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The center console of a small airplane. The vertical black wheel with spherical bumps is the trim-tab control. Moving it upwards changes the 'hands-off' elevator position to a more nose-down position; moving it downwards does the reverse.

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 aero-dynamic 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 further away from the axis of rotation of the control surface than the center of pressure of the control surface, the movement generated by the tab can match the movement generated by the control surface. The position of the control surface on its axis will change until the movements from the control surface and the trim surface balance each other.

Contents

Uses in boats

Boats with planing-type hulls will often have trim tabs attached to the trailing edge of the hull or transom. 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 captain 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).

Uses in aircraft

Many airplanes (including gliders) have trim tabs on their elevators, as a simple method of providing trim in the lateral axis.

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

  • a spring attached to the control system that can be adjusted by the pilot
  • 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 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 is rotated forward, the nose is held down; 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. On some of these, the rudder trim tab is rigid but adjustable on the ground by bending: 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. Other aircraft have hinged rudder trim tabs that the pilot can adjust 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 pilots' workload during continuous manoeuvres (eg: 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.

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 parasite 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.

Trim tab as a metaphor

The engineer Buckminster Fuller is often cited for his use of trim tabs as a metaphor for leadership and personal empowerment. In the February 1972 issue of Playboy, Fuller said:

Gravestone of Buckminster Fuller with the quote "call me trimtab"
Grave of Buckminster Fuller

Something hit me very hard once, thinking about what one little man could do. Think of the Queen Mary -- the whole ship goes by and then comes the rudder. And there's a tiny thing at the edge of the rudder called a trim tab.

It's a miniature rudder. Just moving the little trim tab builds a low pressure that pulls the rudder around. Takes almost no effort at all. So I said that the little individual can be a trim tab. Society thinks it's going right by you, that it's left you altogether. But if you're doing dynamic things mentally, the fact is that you can just put your foot out like that and the whole big ship of state is going to go.

So I said, call me Trim Tab.

The official newsletter of the Buckminster Fuller Institute is called "Trimtab".[2]

References

  1. ^ The Anatomy Of The Aeroplane Darrol Stinton, ISBN 0-632-01876-3
  2. ^ member newsletter
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