For an aircraft to reach its destination, the forces of flight have to be precisely manipulated. To do this, aircraft have control surfaces that can direct airflow in very specific ways.
Flight control surfaces are hinged (movable) airfoils designed to change the attitude of the aircraft during flight. These surfaces are divided into three groups – primary, secondary, and auxiliary.
There are three axes on which an aircraft can turn: the lateral axis, the longitudinal axis, and the vertical axis.
Primary Flight Control Surfaces
The movement of any primary flight control surface causes the aircraft to rotate around the axis of rotation associated with the control surface.
| Primary Control Surface | Aircraft Movement | Axis of Rotation | Type of Stability |
|---|---|---|---|
| Aileron | Roll | Longitudinal | Lateral |
| Elevator/Stabilator | Pitch | Lateral | Longitudinal |
| Rudder | Yaw | Vertical | Directional |
Secondary/Auxiliary Flight Control Surfaces
An aircraft may possess none, one, or a combination of these control surfaces.
| Name | Location | Function |
|---|---|---|
| Flaps | Inboard trailing edge of wings | Extends the camber of the wing for greater lift and slower flight. Allows control at low speeds for short field takeoffs and landings. |
| Leading-edge flap | Inboard leading edge of the wing | Extends the camber of the wing for greater lift and slower flight. Allows control at low speeds for short field takeoffs and landings. |
| Slats | Mid to outboard leading edge of the wing | Extends the camber of the wing for greater lift and slower flight. Allows control at low speeds for short field takeoffs and landings. |
| Slots | The outer leading edge of wing forward of ailerons | Directs air over the upper surface of the wing during a high angle of attack. Lowers stall speed and provides control during slow flight. |
| Spoilers | Upper and/or trailing edge of the wing | Decreases (spoils) lift. Can augment the aileron function. |
| Trim tabs | The trailing edge of primary flight control surfaces | Reduces the force needed to move a primary control surface. |
| Balance tabs | The trailing edge of primary flight control surfaces | Reduces the force needed to move a primary control surface. |
| Anti-balance tabs | The trailing edge of primary flight control surfaces | Increases feel and effectiveness of primary control surface. |
| Servo tabs | The trailing edge of primary flight control surfaces | Assists or provides the force for moving a primary flight control. |
Flight Control Tabs
| Type | Direction of motion in relation to control surface | Activation | Effect |
|---|---|---|---|
| Trim tab | Opposite direction | Set by the pilot from the cockpit. Uses independent linkage. | Statically balances the aircraft in flight. Allows “hands-off” maintenance of flight conditions. |
| Balance tab | Opposite direction | Moves when the pilot moves control surface. Coupled to control surface linkage. | Aids pilot in overcoming the force needed to move the control surface. |
| Servo tab | Opposite direction | Directly linked to flight control input device. Can be primary or backup means of control. | Aerodynamically positions control surfaces that require too much force to move manually. |
| Anti-balance or Anti-servo tab | Same direction | Directly linked to flight control input device. | Increases force needed by the pilot to change flight control position. De-sensitizes flight controls. |
| Spring tab | Opposite direction | Located in line of direct linkage to servo tab. Spring assists when control forces become too high in high-speed flight. | Enables moving control surface when forces are high. Inactive during slow flight. |
