Angle of attack is defined as the angle between which two lines?

• A. Relative wind and the vertical reference line.
• B. The chord line of the airfoil and the relative wind (or flight path).
• C. The wing's leading edge and the aircraft's longitudinal axis.
• D. The fuselage centerline and the relative wind.

Answer: (B)

Angle of attack is the angle between the airfoil’s chord line and the oncoming relative wind. The chord line is the straight line from the wing’s leading edge to its trailing edge, which defines the wing’s orientation in the air. The relative wind is the airflow direction as seen by the wing, effectively the air flowing opposite the airplane’s motion. The lift produced depends on how these two lines meet—the steeper the wing meets the oncoming air (within limits), the greater the lift up to the point where the flow may separate and stall.

Other descriptions aren’t used for this measure: the angle between the relative wind and a vertical reference line isn’t the angle at which the wing meets the air; the angle between the wing’s leading edge and the aircraft’s longitudinal axis describes pitch, not attack; and the angle between the fuselage centerline and the relative wind describes sideslip, not angle of attack.