What is meant by the critical angle of attack?

• A. The minimum angle of attack at which lift begins.
• B. The angle of attack that yields maximum lift coefficient.
• C. The angle where flow becomes subsonic.
• D. The highest angle of attack at which the air passes over the airfoil in a smooth flow.

Answer: (D)

The critical angle of attack is the highest angle at which the airflow remains smoothly attached to the wing’s surface. Up to this point, the air follows the wing shape cleanly, giving lift. Once you exceed this angle, the flow separates from the upper surface, causing a stall and a rapid drop in lift. That’s why choosing the description of the highest angle of attack at which the air passes over the airfoil in smooth flow best captures the idea. The other notions—lift beginning at a minimum angle, the angle for maximum lift coefficient, or flow becoming subsonic—don’t describe the onset of stall.