FAA Powerplant Written Practice Test

What is the angle of attack of a propeller blade measured against?

1. The engine's rotational speed

2. The direction of fuel flow

3. The relative wind

4. The pitch of the propeller

The correct answer is: The relative wind

The angle of attack of a propeller blade is measured against the relative wind, which is the airflow that moves over the blade as the propeller rotates. The relative wind is defined as the direction of the airflow with respect to the propeller blade at any given time. Measuring the angle of attack in relation to relative wind is critical because it directly affects the lift generated by the blade. When the blade encounters the relative wind at a certain angle, it creates a pressure differential that allows the propeller to produce thrust. Therefore, understanding how the angle of the blade relates to the direction of the airflow is essential for efficient propeller operation and performance. In contrast, measuring against factors like the engine's rotational speed, direction of fuel flow, or the pitch of the propeller does not provide the necessary context for understanding how much lift or thrust is being generated by the blades in their operating environment. The concept of angle of attack is fundamentally tied to the blades' interaction with the airflow, making relative wind the most relevant reference.