Trim tab

Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by adjusting the angle of the tab relative to the larger surface.

Changing the setting of a trim tab adjusts the neutral or resting position of a control surface (such as an elevator or rudder). As the desired position of a control surface changes (corresponding mainly to different speeds), an adjustable trim tab will allow the operator to reduce the manual force required to maintain that position—to zero, if used correctly. Thus the trim tab acts as a servo tab. Because the center of pressure of the trim tab is farther away from the axis of rotation of the control surface than the center of pressure of the control surface, the moment generated by the tab can match the moment generated by the control surface. The position of the control surface on its axis will change until the torques from the control surface and the trim surface balance each other.

Boats with outboard engines will often have trim tabs attached to the lower unit of the engine. These are used to adjust the pitch attitude of the boat while underway. Changes in boat speed or weight placement will usually require the trim tabs to be adjusted to keep the boat at a comfortable and efficient pitch attitude. This reduces the work of the engine by reducing the amount of manual control necessary, as well as providing for greater efficiency by keeping the ship in the ideal orientation for the conditions.

Trim tabs are most found on cruisers, sport fishing boats and center console boats ranging from 20 feet and up.

In some sailboats, the trailing edge of the keel has a trim tab which is used to null out rudder forces (lee or weather helm).

All aircraft must have a system for ensuring trim in the pitch axis, though methods other than trim tabs may be used. Alternatives include:

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.

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Wikipedia