Has anyone seen or heard of a propellor with the center hole being maybe a
10" diameter so that the prop could be driven internally by a gear?

what we're discussing here is like a starter ring gear but where the teeth
are inside and has a large enough diameter to accomodate some sort of
driving gear and a structual member not in contact with the ring gear. The
ring gear would then be attached to the prop with the very large center
hole.

Our first thought were the gears inside a auto rear end (connecting drive
shaft to axle shaft) but instead of being at right angles, looking for
driving and driven gears to be inline.

Didn't some old fighterplanes fire through the prop and spinner? How did
that work?

Thanks, Dick