Definition
A plain flap is the simplest type of wing flap, consisting of a hinged section of the wing's trailing edge that pivots downward as a single piece. Lowering it increases the wing's camber (curvature), which raises both lift and drag at a given airspeed, allowing slower flight and steeper descents during approach and landing.
Plain English
The most basic kind of flap. It's just a hinged panel on the back edge of the wing that swings down. Dropping it makes the wing more curved, which lets the airplane fly slower and come down more steeply.
Context Anchor
Seen in slow-speed flight and landing discussions, especially when comparing different flap designs and how they affect lift and drag.
Derivation
"Plain" here means basic or unmodified -- as opposed to split, slotted, or Fowler flaps, which add extra mechanical features. The name simply tells you this is the no-frills version.
Why Pilots Care
They let the aircraft maintain lift at reduced speeds during approach and landing.
Intuition Check
Plain does not mean weak or unimportant here. It means the flap design is simple: one rear wing panel hinges downward.
Example Sentence 1
The Piper Cub uses plain flaps, which hinge straight down from the trailing edge of the wing.
Example Sentence 2
Extending the plain flaps increased both lift and drag as the airplane slowed for landing.