Like Canadian Joy said: If you place the batteries to roughly match the weight of the current engine and fuel tanks, then you keep the re-engineering (and re-building time) to a minimum.
But, like Warren (Gasman) said: The energy density of batteries is much lower than that of fuel. The range of an electric airplane will be a small fraction of the range of a similar hydrocarbon-powered airplane (as will the endurance, unless you fly at ultralight/glider speeds, like 40mph).
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Let’s do a back-of-the-envelope calculation. Say that
you burn 5 gph while you cruise. (That might not be exactly right but it’s gotta be within like 30%, right? Especially if you don’t mind slower speeds like you said). In the engines we commonly see on most single-engine airplanes, 5 gph probably gets you somewhere around
100 to 125 hp, let’s conservatively say 100. (Again, very rough ballpark, give or take like 30%). 100 hp is about 75 kiloWatts (i.e. 75 kiloWatt-hours per hour).
Most batteries can carry about 25 to 40 Watt-hours per pound, but the fancy expensive ones can do 100 Watt-hours per pound, maybe a little more. (Go to the website selling any large battery and they’ll tell you the Watt-hours – or at least the Amp-hours and the Volts, which you multiply together – and probably the weight. Most of the “lightweight” lithium-ion batteries will be in the 30s of Watt-hours per pound.
This one has 42, for example). Say we spare no expense and go with 100 Watt-hours per pound. If we want 100 hp, i.e. 75,000 Watt-hours per hour, then we’d need…
750 pounds of battery per hour of flight. Compare that against the ~5 gallons (~30 pounds) of fuel that you currently need for each hour of flight.
In short: Each pound of battery only carries something like 4% (in my back-of-the-envelope estimate above. In reality it could be a little more, but still, less than 10%) as much energy as a pound of hydrocarbon fuel.
Cars work well as electric vehicles because it's possible to replace their 15-to-20 gallon (i.e. 90-to-120-pound) gas tanks with
giant 1,500-pound batteries. You can't do that in an RV.
Again, this was very hand-wavy, back-of-the-envelope, “This number and that number might be off by 30% or more”… (Experts in batteries, or in the specific fuel consumption of piston engines and how it and the horsepower change with RPM… feel free to weigh in and correct my estimates). But even if we’re off by a factor of two or more, you can see that you’re not going to fly long cross-countries in an electric RV-3.
The
motorgliders and ultralights currently offered as electric airplanes are designs that can fly with 60hp or less. Luckily, that’s roughly the max power output of one of the motors used in Zero electric motorcycles, so that’s
what they used to power the electric
Xenos and
Gull and
Strojnik. What do you think the climb performance of an RV-3 would be with only 60hp? Those electric airplanes, of course, are a lot slower, and have more wingspan (less induced drag), so they climb ok. Still, even at ~50% power (30hp), they need something like 200 lbs of battery per hour of flight, which is not too crazy. (… or, rather, per hour of
powered flight. You can fly longer if you turn the engine off and do some soaring). Some RV-3s do carry 200 lbs of fuel, but I don’t think they could hold altitude with only 30 hp.
Some
ultralights and
motorgliders and
LSAs and
older airplanes – and some remarkable Experimentals like the
Quickie – can take off and climb on a little more than 20 hp and cruise on even less than that. So now you’re in the realm of 100ish pounds of batteries per hour of flight. That’s definitely doable. But an RV needs a few times more power than that, and therefore, a few hundred pounds of battery.
(Yes, replacing a piston engine by a lighter electric motor buys you many "free" pounds for batteries... but not "hundreds". In an RV, if that difference is 100 lbs, that buys you… ~10 minutes of flight time. In an ultralight, if that difference is 20 lbs, that buys you… ~10 minutes of flight time. Again, this is all very approximate, within a factor of 2, just to get an idea of the general trends).
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BTW,
Van himself has an electric motorglider. But that kind of flying is very different from what he designed the RV-3 to do.
The RV-9, 10, and 12 are more efficient than the 3, i.e. they can fly with less power proportionally to their size. The 12 might even not be a total disaster if you put a 60hp electric motor in it, especially if you replaced the wings by bigger (glider-style) ones.
In any case… Given the energy density of batteries at the moment, there is just no way to fly an electric airplane very far. Even if you
do everything you can to design an electric airplane that is optimized for range (battery weight far exceeding the occupant weight, thin low-drag wings optimized for low power and slow speeds and not capable of taking a lot of Gs), the airplane probably could not fly 300 miles or one-hour-plus-reserves (
unless you “cheat” and also get some power during the flight from burning fuel).