just read about people playing around with a combination of counter-rotating props and toroidal props.

So having better efficiancy (like +10% and more) of counter rotation, but trying to reduce the drawbacks of that (more noise and stress due to way more vortices)

IDK, but sounds like a great idea, that will take a long time to get exactly right.

with modern fluid simulation, someone might just be able to pull it off.

But then there's the problems of actually producing props on the cheap AND durable? And what about having that work at different speeds?

This'll get compex fast.

And it might work better for boats than for airplanes.