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AP4ATCO - Aircraft Controls

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References

Objectives

Gain an understanding of:

  • Types and use of aircraft controls

Flight Controls

Aircraft flight controls are the means by which a pilot controls the direction and attitude of an aircraft in flight.

Flight control systems are subdivided into what are referred to as primary and secondary flight controls. Primary flight controls are required to safely control an aircraft during flight and consist of ailerons, elevators (or, in some installations, stabilator) and rudder. Secondary flight controls are intended to improve the aircraft performance characteristics or to relieve excessive control loading, and consist of high lift devices such as slats and flaps as well as flight spoilers and trim systems.

Flight Control Surfaces
B727 Flight Control Surfaces. Source: Wikicommons. Origin: FAA(USA)

Movement of any of the primary flight controls causes the aircraft to rotate around the axis of rotation associated with the control surface. The ailerons control motion around the longitudinal axis (roll), the elevator controls rotation around the lateral axis (pitch) and the rudder controls movement around the vertical axis (yaw).

The most basic flight control systems are mechanical and, although they date back to the earliest aircraft types, are in use in the majority of light, general aviation aircraft. In this design, a collection of mechanical components such as cables, pulleys, rods and chains transmit the movement of the flight deck controls to the appropriate control surface(s). In larger and faster aircraft, the aerodynamic forces become too great for the pilot to overcome without assistance so hydraulic systems are often incorporated to move the flight control surface. In some newer aircraft models, the quest for reduced weight and the associated fuel savings has led designers to replace most of the mechanical components with computers and fiber optics to produce control systems which are referred to as Fly-By-Wire.

Ailerons

Ailerons control movement about the longitudinal axis of an aircraft. This movement is referred to as "roll". The ailerons are attached to the outboard trailing edge of each wing and, when a manual or autopilot control input is made, move in opposite directions from one another. In some large aircraft, two ailerons are mounted on each wing. In this configuration, both ailerons on each wing are active during slow speed flight. However, at higher speed, the outboard aileron is locked and only the inboard or high speed aileron is functional.

Moving the flight deck control wheel or control stick to the right results in the aileron mounted on the right wing to deflect upward while, at the same time, the aileron on the left wing deflects downward. The upward deflection of the right aileron reduces the camber of the wing resulting in decreased lift on the right wing. Conversely, the downward deflection of the left aileron results in an increase in camber and a corresponding increase in lift on the left wing. The differential lift between the wings results in the aircraft rolling to the right. On some aircraft, ailerons are augmented by roll spoilers mounted on the upper surface of the wing.

Elevators

An elevator controls movement about the lateral axis of an aircraft. This movement is referred to as "pitch". Most aircraft have two elevators, one of which is mounted on the trailing edge of each half of the horizontal stabilizer. When a manual or autopilot control input is made, the elevators move up or down as appropriate. In most installations, the elevators move symmetrically but, in some fly-by-wire controlled aircraft, they move differentially when required to meet the control input demands. Some aircraft types have provisions to "disconnect" the right and left elevators from one another in the event of a control surface jam while other types use different hydraulic systems to power the left and right elevator to ensure at least one surface is operational in the event of hydraulic system failure(s).

The elevators respond to a forward or aft movement of the control column or control stick. When the pilot moves the controls forward, the elevator surface is deflected downwards. This increases the camber of the horizontal stabilizer resulting in an increase in lift. The additional lift on the tail surface causes rotation around the lateral axis of the aircraft and results in a nose down change in aircraft attitude. The opposite occurs with an aft movement of the flight deck controls.

Rudder

The rudder controls rotation about the vertical axis of an aircraft. This movement is referred to as "yaw". The rudder is a movable surface that is mounted on the trailing edge of the vertical stabilizer or fin. Unlike a boat, the rudder is not used to steer the aircraft; rather, it is used to overcome adverse yaw induced by turning or, in the case of a multi-engine aircraft, by engine failure and also allows the aircraft to be intentionally slipped when required.

In most aircraft, the rudder is controlled through the flight deck rudder pedals which are linked mechanically to the rudder. Deflection of a rudder pedal causes a corresponding rudder deflection in the same direction; that is, pushing the left rudder pedal will result in a rudder deflection to the left. This, in turn, causes the rotation about the vertical axis moving the aircraft nose to the left. In large or high speed aircraft, hydraulic actuators are often used to help overcome mechanical and aerodynamic loads on the rudder surface.

Rudder effectiveness increases with aircraft speed. Thus, at slow speed, large rudder input may be required to achieve the desired results. Smaller rudder movement is required at higher speeds and, in many more sophisticated aircraft, rudder travel is automatically limited when the aircraft is flown above Manoeuvring Speed to prevent deflection angles that could potentially result in structural damage to the aircraft.

Secondary Flight Control Systems

Trim Systems and Flaps are considered to be a "secondary" flight control systems.

By definition, to "trim" an aircraft is to adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the set attitude without any control input. While all axes of rotation are affected by aerodynamic forces, not all aircraft types are capable of being trimmed in all three axes. Virtually all aircraft designs incorporate some form of pitch axis trim and most have provisions of some description for trimming in the yaw axis. Roll axis trim exists on many aircraft but it is the least frequently encountered installation of the three.

Flaps are a high lift device consisting of a hinged panel or panels mounted on the trailing edge of the wing. When extended, they increase the camber and, in most cases, the chord and surface area of the wing resulting in an increase of both lift and drag and a reduction of the stall speed. These factors result in an improvement in takeoff and landing performance.

Throttle Lever

A throttle lever, more often referred to as a thrust lever or power lever, is the means by which the pilot controls the amount of fuel provided to the engine with which it is associated. There is normally one throttle lever for each engine and, depending upon the flight deck or cockpit configuration, they may be installed on the centre console, side console, on the dash board or mounted on the aircraft ceiling. In some two pilot flight decks, each pilot station has its own set of throttle levers. In some older aircraft, the pilots shared one set of throttles and a second set was installed at the flight engineer station. In both of these cases, the levers are linked and moving one set of levers results in a similar movement of the other. Dependant upon the installation, throttle levers may incorporate provisions for selecting reverse thrust, have a fuel cut-off position or have some means of preventing beta (ground) range selections whilst the aircraft is in flight.

Quiz Questions:

Q1: Ailerons control motion around the:

  1. Longitudinal axis
  2. Lateral axis
  3. Vertical axis

Answer


Q2: Rudder effectiveness increases with aircraft...

  1. Weight
  2. Speed
  3. Skin temperature

Answer


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