What does the tip path plane refer to?

The tip path plane refers to the area of the projected outline of rotor travel. When a helicopter rotor rotates, the tips of the blades move in a circular path, which forms a plane known as the tip path plane. This concept is essential for understanding rotor dynamics and how the rotor interacts with the air around it. Understanding the tip path plane is crucial for pilots, as it impacts factors like lift generation, rotor efficiency, and potential clearance issues with obstacles during flight.

The other definitions, while related to rotor mechanics, do not accurately describe the tip path plane. The space between the rotor blades is not a plane but rather an area that can affect airflow and lift. The height at which the rotor blades operate does not define the plane itself, as the plane pertains to the motion of the tips, not simply their altitude. The distance from the rotor hub to the tips of the blades describes the rotor radius, not the path that the tips take as they rotate. Thus, option A is the precise choice that encapsulates what the tip path plane represents in helicopter aerodynamics.