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An Aeroplane (Airplane in US usage), is defined as: a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight. (ICAO Doc 9110)
Straight and level flight of aircraftIn steady flight, an aircraft can be considered as being acted on by four forces in equilibrium: lift, weight, thrust, and drag. Thrust is the force generated by the engine and acts along the engine's thrust vector. Lift acts vertical to the motion of the aircraft. Drag acts parallel to the motion of the aircraft. Weight acts through the aircraft's centre of gravity, towards the centre of the Earth. In straight and level flight, lift is approximately equal to weight. In addition, if the aircraft is not accelerating, thrust is approximately equal to drag. In straight, climbing flight, thrust exceeds drag, and lift is approximately less than weight. In straight, descending flight thrust is less than drag, and lift is approximately less than weight. In turning flight, lift exceeds weight. Aircraft control and movementThere are three primary ways for an aircraft to change its orientation relative to the passing air. Pitch (movement of the nose up or down), Roll (rotation around the longitudinal axis, that is, the axis which runs along the length of the aircraft) and Yaw (movement of the nose to left or right.) Turning the aircraft (change of heading) requires the aircraft firstly to roll to achieve an angle of bank; when the desired change of heading has been accomplished the aircraft must again be rolled in the opposite direction to reduce the angle of bank to zero. Aircraft control surfacesYaw is induced by a moveable rudder, attached to a vertical fin usually at the rear of the aircraft. Sometimes the entire fin is movable. Movement of the rudder changes the size and orientation of the force the vertical surface produces. Since the force is created a distance behind the centre of gravity this sideways force causes a yawing motion. On a large aircraft there may be several independent rudders on the single fin for both safety and to control the inter-linked yaw and roll actions. It should be realized that using yaw alone is not a very efficient way of executing a level turn in an aircraft and will result in some sideslip. A precise combination of bank and lift must be generated to cause the required centripetal forces without producing a sideslip. Pitch is controlled by the rear part of the tailplane's horizontal stabilizer being hinged to create an elevator. By moving the elevator control backwards the pilot moves the elevator up (a position of negative camber) and the downwards force on the horizontal tail is increased. The angle of attack on the wings increased so the nose is pitched up and lift is generally increased. In micro-lights and hang gliders the pitch action is reversed - the pitch control system is much simpler so when the pilot moves the elevator control backwards it produces a nose-down pitch and the angle of attack on the wing is reduced. The system of a fixed tail surface and moveable elevators is standard in subsonic aircraft. Craft capable of supersonic flight often have a stabilator, an all-moving tail surface. Pitch is changed in this case by moving the entire horizontal surface of the tail. This seemingly simple innovation was one of the key technologies that made supersonic flight possible. In early attempts, as pilots exceeded the critical Mach number, a strange phenomenon made their control surfaces useless, and their aircraft uncontrollable. It was determined that as an aircraft approaches the speed of sound, the air approaching the aircraft is compressed and shock waves begin to form at all the leading edges and around the hinge lines of the elevator. These shock waves caused movements of the elevator to cause no pressure change on the stabilizer upstream of the elevator. The problem was solved by changing the stabilizer and hinged elevator to an all-moving stabilizer - the entire horizontal surface of the tail became a one-piece control surface. Also, in supersonic flight the change in camber has less effect on lift and a stabilator produces less drag. Aircraft that need control at extreme angles of attack are sometimes fitted with a canard configuration, in which pitching movement is created using a forward foreplane (roughly level with the cockpit). Such a system produces an immediate increase in lift and therefore a better response to pitch controls. This system is common in delta-wing aircraft (deltaplane), which use a stabilator-type canard foreplane. A disadvantage to a canard configuration compared to an aft tail is that the wing cannot use as much extension of flaps to increase wing lift at slow speeds due to stall performance. A combination tri-surface aircraft uses both a canard and an aft tail (in addition to the main wing) to achieve advantages of both configurations. A further design of tailplane is the V-tail, so named because that instead of the standard inverted T or T-tail, there are two vertical fins angled away from each other in a V (if they're arranged like a V, at least one of them isn't vertical). To produce yaw like a rudder, the two trailing edge control surfaces move in the same direction. To produce pitch like an elevator, the surfaces move in opposite directions. Roll is controlled by movable sections on the trailing edge of the wings called ailerons. The ailerons move differentially - one goes up as the other goes down. The difference in camber of the wing cause a difference in lift and thus a rolling movement. As well as ailerons, there are sometimes also spoilers - small hinged plates on the upper surface of the wing, originally used to produce drag to slow the aircraft down and to reduce lift when descending. On modern aircraft, which have the benefit of automation, they can be used in combination with the ailerons to provide roll control. The earliest powered aircraft built by the Wright brothers did not have ailerons. The whole wing was warped using wires. Wing warping is efficient since there is no discontinuity in the wing geometry. But as speeds increased unintentional warping became a problem and so ailerons were developed. The actual linkages within the aircraft are discussed in aircraft flight control systems. AerodynamicsA false explanation for lift has been put forward in mainstream books, and even in scientific exhibitions. Known as the equal transit-time fallacy, it states that the parcels of air which are divided by an airfoil must rejoin again; because of the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom. Therefore, because of its higher speed the pressure of the air above the airfoil must be lower. Despite the fact that this "explanation" is probably the most common of all, it is false in that there is no requirement that divided parcels of air rejoin again, and in fact they do not do so. Consider a parcel of air which is divided by an airfoil, one half of the parcel flowing over the upper surface and the other half flowing under the lower surface. The parcel flowing over the upper surface acquires higher speed and reaches the trailing edge of the airfoil before the slower parcel flowing under the lower surface. The two parcels never re-unite. This can be understood using knowledge of circulation and the Kutta-Joukowski theorem. See alsoAircraft Flight Control Systems
A flight control system consists of the flight control surfaces, the respective cockpit controls, connecting linkage, and necessary operating mechanisms to control aircraft in flight. The fundamentals of aircraft controls have been explained in aeronautics. Discussion here centers on the underlying mechanisms of the flight controls. Generally the cockpit controls are arranged like this:
Even when an aircraft uses different kinds of surfaces, such as ruddervators or flaperons, the aircraft will still normally be designed so that the yoke or stick controls pitch and roll in the conventional way, to avoid pilot confusion. In addition to the primary 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 (other types of trim, for rudder and ailerons, are common on larger aircraft but may also appear on smaller ones). Many aircraft have wing flaps, controlled by a switch or stick, which alter the shape of the wing to make it easier to control at slower speeds for takeoff and landing, and some have movable landing gear, also controlled by a switch or stick, which can be retracted to decrease drag while in the air. Many other, more advanced control systems may be available, including cowl flaps (which add drag as well as cooling the engine), slats, spoilers, and air brakes.
Flight control systemsMechanicalMechanical flight control systems are the most basic designs. They were used in early aircraft and currently in small aeroplanes where the aerodynamic forces are not excessive. The flight control systems uses a collection of mechanical parts such as rods, cables, pulleys and sometimes chains to transmit the forces of the cockpit controls to the control surfaces. The Cessna Skyhawk is a typical example. Since an increase in control surface area in bigger and faster aircraft leads to a large increase in the forces needed to move them, complicated mechanical arrangements are used to extract maximum mechanical advantage in order to make the forces required bearable to the pilots. This arrangement is found on bigger or higher performance propeller aircraft such as the Fokker 50. Some mechanical flight control systems use servo tabs that provide aerodynamic assistance to reduce complexity. Servo tabs are small surfaces hinged to the control surfaces. The mechanisms move these tabs, aerodynamic forces in turn move 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. HydromechanicalThe complexity and weight of a mechanical flight control systems increases considerably with size and performance of the aircraft. Hydraulic power overcomes these limitations. With hydraulic flight control systems aircraft size and performance are limited by economics rather than a pilot's strength. A hydraulic flight control systems has 2 parts:
The mechanical circuit links the cockpit controls with the hydraulic circuits. Like the mechanical flight control systems, it is made of rods, cables, pulleys, and sometimes chains.
The hydraulic circuit has hydraulic pumps, 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 servo valves control the movement of the actuators. The pilot's movement of a control causes the mechanical circuit to open the matching servo valves in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces. This arrangement is found in older jet transports and high performance aircraft. Examples include the Antonov An-225 and the Lockheed SR-71. Artificial feel devicesIn mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and can be felt by the pilot. This gives tactile feedback of airspeed and aids flight safety. For example, with the controls of the Avro Vulcan jet bomber, the requisite feedback was achieved by a spring device. The fulcrum of the device was moved in proportion to the square of the airspeed (for the elevators) to give increased resistance at higher speeds. Fly-by-wire
Mechanical and hydraulic flight control systems are heavy and require careful routing of flight control cables through the aircraft using systems of pulley, cranks, wires and, with hydraulically-assisted controls, hydraulic pipes. Both systems often require redundant backup, which further increases weight. Furthermore, both have limited ability to compensate for changing aerodynamic conditions. Dangerous characteristics such as stalling and spinning and Pilot-induced oscillation (PIO) at aft Centres of Gravity can still occur with these systems and depend on the aerodynamics and structure of the aircraft concerned rather than the control system itself. By using electrical control linkages combined with computers, designers can save weight, improve reliability, and use the computers to prevent the undesirable characteristics mentioned above. The words Fly-by-Wire imply only an electrically-signalled control system. The term is generally used, however, in the sense of computer-configured controls. This is where, between the operator and the final control actuator or surface, a computer system is interposed. This modifies the inputs of the pilot (or operator for non-aircraft systems) in accordance with software programmes. These are carefully developed and validated in order to produce maximum operational effect without compromising safety.
Electronic fly-by-wire systems can respond more flexibly to changing aerodynamic conditions, by tailoring flight control surface movements so that aircraft response to control inputs is consistent for all flight conditions. Electronic systems require less maintenance, whereas mechanical and hydraulic systems require lubrication, tension adjustments, leak checks, fluid changes, etc. Furthermore putting circuitry between pilot and aircraft can enhance safety; for example the control system can prevent a stall, or can stop the pilot from overstressing the airframe. A fly-by-wire system literally replaces physical control of the aircraft with an electrical interface. The pilot's commands are converted to electronic signals, and flight control computers determine how best to move the actuators at each control surface to provide the desired response. Those actuators initially are usually hydraulic, but electric actuators have been investigated. The main concern with fly-by-wire systems is reliability. While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers will immediately render the aircraft uncontrollable. For this reason, most fly-by-wire systems incorporate either redundant computers (triplex, quadruplex etc) or some kind of mechanical or hydraulic backup. A "mixed" control system such as the latter is not desirable and modern FBW aircraft normally avoid it by having more independent FBW channels, thereby reducing the possibility of overall failure to minuscule levels that are acceptable to the independent regulatory and safety authority responsible for aircraft design, testing and certification before operational service. AnalogThe fly-by-wire flight control system eliminates the complexity, fragility and weight of the mechanical circuit of the hydromechanical flight control systems and replaces it with an electrical circuit. The cockpit controls now operate signal transducers which generate the appropriate commands. The commands are processed by an electronic controller. The autopilot is now part of the electronic controller. The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves. The valves are operated by the electronic controller. This is the simplest and earliest configuration, an analog fly-by-wire flight control systems, first fitted to the Avro Vulcan in the 1950s. In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This is still used in the Embraer E-Jets family of aircraft and was used in Concorde, the first fly-by-wire airliner. On more sophisticated versions, analog computers replaced the electronic controller. The cancelled supersonic Canadian fighter, the Avro CF-105 Arrow, was built this way in the 1950s. Analog computers also allowed some customization of flight control characteristics, including relaxed stability. This was exploited by the early versions of F-16, giving it impressive maneuverability. DigitalA digital fly-by-wire flight control system is similar to its analog counterpart. However, the signal processing is done by digital computers. The pilot literally can "fly-via-computer". This increases flexibility as the digital computers can receive input from any aircraft sensor. It also increases electronic stability, because the system is less dependent on the values of critical electrical components in an analog controller. 
The computers read positions and forces from the pilot's controls and aircraft sensors. They solve differential equations that move the flight controls to carry out the intentions of the pilot. The program in the digital computers let aircraft designers tailor an aircraft's handling characteristics precisely, within the overall limits of what is possible with the aerodynamics and structure of the aircraft. For example, the software can prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits (the aircraft's envelope) such as the stall, spin or limiting G. Software can also be used to filter control inputs to avoid pilot-induced oscillation. Sidesticks or conventional control yokes can be used to fly such an aircraft. While the side stick offers the advantages of being lighter, mechanically simpler, and unobtrusive, Boeing considered the lack of visual feedback from the side stick a problem, and so uses conventional yokes in the 777 and the upcoming 787. The Airbus series have used side-sticks extensively and the new A380 super-jumbo uses them. In fighter aircraft, side-sticks are smaller such as in the F-16 Falcon, of which many thousands have been produced. As the computers continuously 'fly' the aircraft, pilot workload can be reduced. It is now possible to fly aircraft with relaxed stability. The primary benefit for military aircraft is more responsive flight performance and so-called 'carefree handling' because stalling and spinning and other undesirables can be prevented. Digital flight control systems enabled inherently unstable aircraft such as Lockheed Martin F-117 Nighthawk to fly. A modified NASA F-8C Crusader was the first digital fly-by-wire aircraft, in 1972. At about the same time, in the UK a Hunter fighter was modified at the Farnborough research centre with FBW controls in the right seat, the left seat being for a safety pilot with conventional controls and a FBW cut-out. The US Space Shuttle has digital fly-by-wire controls, first used in free-flight Approach and Landing Tests in 1977. In 1984, the Airbus A320 was the first airliner with digital fly-by-wire controls. In 2005, the Dassault Falcon 7X was the first business jet with fly-by-wire controls. On military aircraft, fly-by-wire improves combat survivability because it avoids hydraulic failure. A common reason behind the loss of military aircraft in combat is damage causing hydraulic leaks leading to loss of control. Most military aircraft have several completely redundant hydraulic systems, but hydraulic lines are often routed together, and can be damaged together. With a fly-by-wire system, wires can be more flexibly routed, are easier to protect and less susceptible to damage than hydraulic lines. The Federal Aviation Administration (FAA) of the United States adopted the RTCA/DO-178B, titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software. Any safety-critical component in a digital fly-by-wire system including control laws and the operation system will have to be certified to DO-178B Level A, which is applicable for potentially catastrophic failures. Nonetheless the top concern for computerized, digital fly-by-wire systems is reliability, even more than analog systems. This is because a computer running software is the only control path between pilot and control surfaces. If the computer software crashes, the pilot cannot control the aircraft. Therefore virtually all fly-by-wire systems are triply or quadruply redundant: they have three or four computers in parallel, and three or four separate wires to each control surface. If one or two computers crash, the others continue working. In addition most early digital fly-by-wire aircraft also had an analog electric, mechanical or hydraulic backup control system. For airliners, redundancy improves safety, but fly-by-wire also improves economy because the elimination of heavy mechanical items reduces weight. Boeing and Airbus differ in their FBW philosophies. In Airbus aircraft, the computer always retains ultimate control and will not permit the pilot to fly outside the normal flight envelope. In a Boeing 777, the pilot can override the system, allowing the plane to be flown outside this envelope in emergencies. The pattern started by Airbus A320 has been continued with the Airbus family and the Boeing 777. The Boeing 787 makes some minor improvements in the control laws, adopting some protections that Airbus has had in place for decades. Aircraft-engine integrationThe advent of FADEC (Full Authority Digital Engine Control) engines permits operation of the flight control systems and autothrottles for the engines to be fully integrated. On modern military aircraft other systems such as autostabilization, navigation, radar and weapons system are all integrated with the flight control systems. FADEC allows maximum performance to be extracted from the aircraft without fear of engine misoperation, aircraft damage or high pilot workloads. In the civil field, the integration increases flight safety and economy. The Airbus A320 and its fly-by-wire brethren are protected from low-speed stall. In such conditions, the flight control systems commands the engines to increase thrust without pilot intervention. In economy cruise modes, the flight control systems adjust the throttles and fuel tank selections more precisely than all but the most skillful pilots. FADEC reduces rudder drag needed to compensate for sideways flight from unbalanced engine thrust. The fuel management controls keep the aircraft's attitude accurately trimmed with fuel weight, rather than drag-inducing aerodynamic trims in the elevators. CarsFly by wire has now become mainstream enough to be used in mass production motor cars. The Toyota Prius Hybrid takes account of pedal action and gear changes to work out how much fuel is required, what CVT ratio to use, and how to apply the electric motor/generator. The concept car the Nissan Pivo also uses Drive by Wire technology to allow it to fully rotate without any hydraulic lines etc getting in the way. See also electronic throttle control. Fly-by-opticsFly-by-optics is sometimes used instead of fly-by-wire because it can transfer data at higher speeds, and it is immune to electromagnetic interference. In most cases, the cables are just changed from electrical to fiber optic cables. Sometimes it is referred to as "Fly-by-light" due to its use of Fiber Optics. The data generated by the software and interpreted by the controller remain the same. Power-by-wireHaving eliminated the mechanical circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits. The hydraulic circuit is replaced by an electrical power circuit. The power circuits power electrical or self-contained electrohydraulic actuators that are controlled by the digital flight control computers. All benefits of digital fly-by-wire are retained. The biggest benefits are weight savings, the possibility of redundant power circuits and tighter integration between the aircraft flight control systems and its avionics systems. The absence of hydraulics greatly reduces maintenance costs. This system is used in the Lockheed Martin F-35 and in Airbus A380 backup flight controls. IntelligentA newer flight control system, called Intelligent Flight Control System, is an extension of modern digital fly-by-wire flight control systems. The aim is to intelligently compensate for aircraft damage and failure during flight, such as automatically using engine thrust and other avionics to compensate for severe failures such as loss of hydraulics, loss of rudder, loss of ailerons, loss of an engine, etc. Several demonstrations were made on a flight simulator where a Cessna-trained small-aircraft pilot successfully landed a heavily-damaged full-size concept jet, without prior experience with large-body jet aircraft. This development is being spearheaded by NASA Dryden Flight Research Center[1]. It is reported that enhancement is mostly a software upgrade to an existing fully computerized digital fly-by-wire flight control systems. PropulsiveMany radio controlled airplanes, especially the low end `toy' models, are designed to be flown with no movable control surfaces at all. Instead, the planes typically have two propellers or ducted fans, one on each wing and the plane is controlled only by this. Usually the planes only have two control channels -- throttle and yaw. In general this results in a plane that flies poorly and is very difficult to fly, though some fly better than others. An example of a plane that is flown in this way is the Air Hogs Dominator. Some model planes are designed this way because it's often cheaper and lighter to control the speed of a motor than it is to actually provide a moving control surface. Full-scale planes are generally not designed without control surfaces like this because 1) it rarely produces good control even under ideal conditions and 2) a loss of engine power would lead to a total loss of flight control and an almost certain crash. Flying with disabled flight control surfacesSeveral aviation incidents have occurred in which the control surfaces of the aircraft became disabled, frequently due to loss of hydraulic systems. An airplane's loss of control surfaces results in its speed and direction being uncontrollable via conventional methods. Aircraft are not designed to be flown in such circumstances (which is why they have redundant hydraulics), but a few pilots have had some success in controlling such aircraft. TechniqueThe basic means of controlling the airplane is by making use of the position of the engine(s). If the engines are mounted under the centre of gravity, as is the case in most passenger jets, then increasing the thrust will raise the nose, while decreasing the thrust will lower it. This control method may call for control inputs that go against the pilot's instinct: when the plane is in a dive, adding thrust will raise the nose and vice versa. Additionally, asymmetrical thrust may be used for directional control: if the left engine is idled and power is increased on the right side this will result in a yaw to the left, and vice versa. If throttle settings allow the throttles to be shifted without affecting the total amount of power, then yaw control can be combined with pitch control. If the plane is yawing, then the wing on the outside of this yaw movement will go faster than the inner wing. This creates higher lift on the faster wing, resulting in a rolling movement, which helps to make a turn. Controlling speed is very difficult with engine control only, and will most likely result in a fast landing. A fast landing would be required anyway if the flaps can not be extended due to loss of hydraulics; loss of hydraulic systems is often the source of the loss of control surfaces. Only passenger jets with an engine mounted on the vertical tail, such as a DC-10, MD-11 or Lockheed Tristar, will be able to control the speed to a higher degree, as this engine is on the fuselage centreline and above the centre of gravity. On the other hand, planes that have two or four engines mounted on the sides of the empennage (as is the case with most business jets) will only have limited benefit from asymmetrical thrust. The biggest challenge for a pilot forced to fly an airplane without control surfaces is to avoid the phugoid instability mode (a cycle in which the aircraft repeatedly climbs and then dives), which requires careful use of the throttle. Because this type of aircraft control is difficult for humans to achieve, some researchers have attempted to integrate this control ability into the computers of fly-by-wire aircraft. Early attempts to add the ability to real airplanes were not very successful, the software having been based on experiments conducted in flight simulators where jet engines are usually modeled as "perfect" devices with exactly the same thrust on each engine, a linear relationship between throttle setting and thrust, and instantaneous response to input. Later, computer models were updated to account for these factors, and planes have been successfully flown with this software installed.[1] However, it remains a rarity on commercial aircraft. Incidents on commercial airliners
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This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Flight Control Systems" |
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