The document summarizes the flight control system of a conventional fixed-wing aircraft. It describes the primary and secondary flight control surfaces and how they are used to control pitch, roll, and yaw. The primary surfaces include elevators, ailerons, and rudders. The secondary surfaces improve lift and handling characteristics and include flaps, slats, spoilers, and trim tabs. The document then provides details on how the different control surfaces are operated through linkages, cables, actuators and other mechanisms connected to the cockpit controls.

1. UNIT 2
FLIGHT CONTROL SYSTEM

2. Flight Control System
• A conventional fixed-wing aircraft
flight control system consists
of flight control surfaces, the
respective cockpit controls,
connecting linkages, and the
necessary operating mechanisms
to control an aircraft's direction
in flight. Aircraft engine controls ar
e also considered as flight
controls as they change speed.
2

3. • The flight controls keep the aircraft at a required attitude during flight.
• Movable control surfaces are installed on the wing and the empennage of
the aircraft.
• Flight control surfaces are classified into primary and secondary flight
control system
• The Primary flight control system provides longitudinal (pitch), directional
(yaw) and lateral (roll) control of the aircraft. The primary flight control
system includes elevator, rudder, aileron.
• The Secondary flight control system improves the lift and handling
characteristics of the aircraft. The secondary flight control system includes
leading edge devices, trailing edge flaps, trim control surfaces, spoilers and
speed brakes.

4. Primary Flight Control System
The primary flight control
surfaces are incorporated
into the wing and the
empennage. They includes
• The elevators on the
horizontal tail for pitch
control
• Rudder on the vertical tail
for yaw control
• Ailerons outboard on the
wing operated differentially
for roll control

5. • In addition to basic primary control surfaces there are other primary control surfaces to meet
the demand of pitch, yaw and roll.
• Canards: pitch control surface can also be placed on the front fuselage for longitudinal
stability and control. It can be fixed surface with a trailing edge control surface or whole
surface can rotate in a controlled way.
• Elevons: is a wing trailing edge surface, which function as an elevator for pitch control and
as an aileron for roll control. It is typically used on tailless delta wing aircraft and fighter
aircraft as a very effective and efficient means of providing pitch and roll control.
• Flaperons: is a wing trailing edge surface, which functions as a flap for overall lift of the
wing and aileron for roll control. This designed are used for short runways.
• Ruddervators: is a combination of rudders and elevators, it is a combination of vertical fin
and the stabilizer into one pair of control. When serving as elevators the surface on each
side of the tail move in same direction (either up or down), while serving as a rudder the
surface move in opposite direction (one up and one down).
• Stabilator: it performs the function of horizontal stabiliser and elevator. These surfaces are
actually the two halves (left and right) of the main horizontal stabiliser. Both halves operate
symmetrically like stabilizer for pitch control and differentially (asymmetrically) for roll control
to augment ailerons. These are primarily used on light aircraft and high performance military
aircraft.

6. Pitch control system
• It is exercised by four elevators located on the trailing edge of the
tail plane
• Elevators are controlled manually, thus aerodynamic loads on
surface are reflected back to pilot
• The control is provided by fore and aft movement of pilot and co-
pilot control column
• Both the control columns are mechanically linked together by a
torque tube. Input command through the control column
transmitted through control cables to the rear cables and then to
the elevator torque tubes by the control rods
• Aircraft that operate in higher speed ranges usually have a
movable horizontal stabiliser.

7. Pitch control system

8. Roll control system
• Roll control is provided by two aileron sections located on the
outboard third of the trailing edge of each wing.
• Moments are generated by the ailerons and also by the spoilers.
• In small aircraft the aileron control system is comprised of push-
pull rods, bell cranks, cables, pulleys, sprockets and roller chains.
• In larger aircraft ailerons are controlled from the control wheels on
the control column through control cables.
• The control wheel and torque shaft drive the control drum at the
base of the control column to operate ailerons.

9. Roll control system

10. Yaw control system
• Yaw moment are generated by a single rudder with a control tab.
• Input command from rudder pedals are transmitted from the front quadrant
by cables to the rear quadrants where cable input gives desired deflection to
the rudder.
• Yaw damping is provided by the yaw damper, the prime function of yaw
damper is to minimize the Dutch roll by providing automatic rudder
displacement. It keeps the aircraft stable.
• The yaw damper is an auto flight system which moves the rudder to reduce
unwanted aircraft yaw motion caused by Dutch roll or turbulence.
• The yaw damper receives the electrical signals from the gyros, as the nose
moves left or right. This signal is sent to yaw damper where it directs the
rudder opposite to the direction of yaw.
• The damper system then provides the necessary rudder movement to
oppose and damp out the yaw.

11. Yaw control system

12. Secondary Flight Control System
• The secondary flight control
surfaces improve the lift and
handling characteristics of the
aircraft.
• The primary control surfaces are
continuously activated to
maintain safe control of the
aircraft. While secondary control
surfaces are generally less
critical from the standpoint of
safe flight and are usually
deployed during certain flight
phase to alter the aerodynamic
configuration.

13. • Following secondary control surface are installed on aircraft. The number
and type of which depends on the speed of the aircraft and its intended
operation.
• Trim control surfaces: is usually used for trimming an aircraft for straight
and level flight without any control input. The small auxiliary control
surfaces are hinged to the trailing edge of the main control surfaces to
produce control moments.
• High Lift devices: most aircraft are equipped with trailing edge flaps,
leading edge flaps or slats to increase the lift. High lift devices provide
higher lift at low speeds for take off and higher lift at low speeds.
• Speed brakes: the main purpose is to add drag to decelerate the aircraft in
flight. Often called as air brakes, drive brakes or drag brakes.
• Spoilers: are used to augment the ailerons, surfaces are raised to spoil lift
and when controlled about longitudinal axis cause a rolling moment in the
direction of raised spoilers.

14. Flaps operation
• The trailing edge flaps increase the wing area and hence these increases the lift
and help to improve take off and landing performance.
• In smaller aircraft flap control system comprises an electric motor and
transmission assembly viz drive pulley, push-pull rods and cables. While in larger
aircraft the operation is mechanically controlled or hydraulically operated.
• During normal operation the flap lever moves a cable system that supplies a
mechanical input to the flap control unit, which sends hydraulic power to the flap
power drive unit.
• The power drive unit moves the flap drive system to operate the flaps. The flap
power drive unit consist of a gearbox, a hydraulic motor and an electric motor.
• The gearbox transfers power from hydraulic and electric motor to the flap torque
tubes. As the torque tube turn, the gearbox moves the follow-up cables that are
attached to the flap control unit.

15. Flap control system

16. Spoiler Operation
• The spoiler supply speed brake control to reduce lift and increase drag
during landing and reject take-off.
• During roll control the flight spoilers on the wing move asymmetrically i.e.
one will move up and other will move down.
• Spoilers are controlled with the control wheels. The control wheels gives
mechanical input to the aileron power control units, which connects the
body quadrant to the spoiler control quadrant and to the spoiler cables.
• The spoiler cables move the flight spoiler quadrant and provide an input to
the flight spoiler actuators.
• When the control wheel is turned clockwise, the spoilers on the right wing
start moving up.
• When the control wheel is turned counter clockwise, the left spoilers move
up.

17. Spoiler operation

18. Speed Brake Control
• During speed brake control, the spoilers on both the wings move
symmetrically.
• Mechanical input from the speed brake lever goes to the speed brake input
quadrant. When the speed brake lever moves up, the speed brake input
quadrant moves the ground spoiler control valve.
• The ground spoiler control valve sends hydraulic power to the ground
spoiler actuator.
• Generally both the ground and flight spoilers move up when the aircraft is
on the ground, the flight spoiler move up when the aircraft is in the air.
• The ground spoilers help reduce lift and increase drag of the aircraft during
landing and reject take-off

19. Speed brake control Operation

20. Leading edge Flap and Slat
• The leading edge devices include Krueger flaps and slats on the leading
edge of each wing.
• During cruise these surfaces are fully retracted, these surfaces extend
during take-off to increase the lift, which permits slower speeds.
• During landing the slats fully extend to increase lift and help prevent stall.
• The leading edge flaps and slats are hydraulically controlled.
• The trailing edge flap power drive unit gives mechanical input to the leading
edge flap and slat control valve. The leading edge flap and slat control valve
sends hydraulic power to the leading edge flap and slat actuators.
• The leading edge flap actuators have two position namely retract and
extend. While leading edge slat actuators have various position.

21. Leading edge flap and slat

22. FLIGHT CONTROL LINKAGE SYSTEM
• A simple flight control system may operated entirely through
mechanical linkages and cables from the control stick to the
control surface
• There are two types of mechanical system:
• Push-pull Rod type: It is well known for their ease of movement. A
sequence of rod link the control surface to the cabin input. Bell crank
lever is necessary to alter the direction of force and to obtain
conventional coupling between stick movement and control surface
deflection.
• Cable-pulley system: Cables are used in place of rods, pulleys are used
to alter the direction of the lines, equipped with idlers to reduce any
slack due to structure elasticity.

23. Push-pull Rods
• Push pull rods eliminate the problem of varying cable tension.
• A single push-pull rod can transfer either tension or compression
loads.
• Installation is simple, don’t required complex system to make it
heavier

24. Cable-pulley
• A cable-pulley system can only handle tension loads.
• Individual cables are lighter than push-pull rods, but it required large number
of fabrication and installation pulleys, brackets and guards and leads to
become heavier.
• The numerous pulleys and higher cable tensions result in a generation of
heavy control pressure because of friction.
• In larger aircraft cable-pulley system is preferred over pus-pull rod system
because its more flexible and allows reaching more remote areas of the
aircraft.
• For pressurised aircraft the control cables pass from pressurised section to
unpressurised section through air pressure seals.

25. • In cable-pulley system a quadrant is usually employed at the base
of the control column to impart force and motion to the cable
system.
• A torque tube is attached to the control surface which changes
linear motion of the cable into rotary motion to deflect control
surface.

26. Fly-by-wire
• Fly-by-wire (FBW) is a system that replaces the conventional
manual flight controls of an aircraft with an electronic interface.
• The movements of flight controls are converted to electronic
signals transmitted by wires (hence the fly-by-wire term).

27. Working
• A fly-by-wire flight control system is that where control inputs from
the pilot are transmitted to the control surfaces by electronic
signals rather than mechanical means.
• The control columns have electronic transducers that sense the
position of the control column and sends to computers, which use
this information to position the control surfaces.
• The signals from the computer are transmitted by wires instead of
control cables and are converted into a hydraulic signal to operate
hydraulic system to move the control surface.

29. Fly-by-Wire

30. • Currently in flight control system hydraulic actuators are installed,
mechanically and electrically signaled control valves are used to
operate actuators (i.e. electro-hydraulic servo ).
• These servo valve is used to convert small electrical voltages to
hydraulic power.
• The position of the actuator is measured by the position transducer i.e
Linear Variable Differential Transformers(LVDTs) to translate linear
motion into electrical signal.
• Rotary variable differential Transformers (RVDTs ) to translate angular
displacement into electrical signal.
• The pilots demand is compared with the LVDT/RVDT feedback
• When feedback signal is equal to the command signal from cockpit, a
null condition is reached and control surface movement stops.

31. • The fly-by-wire system reduces the mechanical linkage problems
like amount of weight, tension and compression loads, friction
• A fly-by-wire is a fault tolerant system as there is no fail safe
state when aircraft is in operation.
• Generally the primary and secondary flight computers are used
for calculation and sending signals to the actuators associated
with the control surfaces.
• The linkages between the flight control computers and the flight
surfaces are arranged so that the surface can be controlled by
multiple independent actuators.
• Each actuator is controlled by different computers, to avoid loss
of single actuator or computer signal will not mean loss of
control of that surface.

32. Auto Pilot
• Auto pilot is used to relieve the pilot from
some of his workload.
• It keeps the aircraft on a pre-selected
magnetic heading and stabilized condition
on both horizontal and lateral axis.
• Basic components are :
• Gyros to sense the aircraft movement
• Servos to move the control surface
• Amplifier to increase the gyro signal to operate
servos
• Controller to allow small movements or correction to
flight control surfaces
• Autopilot uses electrical signals developed
by gyro sensing equipment to fly an aircraft.

33. • The gyros operate the flight instruments to indicate the position of
the aircraft axes in flight.
• Any variation in the instrument is amplified and a signal is sent to
the autopilot servo to move the control surface.
• When the control surface moves a follow up signal is sent and
when the input and output are equal the input is removed.

35. ACTUATORS
• An actuators is a component of a machine that is responsible for
moving and controlling a mechanism or system.
• It takes power from different sources and converts the energy to
facilitate the motion.
• An actuators turns a control signal into mechanical action.
• There are four main types of actuators:
• Hydraulic Actuator
• Pneumatic Actuator
• Electric Actuator
• Mechanical Actuator
• Most actuators produce either linear (straight line), rotary (circular) or
oscillatory motion.

36. • Hydraulic actuators consist of a cylinder or fluid motor that utilizes
hydraulic power to facilitate mechanical process. The mechanical
motion gives an output in terms of linear, rotary or oscillatory motion.
(liquids are nearly incompressible)
• Pneumatic actuators work on the same concept as hydraulic
actuators except compressed gas is used instead of liquid. Energy in
the form of compressed gas is converted into linear or rotary motion,
depending on the type of actuator.
• Electric actuators are devices powered by motors that convert
electrical energy to mechanical torque. The electrical energy is used to
create motion in equipment that require multi-turn valves like gate or
globe valves.
• Mechanical actuators function through converting rotary motion to
linear motion. such as gears, rails, pulley, chain and others are used to
help convert the motion.

37. • Linear Actuators are Actuators that
creates motion in a straight line.
• In linear hydraulic actuators, a typical
set-up is made up of a hollow cylinder
that contains a liquid, usually oil, and a
piston that is inserted in it. When
pressure is applied onto the piston,
objects can be moved by the force
produced.
• Linear actuators are used in machine
tools and industrial machinery, such as
printers, hydraulic car jack and in many
other places where linear motion is
required.

38. • A rotary actuator is an actuator that
produces a rotary motion.
• The simplest actuator is purely
mechanical, where linear motion in one
direction gives rise to rotation. The most
common actuators are electrically
powered others may be powered
pneumatically or hydraulically.
• The motion produced by an actuator may
be either continuous rotation or
movement to a fixed angular position.
• The example can be given as
servomotors, stepper motors etc.

39. ACTUATOR IMPLEMENTATIONS
•Direct drive actuation
• Fly-by-Wire (FBW) actuation
• Electro-Hydrostatic Actuator (EHA)
• Electro-Mechanical Actuator (EMA)

40. Direct drive actuation
• In a direct-drive actuator, the traditional gearbox is removed.
However, it requires the motor in the direct-drive actuator to
be able to produce enough torque at a usable speed

41. Fly-by-Wire (FBW) actuation
• Fly-by-wire flight control actuators receive signals from the flight
control computer and convert them into control surface motion
for optimal flight control and flying qualities.
• Fly-by-wire actuators use signals from the pilot via the aircraft's
flight control system to control flight control surfaces along the
vertical (pitch), longitudinal (roll), and horizontal (yaw) axes for
safe and coordinated flight. Fly-by-wire flight control actuation is
used by helicopters, military aircraft, commercial transports,
regional jets, and business jets.

42. Electro-Hydrostatic Actuator (EHA)
• Electro-Hydrostatic actuators (EHAs), replace hydraulic
systems with self-contained actuators operated solely by
electrical power.
• EHAs eliminate the need for separate hydraulic pumps and
tubing, simplifying system architectures and improving safety
and reliability. actuation combines the advantages of hydraulic
and electric actuation high force capabilities, energy efficiency
and fail-safe options.
• The electro-hydrostatic actuator (EHA) converts power from
electric to hydraulic to mechanic.

43. Electro-Mechanical Actuator (EMA)
• Electromechanical actuators are mechanical actuators where the
control knob or handle has been replaced by an electric motor.
The rotary motion of the motor is converted into linear
displacement.
• Advantages :
• They are more resistant to temperature variations.
• They improve machine performance, easy setup and installation.
• Since there is no oil to change, no leaks to repair, these electric linear
actuators require less maintenance.
• As they do not suffer air leaks, these linear actuators are suitable for
applications that require clean operation as in the case of food, beverage,
packaging, medical industries.

44. END OF CHAPTER 2