FAA Aviation Mechanic Powerplant (AMP)PropellersMedium
A mechanic explains propeller theory to a student regarding the forces acting on a rotating blade. The aerodynamic twisting moment created by the airflow's center of pressure acting ahead of the blade's pitch-change axis tends to move the blade toward which condition?
- AThe full reverse pitch position
- BA lower (finer) blade angle
- CThe fully feathered position only
- DA higher (coarser) blade angle
Show answer & explanationAnswer & explanation
Correct answer: D. A higher (coarser) blade angle
The aerodynamic center of pressure on a propeller blade is normally located ahead of the pitch-change axis, creating a twisting moment that tends to rotate the blade toward a higher (coarser) pitch angle. This is opposite to the centrifugal twisting moment, which tends to move the blade toward a lower (finer) pitch.
Why the other options are wrong
- A. Reverse pitch requires powered actuation, not a natural aerodynamic force.
- B. This describes the centrifugal twisting moment, not the aerodynamic one.
- C. Feathering is a controlled, powered movement, not a natural aerodynamic tendency.
Aerodynamic Twisting Moment
A force created by air pressure acting ahead of the blade's pitch-change axis that tends to twist the blade toward a higher (coarser) pitch angle, opposing the centrifugal twisting moment.
- Center of pressure is ahead of pitch axis
- Tends to increase (coarsen) blade angle
- Opposes centrifugal twisting moment
- Both moments factor into governor/counterweight design
Memory trick: Air pushes forward, blade turns coarse — 'Aerodynamic Aims high.'