FAA Powerplant Written Practice Test

What operational force tends to bend the propeller blades forward at the tips?

1. Torque bending

2. Thrust bending

3. Centrifugal force

4. Lift force

The correct answer is: Thrust bending

Thrust bending is the correct choice because it refers to the force generated by the propeller as it produces thrust to propel the aircraft forward. When the propeller spins, each blade generates a pressure differential that creates lift, similar to an airfoil. This lift acts at a distance from the hub of the propeller, effectively creating a bending moment that pushes the tips of the blades forward. As the aircraft moves forward, the pressure decreases on the upper surface of the blades, and an increase in the angle of attack can occur, further contributing to the bending force. The forward bending at the tips helps counteract the effects of the propeller's rotation and maintains smooth airflow over the blades. Understanding this concept is crucial for recognizing how propeller dynamics affect flight, including performance and structural integrity. Other forces like torque bending refer to the twisting force around the hub, centrifugal force relates to the outward force from the center of rotation, and lift force is generally associated with the vertical lift acting on an aircraft rather than specifically indicating the bending of propeller blades.