Cruise altitude for electric airplanes.
Thread Starter
Cruise altitude for electric airplanes.
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.
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.
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.
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.
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).
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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
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
Only half a speed-brake
A stimulating thought. What would be the optimal descent RPM of an electro-prop? Descent with the power on, recuperating, or even feathered?
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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
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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.
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.
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"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".
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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.
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There would be loss of efficiency by doing this
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.
. 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.
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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!)
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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.
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.
Slow.
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 ?
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 ?
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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.
. U2 aerodynamics would seem to rule in that case.
@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
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
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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,
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,
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Cruise altitude for electric airplanes.
(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.
For solar powered ones that can stay up indefinitely: Above the clouds...and above turbulence, since they're built light with enormous wings.
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."
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."
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.
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).
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).