FAA Powerplant Written Practice Test

Which force causes the propeller blade tips to lag in the opposite direction of its rotation?

1. Torque bending

2. Thrust bending

3. Centrifugal force

4. Aerodynamic drag

The correct answer is: Torque bending

The force that causes the propeller blade tips to lag in the opposite direction of its rotation is related to the concept of torque bending. As the propeller spins, each blade experiences centrifugal forces that attempt to pull it outward. However, due to the angle of attack and the aerodynamic lift generated by the propeller blades, a twisting action occurs. This twisting is a result of the difference in forces acting on the blade as it moves through the air. As the blade rotates, the section that is moving faster (the blade tip) experiences different aerodynamic forces compared to the section near the hub. This difference in forces induces a bending moment that causes the tips of the blades to lag behind the plane of rotation. Consequently, this lagging effect is primarily due to the characteristics of torque and how it influences the blade's behavior under rotation. Understanding torque bending is crucial because it highlights the interaction of aerodynamic forces and the mechanical forces acting on the propeller, ultimately affecting the efficiency and performance of the propeller system.