FAA Powerplant Written Practice Test

What operational force causes the tips of the propeller blades to lag?

1. Torque bending

2. Thrust bending

3. Lift

4. Drag bending

The correct answer is: Torque bending

The operational force that causes the tips of the propeller blades to lag is torque bending. This occurs as a result of the torque generated by the engine. When the engine produces power, it creates a rotational force that is transmitted through the propeller shaft to the blades. Due to inertia, the tips of the blades experience a delay in their acceleration compared to the root of the blades, resulting in a bending effect. As the engine generates torque, the blades are forced to rotate around the hub, but because of the physical properties of the materials and the forces acting on them, the tips lag in their response. This phenomenon primarily manifests as the bending that occurs along the length of the blade, with the maximum lag occurring at the tips. Understanding this can help with recognizing how propeller dynamics influence performance and efficiency. The other forces mentioned, like thrust and lift, do not directly relate to the lagging effect of the blade tips. Thrust is the forward force produced by the propeller exerting pressure on the air, while lift refers to the upward force acting on the blades as they cut through the air. Drag bending, which involves resistance forces acting against the motion, is also not a primary cause of the lagging effect in propeller blades.