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An aileron (French for 'little wing') is a hinged flight control surface usually attached to the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll, or movement around the aircraft's longitudinal axis, which normally results in a change in heading due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.
The aileron was first patented by the British scientist and inventor Matthew Piers Watt Boulton in 1868, based on his 1864 paper On Aërial Locomotion. Even though there was extensive prior art in the 19th century for the aileron and its functional analog, wing warping, in 1906 the United States granted an expansive patent to the Wright Brothers of Dayton, Ohio for the invention of a system of aerodynamic control that manipulated an airplane's control surfaces. Much litigation ensued—as with the earlier automotive Selden patent—over the legal issues of lateral roll control, until World War I compelled the U.S. Government to legislate a legal resolution.
In the present day, ailerons have become highly refined in their designs and performance with multiple types created to suit the various fixed wing aircraft in existence.
The name 'aileron', from French, meaning 'little wing', also refers to the extremities of a bird's wings used to control their flight. It first appeared in print in the 7th edition of Cassell's French-English Dictionary of 1877, with its lead meaning of "small wing". In the context of powered airplanes it appears in print about 1908. Prior to that, ailerons were often referred to as rudders, their older technical sibling, with no distinction between their orientations and functions, or more descriptively as horizontal rudders (in French, gouvernails horizontaux). Among the earliest printed aeronautical use of 'aileron' was that in the French aviation journal L'Aérophile of 1908.
Ailerons came into widespread use about 1915, well after the rudder and elevator flight controls. Although there were previously many conflicting claims over who first invented the aileron and its function, lateral, or roll control, the flight control device was invented and described by the British scientist and metaphysisist Matthew Piers Watt Boulton in his 1864 paper On Aërial Locomotion. He was the first to patent an aileron control system in 1868.
Boulton's description of his lateral flight control system was both clear and complete. It was "the first record we have of appreciation of the necessity for active lateral control as distinguished from [passive lateral stability].... With this invention of Boulton's we have the birth of the present-day three torque method of airborne control" as was praised by Charles Manly. This was also endorsed by C.H. Gibbs-Smith. Boulton's British patent, No. 392 of 1868, issued about 35 years before ailerons were 'reinvented' in France, became forgotten and lost from sight until after the flight control device was in general use.[Note 1] Gibbs-Smith stated on several occasions that if the Boulton patent had been revealed at the time of the Wright brothers' legal filings, they might not have been able to claim priority of invention for the lateral control of flying machines. The fact that the Wright brothers were able to gain a patent in 1906 did not invalidate Boulton's lost and forgotten invention.
Boulton had described and patented ailerons in 1868 and they were not used on manned aircraft until they were employed on Robert Esnault-Pelterie’s glider in 1904, although in 1871 a French military engineer, Charles Renard, built and flew an unmanned glider incorporating ailerons on each side (which he termed ‘winglets’), activated by a Boulton-style pendulum controlled single-axis autopilot device.
The pioneering U.S. aeronautical engineer Octave Chanute published descriptions and drawings of the Wright brothers' 1902 glider in the leading aviation periodical of the day, L'Aérophile, in 1903. This prompted Esnault-Pelterie, a French military engineer, to build a Wright-style glider in 1904 that used ailerons in lieu of wing warping. The French journal L’Aérophile then published photos of the ailerons on Esnault-Pelterie’s glider which were included in his June 1905 article, and its ailerons were widely copied afterward.
The Wright brothers used wing warping instead of ailerons for roll control on their glider in 1902, and about 1904 their Flyer II was the only one able to do a coordinated banked turn. During the early years of powered flight they had better roll control than airplanes that used movable surfaces. From 1908, as aileron designs were refined it became clear that ailerons were much more effective and practical than wing warping. Ailerons also had the advantage of not weakening the airplane's wing structure as did the wing warping technique, which was one reason for Esnault-Pelterie's decision to switch to ailerons.
As aileron design became more sophisticated and efficient, the flight control device entered widespread use starting in 1915. The U.S. Government, frustrated by the lack of its country's aeronautical advances during World War I, enforced a patent pool effectively putting an end to the Wright brothers patent war. The Wright brothers quietly changed their aircraft flight controls from wing warping to the use of ailerons at that time as well.
Others who were previously thought to have been the first to introduce ailerons included:
Regardless of the 1868 Boulton patent and the extensive prior art created by multiple other experimenters, the Wright Brothers' Ohio patent attorney Henry Toulmin filed an expansive patent application, and on May 22, 1906 they were granted U.S. Patent 821393. The patent's importance lay in its claim of a new and useful method of controlling an airplane. The patent application included the claim for the lateral control of aircraft flight that was not limited to wing warping, but through any manipulation of the "....angular relations of the lateral margins of the airplanes [wings].... varied in opposite directions". Thus the patent explicitly stated that other methods besides wing-warping could be used for adjusting the outer portions of an airplane's wings to different angles on its right and left sides to achieve lateral roll control.
Multiple U.S. court decisions favoured the expansive Wright patent, which the Wright Brothers sought to enforce with licensing fees starting from $1,000 per airplane, and astoundingly said to range up to $1,000 per day. According to Louis S. Casey, a former curator of the Smithsonian Air & Space Museum in Washington, D.C., and other researchers, due to the patent they had received the Wrights stood firmly on the position that all flying using lateral roll control, anywhere in the world, would only be conducted under license by them.
The Wrights subsequently became embroiled with numerous lawsuits they launched against every recalcitrant aircraft builder which used lateral flight controls (essentially all manufacturers not paying them their demanded royalties), and the brothers were consequently blamed for playing "...a major role in the lack of growth and aviation industry competition in the United States comparative to other nations like Germany leading up to and during World War I". Years of protracted legal guerrilla warfare ensued with many other aircraft builders until the United States entered World War I, when its government imposed a legislated agreement between all U.S. parties which resulted in royalty payments of 1% to the Wrights.
There are still conflicting claims today over who first invented the aileron. Other 19th century engineers and scientists, including Charles Renard, Alphonse Pénaud, and Louis Mouillard, had described similar flight control surfaces, possibly serving as further inspiration to Boulton aside from Count d'Esterno. Another technique for lateral flight control, wing warping, was also described or experimented with by several people including Jean-Marie Le Bris, John Montgomery, Clement Ader, Edson Gallaudet, D.D. Wells, and Hugo Mattullath. Aviation historian C.H. Gibbs-Smith wrote that the aileron was "....one of the most remarkable inventions... of aeronautical history, which was immediately lost sight of".
In 1906, the Wright brothers obtained a patent, not for the invention of an airplane (which had existed for a number of decades in the form of gliders) but for the invention of a system of aerodynamic control that manipulated a flying machine's surfaces, including lateral flight control, although rudders, elevators and ailerons had been invented long before their efforts began. Irrespective of such controversies it was Boulton, undisputedly, who was the first to patent ailerons, doing so in 1868. The ailerons used by Esnault-Pelterie in 1904 followed Boulton's concept, although it is not known whether he had studied the 1868 patent or if he independently reinvented them.
Pairs of ailerons are typically interconnected so that when one is moved downward, the other is moved upward: the down-going aileron increases the lift on its wing while the up-going aileron reduces the lift on its wing, producing a rolling (also called 'banking') moment about the aircraft's longitudinal axis (which extends from the nose to the tail of an airplane). Ailerons are usually situated near the wing tip, but may sometimes also be situated nearer the wing root. Modern airliners may also have a second pair of ailerons on their wings, and the terms 'outboard aileron' and 'inboard aileron' are used to describe these positions respectively.
An unwanted side effect of aileron operation is adverse yaw—a yawing moment in the opposite direction to the roll. Using the ailerons to roll an aircraft to the right produces a yawing motion to the left. As the aircraft rolls, adverse yaw is caused primarily by the change in drag on the left and right wing. The rising wing generates increased lift, which causes increased induced drag. The descending wing generates reduced lift, which causes reduced induced drag. The difference in drag on each wing produces the adverse yaw. There is also often an additional adverse yaw contribution from a difference in profile drag between the up-aileron and down-aileron.
In a coordinated turn, adverse yaw is effectively compensated by the use of the rudder, which results in a sideforce on the vertical tail that opposes the adverse yaw by creating a favorable yawing moment. Another method of compensation is 'differential ailerons', which have been rigged such that the down-going aileron deflects less than the up-going one. In this case the opposing yaw moment is generated by a difference in profile drag between the left and right wingtips. Frise ailerons accentuate this profile drag imbalance by protruding beneath the wing of an upward-deflected aileron, most often by being hinged slightly behind the leading edge and near the bottom of the surface, with the lower section of the aileron surface's leading edge protruding slightly below the wing's undersurface when the aileron is deflected upwards, substantially increasing profile drag on that side. Ailerons may also be designed to use a combination of these methods.
With ailerons in the neutral position, the wing on the outside of the turn develops more lift than the opposite wing due to the variation in airspeed across the wing span, which tends to cause the aircraft to continue to roll. Once the desired angle of bank (degree of rotation about the longitudinal axis) has been obtained, the pilot uses opposite aileron to prevent the angle of bank from increasing due to this variation in lift across the wing span. This minor opposite use of the control must be maintained throughout the turn. The pilot also uses a slight amount of rudder in the same direction as the turn to counteract adverse yaw and to produce a "coordinated" turn wherein the fuselage is parallel to the flight path. A simple gauge on the instrument panel called the slip indicator, also known as "the ball", indicates when this coordination is achieved.
Particularly on larger or faster aircraft, control forces may be extremely heavy. Borrowing a discovery from boats that extending a control surface's area forward of the hinge lightens the forces needed first appeared on ailerons during World War I when ailerons were extended beyond the wingtip and provided with a horn ahead of the hinge. Known as overhung ailerons, possibly the best known examples are the Fokker Dr.I and Fokker D.VII. Later examples brought the counterbalance in line with the wing to improve control and reduce drag. This is seen less often now, due to the Frise type aileron[clarification needed] which provides the same benefit.
Trim tabs are small movable sections resembling scaled down ailerons located at or near the trailing edge of the aileron. On most propeller powered aircraft, the rotation of the propeller(s) induces a counteracting roll movement due to Newton's third law of motion, in that every action has an equal and opposite reaction. To relieve the pilot of having to provide continuous pressure on the stick in one direction (which causes fatigue) trim tabs are provided to adjust or trim out the pressure needed against any unwanted movement. The tab itself is deflected in relation to the aileron, causing the aileron to move in the opposite direction. Trim tabs come in two forms, adjustable and fixed. A fixed trim tab is manually bent to the required amount of deflection, while the adjustable trim tab can be controlled from within the cockpit so that different power settings or flight attitudes can be compensated for. Some large aircraft from the 1950s (including the Canadair Argus) used free floating control surfaces that the pilot controlled only through the deflection of trim tabs, in which case additional tabs were also provided to fine tune the control to provide straight and level flight.
Spades are flat metal plates, usually attached to the aileron lower surface, ahead of the aileron hinge, by a lever arm. They reduce the force needed by the pilot to deflect the aileron and are often seen on aerobatic aircraft. As the aileron is deflected upward, the spade produces a downward aerodynamic force, which tends to rotate the whole assembly so as to further deflect the aileron upward. The size of the spade (and its lever arm) determine how much force the pilot needs to apply to deflect the aileron. A spade works in the same manner as a horn but is more efficient due to the longer moment arm.
To prevent control surface flutter (aeroelastic flutter), the center of lift of the control surface should be behind the center of gravity of that surface. To achieve this, lead weights may be added to the front of the aileron. In some aircraft the aileron construction may be too heavy to allow this system to work without huge weight increases. In this case, the weight may be added to a lever arm to move the weight well out in front to the aileron body. These balance weights are tear drop shaped (to reduce drag), which make them appear quite different from spades, although both project forward and below the aileron. In addition to reducing flutter, mass balances also reduce the stick forces required to move the control surface in flight.
Used during aviation's pre-war "pioneer era" and into the early years of the First World War, these ailerons were each controlled by a single cable, which pulled the aileron up. When the aircraft was at rest, the ailerons hung vertically down. This type of aileron was used on the Farman III biplane 1909 and the Short 166. One of the disadvantages of this setup was a greater tendency to yaw than even with basic interconnected ailerons. During the 1930s a number of light aircraft used single acting controls but used springs to return the ailerons to their neutral positions when the stick was released.
Engineer Leslie George Frise (1897–1979) of the Bristol Aeroplane Company  developed an aileron shape that is pivoted at about its 25 to 30% chord line and near its bottom surface , in order to decrease stick forces as aircraft became faster during the 1930s. When the aileron is deflected up (to make its wing go down), the leading edge of the aileron dips into the airflow beneath the wing. The moment of the leading edge in the airflow helps to move up the trailing edge, which decreases the stick force. The down-moving aileron also adds energy to the boundary layer. The edge of the aileron directs air flow from the under-side of the wing to the upper surface of the aileron, thus creating a lifting force, which adds to the lift of the wing. This reduces the needed deflection angle of the aileron.
The Frise aileron bonus is often described as its ability to counteract adverse yaw. To do so, the leading edge of the aileron has to be sharp or bluntly rounded, that adds significant drag to the going up aileron and helps the aircraft to yaw (turn) in the desired direction, but adds some unpleasant, non linear effect and or potentially dangerous aerodynamic vibration (flutter). In fact the use of differential aileron movements — as with the famous de Havilland Tiger Moth biplane — is much more efficient to decrease adverse yaw moments
By careful design of the mechanical linkages, the up aileron can be made to deflect more than the down aileron (e.g., US patent 1565097). This helps reduce the likelihood of a wing tip stall when aileron deflections are made at high angles of attack. The idea is that the loss of lift associated with the up aileron carries no penalty while the increase in lift associated with the down aileron is minimized. The rolling couple on the aircraft is always the difference in lift between the two wings. The de Havilland Tiger Moth classic British biplane is one of the best-known aircraft, and one of the earliest, to use differential ailerons.
On the earliest aircraft, such as the Wright Flyer, lateral control was effected by twisting the outboard portion of the wing so as to increase or decrease lift by changing the angle of attack. This had the disadvantages of stressing the structure, being heavy on the controls, and of risking stalling the side with the increased angle of attack during a maneuver. By 1916, most designers had abandoned wing warping in favor of ailerons. Researchers at NASA and elsewhere have been taking a second look at wing warping again, although under new names. The NASA version is the X-53 Active Aeroelastic Wing while the United States Air Force tested the Adaptive Compliant Wing. It is probably significant that none of the aircraft designed after those experiments implemented this type of control.
Spoilers are devices that when extended into the airflow over a wing, disrupt the airflow and reduce the amount of lift generated. A small number of aircraft designs have used spoilers in lieu of, or to supplement ailerons, such as the F4 Phantom II and Northrop P-61 Black Widow, whose entire trailing edge was occupied with full span flaps.
Some aircraft such as the Fokker Spin and model gliders lack any type of lateral control. Those aircraft use a higher amount of dihedral than conventional aircraft. Deflecting the rudder gives yaw and a lot of differential wing lift, giving a yaw induced roll moment. This type of control system is most commonly seen on simpler 2-function (pitch and yaw control) glider models or 3-function (pitch, yaw and throttle control) model powered aircraft, such as radio-controlled versions of "Old Timer" free-flight engine-powered model aircraft.
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