Introduction to flight controls
Where an aeroplane is stable or unstable, it is necessary for the pilot to be able to control it, so that he can manoeuvre it into any desired position.
Aircraft flight controls are the means to maneuver the aircraft into desired direction. Maneuvering of the aircraft from the cockpit involves:
(i) Exercising cockpit controls by the cockpit crew
(ii) Deflection of the control surfaces as a result of the exercise of the cockpit controls
(iii) Controlled movement of the aircraft into desired direction as a result of the actions in a) and b)
There are linkages connecting cockpit controls to the control surfaces. So, deflection of a cockpit control causes deflection of the connected control surface. There are major axes of the aircraft about which defined major movements occur due to deflection of the control surfaces.
This week overviews the control principles. Subsequent weeks elaborates the controls.
2.2 THREE PRIMARY FLIGHT CONTROLS
Three cockpit controls, connecting three primary control surfaces as well as assisted by other control surfaces constitute three primary flight control systems. They are:
(i) Lateral Control system
(ii) Longitudinal Control system and
(iii) Directional Control system
Lateral control is provided by the ailerons, i.e. flaps hinged at the rear of the aerofoils near each wing tip.
Longitudinal control is provided by the elevators, i.e. flaps hinged behind the tail plane.
Directional control is provided by the rudder, i.e. a vertical flap hinged to the stern post.
1.2 MAJOR AXES / MAJOR MOVEMENTS
As in earlier outcome, there are three defined major axes of an aircraft: Longitudinal, lateral and vertical/normal axis. An aircraft in flight is free to rotate about its three axes, and its flight controls are designed to allow the pilot to control its rotation about each axis. Three axes pass through the point of Center of Gravity of the aircraft.
There are three basic movements about three major axes. They are: Rolling, Pitching and Yawing.
Figure 1.1 shows the major axes and major movements about the major axes.
1.3 Control surfaces
1.3.1 The control surfaces of an airplane do no more than modify the aerodynamic shape of the surface to which they are attached. This change in shape changes the lift and drag produced by the surface, with the immediate result of rotating the airplane about one of its three axes. It is this rotation that produces the change in flight path that gives us the maneuver we want Control surfaces are attached to the mainplane, tailplane and Fin.
- Primary Control surfaces
- Secondary Control Surfaces
- Auxiliary Control Surfaces
1.3.2 Primary Control Surfaces: There are three primary control surfaces:
(i) Aileron: There are left hand and right hand ailerons hinged to the trailing edges of the left hand and right hand outboard side of the mainplane (wing)
(ii) Elevator: Elevator is hinged to the trailing edge of the tailplane (horizontal stabilizer)
(iii) Rudder: Rudder is attached to the trailing edge of the fin (vertical stabilizer)
Cockpit Controls and The Primary Control Surfaces: There are three cockpit controls connected to the three primary control surfaces:
(i) Control Wheel : This is like a steering wheel that connects Ailerons. Sideways movement of the control wheel deflects left wing aileron and right wing aileron in opposite direction (upward/downward about their hinge lines at the trailing edges). Deflection of ailerons relates to the basic movement of rolling about the longitudinal axis of the aircraft.
(ii) Control Column: This is a column or stick carrying the control wheel at the top. Movement of the column/stick forward and rearward in the cockpit controls elevator deflecting down/up along the trailing edge of the tail plane surface. These deflections of the elevator control the basic movement of pitching of the aircraft about the lateral axis.
Figure 1.1: Aircraft major axes and major movement
(i) Rudder Pedal: Rudder pedals are foot controls (LH and RH). Depressing forward of these pedals give deflections of rudder surface to the left or right direction about its vertical hinge line at the trailing edge of the fin. These deflections are basically for giving Yawing movement
1.3.3 Secondary control surfaces: These are the control surfaces that help in deflecting the primary flight control surfaces.
Flight controls may be ‘powered’ or ‘power assisted’ and ‘non-powered’. In the non-powered flight controls, flight loads may be felt in hand when pilot operates the control. So, some kinds of ‘tabs’ are hinged to the trailing edge of the primary control surfaces. They assist the pilot in deflecting primary flight control surfaces. Examples
of some tabs used by non-powered flight controls are: Servo tabs, spring tabs, balanced tabs.
1.3.4 Auxiliary Control Surfaces: There are two groups:
- Lift Augmentation Devices/High Lift Devices: These auxiliary surfaces improve the flow characteristics at “stalling speed” and help to avoid stall and loss of lift and to increase lift at low speed (when stall is most likely to occur). Some examples of high lift devices are:
- a) Leading Flap or Krueger
- b) Leading Edge Slat, slot or simply ‘Slat’, slot
- c) Trailing Edge Flap or simply ‘Flap
Cockpit control for high lift devices is the Flap/Slat handle/lever.
- Lift Spoilers: These auxiliary surfaces breaks the flow/destroys lift and thus increases drag intentionally. This is required during approach and landing phase to reduce altitude (for descending) and to reduce the forward speed of the aircraft. After landing, they produce maximum amount of drag giving braking effect to the aircraft, hence they are called ‘speed breakers’.
Cockpit control for lift spoilers is the spoiler/speed brake lever.
Controls and control surfaces are elaborately discussed in subsequent weeks.