Definition
A simple hinged section on the trailing edge of the wing that pivots downward to increase the wing's camber, producing more lift (and more drag) at a given airspeed. It is the most basic type of wing flap.
Plain English
The simplest kind of flap. It's a hinged panel at the back of the wing that swings down to help the airplane fly slower without stalling, mainly during takeoff and landing.
Context Anchor
Seen in discussions of flap effectiveness, slow flight, takeoffs, and landings.
Derivation
"Plain" here means "simple" or "basic" -- this is the original, no-frills flap design, in contrast to more complex types like split, slotted, or Fowler flaps.
Why Pilots Care
Plain flaps give a useful lift increase for landing but also add substantial drag, limiting how far the airplane can glide or climb with them extended.
Analogy
A plain flap is like a simple hinged door on the back of the wing. When it swings down, it changes the wing’s shape and how the air moves around it.
Intuition Check
Plain does not mean unimportant here. It means this is the simplest flap design: one hinged panel that moves downward.
Example Sentence 1
The trainer's plain flaps deflect to 40 degrees, allowing a slower approach speed and a steeper glide path.
Example Sentence 2
Because the airplane had plain flaps, the approach was flown at a higher power setting to maintain airspeed.