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Electric Propulsion

S

Strappe

Member
Searching the forums here and elsewhere, I have not found anything current about building an RV (or other homebuilt) with electric propulsion. The technology is advancing quickly, and several companies (most notably, Pipistrel) are selling complete systems.

I fly an RV-12 built by others, and I'm hoping to begin building one (or maybe something else) in the next year or two. I'm OK with the ROTAX 912 ULS, but what a great advantage to have an electric power plant, if one with enough Kw could be found in a plug-and-play kit. The 40Kw (around 54 HP) Pipistrel kit is too small for most homebuilders' goals.

At this stage of development in the electric propulsion field, I'm more interested in the amazing power to weight ratio, the less expensive maintenance, and the quietness than of being greener when I fly (knowing my electricity will come from one of the dirtiest coal-fired plants in the US).

With batteries, engine and all of the other parts in the Pipistrel kit, it only weighs 165lb.

The 100 HP Rotax ULS in my RV-12 with alternator and exhaust system weighs 137 lbs. It holds 19 gal 91UL that, at 6.35 lbs per gallon, weighs approximately 120.65 lbs, making the the current RV-12 engine and fuel package 258 lbs. Of course, it gets lighter as it burns off fuel.

This is not a fair comparison since the ROTAX makes 46 more horsepower, or nearly twice as much. Presumably the larger electric motor, greater battery capacity to generate at least 70-75 KW might make weight a disadvantage for electric. But, likely not for long. With all major car and most truck companies investing heavily in the technology, it seems a good time to start looking seriously at offering e-options, and even hybrid options for greater endurance.

That electric kit of Pipistrel's is kind of eye-opening. No fluids, tiny little engine that makes 56 HP. Doesn't bolting something like that onto the front (or top, or back) or you project look like a lot of fun?
 
Energy density is the key

When the batteries will supply power for the same length of time as the fuel tanks do, without weighing more than the petro fuel, and the batteries can be recharged in the same time as it takes to refuel with gasoline, then the prospect may seem a lot more attractive.

Still a long time coming IMHO.

Dont forget the problem recharging at outlying airports............
 
At 50-something horsepower, I could see that being a viable source for a small experimental twin, just something to play with and show it off. The technology is only just barely mature enough to use in surface vehicles, it's a long way from viable for aircraft.
 
Strappe said:
The 100 HP Rotax ULS in my RV-12 with alternator and exhaust system weighs 137 lbs. It holds 19 gal 91UL that, at 6.35 lbs per gallon, weighs approximately 120.65 lbs, making the the current RV-12 engine and fuel package 258 lbs. Of course, it gets lighter as it burns off fuel.

This is not a fair comparison since the ROTAX makes 46 more horsepower, or nearly twice as much. Presumably the larger electric motor, greater battery capacity to generate at least 70-75 KW might make weight a disadvantage for electric. But, likely not for long. With all major car and most truck companies investing heavily in the technology, it seems a good time to start looking seriously at offering e-options, and even hybrid options for greater endurance.

That electric kit of Pipistrel's is kind of eye-opening. No fluids, tiny little engine that makes 56 HP. Doesn't bolting something like that onto the front (or top, or back) or you project look like a lot of fun?
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What Mike said..... again.

You compared the horse power and weight of the RV-12 power system to the Pipistrel, but you didn't compare the range capability that the stored battery power has compared to liquid fuel.

That is the main technology improvement that we need to have catch up, to make it viable in aircraft. When it gets to the point that the propulsion system, at a somewhat comparable weight to what we have now, has somewhere close to the same flight duration/range, we will begin to see lots of electric powered aircraft in the air.

It is not a matter of if, but when.
 
rvbuilder2002 said:
When it gets to the point that the propulsion system, at a somewhat comparable weight to what we have now, has somewhere close to the same flight duration/range, we will begin to see lots of electric powered aircraft in the air.
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And don't forget recharge time. I can refuel with mogas or 100LL in five minutes. How long would it take to charge an electrical power source with enough power for a couple of hours' worth of flying?

You're right, though -- it's not if, it's when.
 
Maybe Some Day

I know of several folks building Experimentals that are using electric motors and lithium batteries. They are generally small, single seat aircraft not meant to go very far.

Here are a few rules of thumb based on available technology today:

Motors can generally be had that supply ~9kW/kg. So to replace a 100hp (75kW) Rotax 912 you'd be looking at a bare motor weight of around 18 pounds. This seems pretty reasonable. I've held a 40kW syncronous AC motor in my hand and it was around 10 pounds.

Batteries are what kill you. A good baseline today is that the battery system (cells, battery monitoring, cooling, etc) are about 16 pounds per kW (this is what you get out of a Tesla 100kWh pack). If the Pipistrel system only weighs 165 pounds, and we're subtracting 18 pounds for the motor, that leaves you with 147 pounds of batteries, which would come out to about 9.2kWh of battery. Now you can run your 75kW motor at full power for 7 1/2 minutes...

See the problem?
 
DaleB said:
You're right, though -- it's not if, it's when.
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That's a trick question: the correct answer is never, as taught in that high school physics/chemistry class most of these electric skateboard kids didn't pay attention to.

For every pound of gasoline, Mr. Lycoming burns about fourteen pounds of air (cue in LOP crowd in three..two..one..). That means you don't have to lift 93% of your airplane's chemical fuel, or some 3600 lbs of batteries given any possible (non-magical) chemistry to get the 50-gallon range of a 2000 lbs gross RV-14A.

Meanwhile, isobutanol is already produced at scale--renewably!--at lower cost per gallon than the retail price of avgas. I think I can see the future of aviation fuel with clarity...
 
For every pound of gasoline, Mr. Lycoming burns about fourteen pounds of air. That means you don't have to lift 93% of your airplane's chemical fuel, or some 3600 lbs of batteries given any possible (non-magical) chemistry to get the 50-gallon range of a 2000 lbs gross RV-14A.
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^^^ This ^^^
 
Also, generally speaking, most airplanes are designed around a specific type of powerplant. Changing that so drastically would probably require a wholly new design, from the beginning.

Dave
 
A agree with everything said but just the fact that we are talking about it here is exciting! Shows there are a lot of people thinking about it.
 
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Not so bad...

Aluminum said:
For every pound of gasoline, Mr. Lycoming burns about fourteen pounds of air (cue in LOP crowd in three..two..one..). That means you don't have to lift 93% of your airplane's chemical fuel, or some 3600 lbs of batteries given any possible...
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Actually, for every pound of gas Mr. Lycoming burns 3 pounds of oxygen. All the rest of the air just passes thru. So you carry 25% of the fuel. Since a gas engine is a heat engine, it?s only 40% or so efficient, while electric motors approach 100%. So Lycoming still wins but not as overwhelmingly - if electric storage weight can be brought down, it looks feasible. That?s a big ?if?.
 
Let's say that battery technology advances to the point where there's enough energy in the batteries to give the airplane comparable speed and range to a IC powered one.

What I worry about is what happens if all that energy decides to discharge in a short period of time? Remember gasoline needs oxygen to produce energy, so gas in the tank can't explode unless it's vaporized with the right percentage of oxygen. High density energy stored in a battery doesn't have that limitation and given the right (wrong?) circumstances can discharge very quickly indeed.

We've already seen the small scale example of this when LI-ion batteries decide to violently discharge.

You'd be flying around with the equivalent of many pounds of high explosive.

So not only does the energy density of batteries have to substantially improve, but the proven safety as well.

It will be a long time coming I think.
 
Hey, in the 70s and 80s we thought electric powered cars were impossible too. If you'd described the current crop of batteries back then you'd have been regarded as a crackpot. My cell phone has more processing power, memory and storage than the first several mainframe computers I worked on - heck, the Raspberry Pi I have sitting here on my desk does. There's a good chance my microwave oven does, too. Technology marches on, and we figure out new ways of doing things that would look like magic or sci-fi to our 20-year-past selves.

I don't expect to see viable electric powered airplanes any time soon... but I do expect to see them.
 
I don't think anyone would propose using lots of exploding Li-ion batteries. Lithium iron phosphate, on the other hand, has a good safety record so far.

We know of way too many innovative companies out there working on electric and various hybrid vehicles to think progress won't begin to accelerate rapidly.

Most of my trips in a plane are easily within current considerations of battery range - many are hour-long, low altitude cruises. For fun. For relaxing.

7? minutes? That might miss the point that no one would try to power that size engine with that little battery capacity.

Seems to me that the homebuilding community is almost entirely responsible for many of the advances in light aircraft design and innovation. Isn't that the main force that move us out of the 1940s or 50s in materials as well?
 
When I looked into it the battery density just wasn't there yet for a traditional plane. That's why the few electric airplanes you do see are basically motor gliders. That being said, with some bespoke engineering it's getting close. Check out the Bye Sunflyer.

Some companies aren't waiting around for batteries and are going hybrid electric. Combustion engine drives a generator which in turn drives the electric motor. This sounds inefficient until you look at vertical lift. Getting rid of gearboxes, transmissions, and complicated mechanical flight controls makes it a lot more compelling. Super light motors are everywhere. Now, if I could just figure out where to get a super light generator....?

DEM
 
BobTurner said:
Actually, for every pound of gas Mr. Lycoming burns 3 pounds of oxygen. All the rest of the air just passes thru. So you carry 25% of the fuel. Since a gas engine is a heat engine, it?s only 40% or so efficient, while electric motors approach 100%. So Lycoming still wins but not as overwhelmingly - if electric storage weight can be brought down, it looks feasible. That?s a big ?if?.
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Ha! Neat trick! :cool:

Too bad you'd melt tungsten if you only burned the oxygen. :D

Those other ten pounds are there for cooling and doing work, and even that isn't nearly enough as evidenced by our ROP/LOP hobby.

Batteries also need some of those pounds to cool away the 15% of capacity that escapes as heat during discharge, so the drag advantage isn't as great either.

Open cycle lithium-air batteries could turn the tables in theory, of course. Don't hold your breath though: making those work requires unknown science. You wouldn't want to swap out your fuel tank after a dozen refills.
 
Rallylancer122 said:
When I looked into it the battery density just wasn't there yet for a traditional plane. That's why the few electric airplanes you do see are basically motor gliders. That being said, with some bespoke engineering it's getting close. Check out the Bye Sunflyer.

Some companies aren't waiting around for batteries and are going hybrid electric. Combustion engine drives a generator which in turn drives the electric motor. This sounds inefficient until you look at vertical lift. Getting rid of gearboxes, transmissions, and complicated mechanical flight controls makes it a lot more compelling. Super light motors are everywhere. Now, if I could just figure out where to get a super light generator....?

DEM
Click to expand...
Can't speak to efficiency, but those brushless motors will function as generators. Latest automotive tech is using them as starter/generators.
 
back to original post: viable choices today

Electrics are flying, in the US -- and advancing at the speed of MONEY. I have seen both of these options, up close - and flying. Two completely different approaches: the first an effort at a viable business model, the second a pure researcher/ tech- geek traditional experimental aviation path. If you are serious about building something, you would be hard-pressed to find a better option than Brian Carpenter's EMG-6 [he's an excellent/ patient teacher, as well as very deep into his subject]; but recognize what the goal is: this is not a cross-country cruiser.
And no, i don't have an investment in either one.

http://sunflyer.com/specifications/

https://electricmotorglider.com/
 
keijidosha said:
Pipistrel has a 2 place trainer flying outside the US called the Alpha Electro. Article explains why it is not currently flying in the US.
https://www.flyingmag.com/pipistrels-first-electric-airplane-arrives-in-canada

Useful specs are available in it's POH Pipistrel Alpha Electro POH-165-0040-001_A00.pdf
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Interesting that the FAA won't allow a non reciprocating engine for commercial LSA operations, but what about all those non reciprocating engines operating in the flight levels?
 
bret said:
Interesting that the FAA won't allow a non reciprocating engine for commercial LSA operations, but what about all those non reciprocating engines operating in the flight levels?
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Pretty sure that was bureaucratic sloppiness when they wrote the rule (to keep turbines out of simple, light a/c), not unlike the original version that allowed amphibious a/c but prohibited re- positioning the gear in flight.

It was doubly sloppy since some of the new turbines with FADEC are simpler to operate than piston engines.
 
rv7charlie said:
Pretty sure that was bureaucratic sloppiness when they wrote the rule (to keep turbines out of simple, light a/c), not unlike the original version that allowed amphibious a/c but prohibited re- positioning the gear in flight...
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My understanding it was unintentional as well, so there may be a chance the waiver/exception will be granted. There will be 6 Alpha Electros at flying schools in my local area by end of March.
 
R&D is all well and good. My problem is when the govt is subsidizing at the consumer production level
 
rv7charlie said:
Pretty sure that was bureaucratic sloppiness when they wrote the rule (to keep turbines out of simple, light a/c), not unlike the original version that allowed amphibious a/c but prohibited re- positioning the gear in flight.

It was doubly sloppy since some of the new turbines with FADEC are simpler to operate than piston engines.
Click to expand...

Well at least the FAA got cleverer with the new Part 23 rules. This is a little part of the fuel section, which does allow for electric power sources -

(c) Each fuel storage refilling or recharging system must be designed to -
(1) Prevent improper refilling or recharging;
 
BobTurner said:
Actually, for every pound of gas Mr. Lycoming burns 3 pounds of oxygen. All the rest of the air just passes thru. So you carry 25% of the fuel. Since a gas engine is a heat engine, it’s only 40% or so efficient, while electric motors approach 100%. So Lycoming still wins but not as overwhelmingly - if electric storage weight can be brought down, it looks feasible. That’s a big ‘if’.
Click to expand...

Not exactly true.

You have to produce that electricity some place. Then there is that part about transporting it to the outlet so you can plug your energy storage device into it.

I think if you took all that into account, you might just find that liquid dead dinosaurs are much more efficient and pollute less.

Yes, Tesla's are nice cars but they are coal powered, for the most part.

All that said, it will be pretty cool, when/if they get usable electric planes. The other problem with them is how the FAA figures out how much "fuel on board" is required to reserves.
 
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