Egregoros

Cylindrical batteries: 283wh/kg
Apparently about $10 each, so $83/kWh
36kWh = 127kg (279lb)

A gallon of gas, burned in an aircraft engine, is probably good for about 9.4kWh. So 36kWh is probably equivalent to about 4 gallons of gas.

The clever part is you make a string of batteries and put it inside a carbon fiber tube sealed in with epoxy, and then you have a solid rod. You're not doing anything for tensile strength but for compressive strength this would be amazing.

And tension is the easier force to deal with. Lots of materials like kevlar have incredible tensile strength. Compression is hard because compressive load causes bending. This is quite obvious when you think about it, there exists 400lb test fishing line but you would search to the ends of the earth to find anything as narrow as a fishing line that can hold up 400lb in compression.

But compressive load is really interesting because almost everything has good compressive strength - as long as it's confined inside of a tube. You could fill a carbon fiber tube with playdough and it would have great compressive strength - as long as the playdough is confined and has no holes to ooze out though.

Furthermore, the contents of the tube are placed under "pressure", but not load in the classical sense. Bending and squeezing forces only come into play when an object is not confined.

So making compressive load carriers out of carbon tubes full of batteries seems like a really interesting way to integrate batteries without the entire weight penalty.

> electric was not a viable option

Well, yes and no. You're absolutely right that the weight is bad, it's like 10x as bad. But on the other side of things, something like 2/3rds of the cost of a plane ticket is just going straight to fuel, so if you can find a way to replace that fuel with electricity, you're taking maybe 20-30% off the cost of the flight, so the pot of gold at the end of that rainbow is so rich that you start re-thinking the definition of "viable"...

In terms of actual physics viability, it's "borderline". You can get up in the air for an hour or maybe 2 with electricity, which for a lot of short haul, tourist stuff, etc is potentially enough. And if it's "potentially enough" then it's 100% worth it because of the aforementioned fuel cost...

Now what I'm talking about is that blue thing that runs the length of the wing. That has to be frustratingly strong because the whole airplane hangs off of it. So I'm thinking something like pic 2 with two battery tubes, having periodic spacer tubes, and then the whole thing is wrapped up in carbon fiber like it was a Christmas present - and that outer wrapping prevents the two from racking.

Heart ES-30 is an electric plane (still in development) and it has 200km (124 miles) of range on electric only. However, the outer two engines are turbines, so it can extend flight beyond electric range...

Even if you need to burn gas to complete the flight, using as much electricity as possible is *obviously* a cost savings...

It's not clear from the picture but the wingspan is big for what it is. 31m (101ft) for a 30 seat aircraft, the Boeing 727-800 is 25m (117ft) for 160-180 passengers. Long wings improve efficiency...

So like I said, viability is borderline, but they have something like 250 firm orders from airlines, so like I said, the pot of gold is rich...

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...

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...