A hardpoint is a location on an airframe designed to carry an external or internal load. This includes a station on the wing or fuselage of a civilian aircraft or military aircraft where external jet engine, countermeasures, gun pods, targeting pods or drop tanks can be mounted. In aeronautics, the term station is used to refer to a point of carriage on the frame of an aircraft. A station is rated to carry a certain amount of payload, it is a design number which has taken the rated g-forces of the frame into account. Therefore, point loads on the structure from externally or internally mounted stores, equipment and payload are the weight of the item and any pylons, mounting brackets, etc. multiplied by the maximum load factor which the aircraft will sustain when these items are carried. In civilian aviation a station is used to carry an external engine or a fuel tank; as engines are a fixed installation, operators refer to them with the designation of the engine. Therefore, the term is being used for load points meant for non-fixed installation.
In the military, a station can be called weapons station. Unlike civilian aircraft, NATO aircraft frame strength is required to remain without detrimental deformations at 115 percent of the limit or specified loads, without structural failure at ultimate loads. Most stations on a military aircraft serve to carry weapons. A minor number of stations can serve to carry external fuel tanks; these stations are called a general aeronautic term referring to usage of fuel like wet thrust. The term wet is carried over to the adapters, such as a pylon. Wing stations require pylons to carry objects. Stations on the fuselage may not require a pylon, such as the fuselage stations on the McDonnell Douglas F-15 Eagle, while other aircraft need pylons for certain stations in order to provide clearance for the landing gear retraction sequence or to provide necessary item space. Swing-wing aircraft that mount pylons on the moving portion of the wing must include a mechanism for swiveling the pylon as the wing sweeps fore or aft, in order to keep the pylon and store facing directly forwards at all times.
The F-111's outermost pair of hardpoints do not swivel, can only be used while the wing is extended. This restricts the aircraft to subsonic flight only while these pylons are fitted fitted with fuel tanks during ferry flights; the pylons are automatically jettisoned if the wing sweep moves past 26 degrees, which would mean that the aircraft is accelerating towards transonic speeds. Stations may be numbered for reference or not at all; the numbering is not consistent and may originate from elsewhere like station 559 on the B-52. There is not an order in which numbers are assigned; the order can be for example from left to right or vice versa, or mirrored and from outboard to inboard. The unique centerline station is no exception. A pylon serves to connect the frame of an aircraft to an item or object, being carried; the use of a pylon is necessary to clear the carriage item of control surfaces as well as prevent undesired disturbance of the flow of air toward the wing. Pylons are designed to be aerodynamic to reduce air resistance.
There are many different forms and designs of pylons distinctly termed accordingly like a wedge adaptor or stub wing pylon. Stealth aircraft like the F-22 or F-35 can use jettisonable pylons to retain stealth and reduce drag. While most pylons are part of a modular system, compatible with numerous stores, certain weapons and aircraft can require special pylons or adapters to carry a specific load. For example, in the Vietnam War, the "Wild Weasel" defense suppression version of the F-105 Thunderchief, the F-105G, could carry the usual AGM-45 "Shrike" anti-radiation missile on a standard pylon and launcher, but the newly developed AGM-78 Standard ARM required a specially designed and unique "LAU-78/a" launcher, unique to that missile. NATO suspension equipment and stores are standardized in MIL-STD-8591. A military pylon provides carriage and the ability to jettison external stores – weapons, fuel tanks or other ordnance. Pylons have a modular bay to carry a wider variety of stores; these adaptors can be bomb racks, launchers or other types of support structures each with their own provisions for mounting all other assemblies.
Racks carry and release stores. Racks are either part of, or can be inserted into, the modular bay of a support structure such as a pylon. A rack can mount a store or another piece of suspension equipment, for example, numerous bombs being mounted onto a single pylon, such as was done on F-105 Thunderchief missions over Vietnam, or the large external pylons on the B-52 Stratofortress, which can carry 12 unguided bombs in four triple ejector racks mounted to a single pylon. Alternatively, using the same pylon, but different racks and adapters, 9 air-launched cruise missiles can be carried. Using modular racks and universal adapters makes it much easier to configure the desired load; the store is mounted by locking the store's lugs with L-shaped suspension hooks in the rack. Depending on the mass of the store there can be a single lug or a number of lugs on the store separated by a certain distance; the distances are standardized. For NATO there is the 14-inch suspension for a 30-inch suspension for heavier stores.
Depending on specific stores from 1000 lb upward
Medieval architecture is architecture common in the Middle Ages, includes religious and military buildings. Styles include pre-Romanesque and Gothic. While most of the surviving medieval architecture is to be seen in churches and castles, examples of civic and domestic architecture can be found throughout Europe, in manor houses, town halls, almshouses and residential houses; the Latin cross plan, common in medieval ecclesiastical architecture, takes the Roman basilica as its primary model with subsequent developments. It consists of a nave and the altar stands at the east end. Cathedrals influenced or commissioned by Justinian employed the Byzantine style of domes and a Greek cross, with the altar located in the sanctuary on the east side of the church. Surviving examples of medieval secular architecture served for defense. Castles and fortified walls provide the most notable remaining non-religious examples of medieval architecture. Windows gained a cross-shape for more than decorative purposes, they provided a perfect fit for a crossbowman to safely shoot at invaders from inside.
Crenellated walls provided shelters for archers on the roofs to hide behind when not shooting invaders. While much of the surviving medieval architecture is either religious or military, examples of civic and domestic architecture can be found throughout Europe. Examples include manor houses, town halls and bridges, but residential houses. European architecture in the Early Middle Ages may be divided into Early Christian, Romanesque architecture, Russian church architecture, Norse Architecture, Pre-Romanesque, including Merovingian, Carolingian and Asturian. While these terms are problematic, they nonetheless serve adequately as entries into the era. Considerations that enter into histories of each period include Trachtenberg's "historicising" and "modernising" elements, Italian versus northern and Byzantine elements, the religious and political maneuverings between kings and various ecclesiastic officials. Romanesque, prevalent in medieval Europe during the 11th and 12th centuries, was the first pan-European style since Roman Imperial Architecture and examples are found in every part of the continent.
The term was not contemporary with the art it describes, but rather, is an invention of modern scholarship based on its similarity to Roman Architecture in forms and materials. Romanesque is characterized by a use of round or pointed arches, barrel vaults, cruciform piers supporting vaults. Romansque buildings are known throughout Europe; the various elements of Gothic architecture emerged in a number of 11th and 12th century building projects in the Île de France area, but were first combined to form what we would now recognise as a distinctively Gothic style at the 12th century abbey church of Saint-Denis in Saint-Denis, near Paris. Verticality is emphasized in Gothic architecture, which features skeletal stone structures with great expanses of glass, pared-down wall surfaces supported by external flying buttresses, pointed arches using the ogive shape, ribbed stone vaults, clustered columns and pointed spires. Windows contain stained glass, showing stories from lives of saints; such advances in design allowed cathedrals to rise taller than and it became something of an inter-regional contest to build a church as high as possible.
Variations included these Brick Gothic History of Arabic and Western European domes List of medieval stone bridges in Germany List of medieval bridges in France Architecture of the Tarnovo Artistic School Braun, Hugh, An Introduction to English Mediaeval Architecture, London: Faber and Faber, 1951. "Building the House of God: Architectural Metaphor and The Mystic Ark," Codex Aquilarensis: Revista de arte medieval Fletcher, Banister. ISBN 0-7506-2267-9. Cf. Part Two, Chapter 13. Rudolph, Conrad, "Building-Miracles as Artistic Justification in the Early and Mid-Twelfth Century," Radical Art History: Internationale Anthologie, ed. Wolfgang Kersten 398-410 Rudolph,Conrad, "The Architectural Metaphor in Western Medieval Artistic Culture: From the Cornerstone to The Mystic Ark," The Cambridge History of Religious Architecture, ed. Stephen Murray The stave churches in Norway
A strut is a structural component found in engineering, aeronautics and anatomy. Struts work by resisting longitudinal compression, but they may serve in tension. Part of the functionality of the clavicle is to serve as a strut between the scapula and sternum, resisting forces that would otherwise bring the upper limb close to the thorax. Keeping the upper limb away from the thorax is vital for its range of motion. Complete lack of clavicles may be seen in cleidocranial dysostosis, the abnormal proximity of the shoulders to the median plane exemplifies the clavicle's importance as a strut. Strut is a common name in timber framing for a brace of scantlings lighter than a post. Struts are found in roof framing from either a tie beam or a king post to a principal rafter. Struts may be straight or curved. In the U. K. strut is used in a sense of a lighter duty piece: a king post carries a ridge beam but a king strut does not, a queen post carries a plate but a queen strut does not, a crown post carries a crown plate but a crown strut does not.
Strutting or blocking between floor joists adds strength to the floor system. Struts provide outwards-facing support in their lengthwise direction, which can be used to keep two other components separate, performing the opposite function of a tie. In piping, struts restrain movement of a component in one direction while allowing movement or contraction in another direction. Strut channel made from steel, aluminium, or fibre-reinforced plastic is used in the building industry and is used in the support of cable trays and other forms of cable management, pipes support systems. Bracing struts and wires of many kinds were extensively used in early aircraft to stiffen and strengthen, sometimes to form, the main functional airframe. Throughout the 1920s and 1930s they fell out of use in favour of the low-drag cantilever construction. Most aircraft bracing struts are principally loaded in compression, with wires taking the tension loads. Lift struts came into increasing use during the changeover period and remain in use on smaller aircraft today where ultimate performance is not an issue.
They are applied to a high-wing monoplane and act in tension during flight. Struts have been used for purely structural reasons to attach engines, landing gear and other loads; the oil-sprung legs of retractable landing gear are still called Oleo struts. As components of an automobile chassis, struts can be passive braces to reinforce the chassis and/or body, or active components of the suspension. An example of an active unit would be a coilover design in an automotive suspension; the coilover combines a spring in a single unit. A common form of automotive suspension strut in an automobile is the MacPherson strut. MacPherson struts are purchased by the automakers in sets of four completed sub-assemblies: These can be mounted on the car bodies as part of the manufacturers' own assembly operations. A MacPherson strut combines the primary function of a shock absorber, with the ability to support sideways loads not along its axis of compression, somewhat similar to a sliding pillar suspension, thus eliminating the need for an upper suspension arm.
This means that a strut must have a more rugged design, with mounting points near its middle for attachment of such loads. Another type common type of strut used in air suspension is an air strut which combines the shock absorber with an air spring and can be designed in the same fashion as a coilover device; these come available in most types of suspension setups including beam axle and MacPherson strut style design. Transportation-related struts are used in "load bearing" applications ranging from both highway and off-road suspensions to automobile hood and hatch window supports to aircraft wing supports; the majority of struts feature a bearing, but only for the cases, when the strut mounts operate as steering pivots. For such struts, the bearing is the wear item, as it is subject to constant impact of vibration and its condition reflects both wheel alignment and steering response. In vehicle suspension systems, struts are most an assembly of coil-over spring and shock absorber. Other variants to using a coil-over spring as the compressible load bearer include support via pressurized nitrogen gas acting as the spring, rigid support which provides neither longitudinal compression/extension nor damping.
Cabane strut Chapman strut Jury strut Lift strut Spacers and standoffs Strut bar
An aircraft canopy is the transparent enclosure over the cockpit of some types of aircraft. An aircraft canopy provides a controlled and sometimes pressurized environment for the aircraft's occupants, allows for a greater field of view over a traditional flight deck. A canopy's shape is a compromise designed to minimize aerodynamic drag, while maximizing visibility for pilots and other crewmembers. Early aircraft had no canopies at all; the pilots were exposed to the weather, although most flying was done in good weather. Through World War I most aircraft had no canopy, although they had a small windshield to deflect the prop wash and wind from hitting the pilot in the face. In the 1920s and 1930s, the increasing speed and altitude of airplanes necessitated a enclosed cockpit and canopies became more common. Early canopies were made of numerous pieces of flat glass held in position by muntins; the muntins reduced visibility, problematic for military aircraft. Glass canopies were much heavier than acrylic canopies, which were first introduced shortly before World War II.
The acrylic bubble canopy was used on aircraft such as the Supermarine Spitfire and Westland Whirlwind, which gave better all-round visibility and reduced weight. It is still being used today on most fighter aircraft. In the 1970s, US aviation artist Keith Ferris invented a false canopy to paint on the underside of military aircraft, directly underneath the front of the plane, the purpose of, to confuse an enemy so they do not know in what direction the aircraft is headed; this ruse was inspired by animals and fishes that have similar markings on the head and tail, so they can confuse other creatures. Pilots remain skeptical of this feature, asserting that if the enemy is close enough to see the marking, they are too close to be fooled by it. On many high-performance military aircraft, the canopy is an integral part of the ejection seat system; the pilot cannot be ejected from the aircraft until the canopy is no longer in the path of the ejection seat. In most ejection seat equipped aircraft, the canopy is blown upwards and rearwards by explosive charges.
The relative wind blows the canopy away from the ejection path. However, on some aircraft, such as the McDonnell Douglas AV-8B Harrier II, the pilot may be forced to eject when in a hover, or when going too slow for the relative wind to move the canopy out of the path of the ejection seat. In that situation, the pilot could impact the canopy when ejecting. To overcome that possibility, some aircraft have a thin cord of plastic explosive zig-zagging across the canopy over the pilot's head. In the event of an ejection, the explosive cord is activated first; the ejection seat and pilot is launched through the shattered canopy. Most modern acrylic canopies are vacuum formed. A sheet of acrylic is secured to a female mould the entire assembly is heated in an oven until the acrylic is pliable; the air is removed from the mould and the acrylic sheet is drawn into it, forming the shape of the canopy. The acrylic is trimmed to the appropriate shape and attached to an aluminum or composite frame; some one-off canopies are made in a similar fashion, but since a mould would be too time-consuming to make, the acrylic is heated and vacuum formed until it approximates the shape the builder is seeking.
This type of construction is less precise and each canopy is unique. If multiple canopies will be needed, a mould is always used. Have Glass is the code name for a series of RCS reduction measures for the F-16 fighter, its primary aspect is the addition of an indium-tin-oxide layer to the gold tinted cockpit canopy, reflective to radar frequencies. An ordinary canopy would let radar signals straight through where they would strike the many edges and corners inside and bounce back to the radar source, the reflective layer dissipates these signals instead. Overall, Have Glass reduces an F-16's RCS by some 15 percent; the gold tint reduces glare from the sun to improve the pilot's visibility. The Malcolm Hood is a type of aircraft canopy developed for the Supermarine Spitfire, its concept proved valuable for other aircraft such as the North American Aviation-produced P-51B & C Mustangs as retrofit items, standard on versions of the Vought F4U Corsair, somewhat emulated on the models of the Luftwaffe's Focke-Wulf Fw 190 fighter.
The canopy was manufactured by the British company R Co which gave its name. Instead of taking a straight line between the canopy frames, the hood was bulged outward; this gave the pilot a better view to the rear....the Corsair's initial deficiencies were being worked out on a concurrent basis... The 689th production F4U-1 featured a number of significant changes; the most noticeable was that the cockpit was raised 18 centimeters to improve the pilot's forward view, a bulged canopy, along the lines of the "Malcolm Hood" used on Spitfires, replaced the original "birdcage" framed canopy to provide better all-round field of view. Bubble canopy
The Crusades were a series of religious wars sanctioned by the Latin Church in the medieval period. The most known Crusades are the campaigns in the Eastern Mediterranean aimed at recovering the Holy Land from Muslim rule, but the term "Crusades" is applied to other church-sanctioned campaigns; these were fought for a variety of reasons including the suppression of paganism and heresy, the resolution of conflict among rival Roman Catholic groups, or for political and territorial advantage. At the time of the early Crusades the word did not exist, only becoming the leading descriptive term around 1760. In 1095, Pope Urban II called for the First Crusade in a sermon at the Council of Clermont, he encouraged military support for the Byzantine Empire and its Emperor, Alexios I, who needed reinforcements for his conflict with westward migrating Turks colonizing Anatolia. One of Urban's aims was to guarantee pilgrims access to the Eastern Mediterranean holy sites that were under Muslim control but scholars disagree as to whether this was the primary motive for Urban or those who heeded his call.
Urban's strategy may have been to unite the Eastern and Western branches of Christendom, divided since the East–West Schism of 1054 and to establish himself as head of the unified Church. The initial success of the Crusade established the first four Crusader states in the Eastern Mediterranean: the County of Edessa, the Principality of Antioch, the Kingdom of Jerusalem and the County of Tripoli; the enthusiastic response to Urban's preaching from all classes in Western Europe established a precedent for other Crusades. Volunteers became Crusaders by taking a public vow and receiving plenary indulgences from the Church; some were hoping for a mass ascension into heaven at Jerusalem or God's forgiveness for all their sins. Others participated to satisfy feudal obligations, obtain glory and honour or to seek economic and political gain; the two-century attempt to recover the Holy Land ended in failure. Following the First Crusade there were numerous less significant ones. After the last Catholic outposts fell in 1291, there were no more Crusades.
The Wendish Crusade and those of the Archbishop of Bremen brought all the North-East Baltic and the tribes of Mecklenburg and Lusatia under Catholic control in the late 12th century. In the early 13th century the Teutonic Order created a Crusader state in Prussia and the French monarchy used the Albigensian Crusade to extend the kingdom to the Mediterranean Sea; the rise of the Ottoman Empire in the late 14th century prompted a Catholic response which led to further defeats at Nicopolis in 1396 and Varna in 1444. Catholic Europe was in chaos and the final pivot of Christian–Islamic relations was marked by two seismic events: the fall of Constantinople to the Ottomans in 1453 and a final conclusive victory for the Spanish over the Moors with the conquest of Granada in 1492; the idea of Crusading continued, not least in the form of the Knights Hospitaller, until the end of the 18th-century but the focus of Western European interest moved to the New World. Modern historians hold varying opinions of the Crusaders.
To some, their conduct was incongruous with the stated aims and implied moral authority of the papacy, as evidenced by the fact that on occasion the Pope excommunicated Crusaders. Crusaders pillaged as they travelled, their leaders retained control of captured territory instead of returning it to the Byzantines. During the People's Crusade, thousands of Jews were murdered in what is now called the Rhineland massacres. Constantinople was sacked during the Fourth Crusade. However, the Crusades had a profound impact on Western civilisation: Italian city-states gained considerable concessions in return for assisting the Crusaders and established colonies which allowed trade with the eastern markets in the Ottoman period, allowing Genoa and Venice to flourish; the Crusades reinforced a connection between Western Christendom and militarism. The term crusade used in modern historiography at first referred to the wars in the Holy Land beginning in 1095, but the range of events to which the term has been applied has been extended, so that its use can create a misleading impression of coherence regarding the early Crusades.
The term used for the campaign of the First Crusade was iter "journey" or peregrinatio "pilgrimage". The terminology of crusading remained indistinguishable from that of pilgrimage during the 12th century, reflecting the reality of the first century of crusading where not all armed pilgrims fought, not all who fought had taken the cross, it was not until the late 12th to early 13th centuries that a more specific "language of crusading" emerged. Pope Innocent III used the term negotium crucis "affair of the cross" for the Eastern Mediterranean crusade, but was reluctant to apply crusading terminology to the Albigensian crusade; the Song of the Albigensian Crusade from about 1213 contains the first recorded vernacular use of the Occitan crozada. This term was adopted into French as croisade and in English as crusade; the modern spelling crusade dates to c. 1760. Sinibaldo Fieschi used the terms crux transmarina for crusades in Outremer against Muslims and crux cismarina for crusades in Europe against other enemies of the church.
The Crusades in the Holy Land are traditionally counted as nine distinct campaigns, numbered from the First Crusade of 1095–99 to the Ninth Crusade of 1271–72. This conv
Boeing F/A-18E/F Super Hornet
The Boeing F/A-18E and F/A-18F Super Hornet are twin-engine, carrier-capable, multirole fighter aircraft variants based on the McDonnell Douglas F/A-18 Hornet. The F/A-18E single-seat and F/A-18F tandem-seat variants are larger and more advanced derivatives of the F/A-18C and D Hornet; the Super Hornet has an internal 20 mm M61 rotary cannon and can carry air-to-air missiles and air-to-surface weapons. Additional fuel can be carried in up to five external fuel tanks and the aircraft can be configured as an airborne tanker by adding an external air refueling system. Designed and produced by McDonnell Douglas, the Super Hornet first flew in 1995. Low-rate production began in early 1997 with full-rate production starting in September 1997, after the merger of McDonnell Douglas and Boeing the previous month; the Super Hornet entered service with the United States Navy in 1999, replacing the Grumman F-14 Tomcat, retired in 2006. The Royal Australian Air Force, which has operated the F/A-18A as its main fighter since 1984, ordered the F/A-18F in 2007 to replace its aging F-111C fleet.
RAAF Super Hornets entered service in December 2010. The Super Hornet is an evolutionary redesign of the McDonnell Douglas F/A-18 Hornet; the Super Hornet's unique wing and tail configuration can be traced back to an internal Northrop project P-530, c. 1965. Flying as the Northrop YF-17 "Cobra", it competed in the United States Air Force's Lightweight Fighter program to produce a smaller and simpler fighter to complement the larger McDonnell Douglas F-15 Eagle; the Navy directed that the YF-17 be redesigned into the larger F/A-18 Hornet to meet a requirement for a multi-role fighter to complement the larger and more expensive Grumman F-14 Tomcat serving in fleet defense interceptor and air superiority roles. The Hornet proved to be limited in combat radius; the concept of an enlarged Hornet was first proposed in the 1980s, marketed by McDonnell Douglas as Hornet 2000. The Hornet 2000 concept was an advanced F/A-18 with a larger wing and a longer fuselage to carry more fuel and more powerful engines.
The end of the Cold War led to a period of considerable restructuring. At the same time, U. S. Naval Aviation faced a number of problems; the McDonnell Douglas A-12 Avenger II was canceled in 1991 after the program ran into serious problems. The Navy considered updating an existing design as a more attractive approach to a clean-sheet program; as an alternative to the A-12, McDonnell Douglas proposed the "Super Hornet", an improvement of the successful previous F/A-18 models, which could serve as an alternate replacement for the A-6 Intruder. The next-generation Hornet design proved more attractive than Grumman's Quick Strike upgrade to the F-14 Tomcat, regarded as an insufficient technological leap over existing F-14s. At the time, the Grumman F-14 Tomcat was the Navy's primary air superiority fighter and fleet defense interceptor. Then-Secretary of Defense Dick Cheney described the F-14 as 1960s technology, drastically cut back F-14D procurement in 1989 before cancelling production altogether in 1991, in favor of the updated F/A-18E/F.
The decision to replace the Tomcat with an all-Hornet Carrier Air Wing was controversial. In 1992, the Navy canceled the Navy Advanced Tactical Fighter, which would have been a navalized variant of the Air Force's Lockheed Martin F-22 Raptor; as a cheaper alternative to NATF, Grumman proposed substantial improvements to the F-14 beyond Quick Strike, but Congress rejected them as too costly and reaffirmed its commitment to the less expensive F/A-18E/F. The Super Hornet was first ordered by the U. S. Navy in 1992; the Navy retained the F/A-18 designation to help sell the program to Congress as a low-risk "derivative", though the Super Hornet is a new aircraft. The Hornet and Super Hornet share many characteristics, including avionics, ejection seats, armament, mission computer software, maintenance/operating procedures; the initial F/A-18E/F retained most of the avionics systems from the F/A-18C/D's configuration at the time. The design would be expanded in the Super Hornet with an empty weight greater than the F-15C.
The Super Hornet first flew on 29 November 1995. Initial production on the F/A-18E/F began in 1995. Flight testing started in 1996 with the F/A-18E/F's first carrier landing in 1997. Low-rate production began in March 1997 with full production beginning in September 1997. Testing continued through 1999, finishing with aerial refueling demonstrations. Testing involved 3,100 test flights covering 4,600 flight hours; the Super Hornet underwent U. S. Navy operational tests and evaluations in 1999, was approved in February 2000. With the retirement of the F-14 in 2006, all of the Navy's combat jets have been Hornet variants until the F-35C Lightning II enters service; the F/A-18E single-seat and F/A-18F two-seat aircraft took the place of the F-14 Tomcat, A-6 Intruder, Lockheed S-3 Viking, KA-6D aircraft. An electronic warfare variant, the EA-18G Growler, replaces the EA-6B Prowler; the Navy calls this reduction in aircraft types a "neck-down". During the Vietnam War era, the Super Hornet's roles were performed by a combination of the A-1/A-4/A-7, A-6, F-8/F-4, RA-5C, KA-3
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