What is a movable leading-edge device that extends at high angles of attack to delay airflow separation?

• A. Fixed leading edge
• B. Movable flaps
• C. Flaperon
• D. Movable slat

Answer: (D)

Movable slats are leading-edge devices that extend to high angles of attack to keep the airflow attached over the wing. By creating a narrow gap between the slat and the wing, they energize the boundary layer and re-energize the flow over the upper surface, delaying flow separation and increasing lift at high angles. This helps prevent an early stall and allows a higher critical angle of attack during takeoff, climb, and landing.

A fixed leading edge wouldn’t change with angle of attack, so it wouldn’t delay separation. Movable flaps and flaperons are on the trailing edge and affect lift/drag differently, but they don’t provide the leading-edge energy needed to prevent separation at high AoA. So the device that fits the description is the movable slat.