What is meant by relative wind with regard to an airfoil?

• A. The speed of the wind relative to the airfoil.
• B. The direction the wind strikes the airfoil.
• C. The air pressure on the windward surface.
• D. The airflow velocity around the tail.

Answer: (B)

Relative wind is the airflow the airfoil experiences as it moves through the air—the wind relative to the wing. In practice, you can picture it as the air moving past the wing from the front toward the back, i.e., the wind strikes the leading edge. This direction is what sets the angle of attack and thus how much lift the wing produces. So the best answer captures that the relative wind is about the direction the wind hits the airfoil.

The relative wind does have speed, but the defining aspect in this context is the direction from which the air is coming and hitting the wing. Concepts like the pressure on the windward surface arise from how that wind interacts with the wing at a given angle, and the airflow around the tail is unrelated to the wing’s relative wind.