Aircraft Systems and Instrumentation (10AE848)
Module 1
Airplane Control Systems
Conventional Systems, fully powered flight controls, Power actuated systems, Modern control
systems, Digital fly by wire systems, Auto pilot system active control Technology.
Primary and secondary flight controls:
Primary control surfaces:
On almost all aircraft, the primary flight control surfaces are incorporated into the wing and
the empennage. They typically include -
• The elevators on the horizontal tail for pitch control,
• Rudder on the vertical tail for yaw control, and
• Ailerons outboard on the wings operated differentially for roll control.
All the above mentioned control surfaces are trailing edge type. In addition to the basic
primary control surfaces, there are other primary control surfaces to meet the demand of
pitch, yaw and roll. These are described briefly in the following paras.
Variable Incidence Horizontal Stabiliser
Variable Incidence Horizontal Stabiliser is used for pitch control where the elevator surface
is of inadequate size. The horizontal tail itself is rotated. On large transport aircraft, the
stabiliser is used for pitch trim. Its use eliminates the need for deflecting the elevator for
trimming.
Canards
Pitch control surfaces can also be placed on the front fuselage for the purpose of longitudinal
stability and control, which is referred to as canards. It can be a fixed surface with a trailing
edge control surface or the whole canard surface can rotate in a controlled way.
Elevons
An elevon is a wing trailing edge surface, which functions as an elevator for pitch control and
as an aileron for roll control. Elevon 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
Flaperon is a wing trailing edge surface, which functions as a flap for overall lift of the wing
and aileron for roll control. These types of control surfaces are found on aircraft, which are
designed to operate from short runways.
Ruddervators
Another variation is to combine the vertical fin and the stabiliser into one pair of controls
which form a Vee shape, known as a V-tail. On the V-tail, the combined rudders and
elevators are known as ruddervators. When serving as elevators, the surfaces on each side of
the tail move in the same direction (either up or down). When serving as rudder, the surfaces
move in opposite direction i.e. one up and one down.
Stabilator
Department of Aeronautical Engineering, SIT, Mangaluru Page 1
,Aircraft Systems and Instrumentation (10AE848)
A stabilator performs the function of a horizontal stabiliser and an elevator. These surfaces
are actually the two halves (left and right) of the main horizontal stabiliser. Both halves
operate symmetrically like stabiliser for pitch control and differentially (asymmetrically) for
roll control to augment the ailerons. Stabilators are primarily used on light aircraft and high
performance military aircraft.
Secondary control surfaces
Following secondary control surfaces are installed on aircraft, the number and type of
which depends on the speed of the aircraft and its intended operation.
Trim Control Surfaces
Aerodynamic trim tabs were originally used for trimming an aircraft for straight and level
flight without any control input. These small auxiliary control surfaces are hinged to the
trailing edge of the main control surfaces to produce control moments. Trim tabs can also act
as control tabs. In such a case, the control linkage connects the cable control quadrant
directly to the tab. Thus the tab can be directly moved from the cockpit in the direction
opposite to the direction of the main control surface.
High Lift Devices
Most aircraft are equipped with trailing edge flaps, leading edge flaps or slats to increase the
lift by effectively increasing the wing area and the camber of the airfoil to increase
circulation and delay separation of the airflow. High lift devices primarily provide higher lift
at low speeds for take-off and higher lift at low speeds and low angle of attack for landing.
They also increase the drag to slow the aircraft during descent and landing. These high
lift control surfaces are vanable lift type, which can be classified into following three
categories:
• Dependent on the variation of the wing angle of incidence with reference to the
fuselage (i.e. variable incidence wings).
• The variation of the wing area (i.e. trailing edge flaps).
• The variation of airfoil cross-section (i.e. leading edge flaps and slats).
Speed Brakes
They are often called air brakes, dive brake or drag brakes. They are aerodynamic panels
whose main purpose is to add drag to decelerate the aircraft in flight. They are designed to
pop out of the wing at a near 90° angle to the airflow thereby increasing the drag to a
maximum possible extent.
Spoilers
For lateral control, often more than one set of ailerons or other surfaces are used to meet the
required roll rates. Almost on all large transport aircraft, spoilers are used to augment the
ailerons. These surfaces are raised to spoil lift and. when controlled asymmetrically about the
longitudinal axis, cause a rolling moment in the direction of raised spoiler. Spoilers were first
used to overcome aileron reversal due to aeroelastic effects.
Department of Aeronautical Engineering, SIT, Mangaluru Page 2
, Aircraft Systems and Instrumentation (10AE848)
Flight control linkage system:
The architecture of the flight control system essential for all flight operations has
changed significantly over the years. Soon after the first flight, the flight control surfaces
were operated through a system of push-pull rods, cables and pulleys. Smaller and less
complex aircraft use a simple cable, pulley and push-pull rod system to move the primary
flight control surfaces.
The amount of force required to move the surfaces varies with the speed and angle of
attack of the aircraft. Dynamic forces acting on the primary flight control surfaces during all
phases of the aircraft operation provide feel feedback to the pilot as to what the aircraft is
doing. This dynamic change in control force pressures allows the pilot to develop a feel, the
way aircraft is reacting to the control forces.
As aircraft became larger they have correspondingly larger primary flight control
surfaces needing higher and higher pilot control forces to move them. The introduction of
larger aircraft and increase of the flight envelopes made the pilot effort insufficient in
contrast to the aerodynamic hinge moments generated by the control surface deflection. The
first solution to this problem was the introduction of aerodynamic balances and tabs. Further
increase in the aircraft sizes and flight envelops brought the need of powered systems for
control surface movements. The simple direct cable control of flight surfaces is actuated by
the hydraulic and servo actuators, which provide the power to move the primary flight
control surfaces.
Mechanical Flight Control System: A simple flight control system may be all mechanical
or unboosted i.e. operated entirely through mechanical linkages and cables from the control
stick to the control surface. It is generally used on small aircraft. The linkage from cabin to
control surface can be fully mechanical if the aircraft size and its flight envelop permits. In
this case the hinge moment generated by the surface deflection is low which can be sustained
by the pilot. The primary flight control surfaces 'are moved manually through a series of
push-pull rods, cables, bell cranks, sectors, etc.
The two types of mechanical systems are - push-pull rod type and cable-pulley type.
In the first case a sequence of rods link the control surface to the cabin input (Fig. 1.1). Bell
crank lever is necessary to alter the direction of the force and to obtain the conventional
coupling between stick movement and the control surface deflection. Push-pull controls are
well known for their ease of movement.
Fig. 1.1
Push-pull rods eliminate the problem of varying cable tensions. A single push-pull rod can
transfer either tension or compression loads whereas a cable-pulley system can only handle
Department of Aeronautical Engineering, SIT, Mangaluru Page 3
Module 1
Airplane Control Systems
Conventional Systems, fully powered flight controls, Power actuated systems, Modern control
systems, Digital fly by wire systems, Auto pilot system active control Technology.
Primary and secondary flight controls:
Primary control surfaces:
On almost all aircraft, the primary flight control surfaces are incorporated into the wing and
the empennage. They typically include -
• The elevators on the horizontal tail for pitch control,
• Rudder on the vertical tail for yaw control, and
• Ailerons outboard on the wings operated differentially for roll control.
All the above mentioned control surfaces are trailing edge type. In addition to the basic
primary control surfaces, there are other primary control surfaces to meet the demand of
pitch, yaw and roll. These are described briefly in the following paras.
Variable Incidence Horizontal Stabiliser
Variable Incidence Horizontal Stabiliser is used for pitch control where the elevator surface
is of inadequate size. The horizontal tail itself is rotated. On large transport aircraft, the
stabiliser is used for pitch trim. Its use eliminates the need for deflecting the elevator for
trimming.
Canards
Pitch control surfaces can also be placed on the front fuselage for the purpose of longitudinal
stability and control, which is referred to as canards. It can be a fixed surface with a trailing
edge control surface or the whole canard surface can rotate in a controlled way.
Elevons
An elevon is a wing trailing edge surface, which functions as an elevator for pitch control and
as an aileron for roll control. Elevon 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
Flaperon is a wing trailing edge surface, which functions as a flap for overall lift of the wing
and aileron for roll control. These types of control surfaces are found on aircraft, which are
designed to operate from short runways.
Ruddervators
Another variation is to combine the vertical fin and the stabiliser into one pair of controls
which form a Vee shape, known as a V-tail. On the V-tail, the combined rudders and
elevators are known as ruddervators. When serving as elevators, the surfaces on each side of
the tail move in the same direction (either up or down). When serving as rudder, the surfaces
move in opposite direction i.e. one up and one down.
Stabilator
Department of Aeronautical Engineering, SIT, Mangaluru Page 1
,Aircraft Systems and Instrumentation (10AE848)
A stabilator performs the function of a horizontal stabiliser and an elevator. These surfaces
are actually the two halves (left and right) of the main horizontal stabiliser. Both halves
operate symmetrically like stabiliser for pitch control and differentially (asymmetrically) for
roll control to augment the ailerons. Stabilators are primarily used on light aircraft and high
performance military aircraft.
Secondary control surfaces
Following secondary control surfaces are installed on aircraft, the number and type of
which depends on the speed of the aircraft and its intended operation.
Trim Control Surfaces
Aerodynamic trim tabs were originally used for trimming an aircraft for straight and level
flight without any control input. These small auxiliary control surfaces are hinged to the
trailing edge of the main control surfaces to produce control moments. Trim tabs can also act
as control tabs. In such a case, the control linkage connects the cable control quadrant
directly to the tab. Thus the tab can be directly moved from the cockpit in the direction
opposite to the direction of the main control surface.
High Lift Devices
Most aircraft are equipped with trailing edge flaps, leading edge flaps or slats to increase the
lift by effectively increasing the wing area and the camber of the airfoil to increase
circulation and delay separation of the airflow. High lift devices primarily provide higher lift
at low speeds for take-off and higher lift at low speeds and low angle of attack for landing.
They also increase the drag to slow the aircraft during descent and landing. These high
lift control surfaces are vanable lift type, which can be classified into following three
categories:
• Dependent on the variation of the wing angle of incidence with reference to the
fuselage (i.e. variable incidence wings).
• The variation of the wing area (i.e. trailing edge flaps).
• The variation of airfoil cross-section (i.e. leading edge flaps and slats).
Speed Brakes
They are often called air brakes, dive brake or drag brakes. They are aerodynamic panels
whose main purpose is to add drag to decelerate the aircraft in flight. They are designed to
pop out of the wing at a near 90° angle to the airflow thereby increasing the drag to a
maximum possible extent.
Spoilers
For lateral control, often more than one set of ailerons or other surfaces are used to meet the
required roll rates. Almost on all large transport aircraft, spoilers are used to augment the
ailerons. These surfaces are raised to spoil lift and. when controlled asymmetrically about the
longitudinal axis, cause a rolling moment in the direction of raised spoiler. Spoilers were first
used to overcome aileron reversal due to aeroelastic effects.
Department of Aeronautical Engineering, SIT, Mangaluru Page 2
, Aircraft Systems and Instrumentation (10AE848)
Flight control linkage system:
The architecture of the flight control system essential for all flight operations has
changed significantly over the years. Soon after the first flight, the flight control surfaces
were operated through a system of push-pull rods, cables and pulleys. Smaller and less
complex aircraft use a simple cable, pulley and push-pull rod system to move the primary
flight control surfaces.
The amount of force required to move the surfaces varies with the speed and angle of
attack of the aircraft. Dynamic forces acting on the primary flight control surfaces during all
phases of the aircraft operation provide feel feedback to the pilot as to what the aircraft is
doing. This dynamic change in control force pressures allows the pilot to develop a feel, the
way aircraft is reacting to the control forces.
As aircraft became larger they have correspondingly larger primary flight control
surfaces needing higher and higher pilot control forces to move them. The introduction of
larger aircraft and increase of the flight envelopes made the pilot effort insufficient in
contrast to the aerodynamic hinge moments generated by the control surface deflection. The
first solution to this problem was the introduction of aerodynamic balances and tabs. Further
increase in the aircraft sizes and flight envelops brought the need of powered systems for
control surface movements. The simple direct cable control of flight surfaces is actuated by
the hydraulic and servo actuators, which provide the power to move the primary flight
control surfaces.
Mechanical Flight Control System: A simple flight control system may be all mechanical
or unboosted i.e. operated entirely through mechanical linkages and cables from the control
stick to the control surface. It is generally used on small aircraft. The linkage from cabin to
control surface can be fully mechanical if the aircraft size and its flight envelop permits. In
this case the hinge moment generated by the surface deflection is low which can be sustained
by the pilot. The primary flight control surfaces 'are moved manually through a series of
push-pull rods, cables, bell cranks, sectors, etc.
The two types of mechanical systems are - push-pull rod type and cable-pulley type.
In the first case a sequence of rods link the control surface to the cabin input (Fig. 1.1). Bell
crank lever is necessary to alter the direction of the force and to obtain the conventional
coupling between stick movement and the control surface deflection. Push-pull controls are
well known for their ease of movement.
Fig. 1.1
Push-pull rods eliminate the problem of varying cable tensions. A single push-pull rod can
transfer either tension or compression loads whereas a cable-pulley system can only handle
Department of Aeronautical Engineering, SIT, Mangaluru Page 3
