FAA Powerplant Written Practice Test

The thrust generated by a rotating propeller results from what phenomenon?

1. An increase in air pressure behind the propeller blades

2. An area of decreased pressure immediately in front of the propeller blades

3. The momentum gained by the air

4. The angle of attack of the blades

The correct answer is: An area of decreased pressure immediately in front of the propeller blades

The thrust generated by a rotating propeller primarily results from an area of decreased pressure immediately in front of the propeller blades. When the propeller spins, it accelerates the air behind the blades, creating a high-pressure area behind them and a corresponding low-pressure area in front. This pressure differential is crucial in generating thrust; the air being pushed out at the back pushes the propeller (and the aircraft) forward. While other factors play a role in propeller performance, such as the angle of attack and the momentum transfer to the air (which is relevant to the understanding of how thrust can also be derived), the essential mechanism for creating thrust is related to the pressure changes around the blades. An increase in pressure behind the blades can help illustrate the concept of thrust, but it is the pressure drop in front of the blades that is key in creating that forward force.