What term describes the direction of the wind hitting an airfoil?

The correct term that describes the direction of the wind hitting an airfoil is relative wind. This concept is crucial in aerodynamics and refers to the flow of air as it moves past the wing or airfoil, which is essential for generating lift. Relative wind is determined by the aircraft's motion through the air and the direction of the airflow created by that motion.

In the context of flight, understanding relative wind is key to analyzing the performance of an airfoil, as it directly affects the lift and drag forces acting on the aircraft. The relative wind is always opposite to the direction of the aircraft's flight path, and it changes as the aircraft maneuvers.

Other terms might involve different aspects of wind but do not accurately represent the airflow interacting with the airfoil. For instance, true wind refers to the actual movement of the air relative to the ground, while ground wind describes wind conditions at ground level, which may not affect the aircraft's aerodynamic characteristics in the same way. Effective wind is not a commonly used term in this context. By grasping the significance of relative wind, one gains a better understanding of aircraft performance and maneuverability.