What does trim do in an aircraft?

cherokeeflyer gave great description of how trim , trim tabs work.

larger aircraft, including light twins, turboprops, business jets, military aircraft , airliners use electric motors or hydraulics operate trim function. aircraft hydraulic flight controls, don't have trim tabs @ all. since hydraulic actuator or servo-cylinder doing heavy work @ flight control surface, can trimmed either mechanically or electrically depending on design, moving entire surface enough affect flight path.

here image of elevator trim tab , how works. aileron , rudder tabs work same way, usually, 1 of ailerons , elevators have tab since connected mechanically.

http://www.americanflyers.net/aviationli...

http://www.eaa1000.av.org/pix/albatross/...

airplanes equipped electrically controlled trim have trim switch on control stick grip or 1 of yoke handles. switch trim pitch or roll. separate switch mounted on center console or on panel yaw trim.

http://www.warbirdsite.com/skyhawkgrip3....

yoke pitch trim:

http://www.howardaircraft.com/n5130a-yoke1.jpg

before hydraulics came along aircraft flight controls, large or high performance aircraft difficult fly because there wasn't mechanical advantage deflect huge elevator or aileron. force of airflow on control surfaces higher airspeed increases.

type of trim tab, call servo tab, developed moves in opposite direction of flight control, automatically making easier move big surface. power steering. type of tab can either trimmable or not, depending on manufacturer.

many airplanes have servo tabs on each aileron , elevator, 1 of tabs trimmed (moves separately other tab) use of electric motor.

servo tab:

http://www.airplanetest.com/images/nord/nord10.jpg

edit: failed mention autopilots 'fly' airplane operating trim motors on different flight controls, aircraft equipped type of trim system.

what purpose of trim in aircraft? commercial aircraft.

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