Using the diagram, the critical angle of attack is 20 degrees.

• A. 15 degrees
• B. 20 degrees
• C. 25 degrees
• D. 30 degrees

Answer: (B)

The critical angle of attack is the angle at which airflow can no longer stay attached to the wing, causing the boundary layer to separate and the wing to stall. In the diagram, the stall onset occurs right at 20 degrees, so that angle marks the point where lift can no longer be maintained and stall begins.

Angles lower than that (for example, 15 degrees) are on the attached-flow portion of the lift curve, where increasing AoA still increases lift and no stall has occurred. Angles higher than that (such as 25 or 30 degrees) are beyond the threshold, where flow separation has already built up and lift drops as the stall progresses. Therefore, the value shown in the diagram as the stall onset—the critical angle of attack—is 20 degrees.