[coderenegade]

Aerospace as a discipline has tried just about every propulsion system under the sun. What you're proposing has already been flight tested on an unmanned vehicle, albeit with many smaller props for a larger effective prop disk than the two you're proposing. This is actually better, because electric motors want to use rpm control, so you need to keep the moment of inertia of the propellers low. The efficiency penalty of smaller props is overcome by having many of them arranged closely together to create a large effective prop disk.

For a hybrid system to be worth it, you need to claw back more efficiency than you lose in going from mechanical energy to electrical energy, and then back again. For cars, this is generally the case, because they're always accelerating and decelerating. Their wide operating band means that the engine will always be a game of compromises, which is why sticking a motor and battery in the loop and decoupling the engine from the wheels is beneficial. But planes aren't like that; they go from setpoint to setpoint, and they stay in a given configuration for long periods of time. They have very narrow, highly optimized operating bands, so hybridization just isn't as effective.

[rbanffy]

> For a hybrid system to be worth it, you need to claw back more efficiency than you lose in going from mechanical energy to electrical energy, and then back again.

For short-haul you spend a lot of time climbing and descending in relation to long flights, so the plane spends less time at the optimum the engines were designed for (cruise), so, if the power unit can be at the peak efficiency throughout the flight, with extra energy being supplied by the onboard battery for take-off and the power unit be shut down for descent, we might get to a point where it's economically viable, depending on battery operating costs and weight.

You are right to point out the aerospace industry has tried everything conceivable to see what sticks, but technology evolution sometimes throws us a curveball.

[coderenegade]

It's unlikely; the energy density just isn't there, even for a hybrid system. Because of the way flight profiles work, you're still talking about enough battery to provide meaningful power for half a flight, which probably isn't feasible in the near term.

And it's not like the industry has missed this possibility. I've seen papers on systems as exotic as combining high temperature fuel cells (SOFCs, PCFCs) with the Brayton cycle to achieve a hybrid powerplant. The bar for novel propulsion systems in aerospace is extraordinarily high, higher (imo) than it is for automotive. The exception is unmanned systems, which are new enough and varied enough that there's been an explosion of activity, which has been exciting to see.