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Which of the following aerodynamic balancing device enables big control surface change/movement with relatively small force needed to apply?
  • A
    Anti-balance Tab.
  • B
    Balance Tab.
  • C
    Horn Balance.
  • D
    Servo Tab.

Refer to figure.
The force required to operate the primary control surfaces, even using balance tabs, is often too great to be acceptable. To relieve the pilot of the need to apply so much force to operate the controls servo-tabs are fitted to carry out this function.
The servo-tab is a small tab hinged on the trailing edge of the primary control surface and linked directly to the control column. There is no direct link between the control column and the primary control surface.
The function and operation of the servo-tab is the same as the balance tab and moves the trim tab in the opposite direction to the movement of the primary control surface. However, at low IAS the servo-tab makes the main control surface less effective. The position of a servo-controlled primary control surface cannot be determined until a positive pressure is applied to the servo-tab by the airflow.

Anti-balance Tab: used to increase stick force when they're too light.
Balance Tab: moves when the control surface is moved.

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