Aircraft Flight Control Systems
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A conventional fixed-wing aircraft flight control system consists of
flight control surfaces Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude. Development of an effective set of flight control surfaces was a critical advance in the development of aircraft. Ea ...
, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed. The fundamentals of aircraft controls are explained in
flight dynamics Flight dynamics in aviation and spacecraft, is the study of the performance, stability, and control of vehicles flying through the air or in outer space. It is concerned with how forces acting on the vehicle determine its velocity and attitude ...
. This article centers on the operating mechanisms of the flight controls. The basic system in use on aircraft first appeared in a readily recognizable form as early as April 1908, on
Louis Blériot Louis Charles Joseph Blériot ( , also , ; 1 July 1872 – 1 August 1936) was a French aviator, inventor, and engineer. He developed the first practical headlamp for cars and established a profitable business manufacturing them, using much of th ...
's Blériot VIII pioneer-era monoplane design.


Cockpit controls


Primary controls

Generally, the primary cockpit flight controls are arranged as follows:Langewiesche, Wolfgang
Stick and Rudder: An Explanation of the Art of Flying
McGraw-Hill Professional, 1990, , .
* a
control yoke A yoke, alternatively known as a control wheel or a control column, is a device used for piloting some fixed-wing aircraft.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 563. Aviation Supplies & Academics, 1997. ...
(also known as a control column),
centre stick A centre stick (or center stick in the United States), or simply control stick is an aircraft cockpit arrangement where the control column (or joystick) is located in the center of the cockpit between the pilots or between the pilot's legs. Sin ...
or
side-stick __NOTOC__ A side-stick or sidestick controller is an aircraft control stick that is located on the side console of the pilot, usually on the righthand side, or outboard on a two-seat flightdeck. Typically this is found in aircraft that are equipp ...
(the latter two also colloquially known as a control or
joystick A joystick, sometimes called a flight stick, is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. A joystick, also known as the control column, is the principal cont ...
), governs the aircraft's roll and pitch by moving the
ailerons An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around t ...
(or activating
wing warping Wing warping was an early system for lateral (roll) control of a fixed-wing aircraft. The technique, used and patented by the Wright brothers, consisted of a system of pulleys and cables to twist the trailing edges of the wings in opposite direc ...
on some very early aircraft designs) when turned or deflected left and right, and moves the
elevators An elevator or lift is a cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or decks of a building, vessel, or other structure. They a ...
when moved backwards or forwards * rudder pedals, or the earlier, pre-1919 "rudder bar", to control yaw, which move the
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adverse yaw a ...
; left foot forward will move the rudder left for instance. * throttle controls to control engine speed or
thrust Thrust is a reaction force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can al ...
for powered aircraft. The control yokes also vary greatly amongst aircraft. There are yokes where roll is controlled by rotating the yoke clockwise/counterclockwise (like steering a car) and pitch is controlled by moving the control column towards or away from the pilot, but in others the pitch is controlled by sliding the yoke into and out of the instrument panel (like most Cessnas, such as the 152 and 172), and in some the roll is controlled by sliding the whole yoke to the left and right (like the Cessna 162). Centre sticks also vary between aircraft. Some are directly connected to the control surfaces using cables, others (fly-by-wire airplanes) have a computer in between which then controls the electrical actuators. Even when an aircraft uses variant flight control surfaces such as a V-tail ruddervator, flaperons, or
elevon Elevons or tailerons are aircraft control surfaces that combine the functions of the elevator (used for pitch control) and the aileron (used for roll control), hence the name. They are frequently used on tailless aircraft such as flying wings. A ...
s, to avoid pilot confusion the aircraft's flight control system will still be designed so that the stick or yoke controls pitch and roll conventionally, as will the rudder pedals for yaw. The basic pattern for modern flight controls was pioneered by French aviation figure Robert Esnault-Pelterie, with fellow French aviator
Louis Blériot Louis Charles Joseph Blériot ( , also , ; 1 July 1872 – 1 August 1936) was a French aviator, inventor, and engineer. He developed the first practical headlamp for cars and established a profitable business manufacturing them, using much of th ...
popularizing Esnault-Pelterie's control format initially on Louis' Blériot VIII monoplane in April 1908, and standardizing the format on the July 1909 Channel-crossing
Blériot XI The Blériot XI is a French aircraft of the pioneer era of aviation. The first example was used by Louis Blériot to make the first flight across the English Channel in a heavier-than-air aircraft, on 25 July 1909. This is one of the most fam ...
. Flight control has long been taught in such fashion for many decades, as popularized in
ab initio ''Ab initio'' ( ) is a Latin term meaning "from the beginning" and is derived from the Latin ''ab'' ("from") + ''initio'', ablative singular of ''initium'' ("beginning"). Etymology Circa 1600, from Latin, literally "from the beginning", from a ...
instructional books such as the 1944 work Stick and Rudder. In some aircraft, the control surfaces are not manipulated with a linkage. In ultralight aircraft and motorized hang gliders, for example, there is no mechanism at all. Instead, the pilot just grabs the lifting surface by hand (using a rigid frame that hangs from its underside) and moves it.


Secondary controls

In addition to the primary flight controls for roll, pitch, and yaw, there are often secondary controls available to give the pilot finer control over flight or to ease the workload. The most commonly available control is a wheel or other device to control elevator trim, so that the pilot does not have to maintain constant backward or forward pressure to hold a specific pitch
attitude Attitude may refer to: Philosophy and psychology * Attitude (psychology), an individual's predisposed state of mind regarding a value * Metaphysics of presence * Propositional attitude, a relational mental state connecting a person to a prop ...
(other types of trim, for
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adverse yaw a ...
and
ailerons An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around t ...
, are common on larger aircraft but may also appear on smaller ones). Many aircraft have wing flaps, controlled by a switch or a mechanical lever or in some cases are fully automatic by computer control, which alter the shape of the wing for improved control at the slower speeds used for take-off and landing. Other secondary flight control systems may include slats, spoilers, air brakes and
variable-sweep wing A variable-sweep wing, colloquially known as a "swing wing", is an airplane wing, or set of wings, that may be swept back and then returned to its original straight position during flight. It allows the aircraft's shape to be modified in fli ...
s.


Flight control systems


Mechanical

Mechanical or manually operated flight control systems are the most basic method of controlling an aircraft. They were used in early aircraft and are currently used in small aircraft where the aerodynamic forces are not excessive. Very early aircraft, such as the Wright Flyer I,
Blériot XI The Blériot XI is a French aircraft of the pioneer era of aviation. The first example was used by Louis Blériot to make the first flight across the English Channel in a heavier-than-air aircraft, on 25 July 1909. This is one of the most fam ...
and
Fokker Eindecker The Fokker ''Eindecker'' fighters were a series of German World War I monoplane single-seat fighter aircraft designed by Dutch engineer Anthony Fokker.Boyne 1988 Developed in April 1915, the first ''Eindecker'' ("Monoplane") was the first purpos ...
used a system of
wing warping Wing warping was an early system for lateral (roll) control of a fixed-wing aircraft. The technique, used and patented by the Wright brothers, consisted of a system of pulleys and cables to twist the trailing edges of the wings in opposite direc ...
where no conventionally hinged control surfaces were used on the wing, and sometimes not even for pitch control as on the Wright Flyer I and original versions of the 1909 Etrich Taube, which only had a hinged/pivoting rudder in addition to the warping-operated pitch and roll controls. A manual flight control system uses a collection of mechanical parts such as pushrods, tension cables, pulleys, counterweights, and sometimes chains to transmit the forces applied to the cockpit controls directly to the control surfaces.
Turnbuckle A turnbuckle, stretching screw or bottlescrew is a device for adjusting the tension or length of ropes, cables, tie rods, and other tensioning systems. It normally consists of two threaded eye bolts, one screwed into each end of a small meta ...
s are often used to adjust control cable tension. The
Cessna Skyhawk The Cessna 172 Skyhawk is an American four-seat, single-engine, high wing, fixed-wing aircraft made by the Cessna Aircraft Company.
is a typical example of an aircraft that uses this type of system. Gust locks are often used on parked aircraft with mechanical systems to protect the control surfaces and linkages from damage from wind. Some aircraft have gust locks fitted as part of the control system. Increases in the control surface area, and the higher airspeeds required by faster aircraft resulted in higher aerodynamic loads on the flight control systems. As a result, the forces required to move them also become significantly larger. Consequently, complicated mechanical
gear A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic p ...
ing arrangements were developed to extract maximum
mechanical advantage Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...
in order to reduce the forces required from the pilots.Taylor, 1990. p. 118. This arrangement can be found on bigger or higher performance
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
aircraft such as the
Fokker 50 The Fokker 50 is a turboprop-powered airliner, designed as an improved version of the successful Fokker F27 Friendship. The Fokker 60 is a stretched freighter version of the Fokker 50. Both aircraft were manufactured and supported by Dutch airc ...
. Some mechanical flight control systems use
servo tab __NOTOC__ A servo tab is a small hinged device installed on an aircraft control surface to assist the movement of the control surfaces. Introduced by the German firm Flettner, servo tabs were formerly known as Flettner tabs. Servo tabs are not ...
s that provide aerodynamic assistance. Servo tabs are small surfaces hinged to the control surfaces. The flight control mechanisms move these tabs, aerodynamic forces in turn move, or assist the movement of the control surfaces reducing the amount of mechanical forces needed. This arrangement was used in early piston-engined transport aircraft and in early jet transports. The Boeing 737 incorporates a system, whereby in the unlikely event of total hydraulic system failure, it automatically and seamlessly reverts to being controlled via servo-tab.


Hydro-mechanical

The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft. Hydraulically powered control surfaces help to overcome these limitations. With hydraulic flight control systems, the aircraft's size and performance are limited by economics rather than a pilot's muscular strength. At first, only-partially boosted systems were used in which the pilot could still feel some of the aerodynamic loads on the control surfaces (feedback). A hydro-mechanical flight control system has two parts: *The ''mechanical circuit'', which links the cockpit controls with the hydraulic circuits. Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains. *The ''hydraulic circuit'', which has hydraulic pumps, reservoirs, filters, pipes, valves and actuators. The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit. The actuators convert hydraulic pressure into control surface movements. The electro-hydraulic servo valves control the movement of the actuators. The pilot's movement of a control causes the mechanical circuit to open the matching servo valve in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces. As the actuator moves, the servo valve is closed by a mechanical
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
linkage - one that stops movement of the control surface at the desired position. This arrangement was found in the older-designed jet transports and in some high-performance aircraft. Examples include the
Antonov An-225 The Antonov An-225 Mriya ( uk, Антонов Ан-225 Мрія, lit=dream' or 'inspiration; NATO reporting name: Cossack) was a strategic airlift cargo aircraft designed and produced by the Antonov Design Bureau in the Soviet Union. It was ...
and the Lockheed SR-71.


Artificial feel devices

With purely mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot, allowing tactile feedback of airspeed. With hydromechanical flight control systems, the load on the surfaces cannot be felt and there is a risk of overstressing the aircraft through excessive control surface movement. To overcome this problem, artificial feel systems can be used. For example, for the controls of the RAF's
Avro Vulcan The Avro Vulcan (later Hawker Siddeley Vulcan from July 1963) is a jet-powered, tailless, delta-wing, high-altitude, strategic bomber, which was operated by the Royal Air Force (RAF) from 1956 until 1984. Aircraft manufacturer A.V. Roe an ...
jet
bomber A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching aerial torpedo, torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped ...
and the
RCAF The Royal Canadian Air Force (RCAF; french: Aviation royale canadienne, ARC) is the air and space force of Canada. Its role is to "provide the Canadian Forces with relevant, responsive and effective airpower". The RCAF is one of three environ ...
's Avro Canada CF-105 Arrow supersonic interceptor (both 1950s-era designs), the required force feedback was achieved by a spring device. The fulcrum of this device was moved in proportion to the square of the air speed (for the elevators) to give increased resistance at higher speeds. For the controls of the American
Vought Vought was the name of several related American aerospace firms. These have included, in the past, Lewis and Vought Corporation, Chance Vought, Vought-Sikorsky, LTV Aerospace (part of Ling-Temco-Vought), Vought Aircraft Companies, and Vought Ai ...
F-8 Crusader The Vought F-8 Crusader (originally F8U) is a single-engine, supersonic, carrier-based air superiority jet aircraft built by Vought for the United States Navy and United States Marine Corps (replacing the Vought F7U Cutlass), and for the Fr ...
and the LTV A-7 Corsair II warplanes, a 'bob-weight' was used in the pitch axis of the control stick, giving force feedback that was proportional to the airplane's normal acceleration.


Stick shaker

A stick shaker is a device that is attached to the control column in some hydraulic aircraft. It shakes the control column when the aircraft is approaching stall conditions. Some aircraft such as the
McDonnell Douglas DC-10 The McDonnell Douglas DC-10 is an American trijet wide-body aircraft manufactured by McDonnell Douglas. The DC-10 was intended to succeed the Douglas DC-8, DC-8 for long-Range (aeronautics), range flights. It first flew on August 29, 1970; i ...
are equipped with a back-up electrical power supply that can be activated to enable the stick shaker in case of hydraulic failure.


Power-by-wire

In most current systems the power is provided to the control actuators by high-pressure hydraulic systems. In fly-by-wire systems the valves, which control these systems, are activated by electrical signals. In power-by-wire systems, electrical actuators are used in favour of hydraulic pistons. The power is carried to the actuators by electrical cables. These are lighter than hydraulic pipes, easier to install and maintain, and more reliable. Elements of the
F-35 The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, all-weather stealth multirole combat aircraft that is intended to perform both air superiority and strike missions. It is also able to provide ele ...
flight control system are power-by-wire. The actuators in such an electro-hydrostatic actuation (EHA) system are self-contained hydraulic devices, small closed-circuit hydraulic systems. The overall aim is towards more- or all-electric aircraft and an early example of the approach was the
Avro Vulcan The Avro Vulcan (later Hawker Siddeley Vulcan from July 1963) is a jet-powered, tailless, delta-wing, high-altitude, strategic bomber, which was operated by the Royal Air Force (RAF) from 1956 until 1984. Aircraft manufacturer A.V. Roe an ...
. Serious consideration was given to using the approach on the Airbus A380.


Fly-by-wire control systems

A fly-by-wire (FBW) system replaces manual flight control of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the term ''fly-by-wire''), and flight control computers determine how to move the
actuators An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) ...
at each control surface to provide the expected response. Commands from the computers are also input without the pilot's knowledge to stabilize the aircraft and perform other tasks. Electronics for aircraft flight control systems are part of the field known as
avionics Avionics (a blend of ''aviation'' and ''electronics'') are the electronic systems used on aircraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fitt ...
. Fly-by-optics, also known as ''fly-by-light'', is a further development using
fiber optic cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...
s.


Research

Several technology research and development efforts exist to integrate the functions of flight control systems such as
aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement arou ...
s,
elevators An elevator or lift is a cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or decks of a building, vessel, or other structure. They a ...
,
elevon Elevons or tailerons are aircraft control surfaces that combine the functions of the elevator (used for pitch control) and the aileron (used for roll control), hence the name. They are frequently used on tailless aircraft such as flying wings. A ...
s, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag,
inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law o ...
(for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), and
radar cross section Radar cross-section (RCS), also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy back to the source. ...
for
stealth Stealth may refer to: Military *Stealth technology, technology used to conceal ships, aircraft, and missiles **Stealth aircraft, aircraft which use stealth technology **Stealth ground vehicle, ground vehicles which use stealth technology ** Stea ...
. These may be used in many
unmanned aerial vehicle An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft without any human pilot, crew, or passengers on board. UAVs are a component of an unmanned aircraft system (UAS), which includes adding a ground-based controlle ...
s (UAVs) and 6th generation fighter aircraft. Two promising approaches are flexible wings, and fluidics.


Flexible wings

In flexible wings, also known as "morphing aerofoils", much or all of a wing surface can change shape in flight to deflect air flow much like an
ornithopter An ornithopter (from Greek ''ornis, ornith-'' "bird" and ''pteron'' "wing") is an aircraft that flies by flapping its wings. Designers sought to imitate the flapping-wing flight of birds, bats, and insects. Though machines may differ in form, th ...
. Adaptive compliant wings are a military and commercial effort. The X-53 Active Aeroelastic Wing was a US Air Force,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...
, and
Boeing The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and ...
effort. Notable efforts have also been made by FlexSys, who have conducted flight tests using flexible aerofoils retrofitted to a Gulf stream III aircraft.


Active Flow Control

In active flow control systems, forces in vehicles occur via circulation control, in which larger and more complex mechanical parts are replaced by smaller, simpler fluidic systems (slots which emit air flows) where larger forces in fluids are diverted by smaller jets or flows of fluid intermittently, to change the direction of vehicles. In this use, active flow control promises simplicity and lower mass, costs (up to half less), and
inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law o ...
and response times. This was demonstrated in the Demon UAV, which flew for the first time in the UK in September 2010.


See also

*
Flight envelope protection Flight envelope protection is a human machine interface extension of an aircraft's control system that prevents the pilot of an aircraft from making control commands that would force the aircraft to exceed its structural and aerodynamic operatin ...
*
Flight with disabled controls Several aviation incidents and accidents have occurred in which the control surfaces of an aircraft became disabled, often due to failure of hydraulic systems or the flight control system. Other incidents have occurred where controls were not fun ...
*
Helicopter flight controls A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor bl ...
* HOTAS * Kite control systems * List of airliner crashes involving loss of control *
Matthew Piers Watt Boulton Matthew Piers Watt Boulton (22 September 1820 – 30 June 1894), also published under the pseudonym M. P. W. Bolton, was a British classicist, elected member of the UK's Metaphysical Society, an amateur scientist and an inventor, best ...
, inventor of the
aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement arou ...
(1868) *
Thrust vectoring Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the ...
* Weight-shift control *
Wing warping Wing warping was an early system for lateral (roll) control of a fixed-wing aircraft. The technique, used and patented by the Wright brothers, consisted of a system of pulleys and cables to twist the trailing edges of the wings in opposite direc ...
: an early method for controlling roll


References


Notes


Bibliography

* Spitzer, Cary R. ''The Avionics Handbook'',
CRC Press The CRC Press, LLC is an American publishing group that specializes in producing technical books. Many of their books relate to engineering, science and mathematics. Their scope also includes books on business, forensics and information tec ...
, * Taylor, John W.R. ''The Lore of Flight'', London: Universal Books Ltd., 1990. . * The Arrowheads (Richard Organ, Ron Page, Don Watson, Les Wilkinson). ''Avro Arrow: the story of the Avro Arrow from its evolution to its extinction'', Erin, Ontario, Canada: Boston Mills Press 1980 (revised edition 2004). . * Thom, Trevor. ''The Air Pilot's Manual 4-The Aeroplane-Technical''. 1988. Shrewsbury, Shropshire, England. Airlife Publishing Ltd. * USAF & NATO Report RTO-TR-015 AC/323/(HFM-015)/TP-1 (2001).


External links


Airbus A380 cockpit

Airbus A380 cockpit - a 360-degree Panorama

''Touchdown: the Development of Propulsion Controlled Aircraft at NASA-Dryden'' by Tom Tucker
{{Authority control Flight control systems ja:フライ・バイ・ワイヤ tr:Elektronik kumandalı uçuş vi:Các hệ thống kiểm soát bay