Just the mere fact that an aircraft can get off the ground and stay in the air is an engineering miracle that we often take for granted – albeit, perhaps less so in the past year. While the fixed parts of the fuselage, wings, and stabilizers are essential, the real finesse in maneuvering a jetliner comes from the dynamic parts attached to them – the flight control surfaces. Let’s take a look at what they are and how they work.
Primary and secondary surfaces
The control surfaces are all the dynamic parts on an aircraft that can be manipulated to steer the plane during flight. They are divided into primary and secondary control surfaces. The primary ones on a fixed-wing aircraft include the ailerons, elevators, and the rudder. These are responsible for directing the aircraft.
An aircraft in flight can rotate in three dimensions – horizontal or yaw, vertical or pitch, and longitudinal or roll. The primary control surfaces produce torque, which varies the distribution of aerodynamic force around the airplane.
Secondary control surfaces include spoilers, flaps, slats, and air brakes. These modify the plane’s overall aerodynamics by increasing or reducing the lift or drag that the wings generate.
All surfaces act together to balance the aerodynamic forces that impact an aircraft and to move the plane in different axes in relation to its center of gravity.
The elevators
The elevators raise and lower the aircraft, moving the plane in its transverse axis, producing pitch. Most aircraft have two elevators. They are placed on the trailing edge on each half of the fixed horizontal stabilizer.
Manual or autopilot input moves the elevators up or down as needed by a forward or aft movement of the control column or control stick. If it is moved forward, the elevator deflects downward, which generates an increase in lift for the tail surface. This, in turn, causes the nose of the plane to pivot along the vertical axis and turn downwards. The opposite is true when the control panel is pulled back.
The rudder
The rudder moves the aircraft in its horizontal axis, producing yaw. It sits on the vertical stabilizer or tail fin. It is not used to directly steer the aircraft, as its name might have one believe. Rather it is used to counteract adverse yaw produced by turning the aircraft or to counteract an engine failure on quad jets.
It is also used in order to ‘slip’ and direct the trajectory of the plane before landing during a heavy crosswind approach. The rudder is usually controlled by the left and right rudder pedals in the cockpit.
The ailerons
The ailerons, which is French for ‘little wings,’ are used to tilt the plane from one side to the other, moving it along its longitudinal axis, producing roll. They are attached to the outward edges of the aircraft wings and move in opposite directions from one another to adjust the position of the plane.
When the flight deck control device is moved or turned, one aileron deflects up and the other one downward. This causes one wing to generate more lift than the other, which makes the plane roll and facilitates a curve in the flight path, or what is known as a ‘banked turn.’ The aircraft will continue to turn until an opposite motion returns the plane to straight along the longitudinal axis.
Flaps
The flaps resemble the ailerons, but they sit closer to the fuselage. They change the shape of the aircraft’s wing and are utilized to both generate more lift and increase drag, depending on their angle. Their setting is usually between five and fifteen degrees, depending on the aircraft.
Trailing edge flaps extend and move downwards on the back of the wing. Leading-edge flaps move out and forward on the front of the wing. However, the leading-edge flaps and slats are not individually controlled but respond to the movement of the trailing edge flaps.
Slats and slots
Leading-edge slats extend out from the surface of the front of the wing using hydraulic pressure. Altogether, they can change the shape and size of the wing quite significantly. This lets pilots adapt the amount of drag and lift needed for takeoff and landing procedures.
Slots are openings between the different segments of the flaps. They are aerodynamic features that allow air to flow from beneath the wing to its upper surface. The bigger the surface of the trailing-edge flaps deployed, the more slots are needed.
Spoilers and air brakes
Spoilers and air brakes are used to reduce the lift and slow down the aircraft. They are used on approach and after landing. Spoilers are small panels hinged on the upper surface of the wing and decrease lift by disrupting the airflow.
While spoilers may act as brakes, proper air brakes extend out from the surface into the airstream in order to slow the aircraft down. Most often, they are deployed symmetrically on each side.
Hydraulic circuit
Jet aircraft rely on hydraulics to manipulate the control surfaces. A mechanical circuit links the cockpit control with the hydraulic circuit controlling the dynamic surfaces of the plane. This has hydraulic pumps, reservoirs, filters, pipes, valves, and actuators. This system means that the way an aircraft responds is determined by economics rather than by a pilot’s physical strength.
