Is a Battery-Powered Plane the Future of Air Travel?

[mheslep]

That doesn't seem very useful to me: efficiency-wise, you could do better by starting your decent sooner and making it shallower.

I don't follow. On descent, the aircraft stands to recover energy that would just otherwise be bled off in drag by bringing on the flaps or even airbrakes. Instead prop drag slows the aircraft with electric. Are you describing a method that would entail less time in the air?

 

[mgb_phys]

I don't follow. On descent, the aircraft stands to recover energy that would just otherwise be bled off in drag by bringing on the flaps or even airbrakes. Instead prop drag slows the aircraft with electric. Are you describing a method that would entail less time in the air?

I think the idea is less time with the engine on full power. Instead of flying at full power to the runway an diving steeply while recovering some energy you are better off reducing engine power early and losing altitude gradually.
A bit like taking your foot off the gas well before a red light and cruising to a stop rather than driving at full speed and using regenerative braking at the end.

 

[mheslep]

I think the idea is less time with the engine on full power. Instead of flying at full power to the runway an diving steeply while recovering some energy you are better off reducing engine power early and losing altitude gradually.
A bit like taking your foot off the gas well before a red light and cruising to a stop rather than driving at full speed and using regenerative braking at the end.

Ok I follow, the idea there is to approximate killing power and using the accumulated potential energy to glide all the way in, and I expect that to the extent that is possible it represents the lowest energy profile. But that is not typical (or practical), energy must be bled off (wasted) in drag via turns, flap extensions, air brakes to accommodate the descent.

 

[RonL]

Ok I follow, the idea there is to approximate killing power and using the accumulated potential energy to glide all the way in, and I expect that to the extent that is possible it represents the lowest energy profile. But that is not typical (or practical), energy must be bled off (wasted) in drag via turns, flap extensions, air brakes to accommodate the descent.

I think the prop profile will change a bit as more use of electric power develops, as a plane flies the prop is pulling and when desending you are in essence pushing a wind turbine.

 

[mgb_phys]

It still depends on the amount of power lost by aerodynamic drag.
If when you turn off the engine you lose speed fast enough due to drag then using a regenerative propeller to slow even more isn't possible. It would only be worth it if with no engine you still had to use flaps to dump speed.

I think we covered this in another thread with regenerative braking on bicycles - since most of the power is lost to aerodynamics it isn't worth it.

 

[mheslep]

It still depends on the amount of power lost by aerodynamic drag.
If when you turn off the engine you lose speed fast enough due to drag then using a regenerative propeller to slow even more isn't possible. It would only be worth it if with no engine you still had to use flaps to dump speed...

Flaps are part of it, but I suspect a larger part is bled off in approach turns, which are in part required to do just that.

 

[mgb_phys]

Flaps are part of it, but I suspect a larger part is bled off in approach turns, which are in part required to do just that.

If you have to use active means to lose speed then regenerative braking is in theory worth it - wether it would be worth it in terms of added weight, complexity, reliability etc .

On a similair note the ski resort here now charges you to go down on their cable car - after you hike the 900m up the mountain. That is definitely not fair, with my more than ample mass I am significantly contributing to their power bill.

 

[russ_watters]

Flaps aren't used to bleed speed, they are used to allow the plane to fly at a lower speed. The only thing this would replace is an airbrake.

If I were less lazy, I'd figure out how much kinetic and potential energy is lost in dropping 1000 feet and from 120 to 100 mph...

 

[mheslep]

Flaps aren't used to bleed speed, they are used to allow the plane to fly at a lower speed.

They increase lift and drag.

 

[FredGarvin]

Drag is increased as a byproduct of lift. The purpose of flaps is to increase the angle of attack of the wing while maintaining a constant fuselage orientation and thus lift. This allows the aircraft to maintain the same amount of lift at lower speeds while maintaining a somewhat constant attitude.

 

[mheslep]

Last month in Zurich a Swiss crew unveiled another shot at a pure solar-battery-electric that they plan fly across the Atlantic and then circumnavigate.
http://www.solarimpulse.com/unveiling/documents/unveiling_user.php".
Specs:
Wingspan: 63,40 m
Length: 21,85 m
Height: 6,40 m
Weight: 1 600 Kg
Motor power: 4 x 10 HP electric engines
Solar cells: 11 628 (10 748 on the wing, 880 on the horizontal stabilizer)
Average flying speed: 70 km/h
Take-off speed: 35 km/h
Maximum altitude: 8 500 m (27 900 ft)

Which is 1.4x more power than the old Gossamer whatever.

This is an interesting challenge (the crossing) for a manned aircraft, and a bit risky. But why not just ditch the pilot? Without the pilot and support the problem must be an order of magnitude easier, not to mention that an essentially indefinitely orbiting high altitude aircraft has practical value, but as a transportation device its just a gimmick.

 

[mgb_phys]

not to mention that an essentially indefinitely orbiting high altitude aircraft has practical value,

There have been a bunch of Nasa and secret projects to make a long endurance UAV as a sort of mini-satelite.
If you want to fly in cloud or at night you also need batteries (or fuel cells) and these aren't quite there yet in power/weight. The sunsailor team estimate that you need about 40% efficency panels to fly indefinitely in good conditions (they are in sunny Israel) which is still a little way off.

 

[FredGarvin]

One of the big things the Solar Impulse is relying on is that they gain altitude during the day when they don't have to draw off the batteries. At sunset, they initiate a slight decent and try to extend that for as long as possible to some low altitude point. From then until sunrise, they are on batteries. Kind of risky especially when you have to also think about having to possibly avoid weather issues.

 

[mgb_phys]

I suppose if the aim is just to stay 'up there' then you could hunt for thermals and soar - only using the engine to get to the next thermal.
But if you want to stay over a city to relay TV signals or loiter over a battlefield you are going to need a lot more power.

 

[mheslep]

There have been a bunch of Nasa and secret projects to make a long endurance UAV as a sort of mini-satelite.

Yes, thanks for the NASA reminder: Pathfinder-'95, PF+-'98, Helios (solar+ fuel cells)-'01.

If you want to fly in cloud or at night you also need batteries (or fuel cells) and these aren't quite there yet in power/weight. The sunsailor team estimate that you need about 40% efficency panels to fly indefinitely in good conditions (they are in sunny Israel) which is still a little way off.

Apparently the Solar Impulse crew believes the batteries are there if they are to fly across at 70km/h. Regards energy requirements, I expect a better metric is solar efficiency divided by battery energy density greater than a constant for indefinite flight, so progress in either one gets you there.

Yes here - the 40% referred to manned aircraft.

Q: Commercial Photovoltaic cells still have fairly low efficiency (typically >20%). At what level of efficiency do you believe they will start proving to be more useful in commercial aviation (both manned and unmanned)?
A: Truthfully, Solar cells low efficiency is only half of the problem. The means of energy storage such as rechargeable batteries or fuel cells is the other half. But solar cells for large scale commercial aviation will have to go a long way before they can supply the power that is needed. Even a 40% solar cell covered wing on a sunny day will barely be enough for the cruise power requirements of a small manned aircraft

.
http://thefutureofthings.com/articles/51/solar-uav-to-set-a-new-world-record.html
If 20% panels can now carry a person across the Atlantic w/ good batteries, then I expect 20% panels can keep up a UAV indefinately.

 

[mgb_phys]

If 20% panels can now carry a person across the Atlantic w/ good batteries, then I expect 20% panels can keep up a UAV indefinately.

The solarimpulse is intending to fly to very high altitudes during the day and slowly descend during the night. So the battery just has to generate enough power to reduce the descent rate.
The 40% figure for the UAV is (I think) to give the same performance on battery as during the day - so slightly more than half your solar power goes to charging the battery.

 

[mheslep]

One of the big things the Solar Impulse is relying on is that they gain altitude during the day when they don't have to draw off the batteries. At sunset, they initiate a slight decent and try to extend that for as long as possible to some low altitude point. From then until sunrise, they are on batteries. Kind of risky especially when you have to also think about having to possibly avoid weather issues.

Yes I saw the day/night altitude plan. I don't see that flying at night by itself is risky. The battery energy is definite, the sunrise time is definite. Weather is another issue, though they can fly above most cloud cover ascending during the day, and they don't care about cloud cover when flying lower at night. Otherwise they have the option to simply avoid the more intense weather. I did notice they were concerned about head winds at night. This must mean they intend to chase the sun rise and gain an ~hour of sunlight. If things are that tight then they are certainly at the margin.

 

[mheslep]

I suppose if the aim is just to stay 'up there' then you could hunt for thermals and soar - only using the engine to get to the next thermal.
But if you want to stay over a city to relay TV signals or loiter over a battlefield you are going to need a lot more power.

Why? Based on the energy - time profile claimed for this manned vehicle, then a UAV should weigh ~200kg less without a cabin, have less drag, require no cabin environmental power. Thus I would think a fuel-less loitering UAV immediately possible w/ the same technology. It might have other undesirables, such being slow to move on-off station, or forced off station by weather.

 

[mgb_phys]

and they don't care about cloud cover when flying lower at night. Otherwise they have the option to simply avoid the more intense weather.

They don't have much leeway to avoid weather at night, they are going to descend to a certain altitude by a certain time however bad weather is down there. They also don't have the energy reserves at night to fly a few hundred miles around a large storm system.

Most of these solar powered aircraft also look pretty flimsy to be flying into turbulence.

 

[mheslep]

They don't have much leeway to avoid weather at night, they are going to descend to a certain altitude by a certain time however bad weather is down there. They also don't have the energy reserves at night to fly a few hundred miles around a large storm system.

? They have essentially infinite reserves. Unlike a fueled aircraft, they can afford to fly in any direction they care to. They only need to avoid weather for the night and can choose any compass point w/ out regard for the nearest airport. Granted some 1000 mile wide severe front will cause problems.

Most of these solar powered aircraft also look pretty flimsy to be flying into turbulence.

Agreed, though I wouldn't underestimate. Given the budget one can build very strongly w/ carbon fiber. Edit: yes its carbon fiber honeycomb. Expensive and very strong. And they claim 220Wh/kg in the batteries. That's almost double what's cited for the new EV car batteries. Twice as expensive too I believe.

 

[mheslep]

Wonder if they considered concentrated solar: essentially light pipes directed to higher efficiency cells. 40% multijunction is available, though too heavy to distribute, so use more reflectors, fewer PV cells.