Why battery-powered aircraft will never have significant range

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tspear

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Hybrid is the current best efficiency for cars. The early hypermiling vehicles would use vehicle inertia to store energy, they accelerate at peak engine efficiency, then shut the engine off and coast down. Now, you can just go buy a hybrid battery car tat cycles the engine without varying forward speed. Aircraft don't need a huge surfeit of power like cars do, so hybrid is a lot less benefit. In fact, aircraft engines maximise efficiency at around cruise power. Yes, extra takeoff power is useful, but making the added complexity of hybrid worthwhile is a lot harder. Has anyone else considered JATOs?
That might be true for airlines, but for $500 hamburger runs, I doubt it is true. Or the ratios are way off.
Often I see comments that 2-3 hours is all the bladder for the pilot can handle. As such, a twenty minutes in climb for two hours in cruise is 1/6 of the total time. That is a rather significant portion, and hybrid would very well for this. Further, think about engine heat. Cooling drag is a major factor, so if you can design the cooling to only deal with cruise, and not the higher temps in the climb, you can likely decrease the total drag of the plane.

Tim
 

Jambo

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Hybrids to me don't seem to offer much, at least in the performance aspect. Put the battery weight into engine weight and at the minor cost of mpg you gain in every other performance factor.

The only hybrid configuration that has appeal to me is utilizing the electric portion to power high lift devices such as blown flaps. Specifically if you wanted STOL performance in combination with a decent cruise. I've found the higher designed stall speed, the less the benefit however.
 

blane.c

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Just build the aircraft like a giant coil and fly between high tension power transmission lines. Ya know like when you want to power up the batteries or run out of thermals.
 

flyboy2160

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choppergirl;407084.....I do at least 6 impossible things before breakfast...[/QUOTE said:
LOL Could one of them be deleting some HBA emails? Your inbox is too full to accept more mail. No....wait....you could bend your inbox........
 

proppastie

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Just build the aircraft like a giant coil and fly between high tension power transmission lines. Ya know like when you want to power up the batteries or run out of thermals.
that would work, but I am not sure how healthy it is especially when you get real close
 

pictsidhe

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Uhmmm, that is true for 'standard' aircraft, designed around standard engines, operated in a standard way.

Not so much the case for motorgliders which might want decent power for takeoff and climb, but can cruise on a very small fraction of that power.

Hybrid has other potential advantages:
- much better reliability/safety (electric motors/controllers much simpler than IC engines/fuel systems, greatly reducing risk of engine failure in critical phases of flight)
- possibility of novel/more efficient configurations (e.g. distributed electric propulsion)
- reduced cost of (short) flights (if just going for short flight, it might be mostly covered by batteries...)

One would hope that the mechanical simplicity of electric motors would also put a downward pressure on engine/motor prices ... if the volume climbs ... one can hope
You're right about motorgliders. My evolving 103 design has very low cruise power requirement, but if I want 500fpm, a lot more power needed. Not losing too much engine efficiency at a small fraction of max power is a conundrum. Most aircraft aren't like motorgliders, though...
 

pictsidhe

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Two engines will do it. If the goal is low cruise power like the Rutan Voyager.
Hmm, going to contemplate that. Have an over running clutch and there's no need for feathering. Twin engines are a possibility anyway, but I hadn't thought of shutting one down for cruise.
 

BBerson

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A stopped prop is normally less drag than freewheeling. The prop can be feathered or stowed by folding.
My Grob has a feathering prop and it hardly helps as far as I can tell.
 

pictsidhe

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A stopped prop is normally less drag than freewheeling. The prop can be feathered or stowed by folding.
My Grob has a feathering prop and it hardly helps as far as I can tell.
I'm thinking of hooking both engines to the same prop. Cruise now has an oversized prop as well as better BSFC.
 

karoliina.t.salminen

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Cirrus SR22 type airframe is not good example, because you would not really go anywhere with Cirrus at 80 kts since even best L/D speed is much higher than 80 kts on Cirrus. I understand your comparison is not necessarily to exactly Cirrus, but maybe aicraft with cockpit as roomy as Cirrus has. However, I am not fully convinced that the Cirrus size cockpit will make the aircraft automatically to have best L/D of 10:1 or less (or SR22 with windmilling prop is even worse, gliding it feels like nose is really much down and it is not going anywhere but just about below the plane). If we can make the compromise of flying only 80-85 kts, then we have to maximize cruise L/D for that design point to be 30:1 or better. That will require more aerodynamic shape for cockpit most probably than the Cirrus has, but 30:1 should be possible at 85 kts as evidenced by Pipistrel Taurus G4. Taurus G4 has circa 400 kilometer range. I think that is acceptable compromise for electric airplane with today’s batteries. One could not fly the Pond with it obviously, a range extending generator and fuel would be required, but 400 km range is practical for most flights people actually do in at least here in Finland just for fun. Obviously the long wings will cause problems with hangar space etc. But maybe it could be like sailplane that the long wings would be easy to remove and put to a trailer for winter storage whereas the center section only would be stored in hangar. Not as capable than eg Diamond DA40, but in return, flying it would be almost free vs. 100 eur/h to AVGAS alone (ie 100 pilot hours per year here cost 10000 eur in fuel alone excluding maintenance etc. costs which are considerably higher than on electric plane).

Another interesting category for electric flight would be electric VTOL. It requires a lot of power for short period of time. But since there would be a lot of power available, in realm of 1000 hp from elctric motors realistically (similar to flooring accelerator pedal on Tesla P100D), that power would be only needed for maybe only for 20-30 seconds before transition to horizontal flight as helicopter feature hover is not necessary, and even should not be done, on VTOL. Just enough to clear the trees. The battery pack would need to be lighter than on the 400 km variant. However. I think a practical range of 150 km (Nissan Leaf range) would be just fine for airplane like this and >200 km would be luxury. Because this plane would not require airport for its operations, and locations of airports would not matter. Many consider Hyundai Ioniq EV to be adequate electric car even though it only realistically covers 200 km range. It is not that bad nor impractical if you have charge plugs in both ends of the travel. And the short hopping electric VTOL definately would not need four seats, two would be enough. Joby S2 is a sufficient concept in many ways. It would satisfy my daily commute via air very easily. With the range limitation, the VTOL would be actually safer than normal airplane, in case of nearly running out of energy, the plane could emergency autoland vertically instead of crashing into woods.

Considering the short range, a flight speed of 85 kts would not be that bad, it would not be so long sitting in airplane. As slow as C152 yes, but fraction of energy the C152 uses. On ramp it would use two parking spaces instead of one, but airport fees are typically based on weight and not how large it is.
 
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karoliina.t.salminen

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The non-VTOL would be practical if there was fast chargers installed on every air field. Current state is that not even standard plug is available, so room for improvement on this enabler is plenty. But if fast chargers existed, it would be fine for student flights. In here there is some government rule to leave one hour resting time between flights (I know it is too long and based exactly on what, some armchair psychology maybe instead of real science), during the hour wait, the fast charging of the plane would be plausible and practical.
 

karoliina.t.salminen

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Also interesting comparison of expenses: 100 hour pilot time on AVGAS GA almost equals the OEM price of the battery pack. At the price of fuel, you could buy new batteries. How kewl is that?
 

karoliina.t.salminen

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One idea for the 400 km electric plane. It could have part of battery pack replaceable with generator engine. In short distance flights it would be full electric, and on long distance trip needed, the battery module is taken out and engine is installed in its place. It would not necessarily need to be two planes. If the engine is a self-contained generator module, it would not require week of mechanic’s work to swap it. None of the engine instruments would be wired to the cockpit, but instead the engine would have all the electronics on its module, which would be interfaced to cockpit using just Ethernet.
 

DonEstenan

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Maybe the things will really move forward when several technologies synergestically combine:
1) somewhat increased energy density of batteries. 400wh/kg is not THAT far away from 250wh/kg
2) a configuration with large laminar flow fuselage
- large for comfort, significantly increases the appeal for wider population
- laminar flow to reduce the drag cost of this comfort
- hence, move the props to wings/back
- made much easier by having electric motors
3) readily available lowish-cost reasonable size electric motors/controllers
- probably coming from car/RC world
- electric should also be much easier to use/combine several smaller motors (seems that IC engine cost rises much faster than its power), either powering the same prop or multiple props
4) perhaps open-source code to control the multiple props/battery management, available to all builders for free/minimal cost
5) maybe boundary layer suction (with all that electricity available to power fans), at least in the aft part of the fuselage if not wings...
6) with shute already well accepted 'answer' to engine failure, perhaps configurations not tolerating well engine failure (blown wings) might become accepted (shute + much better reliability of e-motors)

I believe that once it becomes possible for an (average?) homebuilder to use (several?) low-cost (RC?) smallish e-motors to replace WV/Rotax IC engine, resulting in a cheaper, more reliable and significantly cheaper to operate alternative, even if the endurance is limited to 1-2h of motor-gliderish flight, the things will move forward and gain significant momentum.
 

autoreply

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I just added a solar powered calculation to my spreadsheet. it's quite feasible.
A quick look on ebay turned up some 125mm mono cells putting out 3.4W. They are 165um thick; about 2 sheets of paper. Silicon that thin will bend a bit, such as over the top surface of a wing... I came up with a weight of 6grams each. 1hp of cells is going to weigh 1.3kg. The current best commercial flexible frontsheet is 3M ultrabarrier, 204um thick. That'll be a bit lighter than the cells, say 1kg for 1hp Then we need to encapsulate the cells in EVA and add a backsheet. Lets say, with some encapsulating experiments and relying on a carbon substrate for strength (wing skin), 1hp of cells weighs 5kg. How much power does a modest glider need to stay up? How much ballast is it carrying? Batteries are not be there yet, but solar is on!
My 103 design currently has 15m2 of wing area. With 80% cell coverage, that'd be a free 3.6hp, for about 20kg extra weight. More span, Igor!
The problem is speed. Solar powered aircraft will always remain slow. That too is simple physics.

If we look up the state of the art, you're talking about cruise speeds in the 30-35 mph range give or take. Those are aircraft with solar cells of around 20% efficiency. Theoretical PV efficiency is on the order of 75% efficiency. Since power required goes up with the cube of the speed, even a theoretically perfect solar aircraft will cruise at a modest 45-50 mph.



Two considerations;
1/ "never" is a very long time and things may change. I remember lead batteries being wet and very spillable but now they are sealed and about 10x the capacity for mass. I remember torch batteries that eat through the zinc container and rotted the contacts -now it's alkaline batteries with more power and almost leak proof.
I picked the "never" for a good reason, next to being sure to lead to a lot of discussion. The point is that we are up to fundamental limits, not some artificial barrier in progress. Over the last 30 years we have only seen a two-fold increase in energy density in batteries, though they've gotten much more practical. We are fairly close to the theoretical limitations and those we will never surpass. Just like a prop can't be >100% efficient, a piston engine can't exceed 60-something percent thermal efficiency etc.

If somebody claims doubling the efficiency of a prop or a combustion engine, such claims are quickly refuted to the mad-house. Yet, there is widespread belief in the enormous potential increase in battery energy density. That hasn't happened anywhere to the extent most people think (two-fold since the cold war roughly) and we are hitting solid physical barriers very soon.



Don't get me wrong. Battery powered and solar panel powered aircraft are practical and feasible today and I certainly believe they're the future. Just not for going places unless you consider a FAR103 design adequate.
 

tspear

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The problem is speed. Solar powered aircraft will always remain slow. That too is simple physics.

If we look up the state of the art, you're talking about cruise speeds in the 30-35 mph range give or take. Those are aircraft with solar cells of around 20% efficiency. Theoretical PV efficiency is on the order of 75% efficiency. Since power required goes up with the cube of the speed, even a theoretically perfect solar aircraft will cruise at a modest 45-50 mph.





I picked the "never" for a good reason, next to being sure to lead to a lot of discussion. The point is that we are up to fundamental limits, not some artificial barrier in progress. Over the last 30 years we have only seen a two-fold increase in energy density in batteries, though they've gotten much more practical. We are fairly close to the theoretical limitations and those we will never surpass. Just like a prop can't be >100% efficient, a piston engine can't exceed 60-something percent thermal efficiency etc.

If somebody claims doubling the efficiency of a prop or a combustion engine, such claims are quickly refuted to the mad-house. Yet, there is widespread belief in the enormous potential increase in battery energy density. That hasn't happened anywhere to the extent most people think (two-fold since the cold war roughly) and we are hitting solid physical barriers very soon.



Don't get me wrong. Battery powered and solar panel powered aircraft are practical and feasible today and I certainly believe they're the future. Just not for going places unless you consider a FAR103 design adequate.
The problem is how you define battery. Most of the energy tech I follow is not a traditional battery per se. e.g. Fuel Cells, Zinc slurry, or solid state batteries....
As for Li based solutions, yes you are correct we approaching the max density for the current Li technology.

Tim
 
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