From the fact that this question comes up regularly on VAF and on other places like Facebook... I don't think most people appreciate just how low the energy density is on batteries (even the best ones), or how over-powered RVs are (especially the aerobatic ones) when compared with the airplanes that are being electrified.
Let’s do a back-of-the-envelope calculation, taking something like an RV-8 or RV-14.
Say that you burn 8 gph while you cruise
. (That might not be exactly right but it’s gotta be within like 30%, right?). In the engines we commonly see on most single-engine airplanes, 8 gph probably gets you somewhere around 150 hp
. (Again, very rough ballpark, give or take like 30%). 150 hp is about 110 kiloWatts (i.e. 110 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, which is on the high end of what you can buy without spending a LOT more money).
Say we spare no expense and go with 100 Watt-hours per pound. If we want 150 hp, i.e. 110,000 Watt-hours per hour, then we’d need… 1,100 pounds of battery per hour of flight.
And, again, remember that this weight doubles for "reasonable" (i.e. not super high tech) batteries like EarthX ones.
Compare that against the ~8 gallons (~50 pounds) of fuel that you currently need for each hour of flight.
In short: Each pound of battery only carries something like 2% to 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. (And even if you did, you'd only be able to fly for about an hour).
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 for very long at all in an electric RV-8.
(Edit: Removed paragraph about 60hp motorcycle engines. You can still see it here).
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).
BTW, Van himself has an electric motorglider
. But that kind of flying is very different from what he designed the RV-8 to do.
In any case… Given the energy density of batteries at the moment, there is just no way to fly an electric airplane very far. More than 200 miles is almost impossible, even with an airplane designed specifically for that purpose.
(And yes, if you have a generator that burns fuel and drives an electric motor, then sure, you can have a lot more endurance and range than if all the energy you carry is in a battery... but then that's about the same as just powering the propeller by a piston engine, isn't it?)