What is true regarding the blade angle along the length of the propeller blade?

The blade angle of a propeller is designed to vary along the length of the blade primarily to optimize performance at different operating conditions. Specifically, it decreases from the root to the tip. This design is important because the rotational speed and the amount of lift generated by the blade need to be balanced to ensure efficient propulsion.

At the root of the propeller blade, where it attaches to the hub, a larger blade angle is necessary to generate sufficient thrust due to the lower tip speed. As you move toward the tip, the tip speed increases, allowing for a smaller blade angle without compromising effectiveness. This angle reduction from root to tip helps maintain efficient airflow over the blades and prevents stall at high speeds. The variation in angle along the blade results in improved overall performance of the propeller, allowing it to effectively convert the engine's power into thrust.

Changes in engine RPM can affect the angle of attack and performance, but the fundamental design of having a decreasing blade angle is intrinsic to the structure of the propeller itself. Thus, the correct answer reflects this essential aerodynamic principle in propeller design.