Definition
The position of a flight control surface (such as the rudder or elevator) when it is aligned with the surrounding airflow, producing minimum aerodynamic drag and exerting no control force on the airplane.
Plain English
The control surface is sitting flush with the air flowing past it, so the air slides over it smoothly and the surface isn't pushing the airplane in any direction.
Context Anchor
Seen in discussions of how ailerons, elevators, and rudders create movement when they are moved away from their neutral position.
Derivation
From 'streamline' — the smooth, unbroken path that air or water follows around an object. A surface in its 'streamlined position' lies along that natural flow line, so the air isn't disturbed.
Why Pilots Care
Keeping surfaces in the streamlined position prevents unnecessary drag and keeps control inputs precise and efficient.
Analogy
Like holding your flat hand out a car window edge-on to the wind: the air slips past with almost no push. Tilt the hand and it suddenly grabs the air — that's no longer streamlined.
Intuition Check
Streamlined does not just mean sleek or efficient here. It means lined up in the neutral position, with the control surface not deflected to command a change.
Example Sentence 1
With the rudder in the streamlined position, the airplane tracks straight and the pilot feels no pressure on the pedals.
Example Sentence 2
In steady cruise, all primary flight controls should remain in the streamlined position unless small corrections are needed.