If there is an odd number of blades and there is no equal-weight moment blade available, how many blades can be replaced as a set?

• A. Five blades
• B. One blade
• C. Seven blades
• D. Three blades

Answer: (D)

Rotor balance depends on canceling the moments produced around the center of the circular blade pattern. If there’s an odd number of blades and no equal-weight moment blade to offset a replacement, the safest way to keep the net moment zero is to replace three blades that are evenly spaced around the circle (every 120 degrees). Three equal weights at those positions sum to zero vector, so their combined effect on balance is neutral.

Replacing a single blade cannot be balanced because there’s no opposite blade to counteract it, which would be required to cancel the moment. Replacing five or seven blades would require those replacements to be evenly spaced around the circumference to achieve zero net moment, and with an odd blade count and no equal-weight counterpart, that symmetric spacing isn’t guaranteed. Hence three blades is the largest safe set to replace under these conditions.