Aircraft elevator trim is an adjustable surface that applies a force on a control surface to keep it in position and requires less manual input from the pilot to maintain a desired flight attitude. The elevator trim is usually a small separate part of the elevator - called a trim tab - that helps the pilot move the elevator to the required position and keep it there, thereby reducing the amount of control pressure needed to maintain the correct aerodynamic force on the tail to balance the aircraft. By adjusting the angle of the horizontal stabilizer, the elevator trim changes the aircraft's pitch.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is elevator trim in aircraft?
Elevator trim is a type of aircraft trim that reduces the amount of control pressure needed to keep the elevator in a trimmed position. It is located on the trailing edge of the elevator and consists of a small, adjustable secondary flight control surface called an elevator trim tab. This tab sticks out into the free air stream, deflects in the opposite direction to the elevator, and positions the elevator so that the aircraft remains at the desired pitch attitude without constant pilot input. Proper technique for trimming the elevator is to first establish the desired pitch attitude with elevator control and then adjust the elevator trim to relieve the control pressure. This technique is particularly important to have the elevator trimmed properly for the correct speed on final approach. Elevator trim is used during climbs, cruise, descents, and landings, and is the most common type of trim system installed on aircraft.
By balancing the aerodynamic force on the tail, the elevator trim balances the elevator down spring and the torques produced by the wing and tail. When there is no rotation about the center of gravity, the aircraft is said to be trimmed, meaning the torques from the wing and tail lift are equal. This condition relieves the pilot of the need to maintain steady back or forward pressure on the yoke, thereby reducing pilot workload and fatigue.
How does elevator trim work on a plane?
Elevator trim works by redirecting airflow over a secondary tab attached to the elevator. The tab deflects in the opposite sense to the desired pitch change and the redirected airflow then forces the entire elevator surface into the new position. Trim reduces the effort required to keep the movable control surface in any given position at a certain airspeed, so the pilot does not have to hold continuous pressure on the yoke.
What is the function of elevator trim on an aircraft? The function of elevator trim is to maintain the desired pitch attitude and to relieve control pressure so the aircraft will stay on the intended flight path without continuous pilot input. This reduces pilot fatigue and prevents distraction from other duties. Correctly applied trim also reduces drag, improves fuel efficiency and lessens wear on control-surface linkage and bearings.
The purpose of the trim system is to set the neutral position of the elevator so the aircraft will hold the selected pitch attitude hands-off. Once the pilot first establishes the desired pitch attitude with the elevator control, he or she adjusts the trim wheel, lever or electric switch so that the tab or, in stabilator aircraft, the entire stabilator, moves to cancel the stick force. Thereafter the airflow itself keeps the elevator in that deflected position.
How does elevator trim work on a plane-pitch? To command nose-up pitch the trim tab deflects downward, the airflow striking the downward-tab pushes the elevator upward and the nose rises. Conversely, nose-down trim deflects the tab upward, airflow pushes the elevator downward and the nose lowers. Whether the airplane uses a separate tab or a one-piece stabilator, the principle is identical: the trim re-sets the neutral point of the horizontal surface so the aircraft maintains the chosen pitch attitude with no pilot effort.
