The feature that re-energizes the boundary layer by allowing air to pass through a slot is characteristic of which flap type?

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

Answer: (C)

A slot between the flap and the wing allows air from below the wing to flow to the top surface, energizing the boundary layer. This keeps the airflow attached at higher angles of attack and increases lift. Among the flap types, only the slotted flap incorporates this slot feature, enabling that air passage. Plain flaps have no slot, so there’s no boundary-layer energization through a gap. Split flaps create a gap below the flap but don’t provide a slot to feed air across the top surface. Fowler flaps increase wing area and camber, and while some designs may involve gradual gaps, the defining slot that energizes the boundary layer is the hallmark of a slotted flap.