Trim tabs are small, secondary flight control surfaces attached to the trailing edge of a larger, primary control surface like an elevator or rudder. Their function is to relieve the pilot by reducing the control pressures needed to hold the aircraft on its intended flight path. By doing so, the trim tab counteracts the aerodynamic force on the control surface and stabilises the aircraft in the associated axis of rotation. On many small aircraft, ground-adjustable or fixed metal trim tabs are fitted to the rudder, while the horizontal elevators carry variable-position tabs that, once set, remain fixed unless readjusted by an independent control.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are trim tabs on an aircraft?
Trim tabs are small, secondary flight control surfaces that are attached to the trailing edge of a larger, primary control surface. They are used to control the trim of the controls, which helps you reduce or eliminate the need to place pressure on the yoke or rudder to keep your airplane flying straight and level.
Trim tabs are small movable surfaces hinged to the trailing edge of a larger primary control surface like an elevator, rudder or aileron. Because they are secondary flight controls, they do not command the aircraft but instead apply a precise aerodynamic force that holds the parent surface in a chosen deflected position. By creating this force, the trim tab is like a control surface for the control surface, relieving the pilot from continuously pushing or pulling on the yoke, stick or rudder pedals.
A fixed tab, sometimes called a ground-adjustable trim tab, is a small plate bent slightly to one side before flight. Once set, it overcomes unwanted roll or yaw tendencies of the aircraft and is unaffected by speed. In flight, adjustable trim tabs are controlled by wheels, switches or screws in the cockpit, letting the pilot move the tab and thus reposition the larger control surface until control pressures are balanced.
Where are trim tabs located on a plane?
Trim tabs are located at the trailing edge of a control surface. On the rudder, a permanent anti-torque rudder trim tab is mounted at the far rear of the aircraft. Trim tabs can be located on the ailerons. The elevator trim tab is attached to the trailing edge of the elevator and is located almost on the longitudinal axis. Cessna 172 has pitch trim tabs on one side of the elevator only.
Rudder trim tab is a small surface located at the trailing edge of the rudder. An adjustment in trim slightly changes the direction of the entire rudder, thereby pushing the rudder to the right. Applying left rudder trim can correct a left yaw tendency during descents and applying the right rudder can correct a right yaw tendency.
Elevator trim tabs reduce control pressure. Rolling the little black wheel backward makes the aircraft nose go up and rolling it forward makes the aircraft nose go down. Elevator trim is electrically driven or automatically controlled by an autopilot.
What do trim tabs do on a plane?
Trim tabs reduce the pressure required on the control pedals and the yoke, so the pilot no longer has to excessively control the stick or rudder to hold an attitude. By deflecting a small surface hinged to the trailing edge of the main control, the tab causes the local airflow to push the larger surface toward the desired position. Once that position is reached, the tab holds it with minimal input. Elevator trim therefore relieves the amount of control pressure needed to keep the nose up or down, and rudder trim offsets the left-turning tendency created by torque and p-factor in single-engine climbs, keeping the ball centred without a tired foot. A correctly applied tab leaves the pilot free to scan instruments, navigate and communicate, because aircraft trim reduces control pressures felt by the pilot and, in turn, lowers both physical and mental workload.
Trim tabs do not, however, maintain pitch stability by themselves. They only re-position the control surface so the aerodynamic moment balances the existing load. The aircraft's inherent longitudinal stability still depends on the relative position of the wing and tail. Once the tab has been set, any change in speed, weight or configuration alters the trimmed state, so the pilot must re-trim. In effect, the tab is a workload-reducer, not an automatic stabiliser.
What is the direction of trim tab movement?
A trim-tab always moves opposite to the control-surface it serves. Moving the tab up bends the elevator down, and conversely when you apply nose-up trim, the tab itself moves down. This forces the elevator to rise, the tail therefore drops and the nose climbs. Any single motion of the tab drives the flight-control in the opposite direction.
What are the types of trim tabs in aircraft?
The types of trim tabs in aircraft are detailed below.
- Plain Trim Tabs: In a plain trim tab, a trim tab is attached to the trailing edge of an elevator, and is operated by moving a small control wheel in the cockpit. It is one of the most common types of tabs used in small single-engine airplanes. When the tab is moved up or down, it sticks out into the free air stream, and deflects the elevator in the opposite direction.
- Balance Trim Tabs: Balance trim tabs are attached to the control surface linkage, so when the control surface is moved in one direction, the balance tab moves in the opposite direction. By moving the balance tab in the opposite direction, the control load on the yoke is significantly reduced, making the airplane easier to fly.
- Antiservo Trim Tabs: Antiservo trim tabs are similar to balance tabs but they move in the opposite direction. Thus, when the elevator or stabilator moves up, the antiservo tab moves in the same direction. In small aircraft, antiservo tabs increase the control feel, and help prevent over-controlling the aircraft's pitch.
- Ground Adjustable Trim Tabs: Ground adjustable tabs are nonmovable tabs attached to the rudder. These tabs are bent in one direction or the other while on the ground to apply a trim force to the rudder.
What is the difference between a trim tab and a balance tab?
A balance tab looks similar to a trim tab, but it operates differently. While a trim tab is set and then left at a fixed angle to hold the control surface in the desired position, a balance tab is attached to the control-surface linkage and moves every time the pilot moves that surface. When the control surface is moved in one direction, the balance tab moves in the opposite direction. This opposite motion deflects part of the airflow so that the control surface is exposed to a greater force in the pilot's favour, and the heavy hinge-moments are reduced.
What is the difference between a trim tab and a servo tab?
A trim tab is fixed in position relative to the elevator once it is adjusted and cannot move independently. A servo tab is not fixed in position in relation to the elevator and it continues to move whenever the pilot moves the flight controls.
A servo tab is a small tab on a control surface that connects directly to the flight controls. Because of this direct link, only the servo tab moves when the pilot moves the flight controls on the ground. The control surface itself does not move. In flight, the servo tab uses aerodynamic force to drive the control surface, reducing pilot control forces.
How to perform aileron trim tab adjustment?
To perform aileron rim tab adjustment, begin every adjustment by setting the airplane in the desired attitude: get the aircraft in correct yaw, pitch and bank attitude, then use trim to relieve pressures. Next, move the aileron trim wheel in small increments. Each movement of the wheel, crank, or electric switch changes the tab angle, and opposite air deflection pushes the aileron in the desired direction, so stick pressure disappears. Trim tab adjustments are made in small increments, followed immediately by a check flight to determine the result. If the airplane is a Cirrus SR22, the cockpit control does not bend a tab. Instead, the aileron trim moves the entire aileron. On cantilever 177 and 210 series, ground-adjustable tabs on each aileron are set by a pilot or engineer for cruise conditions. Pliers assist in making the bend, and fixed tabs are adjusted in small increments. Whatever the type, consult the POH for guidance on trim usage and keep in mind that single-engine Cessnas were not designed with pilot flight-controllable aileron trim, so any modification must be done on the ground.
Do fighter jets have trim tabs?
High-performance fighters usually do not have trim tabs. Instead, they build the trim function into the moving surface itself. Stabilator trim on these aircraft moves the entire stabilator, so one large surface both pitches the airplane and relieves control pressures. This design keeps the tail surface smooth, free of small projecting tabs that add drag at supersonic speed. Pilots command changes through electric trim controls, identical in feel to the systems in many newer aircraft.
The same arrangement appears on the rudder and, where fitted, on the ailerons. Some aircraft aileron trim does not have trim tabs but the whole panel tilts a few degrees to cancel unwanted roll. Because fighter airframes change balance as fuel burns or stores are released, rapid trim adjustment is vital.
