Elevon
Control surfaces combining elevator and aileron functions.
Elevons are control surfaces on an aircraft that handle both pitch and roll, merging the jobs of an elevator and an aileron. They are common on tailless designs like flying wings. If a separate tail surface performs a similar combined role, it is called a stabilator, though stabilators are sometimes used only for pitch, as seen on the Piper Cherokee. Elevons sit on the trailing edge of each wing. Moving them both up or both down creates a pitching force—nose up or nose down. Moving them in opposite directions—one up, one down—produces a rolling force. These effects can be applied at the same time by setting the surfaces to different positions, such as one wing’s elevons fully down and the other’s partly down. The pilot controls the aircraft as if using separate aileron and elevator surfaces, through a yoke or stick. The inputs are mixed mechanically or electronically to set each elevon’s correct position.
Operational aircraft
One of the first operational aircraft to use elevons was the Avro Vulcan, a strategic bomber for the Royal Air Force’s V-force. The original B.1 variant had no elevons; it used four inboard elevators and four outboard ailerons on its delta wing. The redesigned B.2 variant replaced all those surfaces with eight elevons. At slow speeds, these elevons worked closely with the aircraft’s six electrically actuated three-position airbrakes. Another early user was the Convair F-102 Delta Dagger, a U.S. Air Force interceptor. A few years later, Convair built the B-58 Hustler, an early supersonic bomber, also fitted with elevons. Perhaps the most iconic aircraft with elevons was the Aérospatiale/BAC Concorde, a British-French supersonic passenger airliner. Designers had to maintain precise directional control at supersonic speeds and address the twisting forces from banks and turns. They managed these issues by adjusting the active ratio between inboard and outboard elevons as speed changed. At high speeds, only the innermost elevons—attached to the stiffest part of the wing—were active. The Space Shuttle Orbiter had four elevons on the trailing edges of its delta wing, but they only worked during atmospheric flight, such as its controlled descent to Earth. Outside the atmosphere, attitude control came from the Reaction Control System, 44 compact liquid-fueled rocket thrusters managed by a fly-by-wire flight control system.
- First operational aircraft
- Avro Vulcan
Lore & Background
One of the first operational aircraft to utilise elevons was the Avro Vulcan, a strategic bomber operated by the Royal Air Force's V-force. The original production variant of the Vulcan, designated as the B.1, did not have any elevons present; instead, it used an arrangement of four inboard elevators and four outboard ailerons along its delta wing for flight control. The Vulcan received elevons on its extensively redesigned second variant, the B.2; all of the elevators and ailerons were deleted in favour of eight elevons. When flown at slow speeds, the elevons operated in close conjunction with the aircraft's six electrically actuated three-position airbrakes. Another early aircraft to use elevons was the Convair F-102 Delta Dagger, an interceptor operated by the United States Air Force. A few years after the F-102's introduction, Convair built the B-58 Hustler, an early supersonic bomber, which was also equipped with elevons. Perhaps the most iconic aircraft fitted with elevons was the Aérospatiale/BAC Concorde, a British–French supersonic passenger airliner. In addition to the requirement to maintain precise directional control while flying at supersonic speeds, designers were also confronted by the need to appropriately address the substantial forces that were applied to the aircraft during banks and turns, which caused twisting and distortions of the aircraft's structure. The solution applied for both of these issues was via management of the elevons; specifically, as the aircraft speed varied, the active ratio between the inboard and outboard elevons was adjusted considerably. Only the innermost elevons, which are attached to the stiffest area of the wings, would be active while Concorde was flown at high speeds. The Space Shuttle Orbiter was furnished with elevons, although these were only operable during atmospheric flight, which would be encountered during the vehicle's controlled descent back to Earth. There were a total of four elevons affixed to the trailing edges of its delta wing. While flown outside of atmospheric flight, the Shuttle's attitude control was instead provided by the Reaction Control System (RCS), which consisted of 44 compact liquid-fueled rocket thrusters controlled via a sophisticated fly-by-wire flight control system. The Northrop Grumman B-2 Spirit, a large flying wing operated by the United States Air Force as a strategic stealth bomber, also used elevons in its control system. Northrop had opted to control the aircraft via a combination of split brake-rudders and differential thrust after assessing various different means of exercising directional control with minimal infringement on the aircraft's radar profile. Four pairs of control surfaces are positioned along the trailing edge of the wing; while most surfaces are used throughout the aircraft's flight envelope, the inner elevons are normally only ever applied while being flown at slow speeds, such as on approach to landing. To avoid potential contact damage during takeoff and to provide a nose-down pitching attitude, all of the elevons remain drooped during takeoff until a high enough airspeed has been attained. The B-2's flight surfaces are automatically adjusted and repositioned without pilot input to do so, these changes being commanded by the aircraft's complex quadruplex computer-controlled fly-by-wire flight control system in order to counteract the inherent instability of the flying wing configuration.
Reader's Guide
Elevons are significant because they enable pitch and roll control on tailless aircraft such as flying wings, where separate elevators and ailerons are not feasible. The article describes their use on several notable operational aircraft, including the Avro Vulcan B.2, which replaced separate elevators and ailerons with eight elevons; the Convair F-102 Delta Dagger and B-58 Hustler; the Concorde, which adjusted the active ratio between inboard and outboard elevons with speed; the Space Shuttle Orbiter, which used four elevons only during atmospheric descent; and the Northrop Grumman B-2 Spirit, which used four pairs of control surfaces including inner elevons primarily at slow speeds. The article also notes that elevons reduce camber when moved up in unison, decreasing maximum lift and efficiency, which is a drawback. Research efforts such as flexible wings and fluidics aim to integrate control functions with less mass, cost, drag, inertia, complexity, and radar cross section. The legacy of elevons is their role in enabling the flight of iconic delta-wing and flying-wing aircraft, with control inputs from the pilot mixed mechanically or electronically to position the elevons appropriately.
Did You Know?
- The Avro Vulcan B.2 used eight elevons, replacing the four inboard elevators and four outboard ailerons of the B.1.
- On the Concorde, only the innermost elevons were active at high speeds, as they were attached to the stiffest area of the wings.
- The Space Shuttle Orbiter's four elevons were only operable during atmospheric flight; outside the atmosphere, attitude control was provided by 44 RCS thrusters.
- The B-2 Spirit's inner elevons are normally only used at slow speeds, such as on approach to landing.
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