I could think of a pretty much harmless blade design that MIGHT work (soft blades), but they'd be woefully inefficient AND they would only work for those super-

I could think of a pretty much harmless blade design that MIGHT work (soft blades), but they'd be woefully inefficient AND they would only work for those super-ultra-light micro/nano drones. Anything else is not harmless. Even a tiny quadcopter can easily cut your skin, because the plastic is hard and sharp. Think of it as one of those plastic picnic knives, except not that sharp --- BUT rotating very, very fast.