A yoke, alternatively known as a control wheel is a device used for piloting some fixed-wing aircraft. The pilot uses the yoke to control the attitude of the plane in both pitch and roll. Rotating the control wheel controls the ailerons and the roll axis. Fore and aft movement of the control column controls the pitch axis; when the yoke is pulled back the nose of the aircraft rises. When the yoke is pushed forward the nose is lowered; when the yoke is turned left the plane rolls to the left and when it is turned to the right the plane rolls to the right. Small to medium-size aircraft limited to propeller driven, feature a mechanical system whereby the yoke is connected directly to the control surfaces with cables and rods. Human muscle power alone is not enough for larger and more powerful aircraft, so hydraulic systems are used, in which yoke movements control hydraulic valves and actuators. In more modern aircraft, inputs may first be sent to a fly-by-wire system, which sends a corresponding signal to actuators attached to the control surfaces.
Yokes may feature a stick shaker, designed to help indicate the onset of stall, or a stick pusher, which assists in stall recovery. Yokes come in a variety of the most common being of a "U" or "W" design; some aircraft use an "M" style, such as the Concorde. There are some rarer archaic styles, such as circular designs much like a steering wheel. In larger aircraft they are mounted on a post protruding vertically from the floor, referred to as a control column. In most other planes, they are mounted on a horizontal tube. In the case of the Cirrus SR20 and Cirrus SR22, although the control looks like a side stick, it works like a yoke handle; the Cessna 162 uses a similar approach. Side-sticks and centre-sticks are better for making rapid control inputs and dealing with high g-forces, hence their use in military and aerobatic aircraft. However, yokes are less sensitive thanks to a larger range of motion and provide more visual feedback to the pilot. Most yokes are connected and will both move together, thus providing instant indication to the other pilot when one makes a control input.
This is in contrast to some fly-by-wire control sticks that allow each pilot to send different, sometimes conflicting, inputs though competing inputs are signaled on Airbus FBW craft. Yokes take up more room than side-sticks in the cockpit, may obscure some instruments. A yoke, unlike a side-stick, may be used comfortably with either hand; this can be useful if one needs to manipulate other controls in the cockpit. This advantage is shared with the centre-stick; the yoke incorporates other key functions such as housing thumb or finger buttons to enable the radio microphone, disengage the autopilot, trim the aircraft. In addition, there may be a checklist, or chronometer located in the yoke's centre. Yokes are not used on all aircraft. Helicopters use the majority of military fighter aircraft use a centre or side-stick; some light aircraft use a stick. The latest Airbus family of passenger jets use a side-stick, not unlike a joystick, to actuate control surfaces. There are computer input devices designed to simulate a yoke, intended for flight simulators.
Aircraft flight control system HOTAS, Hands On Throttle-And-Stick Joystick, computer input device
The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, other novel aircraft technologies. The X-29 was developed by Grumman, the two built were flown by NASA and the United States Air Force; the aerodynamic instability of the X-29's airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, to reduce weight; the aircraft first flew in 1984, two X-29s were flight tested through 1991. Two X-29As were built by Grumman from two existing Northrop F-5A Freedom Fighter airframes after the proposal had been chosen over a competing one involving a General Dynamics F-16 Fighting Falcon; the X-29 design made use of the forward fuselage and nose landing gear from the F-5As with the control surface actuators and main landing gear from the F-16. The technological advancement that made the X-29 a plausible design was the use of carbon-fiber composites.
The wings of the X-29, made of graphite epoxy, were swept forward at more than 33 degrees. The Grumman internal designation for the X-29 was "Grumman Model 712" or "G-712"; the X-29 is described as a three surface aircraft, with canards, forward-swept wings, aft strake control surfaces, using three-surface longitudinal control. The canards and wings result in reduced trim drag and reduced wave drag, while using the strakes for trim in situations where the center of gravity is off provides less trim drag than relying on the canard to compensate; the configuration, combined with a center of gravity well aft of the aerodynamic center, made the craft inherently unstable. Stability was provided by the computerized flight control system making 40 corrections per second; the flight control system was made up of three redundant digital computers backed up by three redundant analog computers. Each of the three would "vote" on their measurements, so that if any one was malfunctioning it could be detected.
It was estimated that a total failure of the system was as unlikely as a mechanical failure in an airplane with a conventional arrangement. The high pitch instability of the airframe led to wide predictions of extreme maneuverability; this perception has held up in the years following the end of flight tests. Air Force tests did not support this expectation. For the flight control system to keep the whole system stable, the ability to initiate a maneuver needed to be moderated; this was programmed into the flight control system to preserve the ability to stop the pitching rotation and keep the aircraft from departing out of control. As a result, the whole system as flown could not be characterized as having any special increased agility, it was concluded that the X-29 could have had increased agility if it had faster control surface actuators and/or larger control surfaces. In a forward swept wing configuration, the aerodynamic lift produces a twisting force which rotates the wing leading edge upward.
This results in a higher angle of attack. This aeroelastic divergence can lead to structural failure. With conventional metallic construction, a torsionally stiff wing would be required to resist twisting; the X-29 design made use of the anisotropic elastic coupling between bending and twisting of the carbon fiber composite material to address this aeroelastic effect. Rather than using a stiff wing, which would carry a weight penalty with the light-weight composite, the X-29 used a laminate which produced coupling between bending and torsion; as lift increases, bending loads force the wing tips to bend upward. Torsion loads attempt to twist the wing to higher angles of attack, but the coupling resists the loads, twisting the leading edge downward reducing wing angle of attack and lift. With lift reduced, the loads are reduced and divergence is avoided; the first X-29 took its maiden flight on 14 December 1984 from Edwards AFB piloted by Grumman's Chief Test Pilot Chuck Sewell. The X-29 was the third forward-swept wing jet-powered aircraft design.
On 13 December 1985, a X-29 became the first forward-swept wing aircraft to fly at supersonic speed in level flight. The X-29 began a NASA test program four months after its first flight; the X-29 proved reliable, by August 1986 was flying research missions of over three hours involving multiple flights. The first X-29 was not equipped with a spin recovery parachute, as flight tests were planned to avoid maneuvers that could result in departure from controlled flight, such as a spin; the second X-29 was involved in high angle-of-attack testing. X-29 number two was maneuverable up to an angle of attack of about 25 degrees with a maximum angle of 67° reached in a momentary pitch-up maneuver; the two X-29 aircraft flew a total of 242 times from 1984 to 1991. The NASA Dryden Flight Research Center reported that the X-29 demonstrated a number of new technologies and techniques, new uses of existing technologies, including the use of "aeroelastic tailoring to control structural divergence", aircraft control and handling during extreme instability, three-surface longitudinal control, a "double-hinged trailing-edge flaperon at supersonic speeds", effective high angle of attack control, vortex control, demonstration of military uti
The National Aeronautics and Space Administration is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research. NASA was established in 1958; the new agency was to have a distinctly civilian orientation, encouraging peaceful applications in space science. Since its establishment, most US space exploration efforts have been led by NASA, including the Apollo Moon landing missions, the Skylab space station, the Space Shuttle. NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle, the Space Launch System and Commercial Crew vehicles; the agency is responsible for the Launch Services Program which provides oversight of launch operations and countdown management for unmanned NASA launches. NASA science is focused on better understanding Earth through the Earth Observing System. From 1946, the National Advisory Committee for Aeronautics had been experimenting with rocket planes such as the supersonic Bell X-1.
In the early 1950s, there was challenge to launch an artificial satellite for the International Geophysical Year. An effort for this was the American Project Vanguard. After the Soviet launch of the world's first artificial satellite on October 4, 1957, the attention of the United States turned toward its own fledgling space efforts; the US Congress, alarmed by the perceived threat to national security and technological leadership, urged immediate and swift action. On January 12, 1958, NACA organized a "Special Committee on Space Technology", headed by Guyford Stever. On January 14, 1958, NACA Director Hugh Dryden published "A National Research Program for Space Technology" stating: It is of great urgency and importance to our country both from consideration of our prestige as a nation as well as military necessity that this challenge be met by an energetic program of research and development for the conquest of space... It is accordingly proposed that the scientific research be the responsibility of a national civilian agency...
NACA is capable, by rapid extension and expansion of its effort, of providing leadership in space technology. While this new federal agency would conduct all non-military space activity, the Advanced Research Projects Agency was created in February 1958 to develop space technology for military application. On July 29, 1958, Eisenhower signed the National Aeronautics and Space Act, establishing NASA; when it began operations on October 1, 1958, NASA absorbed the 43-year-old NACA intact. A NASA seal was approved by President Eisenhower in 1959. Elements of the Army Ballistic Missile Agency and the United States Naval Research Laboratory were incorporated into NASA. A significant contributor to NASA's entry into the Space Race with the Soviet Union was the technology from the German rocket program led by Wernher von Braun, now working for the Army Ballistic Missile Agency, which in turn incorporated the technology of American scientist Robert Goddard's earlier works. Earlier research efforts within the US Air Force and many of ARPA's early space programs were transferred to NASA.
In December 1958, NASA gained control of the Jet Propulsion Laboratory, a contractor facility operated by the California Institute of Technology. The agency's leader, NASA's administrator, is nominated by the President of the United States subject to approval of the US Senate, reports to him or her and serves as senior space science advisor. Though space exploration is ostensibly non-partisan, the appointee is associated with the President's political party, a new administrator is chosen when the Presidency changes parties; the only exceptions to this have been: Democrat Thomas O. Paine, acting administrator under Democrat Lyndon B. Johnson, stayed on while Republican Richard Nixon tried but failed to get one of his own choices to accept the job. Paine was confirmed by the Senate in March 1969 and served through September 1970. Republican James C. Fletcher, appointed by Nixon and confirmed in April 1971, stayed through May 1977 into the term of Democrat Jimmy Carter. Daniel Goldin was appointed by Republican George H. W. Bush and stayed through the entire administration of Democrat Bill Clinton.
Robert M. Lightfoot, Jr. associate administrator under Democrat Barack Obama, was kept on as acting administrator by Republican Donald Trump until Trump's own choice Jim Bridenstine, was confirmed in April 2018. Though the agency is independent, the survival or discontinuation of projects can depend directly on the will of the President; the first administrator was Dr. T. Keith Glennan appointed by Republican President Dwight D. Eisenhower. During his term he brought together the disparate projects in American space development research; the second administrator, James E. Webb, appointed by President John F. Kennedy, was a Democrat who first publicly served under President Harry S. Truman. In order to implement the Apollo program to achieve Kennedy's Moon la
AEA June Bug
The June Bug was an early US aircraft designed and flown by Glenn H. Curtiss and built by the Aerial Experiment Association in 1908; the June Bug is famous for winning the first aeronautical prize, the Scientific American Cup awarded in the United States. A solid silver sculpted trophy, $25,000 in cash, would be awarded to whoever made the first public flight of over 1 kilometer. Glenn Curtiss had a hobby of collecting trophies, he and the Aerial Experiment Association built the June Bug with hopes of winning the Scientific American Cup. Aerodrome #3 included the used aileron steering system, but a shoulder yoke made it possible for the pilot to steer by leaning from side to side; the varnish that sealed the wing fabric cracked in the heat, so a mixture of turpentine and gasoline was used. The June Bug had yellow wings because yellow ochre was added to the wing mixture in order to make the aircraft show up better in photographs, due to the orthochromic-form monochrome photographic techniques of that time.
It was named by Dr. Alexander Graham Bell after the common Phyllophaga, a beetle known colloquially in North America as the "June bug," because June bugs were observed to fly to aircraft: they have large stiff outer wings for gliding, more delicate smaller propeller-like wings that do the actual propulsion; the June Bug was tested in New York, at Stony Brook Farm. Curtiss flew it on three out of four tries on June 21, 1908, with distances of 456 ft, 417 ft, 1,266 ft at 34.5 mph. On June 25, performances of 2,175 ft and 3,420 ft were so encouraging that the Association contacted the Aero Club of America about trying for the Scientific American Cup; the Aero Club contacted the Wright brothers. Orville wrote to decline the opportunity on June 30, as the Wrights were busy completing their deal with the United States government; the message was received by July 1, Curtiss took to the air as requested on July 4. The flight was a great public event; the event was overseen by a delegation of 22 notable members of the Aero Club, headed by Alan R. Hawley.
Families came as early as 5 a.m. to claim a spot on the grassy hill, along with reporters, a motion picture film crew. The June Bug became the first airplane in the United States to perform in a movie. A thunderstorm began, umbrellas popped up around the hillside; the town of Hammondsport was nearly empty. The nearby Pleasant Valley Wine Company generously opened its doors and offered generous free samples to all who were there. Charles M. Manly, who had unsuccessfully tested the Langley Aerodrome in 1903, measured out the 1 km and 20 ft distance with plenty of volunteer help; the June Bug took one false start, going not far enough. On the second try, the airplane flew 5,360 ft in 1 minute 40 seconds, winning the trophy and a US$25,000 cash prize, it was such an amazing sight that one woman watching was hit by a train on nearby tracks and suffered two broken ribs. After the flight, the wine cellars reopened their doors with free champagne for all. Amidst the publicity following the flight, the Wrights sent a warning to Curtiss that they had not given permission for the use of "their" aircraft control system to be used "for exhibitions or in a commercial way".
In fact, none of the AEA's aircraft used a wing-warping system like the Wrights' for control, relying instead on triangular ailerons designed by Alexander Graham Bell, which he patented in December 1911. However, in 1913 a court ruled. Three years previous to the June Bug's flight, the Wrights had made flights of up to 24 miles without official witnesses; however the Wrights would have been required to install wheels and dispense with a catapult launch to compete for the 1908 prize. From October to November, the June Bug was modified by adding floats to it in an attempt to create a seaplane. Renamed Loon, attempts to fly it began on Keuka Lake on November 28. Although the aircraft could achieve speeds of up to 29 mph on the water, it could not take off, on January 2, 1909, one of the floats filled with water, the Loon unexpectedly sank, it was recovered, but rotted away in a nearby boathouse. A replica of the June Bug was flown in 1976 by Mercury Aircraft of Hammondsport. Data from General characteristics Length: 27 ft 5 in Wingspan: 42 ft 6 in Powerplant: 1 × Curtiss B-8 V-8 air-cooled piston engine, 40 hp Performance Maximum speed: 39 mph Related development Curtiss Golden Flyer Notes Bibliography Aircraft Ab - Ak
An aircraft stabilizer is an aerodynamic surface including one or more movable control surfaces, that provides longitudinal and/or directional stability and control. A stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are hinged, or it can itself be a movable surface such as a stabilator. Depending on the context, "stabilizer" may sometimes describe only the front part of the overall surface. In the conventional aircraft configuration, separate vertical and horizontal stabilizers form an empennage positioned at the tail of the aircraft. Other arrangements of the empennage, such as the V-tail configuration, feature stabilizers which contribute to a combination of longitudinal and directional stabilization and control. Longitudinal stability and control may be obtained with other wing configurations, including canard, tandem wing and tailless aircraft; some types of aircraft are stabilized with electronic flight control. A horizontal stabilizer is used to maintain the aircraft in longitudinal balance, or trim: it exerts a vertical force at a distance so the summation of pitch moments about the center of gravity is zero.
The vertical force exerted by the stabilizer varies with flight conditions, in particular according to the aircraft lift coefficient and wing flaps deflection which both affect the position of the center of pressure, with the position of the aircraft center of gravity. Transonic flight makes special demands on horizontal stabilizers. Another role of a horizontal stabilizer is to provide longitudinal static stability. Stability can be defined only; this maintains a constant aircraft attitude, with unchanging pitch angle relative to the airstream, without active input from the pilot. Ensuring static stability of an aircraft with a conventional wing requires that the aircraft center of gravity be ahead of the center of pressure, so a stabilizer positioned at the rear of the aircraft will produce lift in the downwards direction; the elevator serves to control the pitch axis. The upwash and downwash associated with the generation of lift is the source of aerodynamic interaction between the wing and stabilizer, which translates into a change in the effective angle of attack for each surface.
The influence of the wing on a tail is much more significant than the opposite effect and can be modeled using the Prandtl lifting-line theory. In the conventional configuration the horizontal stabilizer is a small horizontal tail or tailplane located to the rear of the aircraft; this is the most common configuration. On many aircraft, the tailplane assembly consists of a fixed surface fitted with a hinged aft elevator surface. Trim tabs may be used to relieve pilot input forces. Most airliners and transport aircraft feature a large, slow-moving trimmable tail plane, combined with independently-moving elevators; the elevators are controlled by the pilot or autopilot and serve to change the aircraft’s attitude, while the whole assembly is used to trim and stabilize the aircraft in the pitch axis. Many supersonic aircraft feature an all-moving tail assembly named stabilator, where the entire surface is adjustable. Variants on the conventional configuration include the T-tail, Cruciform tail, Twin tail and Twin-boom mounted tail.
Three-surface aircraft such as the Piaggio P.180 Avanti or the Scaled Composites Triumph and Catbird, the tailplane is a stabilizer as in conventional aircraft. Some earlier three-surface aircraft, such as the Curtiss AEA June Bug or the Voisin 1907 biplane, were of conventional layout with an additional front pitch control surface, called "elevator" or sometimes "stabilisateur". Lacking elevators, the tailplanes of these aircraft were not what is now called conventional stabilizers. For example, the Voisin was a tandem-lifting layout with a foreplane, neither stabilizing nor lifting. In the canard configuration, a small wing, or foreplane, is located in front of the main wing; some authors call it a stabilizer or give to the foreplane alone a stabilizing role, although as far as pitch stability is concerned, a foreplane is described as a destabilizing surface, the main wing providing the stabilizing moment in pitch. In unstable aircraft, the canard surfaces may be used as an active part of the artificial stability system, are sometimes named horizontal stabilizers.
Tailless aircraft lack a separate horizontal stabilizer. In a tailless aircraft, the horizontal stabilizing surface is part of the main wing. Longitudinal stability in tailless aircraft is achieved by designing the aircraft so that its aerodynamic center is behind the center of gravity; this is done by modifying the wing design, for example by varying the angle of incidence in the span-wise direction (wing washout or twist
A fighter aircraft is a military aircraft designed for air-to-air combat against other aircraft, as opposed to bombers and attack aircraft, whose main mission is to attack ground targets. The hallmarks of a fighter are its speed and small size relative to other combat aircraft. Many fighters have secondary ground-attack capabilities, some are designed as dual-purpose fighter-bombers; this may be for national security reasons, for advertising purposes, or other reasons. A fighter's main purpose is to establish air superiority over a battlefield. Since World War I, achieving and maintaining air superiority has been considered essential for victory in conventional warfare; the success or failure of a belligerent's efforts to gain air superiority hinges on several factors including the skill of its pilots, the tactical soundness of its doctrine for deploying its fighters, the numbers and performance of those fighters. Because of the importance of air superiority, since the early days of aerial combat armed forces have competed to develop technologically superior fighters and to deploy these fighters in greater numbers, fielding a viable fighter fleet consumes a substantial proportion of the defense budgets of modern armed forces.
The word "fighter" did not become the official English-language term for such aircraft until after World War I. In the British Royal Flying Corps and Royal Air Force these aircraft were referred to as "scouts" into the early 1920s; the U. S. Army called their fighters "pursuit" aircraft from 1916 until the late 1940s. In most languages a fighter aircraft is known as hunting aircraft. Exceptions include Russian, where a fighter is an "истребитель", meaning "exterminator", Hebrew where it is "matose krav"; as a part of military nomenclature, a letter is assigned to various types of aircraft to indicate their use, along with a number to indicate the specific aircraft. The letters used to designate a fighter differ in various countries – in the English-speaking world, "F" is now used to indicate a fighter, though when the pursuit designation was used in the US, they were "P" types. In Russia "I" was used, while the French continue to use "C". Although the term "fighter" specifies aircraft designed to shoot down other aircraft, such designs are also useful as multirole fighter-bombers, strike fighters, sometimes lighter, fighter-sized tactical ground-attack aircraft.
This has always been the case, for instance the Sopwith Camel and other "fighting scouts" of World War I performed a great deal of ground-attack work. In World War II, the USAAF and RAF favored fighters over dedicated light bombers or dive bombers, types such as the Republic P-47 Thunderbolt and Hawker Hurricane that were no longer competitive as aerial combat fighters were relegated to ground attack. Several aircraft, such as the F-111 and F-117, have received fighter designations though they had no fighter capability due to political or other reasons; the F-111B variant was intended for a fighter role with the U. S. Navy, but it was cancelled; this blurring follows the use of fighters from their earliest days for "attack" or "strike" operations against ground targets by means of strafing or dropping small bombs and incendiaries. Versatile multirole fighter-bombers such as the McDonnell Douglas F/A-18 Hornet are a less expensive option than having a range of specialized aircraft types; some of the most expensive fighters such as the US Grumman F-14 Tomcat, McDonnell Douglas F-15 Eagle, Lockheed Martin F-22 Raptor and Russian Sukhoi Su-27 were employed as all-weather interceptors as well as air superiority fighter aircraft, while developing air-to-ground roles late in their careers.
An interceptor is an aircraft intended to target bombers and so trades maneuverability for climb rate. Fighters were developed in World War I to deny enemy aircraft and dirigibles the ability to gather information by reconnaissance over the battlefield. Early fighters were small and armed by standards, most were biplanes built with a wooden frame covered with fabric, a maximum airspeed of about 100 mph; as control of the airspace over armies became important, all of the major powers developed fighters to support their military operations. Between the wars, wood was replaced in part or whole by metal tubing, aluminium stressed skin structures began to predominate. On 15 August 1914, Miodrag Tomić encountered an enemy plane while conducting a reconnaissance flight over Austria-Hungary; the Austro-Hungarian aviator waved at Tomić, who waved back. The enemy pilot took a revolver and began shooting at Tomić's plane. Tomić fired back, he swerved away from the Austro-Hungarian plane and the two aircraft parted ways.
It was considered the first exchange of fire between aircraft in history. Within weeks, all Serbian and Austro-Hungarian aircraft were armed; the Serbians equipped their planes with 8-millimetre Schwarzlose MG M.07/12 machine guns, six 100-round boxes of ammunition and several bombs. By World War II, most fighters were all-metal monoplanes armed with batteries of machine guns or cannons and some were capable of speeds approaching 400 mph. Most fighters up to this point had one engine.
Angle of attack
In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the oncoming flow; this article focuses on the most common application, the angle of attack of a wing or airfoil moving through air. In aerodynamics, angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is defined; the chord line of the root of the wing is chosen as the reference line. Another choice is to use a horizontal line on the fuselage as the reference line; some authors do not use an arbitrary chord line but use the zero lift axis where, by definition, zero angle of attack corresponds to zero coefficient of lift. Some British authors have used the term angle of incidence instead of angle of attack.
However, this can lead to confusion with the term riggers' angle of incidence meaning the angle between the chord of an airfoil and some fixed datum in the airplane. The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases; as the angle of attack of a fixed-wing aircraft increases, separation of the airflow from the upper surface of the wing becomes more pronounced, leading to a reduction in the rate of increase of the lift coefficient. The figure shows a typical curve for a cambered straight wing. Cambered airfoils are curved such. A symmetrical wing has zero lift at 0 degrees angle of attack; the lift curve is influenced by the wing shape, including its airfoil section and wing planform. A swept wing has a lower, flatter curve with a higher critical angle; the critical angle of attack is the angle of attack. This is called the "stall angle of attack".
Below the critical angle of attack, as the angle of attack decreases, the lift coefficient decreases. Conversely, above the critical angle of attack, as angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge. At the critical angle of attack, upper surface flow is more separated and the airfoil or wing is producing its maximum lift coefficient; as angle of attack increases further, the upper surface flow becomes more separated and the lift coefficient reduces further. Above this critical angle of attack, the aircraft is said to be in a stall. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed; the airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor, the center of gravity of the aircraft and other factors.
However the aircraft always stalls at the same critical angle of attack. The critical or stalling angle of attack is around 15° - 20° for many airfoils; some aircraft are equipped with a built-in flight computer that automatically prevents the aircraft from increasing the angle of attack any further when a maximum angle of attack is reached, regardless of pilot input. This is called the'angle of attack limiter' or'alpha limiter'. Modern airliners that have fly-by-wire technology avoid the critical angle of attack by means of software in the computer systems that govern the flight control surfaces. In takeoff and landing operations from short runways, such as Naval Aircraft Carrier operations and STOL back country flying, aircraft may be equipped with angle of attack or Lift Reserve Indicators; these indicators measure the angle of attack or the Potential of Wing Lift directly and help the pilot fly close to the stalling point with greater precision. STOL operations require the aircraft to be able to operate close to the critical angle of attack during landings and at the best angle of climb during takeoffs.
Angle of attack indicators are used by pilots for maximum performance during these maneuvers since airspeed information is only indirectly related to stall behaviour. Some military aircraft are able to achieve controlled flight at high angles of attack, but at the cost of massive induced drag; this provides the aircraft with great agility. A famous military example is sometimes thought to be Pugachev's Cobra. Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft is not capable of either aerodynamic directional control or maintaining level flight until the maneuver ends; the Cobra is an example of supermaneuvering as the aircraft's wings are well beyond the critical angle of attack for most of the maneuver. Additional aerodynamic surfaces known as "high-lift devices" including leading edge wing root extensions allow fighter aircraft much greater flyable'true' alpha, up to over 45°, compared to about 20° for aircraft without these devices; this can be helpful at high altitudes where slight maneuvering may require high angles of attack due to the low density of air in the upper atmosphere as well as at low speed at low altitude where the margin between level flight AoA and stall AoA is reduced.
The high AoA capability of the aircraft provides a buff