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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.
Uses in boats
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.
Uses in aircraft
All aircraft must have a system for ensuring trim in the pitch axis, though methods other than trim tabs may be used. Alternatives include:
- a spring included in the control system that can be adjusted by the pilot
- in the case of the elevator, an all-moving horizontal stabilizer, called a stabilator, the position of which can be adjusted in flight by a servo tab or an anti-servo tab.
- On some aircraft (e.g. Concorde, McDonnell Douglas MD-11), fuel may be shifted to tanks in the tail during cruise to reposition the center of gravity in order to reduce trim drag. Maintaining the cg near the most aft limit for cruise improves cruise efficiency.
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.
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 (when viewed from behind) to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of many prop-driven aircraft. Some aircraft have hinged rudder trim tabs that the pilot can adjust in flight.
When a servo 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. 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 maneuvers (e.g. 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.
Both elevator trim and pitch trim affect the small trimming part of the elevator on jet airliners. The former is supposed to be set in a certain position for a longer time, while the pitch trim (controlled with the landing pilot's thumb on the yoke or joystick, and is thereby easy to maneuver) is used all the time after the flying pilot has disabled the autopilot, especially after each time the flaps are lowered or at every change in the airspeed, at the descent, approach and final. Elevator trim is most used for controlling the attitude at cruising by the autopilot.
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 parasitic 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.
On military aircraft during wartime, trim tabs often served as unintentional backup control systems for aircraft with damaged controls. Since trim tabs are usually controlled by their own dedicated system of control cables, rods, and/or hydraulic lines, aircraft that had suffered loss of primary controls could often be flown home "on the trim tabs", or by using trim adjustment as a replacement for the non-working primary controls. Such control is effective, if slower and more limited than primary controls, but it does allow the aircraft to be controlled and directed. In other cases, such as an engine being shot out, or damage causing asymmetric drag, trim tabs were invaluable for allowing the pilot to fly the aircraft straight without having to apply a constant force on the stick or rudder to keep the aircraft flying straight. Trim tabs were also important for aircraft such as bombers, who often underwent rapid changes in center-of-gravity when the bombload was dropped, requiring a hand ready on the trim-adjusting wheel to counteract the tendency of the aircraft to pitch up or down. Undertaking high-speed dives or deploying flaps also generally necessitated pitch trim adjustment, as aircraft of the era had different pitch tendencies at different airspeeds, and flaps could change the center of lift. Even burning fuel off could require periodic trim adjustment during a long flight, as it was difficult to ensure that all fuel tanks were equally near the center of gravity (careful selection of various fuel tanks to ensure even consumption from all tanks helped negate this as well). An extreme example would be later P-51 Mustangs, which were given a large long-range fuel tank behind the cockpit to allow long range missions to Berlin. Since the aircraft was not designed to carry this weight so far back in the fuselage, the trim had to be adjusted far forward during the first leg of the flight, and slowly adjusted back as the fuel was used, to avoid causing excess drag, and thus excessive fuel consumption, due to a nose-high attitude on the many-hour flight to Berlin.
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:
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.— Buckminster Fuller
- Applied Aerodynamics at the Douglas Aircraft Company-A Historical Perspective, Schaufele Roger D., AIAA 99-0118, 37th AIAA Aerospace Sciences Meeting and Exhibit
- Evolution Of The Airliner, Whitford Ray, The Crowood Press Ltd. Marlborough, ISBN 978 1 861268709, p.62
- The Anatomy Of The Aeroplane Darrol Stinton, ISBN 0-632-01876-3
- Trimtab, the monthly digital news service of the Buckminster Fuller Institute