Definition
The aerodynamic force produced by a trim device (such as a trim tab, ground adjustable tab, or anti-servo tab) that holds a flight control surface in a desired position, relieving the pilot of the steady control pressure that would otherwise be required to maintain that position.
Plain English
The push or pull, created by a small adjustable surface, that keeps a control surface where you want it so you don't have to keep holding pressure on the yoke, stick, or rudder pedals.
Context Anchor
Seen in flight control discussions, especially when ground adjustable tabs are used to reduce unwanted control pressure in cruise flight.
Derivation
Trim' comes from the Old English 'trymman,' meaning to set in order or arrange properly. In aviation, to 'trim' an aircraft is to adjust it so it flies steady on its own. The 'force' is simply the aerodynamic push the trim device generates to hold things in place.
Why Pilots Care
Eliminating trim force allows hands-off flight, reducing pilot fatigue and improving stability.
Intuition Check
Trim force does not mean engine power or the strength of the trim tab. It means the push or pull the pilot feels in the flight controls while trying to hold the airplane in the desired condition.
Example Sentence 1
After leveling off in cruise, the pilot adjusted the elevator trim until the trim force held the nose steady without any back-pressure on the yoke.
Example Sentence 2
As airspeed increased in the descent, the pilot noticed rising trim force and reset the tab.