Egregoros

NMC cells and next gen solid state are good for about 1000-1500 charge cycles, so that probably comes out to about 1000-2000 hours of flight time, and gonna be at least 3 or 4 years of intensive flying. And by that time, you probably want to replace the batteries with newer (solid state) batteries in order to boost (double) your energy storage...

After some more thought, I actually think it might be better to have installable battery modules, like, in the wing. Because then you can do the math as follows:

Suppose I have 700kg of useful load (this is the case with a Velocity XL). I put a Yamaha Apex 998 engine which weighs 55kg and can be boosted up to 300hp - if you do not care about reliability whatsoever...

Lets say the engine, prop, and all of that comes to 70kg. Then you add 2 additional electric props totaling 30kg, so all in it's 100kg.

Now you're at 600kg remaining, you have an 80kg pilot and passenger, and some baggage totaling 200kg.

Now 400kg left. You need 50kg of fuel to get to your destination on gas engine alone.

Now you have 350kg left, if you have 3x 50kg modules in each wing and 1 in the center, you use all the weight you can carry.

If you need more fuel, more payload, my predictions too rosy... Whatever, remove the center battery module.

Need 50kg more, put the center one back and remove one from each wing.

And this way, you just add the right amount of battery in 50kg increments in order to always be at max weight.

(just double all of the numbers if you think in pounds)

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Now flying it:
1. Always have enough fuel to get to your destination in engine alone.
2. Run your engine and electric both at takeoff
3. Once in cruise, go to engine-only, UNTIL you are within battery reach of your destination airport, then cut the engine.
4. Start the engine before landing so you have extra power in case you need a go-around.

So short hops you will only use the engine during takeoff and landing.

And this whole design idea really depends on *removable* batteries...

Ahh, so gotta build one from spare parts. Works for EAB but not so much for production...

My theory (above) is to use a small/overboosted engine like that with an electric system and modular batteries so you can choose the # of modules to carry based on your load and fuel needs, and you always have 2 redundant powerplants (important since you're not being particularly nice to your engine).

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If you've got 7x 50kg battery modules, that's gonna be roughly 100Kwh of power with current battery tech.

A gallon of gas is good for about 9.4Kwh at the crankshaft, so that's about 10 gallons of gas that you're saving - which probably amounts to about 140–180 NM of electric range (using the Velocity XL as an example).

So if you're just out fooling around, there's no reason you wouldn't do it all-electric. If you're doing a cross-country trip, then you decide how much fuel you need. If you don't want to fly more than 4 hours at a time, you probably do 50 gallons of gas and that leaves you weight for 4 modules - which buys you about 85 miles of emergency electric range.

More weight, fewer modules, you can decide what you want to do...

Yes but not really. Those with Sport Pilot certificates can only fly single engine birds, and a max of two aboard. And last time I checked, one needs a multi-engine rating to fly anything with more than one engine...even if that means two or more motors driving a single propeller.

The big debate going on right now is hybrid powertrains, like the Rotax-based system. The ICE engine is continuously running, therefore able to keep the aircraft flying, with the electric in parallel simply augmenting...but the electric motor has the ability to at least extend the aircraft's glide range in the event of ICE power loss.

Man I wish I could discuss what that group is doing!

Interchangeable modules in cars are not all that interesting. I went down this rabbit hole a few months ago.

The problem is who owns the module? And whoever that is, they have to tolerate their modules moving from point A to point B.

Then the electrical interface for the module is another giant headache because realistically it needs to connect when the module slots in. And that means the plug is rigidly attached to the battery and the other side rigid to the vehicle, so it's gonna be subject to a bunch of vibration, load, etc which is a disaster for designing a plug.

And then at the end of the day, you're saving 5 or 10 minutes of fast charging time to swap a module instead. And fast charging is only getting faster.

But on an aircraft, the owner of the modules is the aircraft owner. There's no compatibility challenge because they buy the aircraft and modules together. You install them by hand before flight, so there's no problem with plugging in a cable when you install it - which eliminates the need for a complex vibration resistant plug.

And battery modules in an aircraft have a different objective than in a car: The goal in an aircraft is to be able to remove battery weight to keep the aircraft under its design maximum while adding payload. PARTICULARLY if you have a combustion engine in addition to electric.

Longer flight -> more fuel needed -> less batteries
More passengers -> less batteries
Short flight, few passengers -> more batteries, save money on fuel...

@cjd @Kalogerosstilitisrefugee I missed your rabbit hole diving, but the batteries ownership issue would be handled between those few airports where these short but frequent trips would utilize this planes. Not a huge issue imo.

About the electrical interface, simply bolting down the batteries to the frames and having cables doing the connection seems like a reasonable approach.
You could even eject the whole battery frame in case of an oopsie.

Maybe I'm overlooking some important details here, but those 2 look entirely doable.

I wouldn't be too worried about actually swapping batteries in a plane. Modern batteries with cooling can charge in around 10 minutes, and you really never spend less than that fussing around between each flight.

And swappable batteries PROBABLY only make sense if you have battery + engine, because you're looking to minimize fuel burn so if you're the only one going up, you take 600lb of additional battery and save that much fuel.

Ruixiang RX4E is a fully electric 4-seater which claims 90 minutes of autonomy. Definitely aimed at flight schools, flying clubs, and guided tour stuff. The battery is removable but there's only one of them and it has coolant lines so you don't want to remove it each flight.

You're not doing any cross country flights (unless your country is Luxembourg) but if you're a flight school or a tour guide, you never ever to do that anyway - and replacing 40$/hr of gas into 5$/hr of electricity, you might be able to cut 20% off the price you charge the customer.

It's not really greenwashing, between Ukraine and Iran, all the money for eco-fantasy has more or less dried up.

The economics is this: If your plane is not loaded to max weight, every ton of batteries you add give you about 30 gallons of fuel equivalent, and every gallon of fuel is about 8 bucks. So if you're about 3 tons under-weight because you don't have payload, you can save $700 worth of fuel on a trip.

If you're crossing an ocean or something, that's just worthless, but if you're doing some regional thing like 150 miles feeding into a major airport, that could be 1/3 of your ticket price.