Uses in Aircraft
Many airplanes (including gliders) have trim tabs on their elevators, as a simple method of providing trim in the pitch axis.
All aircraft must have a system for ensuring trim in the pitch axis, though methods other than trim tabs may be used. Alternatives include:
- a spring included in the control system that can be adjusted by the pilot
- in the case of the elevator, an all-moving horizontal stabilizer whose position can be adjusted in fligh by a servo tab or an anti-servo tab.
- On some aircraft, some fuel weight may be shifted between tanks during the flight to reduce aerodynamic lift induced drag
Elevator trim frees the pilot from exerting constant pressure on the pitch controls. Instead, the pilot adjusts a longitudinal trim control (often in the form of a wheel) to cancel out control forces for a given airspeed / weight distribution. Typically, when this trim control (wheel or lever) is rotated or moved forward, the nose is held down; conversely, if the trim control is moved back, the tail becomes "heavy." Many newer aircraft, especially jet aircraft, have electric trim controls.
Many airplanes also have rudder and/or aileron trim systems. On some of these, the rudder trim tab is rigid but adjustable on the ground by bending: it is angled slightly to the left (when viewed from behind) to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of many prop-driven aircraft. Some aircraft have hinged rudder trim tabs that the pilot can adjust in flight.
When a servo tab is employed, it is moved into the slipstream opposite to the control surface's desired deflection. For example, in order to trim an elevator to hold the nose down, the elevator's trim tab will actually rise up into the slipstream. The increased pressure on top of the trim tab surface caused by raising it will then deflect the entire elevator slab down slightly, causing the tail to rise and the aircraft's nose to move down. In the case of an aircraft where deployment of high-lift devices (flaps) would significantly alter the longitudinal trim, a supplementary trim tab is arranged to simultaneously deploy with the flaps so that pitch attitude is not markedly changed.
The use of trim tabs significantly reduces pilots' workload during continuous maneuvers (e.g.: sustained climb to altitude after takeoff or descent prior to landing), allowing them to focus their attention on other tasks such as traffic avoidance or communication with air traffic control.
Both elevator trim and pitch trim affect the small trimming part of the elevator on jet airliners. The former is supposed to be set in a certain position for a longer time, while the pitch trim (controlled with the landing pilot's thumb on the yoke or joystick, and is thereby easy to maneuver) is used all the time after the flying pilot has disabled the autopilot, especially after each time the flaps are lowered or at every change in the airspeed, at the descent, approach and final. Elevator trim is most used for controlling the attitude at cruising by the autopilot.
Beyond reducing pilot workload, proper trim also increases fuel efficiency by reducing drag. For example, propeller aircraft have a tendency to yaw when operating at high power, for instance when climbing : this increases parasite drag because the craft is not flying straight into the apparent wind. In such circumstances, the use of an adjustable rudder trim tab can reduce yaw.
Read more about this topic: Trim Tab