RE: Prop on an OS 91 ducted fan
It depends on the plane for size and pitch and it will take testing to find the right one for your app. But getting the prop is your problem. You can’t use an off the shelf APC prop, it’s not safe. Some guy are getting a much larger APC prop for the large hub size and carving it down. Some are using a fine grain hard wood prop. Both can be dangerous.
Carbon is the way to go, guys are using APC props as a pattern to make their own molds. I can't recommend anyone that makes them. Sorry