FAA Powerplant Written Practice Test

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

1. Thrust bending

2. Torsional force

3. Centrifugal force

4. Drag force

The correct answer is: Thrust bending

The operational force that causes propeller blades to bend forward at the tips is known as thrust bending. When the propeller generates thrust during flight, the blades experience a distribution of aerodynamic forces that primarily act in a direction parallel to the plane of rotation. As these forces exert themselves on the blades, they create a bending moment that pushes the tips forward. This bending is most pronounced at the tips because of the increased aerodynamic load relative to the root of the blade, where it is more securely attached to the hub. As the engine operates and the propeller accelerates, the thrust generated causes the blades to flex in a characteristic manner, contributing to the overall performance and efficiency of the propeller. The other forces listed do not specifically account for the bending observed at the tips. Torsional force deals with the twisting of the blades rather than bending. Centrifugal force acts outward from the center of rotation, causing the blades to want to fly off outward, but this does not directly cause forward bending at the tips. Drag force acts opposite to the direction of motion, contributing to resistance but not directly affecting the bending of the blades in the manner that thrust bending does.