Is it OK to cut the tips of a 7x4P prop to get a 5.5x4? Will it give the same performance as an the original?

From : Don Stackhouse

No. The cut-down 7-4 will have a different blade shape, with different thickness, camber and chord. Depending on the blade designs, the actual pitch angles at the tip may be different as well. The chord, pitch and airfoil shape at the tip will probably be too high for efficient operation at 5.5 inches.

In the original 7-4 prop, that 5.5 inch diameter location is about where most of the thrust is made; at 5.5 inches, that region is now the tip, and the lift there is zero. The outboard airfoils will therefore be all wrong for the local requirements, attempting to make too much thrust too close to the tip, resulting in poor efficiency. It will still move air around, but not as efficiently as a prop that was designed and manufactured for that diameter. OTOH, if you're making a ducted propeller (where the lift at the tip of the blade is NOT zero), then the cut-down 7-4 might do somewhat better.

Props are like shoes, they have to fit the application. If you put a size 11 shoe on a size 9 foot, it's not likely to be very comfortable. If you cut the toe end of the shoe off to make it the same length as a size 9, it's still not likely to be very comfortable, plus you're going to get cold toes.

Don Stackhouse
DJ Aerotech