Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft. The angle of the tab relative to the larger surface can be adjusted to null out hydro- or aero-dynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the pilot to constantly apply a control force.
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 during flight (in the case of aircraft), a pilot-adjustible trim the trim tab allows the pilot to reduce (to zero, when used correctly) the manual force required to maintain such position.
The trim tab acts as a servo tab. Because the center of pressure of the trim tab is further 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 moments from the control surface and the trim surface balance each other.
Uses in aircraft
Many airplanes (including gliders) have trim tabs on their elevators, this being a simple method of providing trim in the lateral axis.
All aircraft must have a system for ensuring trim in the lateral axis, sometimes using other methods. Alternatives to trim tabs include:
- a spring that can be adjusted by the pilot and that is attached to the control system
- in the case of the elevator, an all-moving horizontal stabilizer whose position can be adjusted in flight by the pilot.
Elevator trim frees the pilot from constantly adjusting the pitch controls. A longitudinal trim control (often in the shape of a wheel) is adjusted by the pilot to cancel out control forces for a given airspeed or weight distribution. When the trim control is rotated forward the nose is held down and conversely if the trim wheel 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. Some aircraft have a rudder trim tab that is rigid but adjustable by bending on the ground. It is angled slightly to the left to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of some prop-driven aircraft. Many aircraft also have control wheels inside the cockpit so the pilot can adjust a hinged rudder trim while in flight.
When a trim 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.[1] 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 the workload on pilots, since their attention can be focused on tasks other than control input during continuous manoeuvres (ie: sustained climb on takeoff to high altitude, descent prior to landing), such as traffic avoidance, or communication with air traffic control.
Not only does keeping an aircraft trimmed properly reduce pilot workload, it also increases fuel efficiency by reducing drag. For example, if an aircraft is climbing it may have a tendency to yaw which increases parasite drag because the craft is not flying straight into the flight path. The yaw may be able to be reduced by use of the rudder trim tab.
References
- ^ The Anatomy Of The Aeroplane Darrol Stinton, ISBN 0-632-01876-3
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