To Robert McHam, reply #6:
There used to be 80:1 geared bench grinders. I think most had a slip dog for the handle - push handle onto shaft to engage so you could spin it up, slide handle out of contact and the wheel spun free and coasted down.
Many of these were converted to drive a flywheel and rubber lined cup (as on today's electric starters). The usual grinder had a clamp base like some small clamp-on bench vises. Clamped onto the field box, you could spin up the flywheel and cup, free it to coast, and bump the spinner into the rubber.
Most that I saw, and saw pix of, were very handy for speed models with 'unorthodox' timing, props and fuels...
Modified to add: I like this way better than using a wrench socket on the prop nut, or another "green" thing sometimes suggested: chuck a spindle fitted with a short piece of garden hose into the egg-beater type hand drill... Tried that once.
JUST once...