Trim tabs are variously integrated into the
rudder,
elevators, and
ailerons of a fixed-wing aircraft. As such, they are elements of an aircraft's system for allowing the pilot to control and maintain
airspeed with a minimum of inputs and mental concentration. Many newer aircraft, especially jet aircraft, have electric trim controls. Elevator trim frees the pilot from exerting constant force on the pitch controls, by adjusting trim control (often in the form of a vertical wheel) to cancel out control forces for a given airspeed and weight distribution. Typically, when this wheel is rotated up (or lever raised) the aircraft's nose pitches down; rotating it down (or depressing the lever) lowers the tail and raises the nose. Many airplanes also have rudder or aileron trim systems. On some, the rudder trim tab is hinged and adjustable during flight; on others it is only adjustable on the ground (to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of many prop-driven aircraft). Most fixed-wing aircraft have a trim tab on the elevator. However, alternative means of controlling the speed and attitude of the aircraft are sometimes used, including: • 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, called a
stabilator, the position of which can be adjusted in flight by a
servo tab or an anti-servo tab. • On some aircraft (e.g.
Concorde,
McDonnell Douglas MD-11), fuel may be shifted to tanks in the tail during cruise to reposition the center of gravity in order to reduce trim drag. Maintaining the center of gravity near the aft-most limit for cruise improves cruise efficiency. 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 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
parasitic 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.
Military On military aircraft during wartime, trim tabs often served as unintentional backup control systems for aircraft with damaged controls. Since trim tabs are usually controlled by their own dedicated system of control cables, rods, and/or hydraulic lines, aircraft that had suffered loss of primary controls could often be flown home "on the trim tabs", or by using trim adjustment as a replacement for the non-working primary controls. Such control is effective, if slower and more limited than primary controls, but it does allow the aircraft to be controlled and directed. In other cases, such as engine failure or damage causing asymmetric drag, trim tabs were invaluable for allowing the pilot to fly the aircraft straight without having to apply a constant force on the stick or rudder to keep the aircraft flying straight. Trim tabs were also important for aircraft such as bombers, which often underwent rapid changes in center-of-gravity when the bombload was dropped, requiring a hand ready on the trim-adjusting wheel to counteract the tendency of the aircraft to pitch up or down. Undertaking high-speed dives or deploying flaps also generally necessitated pitch trim adjustment, as aircraft of the era had different pitch tendencies at different airspeeds, and flaps could change the
center of pressure. Consumption of fuel could require periodic trim adjustment during a long flight, as it was difficult to ensure that all fuel tanks were equally near the center of gravity. An extreme example was the later
P-51 Mustang, which was given a large fuel tank behind the cockpit to allow long-range missions; as fuel from this tank was consumed it was necessary for regular adjustment of the elevator trim. == As a metaphor ==