The thing that limits the diameter of the fan blade is the chassis. The hole for the fan will be in the center of the chassis and the narrowest spot is 7 inches total. So if I had to make a hole that's about 5 inch in diameter for a 5" inch fan blade, I'd only have 1 inch of material left on either side. That would make the chassis weak at that spot. I'd rather have a blade that is 3-4 inches in diameter and have 1.5-2 inches of material left on the sides.

Also, I picked a ducted fan blade instead of a prop because fans are better suited for moving air. I don't disagree that a prop can produce a lot of air, but I think the a fan blade can move more air per revolution. This also adds more resistance for the engine to turn against. RC Car engines are not designed to run screaming with little or no load. I'm looking for maximum air flow per revolution to get the heavy hovercraft aloft.

Thanks for taking the time to read about my silly project.

-Rob