not if its too hard to make the air compressors dual purpose: high volume low pressure for cabin air, high pressure low volume for tank air.

Anyway, with modern turbojet planes is it is much easier and fuel efficient to fly M .8 at FL 32+ than 12k. Drag being a drag with still be true no matter the propulsive force... in fact, electrics should have an advantage considering they can make their full power at altitude versus SL minus the any cooling capacity loss. Combustion engines lose much more power, but the reduction in drag more than offsets this. The main stumbling block for electrics now is the ability to carry enough juice for flights lasting more than an hour. I somehow doubt that battery tech will magically achieve a 2x energy density improvement in any time less than a couple of decades.
However, for those missions of an hour or less, like short regional flights or training flights, I think electrics will be great.

[EEngr]

electrics should have an advantage considering they can make their full power at altitude

Not quite. The power (thrust) that an airplane engine produces is a function of the mass air flow through the engine, fan or propeller being accelerated. The higher you go, the fewer pounds of air per second pass through the system, regardless of power source. On the other hand, the work done and fuel consumption (of whatever kind you choose) goes down along with this reduction in power production. Fortunately, so does drag and the thrust needed to offset it.

The wizards of aerodynamics and engine design work together to make all these requirements fit together in the most efficient way possible.

[EMIT]

As reply to Lord Lucan, post 4.

Recovering energy???

Yet another conceptual mistake, such as you find everywhere in the green madness of todays world!

A car driver that approaches a red traffic light at 50 mph and then brakes to stop the car is wasting the kinetic energy that is in the car - he should have started coasting early enough to come to a stop right at the traffic light without any braking. Because this method is impractical in normal traffic, hey, an electric car can use the motor as a generator to convert the kinetic energy back to stored electric power in the battery.

An airplane that descends from cruise level to landing does use the coast technique already, the engines are already just idling, it cannot be done more efficiently. If you want to “brake” by using the propellor to drive the e-motor as a generator, your aircraft will have more drag, thus descend more steeply than the idle glide path. You will have to expend (=waste) energy either before starting descent, by staying level longer, or by flying level at the bottom of the descent.

There is no such thing as a free lunch!

[david1300]

As reply to Lord Lucan, post 4.

Recovering energy???

Yet another conceptual mistake, such as you find everywhere in the green madness of todays world!

A car driver that approaches a red traffic light at 50 mph and then brakes to stop the car is wasting the kinetic energy that is in the car - he should have started coasting early enough to come to a stop right at the traffic light without any braking. Because this method is impractical in normal traffic, hey, an electric car can use the motor as a generator to convert the kinetic energy back to stored electric power in the battery....!

I think you've got that wrong. The electric car braking is not merely a function of mechanical braking - it is a regenerative braking system that does recover energy in the braking phase.
https://auto.howstuffworks.com/auto-parts/brakes/brake-types/regenerative-braking.htm

[EMIT]

Hi David1300,
did you see the following part of the text that you quoted?

Quote
hey, an electric car can use the motor as a generator to convert the kinetic energy back to stored electric power in the battery.
Unquote

The folly is in people just blindly projecting that sort of ‘technique’ onto aviation.

[Denti]

The folly is in people just blindly projecting that sort of ‘technique’ onto aviation.

I do agree to a large part with you, however, why not think speedbrakes differently? Simply wasting energy into sound/vibrations is not very efficient either. Especially when the planes do become much more aerodynamically efficient and therefore glide too well for ATC prescribed profiles.

[meleagertoo]

Well, you gonna be waiting an awful long time before anyone comes up with an aeroplane that can manage any sort of credibly useful performance to compare with conventionally fueled ones.
Even longer for airline size and performance, barring a revolutionary and to-date unanticipated breakthrough in battery technology or realisation of fuel cells.
It's a simple matter of energy density - how much energy can be stored in the space available. Currently batteries lag two orders of magnitude behind fossil fuels in this.
LiIon batteries currently stand in the range of .4 to .8 MJ/Kg. Jet A1 is 48. That's one hundred times more.
Certainly some necessarily small future aerodynamic and weight effciencies will narrow the gap slightly (small because those sciences are pretty much at peak already)
Batteries got some catching up to do!

[EMIT]

Then it might be time to indoctrinate ATC with proper procedures that do not force us pilots to waste energy with use of speedbrakes.

[Denti]

Well, you gonna be waiting an awful long time before anyone comes up with an aeroplane that can manage any sort of credibly useful performance to compare with conventionally fueled ones.
Even longer for airline size and performance, barring a revolutionary and to-date unanticipated breakthrough in battery technology or realisation of fuel cells.
It's a simple matter of energy density - how much energy can be stored in the space available. Currently batteries lag two orders of magnitude behind fossil fuels in this.
LiIon batteries currently stand in the range of .4 to .8 MJ/Kg. Jet A1 is 48. That's one hundred times more.
Certainly some necessarily small future aerodynamic and weight effciencies will narrow the gap slightly (small because those sciences are pretty much at peak already)
Batteries got some catching up to do!
True to some degree. However, only around 45% at best of the energy stored in the Jet A1 will eventually be translated into forward force. Electric motors on the other hand are at an efficiency of +95% of using the energy in the battery. Of course that does not close the gap yet in aviation, so yes, battery technology has ways to go.

On the car side however, it is already very close. As those engines are rarely if ever used at their optimal point of specific fuel usage, they have an average efficiency of around 17% for gasoline cars and around 20% for diesel ones. On the other hand, a 100kWh battery, the largest size currently available for cars, has only the energy that around 11 litre of gasoline have. So yes, in that application electrification is much more sensible currently.

[Bend alot]

There is also the possibility that the battery packs could be quite small in large airliners.

Telephones use to be fixed to the wall and get its power from that plug, we then had cordless phones that had a battery in the handset and would charge in a cradle at the end of the call. Years later we had "mobile phones" that had a battery pack the size of a brick, these eventually became very small and truly mobile and you plug the charger in once or so a day. Phones then became smart (and bigger again) and use more power again needing plugging into the charge a couple of times a day. From a few years ago many phones do not need to be "plugged" in to charge simply place them on a charging pad.

So it seems a new system will be required for large airliners, as the current battery and fuel cell technologies will not work.

But if we can charge during flight we might only need enough battery storage equivalent to that of the current twin engine ETOPS range.

So if there is a big breakthrough in wireless energy transfer, then heated/cooled pressurised airliners can certainly be a reality.

Will it be mini lightning strikes at set locations like in Back to the Future along flight paths or a more steady continuous charge from lasers tracking the flight from space? I do not have any idea, but I don't expect that a electric powered aircraft will carry "batteries" with the capacity for the entire flight.

As for the height, similar to now I expect.

Wireless Energy Transfer (http://large.stanford.edu/courses/2010/ph240/ma1/)

[Junkflyer]

The reality of battery power required, storage and recharging comparing a phone, computer or car is not even in the same world in comparison to a 121 jet.
A Tesla battery will take a 3,000 pound car a couple hundred miles or so. To take a hundred thousand pound aircraft to 30,000 feet and a few hours of flight or more requires a huge (heavy) amount of battery power/weight.
Also the ability to recharge or relace the cells for a rather quick turnaround is required. The fact that fuel burn and weight loss makes an aircraft more economic over time is lost on battery powered aircraft as well.

[Bend alot]

The reality of battery power required, storage and recharging comparing a phone, computer or car is not even in the same world in comparison to a 121 jet.
A Tesla battery will take a 3,000 pound car a couple hundred miles or so. To take a hundred thousand pound aircraft to 30,000 feet and a few hours of flight or more requires a huge (heavy) amount of battery power/weight.
Also the ability to recharge or relace the cells for a rather quick turnaround is required. The fact that fuel burn and weight loss makes an aircraft more economic over time is lost on battery powered aircraft as well.
And short sighted people never make leaps and bounds.

Although Tesla had the idea of wireless transfer of power on a large magnitude 100ish years ago - he never thought of supplying 10 GW (Many, many, many houses) base load solar power from space 36,000 km away.

A) they will be different batteries than that of now.
B) the take off (launch) of aircraft may be very different - zero power consumed till after take off, dodgem cars or catapult launch or some new thing.
C) The ability to recharge or replace the cells may not be required - they may just be the emergency back up.

D) If your batteries/cells are only back up and are lighter than current fuel burnt to carry such reserve & its weight, fuel then becomes the economic problem to be carried prior to take off in comparison.

F) the phone was a tech advancement example not a power consumption one.

The Japanese a number of years ago now transferred wireless power, enough to boil a kettle with accuracy over 55 meters. That I assume charges a few phones.

https://link.springer.com/article/10.1186/s40309-018-0139-7

[pattern_is_full]

Bend alot: I quite agree that there can be shortsightness. But that cuts both ways. One has to be longsighted enough to see the problems and solve them ahead of time.

A) Will there be new batteries? Very probably. Will they mean compressing more and more joules into smaller and smaller packages? Yep. Three numbers to keep in mind: 7-8-7. Cellphone and laptop batteries have already started small on-board fires, and then there is UPS 6 - brought down by autoignition of a cargo pallet of 81,000 lithium batteries. https://en.wikipedia.org/wiki/UPS_Airlines_Flight_6

B) Good idea - in principle. But - voltage-carrying taxiways? "Third rails" alongside? Overhead wires? We'll assume linear accelerators can be tuned to be less violent than carrier launches. ;)

How much power will "beamed power" beams have to transmit to hold up x-many kilos of aircraft, and x-many kilos of passengers? At what point do they become thousands of directed-energy weapons criss-crossing the skies? https://en.wikipedia.org/wiki/Directed-energy_weapon

Good (longsighted) engineering is not just having a bright idea - it is poking all the holes in that idea yourself, and then figuring out how to plug them.

[BluSdUp]

Dear All
I am skeptical.
But here in Norway we love all electrical , considering we get all our Hydro as they say in Canada, from hydro- electric generators.
And God knows we have enough rain to fill the magazines.
Anyway
Avinor ,the State owned airport owner ( 95% of the public airports) have declared that they support and will aim to have some of the shorthaul TP traffic over on EL operated planes within a short time ( 7- 10 years , I forget).
This has then taken the form of the CEO ,Cpt Falck Pettersen putt putting around in his private EL twoseater , charming any journalist and politician in sight.
I love it!
I think it is some years ahead, but let think Prius!
Been around for ages!
Problems getting it up? Think JATO bottles C130! Or equivalent.What do I know!
I just know that once the need is, there is a way!
After all we had wood fired cars during the war!

Just to stick to what I do know.
My 737 has 7800kg of fuel in the wings AND two wopping big engines that are ca 2500ks each, never to mention all the plumbing and doohickkys that a jet engine needs!
Simply taking a CFM 56-27k with 70% bypass and stuffing in a EL motor with no core thrust gives you close to a 22k derated on. And I am sure that a somewhat more elegant model would take 5 minutes for an Engineer to dream up. Doubtfully done in A and B cafeteria!
So for starter we have ca 10 tons of SALT battery fixed in the wing which gets, say 1 hrs plus IFR reserve.
I am starting to think this is possible.
Helmet On

[Bend alot]

Bend alot: I quite agree that there can be shortsightness. But that cuts both ways. One has to be longsighted enough to see the problems and solve them ahead of time.

A) Will there be new batteries? Very probably. Will they mean compressing more and more joules into smaller and smaller packages? Yep. Three numbers to keep in mind: 7-8-7. Cellphone and laptop batteries have already started small on-board fires, and then there is UPS 6 - brought down by autoignition of a cargo pallet of 81,000 lithium batteries. https://en.wikipedia.org/wiki/UPS_Airlines_Flight_6

B) Good idea - in principle. But - voltage-carrying taxiways? "Third rails" alongside? Overhead wires? We'll assume linear accelerators can be tuned to be less violent than carrier launches. ;)

How much power will "beamed power" beams have to transmit to hold up x-many kilos of aircraft, and x-many kilos of passengers? At what point do they become thousands of directed-energy weapons criss-crossing the skies? https://en.wikipedia.org/wiki/Directed-energy_weapon

Good (longsighted) engineering is not just having a bright idea - it is poking all the holes in that idea yourself, and then figuring out how to plug them.

A) those pesky batteries, good thing that Avgas and Jet A1 has never caused any issues on aircraft or deaths.
B) Maglev or something new.

The thrust required for a set speed to overcome the drag is what is needed to be known, once that is sorted the wings will hopefully hold up x kilos of aircraft and passengers - in a similar way to a conventional aircraft.

I would rather to let the scientists currently working on the "idea" to continue as I am not very good with plugs and they seem better funded than me.

[Bend alot]

Dear All
I am skeptical.
But here in Norway we love all electrical , considering we get all our Hydro as they say in Canada, from hydro- electric generators.
And God knows we have enough rain to fill the magazines.
Anyway
Avinor ,the State owned airport owner ( 95% of the public airports) have declared that they support and will aim to have some of the shorthaul TP traffic over on EL operated planes within a short time ( 7- 10 years , I forget).
This has then taken the form of the CEO ,Cpt Falck Pettersen putt putting around in his private EL twoseater , charming any journalist and politician in sight.
I love it!
I think it is some years ahead, but let think Prius!
Been around for ages!
Problems getting it up? Think JATO bottles C130! Or equivalent.What do I know!
I just know that once the need is, there is a way!
After all we had wood fired cars during the war!

Just to stick to what I do know.
My 737 has 7800kg of fuel in the wings AND two wopping big engines that are ca 2500ks each, never to mention all the plumbing and doohickkys that a jet engine needs!
Simply taking a CFM 56-27k with 70% bypass and stuffing in a EL motor with no core thrust gives you close to a 22k derated on. And I am sure that a somewhat more elegant model would take 5 minutes for an Engineer to dream up. Doubtfully done in A and B cafeteria!
So for starter we have ca 10 tons of SALT battery fixed in the wing which gets, say 1 hrs plus IFR reserve.
I am starting to think this is possible.
Helmet On
I would think a 737 would have a fuel capacity much higher at around 16,000 kg (around 20,000 lts x 0.8)

Engines are around 2,000 kg but extras will be around the 2,500 kg mark.

Less cowling structure to contain a fan blade than a high energy turbine blade would reduce the 2,500 kg mark a vast amount.

[oggers]

the lower the altitude the higher the drag and the more thrust required. The reason we normally cruise at 30k and above is simple - the reduction in drag means less fuel (energy) is required to fly a given distance...That is simple physics

The simple physics do not support the statement. Max L/D gives range speed at fixed angle of attack, drag, and therefore thrust required, regardless of altitude because as density decreases you have to speed up to get the lift back. End result is lift and drag back to where they started. As TAS is higher, power required is also now higher. Nonetheless you get there in a shorter time proportional to the extra power. Therefore, in terms of drag, you require the same energy at all altitudes. But you save time if at high altitude, which is reason enough to do it.

It is when you consider efficiency of the propulsion system that fuel, as opposed to tiime, may be saved by operating higher. The piston engine works most efficiently with a wide open throttle. Therefore best range for a piston is found at the critical altitude where the torque required for range speed can only just be achieved with a wide open throttle. Meanwhile, for a turbine the best TSFC occurs at about 85% RPM and at the tropopause so that is where you want thrust available to equal thrust required for the cruise.

Clearly, if you go to an electric motor then air density does not affect the motor output. However air density will still impact on the thrust available from the prop. Therefore it seems to me that for public transport, electric aircraft will be limited to the 20-30k altitudes unless ducted fans raise the ceiling.

[Bend alot]

The simple physics do not support the statement. Max L/D gives range speed at fixed angle of attack, drag, and therefore thrust required, regardless of altitude because as density decreases you have to speed up to get the lift back. End result is lift and drag back to where they started. As TAS is higher, power required is also now higher. Nonetheless you get there in a shorter time proportional to the extra power. Therefore, in terms of drag, you require the same energy at all altitudes. But you save time if at high altitude, which is reason enough to do it.

It is when you consider efficiency of the propulsion system that fuel, as opposed to tiime, may be saved by operating higher. The piston engine works most efficiently with a wide open throttle. Therefore best range for a piston is found at the critical altitude where the torque required for range speed can only just be achieved with a wide open throttle. Meanwhile, for a turbine the best TSFC occurs at about 85% RPM and at the tropopause so that is where you want thrust available to equal thrust required for the cruise.

Clearly, if you go to an electric motor then air density does not affect the motor output. However air density will still impact on the thrust available from the prop. Therefore it seems to me that for public transport, electric aircraft will be limited to the 20-30k altitudes unless ducted fans raise the ceiling.
Since a blade throw is a big safety issue - I expect ducted fan to be the norm certainly on engines closer to fuselage.

I will not buy into the rest of your statement other to say "I do not agree" WOT well I will be.

[VinRouge]

My bet will be initially a hybrid design, with all engines operating for high drag regimes and in flight shutdown once at cruise condition. We may see a single donk providing generator output for 2 electric propulsion units. This design would lend itself to a return to 3 engines or twin electric, single turbo propulsion, which could also provide pressurisation and heating via heat recovery. The benefits are a much higher propulsive efficiency over jet thrust, operation at design point, heat recovery put to purpose for what would be otherwise wasted energy plus a greater contingency. You could even shut down the propulsion unit and run off of batteries whilst holding! The dawn of geared turbofans lends itself towards this, albeit with much higher gear ratios. The important tech is the ability to handle the enormous torque required to be used to generate electricity for 2 electric propulsion units.

I think all electric is a pipe dream. Too much power required for the likes of pressurisation, heating and ovens to go all electric. We are struggling to get past 100WHr/KG with sulphur battery technology at present so I think hybrid has to be the most likely commercially usable solution. We will still get a significant reduction in CO2 usage.

[macpacheco]

I expect the first generation of electric transport aircraft to be optimized for turboprop like speeds and similar cruising altitudes.
Such aircraft should be a little bit on the heavy side, so they won't be able to exceed 30k ft, specially considering the battery mass is fixed.

Once battery energy density improves substantially, say 100% to 150% better than today, it should be possible to design a transport aircraft optimized for ultra high altitude cruise.
Perhaps as high as FL510.
It needs a wing with a fairly large AoA range, able to achieve high subsonic mach, compensating for the very thin air by increasing AoA.
I expect such an aircraft to have at least 4 engines, perhaps up to 8.
Electric engines are simpler, using more engines with a fan like propulsion system allows for high rpm and modest fan width.
No need to have just 2 bigger engines.
If the aircraft can keep its max thrust all the way up to FL510, why not go up there ? The only limiting factor is having wings that produce enough lift.

The lithium cells used in a Tesla Model 3 have about twice the energy density of those used in the first Tesla Roadster. And the technology continues to improve.

We already have new Lithium chemistries such as Lithium CO2 being tested in labs that promise 7x the energy density of common Lithium Ion cells.
If they manage to delived just 3.5x the energy density of the best cells Tesla has today, that should be enough to produce practical 1000nm IFR range transport jets.

[Winemaker]

Well I think I've solved it. Simply stick some RATs on the back of the wings to generate electricity while the plane is flying! With 95% efficiency one would have enormous range and greatly reduce the battery size!

[Ian W]

Well I think I've solved it. Simply stick some RATs on the back of the wings to generate electricity while the plane is flying! With 95% efficiency one would have enormous range and greatly reduce the battery size!
Don't forget the solar cells on the top of every aircraft surface.

Battery technology does not have the energy density required for aircraft to climb to, let alone operate at, common fixed wing freighter levels and carry any useful payload.

[Pugilistic Animus]

A perpetual motion machine of the first kind

[Pugilistic Animus]

The best idea would be to have engines turn generators that would in turn charge the batteries that run the engines:}