Which flap type has upper and lower surfaces separate, with the lower surface operating like a plain flap, but the upper surface stays immobile or moves only slightly?

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

Answer: (B)

This question is about how separating the upper and lower trailing-edge surfaces changes the way a flap affects the wing. In a split flap, only the lower surface moves downward like a plain flap, while the upper surface stays fixed or moves only slightly. This creates a distinct gap between the two surfaces as the flap deflects.

Because the lower surface is deflected and the upper surface remains largely stationary, airflow is disrupted more and a larger wake is formed. The result is a big increase in drag with only a modest gain (or even a smaller gain) in lift compared to other flap types. The separation between the surfaces makes the flap less efficient at producing lift for a given deflection, but it’s effective when extra drag (for descent or short-field landing) is desirable.

Other flap designs don’t create that separation pattern: plain flaps move both surfaces together, slotted flaps use a gap to energize the flow and boost lift, and Fowler flaps extend backward to increase wing area and camber.