What is the primary purpose of feathering propeller blades?

Feathering propeller blades is primarily intended to reduce drag and vibration, especially when an engine fails or is shut down. When a propeller is feathered, the blades are rotated to align with the relative airflow, effectively minimizing their resistance to motion through the air. This action is crucial in multi-engine aircraft. In the event of an engine failure, feathering helps maintain a better aerodynamic profile, thereby decreasing the drag that would otherwise be produced by a windmilling propeller. This not only assists in maintaining controlled flight but also helps in improving overall aircraft performance and efficiency by allowing the pilot to manage the situation more effectively.

While other options may touch on aspects of propeller operation, they do not capture the primary purpose of feathering as it relates to aerodynamics and the handling of failed engines. Increasing thrust or improving engine efficiency are not direct outcomes of feathering, as feathering is about minimizing adverse effects following engine issues rather than enhancing performance under normal operations. Similarly, enhancing aeroacoustic performance is more about noise reduction rather than the primary aerodynamic benefits achieved through feathering.