FAA Aviation Mechanic Powerplant (AMP)PropellersEasy
A mechanic is selecting a replacement propeller for a high-horsepower engine installed on an aircraft with limited ground clearance. Why might a four-blade propeller be preferred over a two-blade propeller that absorbs the same engine power?
- AFour blades automatically increase the maximum allowable RPM of the engine
- BFour blades eliminate the need for a constant-speed governor
- CFour-blade propellers never require dynamic balancing
- DA four-blade propeller can absorb the same power with a smaller diameter, reducing ground clearance problems
Show answer & explanationAnswer & explanation
Correct answer: D. A four-blade propeller can absorb the same power with a smaller diameter, reducing ground clearance problems
Adding more blades increases total blade area (solidity) so the propeller can absorb the required horsepower with a smaller diameter, which is useful when ground clearance is limited. RPM limits, balancing requirements, and governor need are unrelated to blade count alone.
Why the other options are wrong
- A. RPM limits are set by engine design, not blade count.
- B. A governor is still required for constant-speed control regardless of blade number.
- C. All propellers, regardless of blade count, require balancing.
Blade Count vs. Diameter
Increasing the number of propeller blades increases total blade area (solidity), allowing the propeller to absorb the same engine power with a smaller diameter.
- More blades = more solidity
- Smaller diameter possible for same power absorption
- Useful when ground clearance is limited
- Common on high-horsepower turboprops and radials
Memory trick: More blades, less room needed — pack the power into a smaller circle.