Definition
A movable, vertical control surface hinged to the trailing edge of the vertical stabilizer (fin) at the tail of an aircraft. The rudder is controlled by the pilot's foot pedals and is used to rotate the aircraft about its vertical (yaw) axis, swinging the nose left or right.
Plain English
The hinged flap on the back of the tail fin that the pilot moves with the foot pedals to swing the nose of the aircraft left or right.
Context Anchor
Pilots use the rudder through the foot pedals during takeoff, landing, turns, and any time the airplane needs left-right nose control.
Derivation
From Old English 'rother,' meaning a steering oar — the paddle once used at the stern of a boat to steer it. Aviation borrowed the word directly from boating, since the function is the same: a flat surface at the tail that swings to point the vehicle left or right.
Why Pilots Care
Proper rudder use prevents slips and skids, maintains runway alignment in crosswinds, and counters adverse yaw or asymmetric thrust.
Analogy
Works like the rudder on a boat, swinging the nose left or right through the air.
Intuition Check
Do not think of the rudder as the airplane’s main turning control like a steering wheel in a car. In normal flight, it controls left-right nose movement and is used with the other controls to keep the airplane properly aligned.
Example Sentence 1
On the takeoff roll, the student applied right rudder to counter the airplane's tendency to yaw to the left.
Example Sentence 2
Applying rudder opposite the aileron input helped coordinate the turn and eliminate the slip.