Step 8: THE PROPELLER
First slide the prop onto the crank without the hub to see if it fits inside the duct without binding. A duct is most efficient when the prop fits closely inside the duct. Be very careful when removing tip material, it's easy to cutoff but hard to put back . Once you have the tip clearance set you can begin to sand the blades with a power sander and finish by hand. Once you get it smooth you MUST balance it to avoid vibration.
Hang the prop on a rod held in a vise to see which blade is heavier than the other. Don't short cut by shortening the tips. Just keep sanding the blades. Most props are pretty close when they arrive so it is not an undaunted task. Once it is balanced give it a couple coats of paint to preserve it. It's a good idea to paint a white band at the tips for safety's sake. By now the foam is hard so you can cut the bottom of the duct with a sabre saw to conform with the openings in the hull for lift air. Now is a good time to trial fit the prop and hub to the motor to make sure it runs true inside the duct. The motor should be bolted down securely. Turn the prop over slowly by hand. Check for at least 1/8" clearance all around. You can adjust by putting small shims under the motor mounts if needed.