Which flap type features a slot between the flap and the wing that re-energizes the boundary layer and delays stall?

• A. Plain flap
• B. Split flap
• C. Slotted flap
• D. Fowler flap

Answer: (C)

A slot between the flap and the wing energizes the boundary layer and delays stall. The gap allows high‑velocity air from beneath the wing to flow onto the upper surface of the flap, re-energizing the boundary layer there. This extra energy helps the airflow stay attached to the surface as the flap is extended, reducing the adverse pressure gradient and allowing more lift at higher angles of attack. As a result, the wing can reach a higher lift coefficient before flow separation occurs, lowering stall speed. Other flap designs lack this slot, so their boundary layer isn’t re-energized in the same way, making stall occur sooner or at lower lift gains.