[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 6-10, FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 1 Page 1-32]
Common types of trim systems include :
- trim tabs (trim off the force to realise “hands off” flying)
- balance tabs (reduce the control force)
- servo tabs (on large aircraft as a backup method to move the control surface)
- antiservo tabs (on trailing edge of stabilator to decrease the sensitivity of surface control)
- ground adjustable tabs (prevent skid during normal flight)
- adjustable stabilizer
Trim tabs are used to relieve the pilot of the need to maintain constant pressure on the flight controls – to realise hands-off flying.
Trim tab is the most common installation on small aircraft. It is attached to the trailing edge of the elevator, and most of them are manually operated by a small, vertically mounted control wheel. However, a trim crank may be found in some aircraft. The flight deck control includes a trim tab position indicator.
Trim tab has opposing directional movement to the elevator. Placing the trim control wheel in the full forward position (nose-down position) moves the trim tab to its full up position. With the trim tab up and into the airstream, the airflow over the horizontal tail surface tends to force the trailing edge of the elevator down. This causes the tail of the aircraft to move up and the nose to move down.
The normal trim procedure is to continue trimming until the aircraft is balanced and the nose-heavy condition is no longer apparent. Pilots normally establish the desired power, pitch attitude, and configuration first, and then trim the aircraft to relieve control pressures that may exist for that flight condition. As power, pitch attitude, or configuration changes, retrimming is necessary to relieve the control pressures for the new flight condition.
Balance tabs are used in some aircraft with excessively high control forces. The balance tab aids the pilot to move the control more easily and hold the control surface in position.
The balance tabs look like trim tabs and are hinged in approximately the same places as trim tabs. The essential difference between the two is that the balancing tab is coupled to the control surface rod so that when the primary control surface is moved in any direction, the balance tab automatically moves in the opposite direction.
In more simply words, balance tab is putting moving control surface and trimming at the same time.
Servo Tabs (flight tab on large aircraft)
A servo tab is a small portion of a flight control surface that deploys in such a way that it helps to move the entire flight control surface in the direction that the pilot wishes it to go. A servo tab is a dynamic device that deploys to decrease the pilots work load and de-stabilize the aircraft.
Servo tabs are sometimes referred to as flight tabs and are used primarily on large aircraft. Large control surfaces usually are deflected out of the neutral position by hydraulic actuators. In the case of hydraulic system failure(s), manual linkage to a servo tab can be used to deflect it. This, in turn, provides an aerodynamic force that moves the primary control surface.
Servo tabs are very similar in operation and appearance to the trim tabs previously discussed – control surface is moving in opposite to Servo tabs.
Antiservo Tabs (at the trailing edge of stabilator)
Antiservo tabs move in the same direction as the trailing edge of the stabilator. In addition to decreasing the sensitivity of the stabilator, an antiservo tab also functions as a trim device to relieve control pressure and maintain the stabilator in the desired position.
When the trailing edge of the stabilator moves up, the linkage forces the trailing edge antiservo tab moving up; When the stabilator moves down, the tab also moves down.
Ground Adjustable Tabs
Many small aircraft have a nonmovable metal trim tab on the rudder. It is used to prevent skids during normal cruising flight.
Be noticed that the correct displacement is determined by trial and error on ground.
Read More –
[Ref Aviation Maintenance Technician Handbook – Airframe Volume 1 Page 2-11, 2-12]
Trim Tab v.s Balance Tab v.s Servo Tab
It is quite difficult to understand the difference between these three tabs. Now, let’s study it a little bit deeper.
Trim tabs can be used to correct any tendency of the aircraft to move toward an undesirable flight attitude. Their purpose is to enable the pilot to trim out (force) in any unbalanced condition, without exerting any pressure on the primary controls – moving of the trim tabs cause the control surface movement.
Balance tabs are designed to move in the opposite direction of the primary flight control. Thus, aerodynamic forces acting on the tab assist in moving the primary control surface – to reduce the force need to move the control.
Servo tabs (flight tabs) are used primarily on the large aircraft with large main control surfaces. They aid in moving the main control surface and holding it in the desired position. Only the servo tab moves in response to movement by the pilot of the primary flight controls – moving the servo tabs cause the control surface moving in opposite direction.