A flight control system is used in order to steer an aircraft during flying. Cockpit controls, hydraulically or electrically operated actuators, computers and sensors all of these equipment when put together makes the complete aircraft control system. Aircraft control surfaces are flaps that deflects the air during the flight of an aircraft. A flying aircraft can make three movements freely and they are Pitch, roll and yaw.
Three axis of rotation
1. Lateral axis
Imaginary line drawn crosswise from one wing tip to other wing tip is lateral axis. Nose up and down movement of aircraft around lateral axisis the pitch.
2. Longitudinal axis
Imaginary line drawn through the center line of fuselage from nose to tail of the aircraft is the longitudinal axis. Rotational movement of aircraft around longitudinal axis is the roll.
3. Vertical axis
Imaginary line passes vertically through the center of gravity of the aircraft is the vertical axis. Side ways movement of aircraft around vertical axis is the yawing.
Primary control surfaces
Movement of aircraft is controlled around three of it's axis by three control surfaces and those control surfaces are known as primary flight controls of an aircraft.
1. Elevator controls the pitch
Elevator is mounted on trailing edge of horizontal stabilizer on each side of the tail plane. They move together up or down when pilot moves the control column (control stick) forward or backward. To climb an aircraft pilot pulls the stick backward making the elevator to raise up. Raised elevator decreases the lift and increases the drag to the air flow above the tail plane causing aircraft tail to be pushed down and nose to go up. When aircraft nose is moved up for a climbing attitude the wing airfoil meets the relative air flow with an increased angle of attack and in turn producing increased lift for aircraft to climb. On the other hand to climb down pilot pushes the stick forward there by elevators moves down causing aircraft tail to go up and in turn aircraft nose goes down. This sea-saw like movement between nose and tail of aircraft around its lateral axis is known as pitching. The elevator therefore is used to pitch the aircraft up or down causing it to climb up or climb down.
2. Aileron controls the roll
Aileron is located at the out ward of each wing trailing edges. They deflects opposite to each other when right side aileron moves up left side aileron moves down. When aileron deflects downward the lift on that side of the wing increases and when aileron deflects upward the lift on the opposite side decreases. To roll (bang) the aircraft to the right pilot moves the control stick to the right. The right Aileron deflects upward which decreases lift on the right side making the right wing to fly low. Opposite happens on the left side where the aileron deflects downward to increase the lift causing left wing to fly high in turn right bang becomes possible to the aircraft. Similarly when the stick is moved to the left, the left wing fly down and right wing flies up. This rotational movement of aircraft on its longitudinal axis is known as the roll or the bang of the aircraft. Ailerons are therefore used to roll or rotate the aircraft left or right.
3. Rudder controls the yaw
Rudder is a single control flap mounted to the trailing edge of the vertical stabilizer fin. Rudder swings left or right in accordance with the right or left pedals stepped by the pilot. When pilot wants to move the nose of the aircraft to the right, the right pedal is stepped and rudder in turn swings to the right. Air flowing on the vertical stabilizer now pushes the aircraft tail to the left in turn aircraft nose is moved to the right. Similarly when left pedal is stepped aircraft nose moves to the left. This movement achieved on the vertical axis of the aircraft is known as yawing.
Secondary flight controls
In addition to the primary flight controls most airplanes uses another set of controls known as secondary controls. Flaps, slats, spoilers, trim tabs and speed breakers all mounted on aircraft wings are said to be the secondary flight controlsof the aircraft.
The devices used for producing high lift are the flaps and slats.Flaps are air foiled profiles installed on to the inboard of each wing trailing edge. They are hydraulically operated to extend or retract. The camber of wing airfoil increases when flaps are lowered around 12 deg causing to produce high lift during take off. Flaps also produces high drag and high lift when they are lowered 30 deg down. This in turn helps the aircraft to land smoothly as well as to reduce landing distance.There are different types of flaps like Slotted flap, Fowler flap, Plain flap, Split flap etc. Fowler type flaps are commonly used on large airliners.
Slats are installed at the leading edge of the wing. Lowering of flap towards the rear and extending slats towards the front increases wing total area and also changes the overall shape of the wing to more curved and cambered. This situation generates high lift during aircraft take off. The slats may be fixed or retractable, but flaps are embedded in the wing trailing edge allowing for a minimal amount of drag while the aircraft is in normal cruise flight.
You will also watch during flight small metal panels comes up occasionally from the wing surface and fall back to its hidden lying down positions. These are spoilers, they indeed slow down the speed of the aircraft and correct the lift during flight by creating additional drag on to the wing. During landing they are fully extended up to keep the wing pushed downward against the high lift generated by fully extended flaps.
4. Trim tabs
Is a small adjustable hinged surface on trailing edge of ailerons, elevator and rudder. Some times they are found on rudder or elevators only. Trim tab is moved in direction opposite of the primary control on which they are mounted. Minor attitude corrections of aircraft during flight can be achieved by using this trimming devices instead of altering the settings of the primary controls. Trim tabs are very useful to the pilot engaged in long flights and they are considered labour saving controls.
5. Speed breakers
Speed breakers or air brakes are simple control surfaces used to increase drag during aircraft landing. They are mostly mounted above wing surface or at the sides of the fuselage. Hydraulic actuators shoots them out from its hiding places in to air stream to act like an air brake.
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