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.

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).

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

A stimulating thought. What would be the optimal descent RPM of an electro-prop? Descent with the power on, recuperating, or even feathered?

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.

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".

;)

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.

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.

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.

Slow.
 

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.

@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

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,

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.

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.

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).