One of the major weak spots with multi-rotors is the exposed propeller blades, they break and they hurt things.
I know people have designed guards and experimented with ducted fans, but has anyone done work with a boundary layer turbine arrangement? I've been kicking this idea around in the back of my head for some time now and just recently saw a ceiling fan project that uses this effect.
I always imagined a nested conical frustum stack (imagine lamp shades stacked on top of one another with a small gap between each). As the assembly spins, air is accelerated from the center down and out at an acute angle relative to the vertical axis, whereas the above ceiling fan example moves the air perpendicular to the rotation axis. I know it will blow air, but will it move enough to provide any useful thrust?
Now if I could just get a hold of some graphene and aerogel lampshades to experiment with...