Electric Propulsion

[Strappe]

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?

[Mike S]

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............

[PilotjohnS]

What Mike said. And probably not available as a retro fit.

[airguy]

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.

[rvbuilder2002]

Originally Posted by Strappe

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?

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.

[DaleB]

Originally Posted by rvbuilder2002

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.

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.

[krwalsh]

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?

[Aluminum]

Originally Posted by DaleB

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

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...

[bkervaski]

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.

^^^ This ^^^

[David Paule]

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