Canard (aeronautics)

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This article is about the airframe configuration. For other uses, see Canard (disambiguation).

Canards (lateral surfaces in blue) on the Saab Viggen

In aeronautics, a canard (French for "duck") is a fixed-wing aircraftconfiguration in which a small horizontal surface, also named the canard or foreplane, is positioned forward of the main wing in contrast to the conventional position at the tail.^[1]^[2]^[3] Because of this it is sometimes described as "tail-first".^[4]

The term "canard" arose in France. The appearance of the Santos-Dumont 14-bis of 1906 reminded the French public of a flying duck (Fr. canard),^[5] and later the Fabre Hydravion of 1910 was named "Le Canard".^[6] Thereafter all aeroplanes with a foreplane were known as canards.^[7]

History

Pioneer yearsThe Wright Brothers began experimenting with the foreplane configuration around 1900. Their first kite included a front surface for pitch control and they adopted this configuration for their first Flyer. They were aware that Otto Lilienthal had been killed in a glider with an aft tail, due to a lack of pitch control. They expected a foreplane to be a better control surface, in addition to being visible to the pilot in flight.

Many pioneers initially followed the Wrights' lead. For example the Santos-Dumont 14-bis aeroplane of 1906 had no tail but small control surfaces in the front. The Fabre Hydravion of 1910 was the first floatplane to fly and had a foreplane. It was named "Le Canard".

But canard behaviour was not properly understood and other European pioneers were establishing the tailplane — among them, Louis Blériot — as the "conventional" design. Some – including the Wrights – experimented with both fore and aft planes on the same aircraft, now known as the three surface configuration.

After 1911, few canard types would be produced for many decades. In 1914 W.E. Evans commented that "the Canard type model has practically received its death-blow so far as scientific models are concerned."^[8]

1914 to 1945[edit]

In 1917 de Bruyère constructed his C.1 biplane fighter. It featured a single (monoplane) canard foreplane with both conventional and ventral tail fins behind which was the rear-mounted pusher propellor. The tip sections of the upper wings were movable and acted as ailerons. The C.1 rolled over and crashed on its first flight.^[9]

First flown in 1927, the experimental Focke-Wulf F 19 "Ente" (duck) was more successful. Two examples were built and although one crashed for unrelated reasons, the second example continued flying until 1931.

Just before and during World War II some more experimental canard fighters were flown, including the Ambrosini SS.4, Curtiss-Wright XP-55 Ascender and Kyūshū J7W1 Shinden, but no production aircraft were completed. The Shinden was ordered into production "off the drawing board" but hostilities ceased before any other than prototypes had flown.

Just after the end of World War II in Europe in 1945, what may have been the first canard designed and flown in the Soviet Unionappeared as a test aircraft, the lightweight Mikoyan-Gurevich MiG-8 Utka. It was reportedly a favorite among MiG OKB test pilots for its docile, slow-speed handling characteristics and flew for some years, being used as a testbed during development of the (conventional) MiG-15.

The canard revival[edit]

From the 1950s, American designers and especially North American experimented with supersonic canard delta designs, with some such as the North American XB-70 Valkyrie and the Soviet equivalent Sukhoi T-4 flying in prototype form. But it was not until 1967 that the Swedish Saab 37 Viggen became the first canard aircraft to enter production. This spurred many designers, and canard surfaces sprouted on a number of designs derived from the popular Dassault Mirage delta-winged jet fighter. These included variants of the French Dassault Mirage III, Israeli IAI Kfir and South African Atlas Cheetah. The canard delta remains a popular configuration for combat aircraft.

The Viggen also inspired Burt Rutan to create a two seater homebuilt canard design, accordingly named VariViggen (1972). Rutan's next two canard designs, the VariEze and Long-EZ had longer-span swept wings. These designs were radically different from anything seen before ^[10] and were also very successful with many examples built.^[11] The 1980s saw the spreading of Rutan's ideas to other designers, including executive canards such as the OMAC Laser 300, Avtek 400 and Beech Starship.

The development of fly-by-wire and artificial stability produced a new generation of military canard designs. The Saab JAS 39 Gripenmultirole fighter flew in 1988 and was adopted by a number of national air forces. Others followed. Types which would follow it into operational service included the Eurofighter Typhoon in 1994 and the Chinese Chengdu J-10 in 1998.

Modern canards[edit]

Static canard designs can have issues with stability and behaviour in the stall. Modern computerized controls began to turn the complex interactions in airflow between the canard and the main wing from stability concerns into maneuverability advantages.^[12] Some canard aircraft designs have trim advantages that allow them to better adjust for center of mass changes due to load changes or fuel use, and for aerodynamic center changes when shifting between subsonic and supersonic flight.

Design principles[edit]

A canard foreplane may be used for various reasons such as lift, (in)stability, trim, flight control, or to modify airflow over the main wing. Design analysis has been divided into two main classes, for the lifting-canard and the control-canard.^[13] These classes may follow the close-coupled type or not, and a given design may provide either or both of lift and control.

Rutan Long-EZ, with high aspect ratio lifting-canard

Lift[edit]

In the lifting-canard configuration, the weight of the aircraft is shared between the wing and the canard. It has been described as an extreme conventional configuration but with a small highly-loaded wing and an enormous lifting tail which enables the centre of mass to be very far aft relative to the front surface.^[14]

A lifting canard generates an upload, in contrast to a conventional aft-tail which generates negative lift that must be counteracted by extra lift on the main wing. As the canard lift appears to increase the overall lift capability of the aircraft, this may appear to favor the canard layout. In particular, at takeoff the wing is most heavily loaded and where a conventional tail exerts a downforce worsening the load, a canard exerts an upward force relieving the load. This allows a smaller main wing.

However, the foreplane downwash effect on the wing lift distribution is unfavorable for the canard concept, so the difference in overallinduced drag is actually not obvious, and depends on the details of the configuration.^[12]^[14]^[15] Also, pitch stability requirements dictate that the canard must stall before the wing, so the wing can never reach its maximum lift capability. Hence, the wing must then be larger than on the conventional configuration, which increases its area, weight and profile drag.^[12]^[15]

A danger associated with an insufficiently-loaded canard—i.e. when the center of gravity too far aft—is that when approaching stall, the main wing may stall first. This causes the rear of the craft to drop, deepening the stall and sometimes preventing recovery.^[16]

With a lifting-canard type, the main wing must be located further aft of the center of gravity than a conventional wing, and this increases the downward pitching moment caused by the deflection of trailing-edge flaps. Small, highly-loaded canards do not have sufficient extra lift available to balance this moment, so lifting-canard aircraft cannot readily be designed with powerful trailing-edge flaps.^[13]

Control[edit]

The control canard on an RAF Typhoonin flight

In a control-canard design, most of the weight of the aircraft is carried by the wing and the canard is used primarily for longitudinal control during maneuvering. Thus, a control-canard mostly operates only as a control surface and is usually at zero angle of attack, carrying no aircraft weight in normal flight. Modern combat aircraft of canard configuration typically have a control-canard. In modern combat aircraft, the canard is usually driven by a computerizedflight control system.^[13]

One benefit obtainable from a control-canard is the avoidance of pitch-up. An all-moving canard capable of a significant nose-down deflection will protect against pitch-up. As a result, the aspect ratio and wing-sweep of the wing can be optimized without having to guard against pitch-up.^[13]

They are used to intentionally destabilize some combat aircraft in order to make them more manoeuvrable. In this case, electronic flight control systems use the pitch control function of the canard foreplane to create artificial static and dynamic stability.^[12]^[15]

Stability[edit]

A canard foreplane may be used as a horizontal stabiliser, whether stability is achieved statically^[17]^[18]^[19] or artificially (fly-by-wire).^[20]

Being placed ahead of the center of gravity, a canard foreplane acts directly to reduce Longitudinal static stability (stability in pitch). Nevertheless, a canard stabilizer may be added to an otherwise unstable design to obtain overall static pitch stability.^[17] To achieve this stability, the change in canard lift coefficient with angle of attack (lift coefficient slope) should be less than that for the main plane.^[21] A number of factors affect this characteristic.^[13]

For most airfoils, lift slope decreases at high lift coefficients. Therefore, the most common way in which pitch stability can be achieved is to increase the lift coefficient (so the wing loading) of the canard. This tends to increase the lift-induced drag of the foreplane, which may be given a high aspect ratio in order to limit drag.^[21] Such a canard airfoil has a greater airfoil camber than the wing.

Another possibility is to decrease the aspect ratio of the canard,^[22] with again more lift-induced drag and possibly a higher stall angle than the wing.

A design approach used by Burt Rutan is a high aspect ratio canard with higher lift coefficient (the wing loading of the canard is between 1.6 to 2 times the wing one) and a canard airfoil whose lift slope is non-linear (nearly flat) between 14° and 24°.^[23]

Another stabilisation parameter is the power effect. In case of canard pusher propeller: "the power-induced flow clean up of the wing trailing edge" ^[23] increases the wing lift slope. Conversely, a propeller located ahead of the canard (increasing the lift slope of the canard) has a strong destabilising effect.^[24]

Wright Flyer stability[edit]

The first powered airplane to fly, the Wright Flyer, a lifting-canard (although conceived as a control-canard),^[25] was "highly unstable" and barely controllable.^[26] Following the first flight, the Wright Flyers had some ballast added to the nose to move the center of gravity forward and reduce pitch instability. However the basics of pitch stability of the canard configuration were not understood by the Wright Brothers. F.E.C. Culick stated, "The backward state of the general theory and understanding of flight mechanics hindered them ... Indeed, the most serious gap in their knowledge was probably the basic reason for their unwitting mistake in selecting their canard configuration".^[27]

Close coupling[edit]

Saab 37 Viggen of the Swedish Air Force

In the close-coupled canard, the foreplane is located just above and forward of the wing. At high angles of attack (and therefore typically at low speeds) the canard surface directs airflow downward over the wing, reducing turbulence which results in reduced drag and increased lift.^[28] Typically the foreplane creates a vortex which attaches to the upper surface of the wing, stabilising and re-energising the airflow over the wing and delaying or preventing the stall.

The canard foreplane may be fixed as on the IAI Kfir, have landing flaps as on the Saab Viggen, or be moveable and also act as a control-canard during normal flight as on theDassault Rafale.

A close-coupled canard has been shown to benefit a supersonic delta wing design which gains lift in both transonic flight (such as forsupercruise) and also in low speed flight (such as take offs and landings).^[29]

Stealth[edit]

Canard aircraft are sometimes said to have poor stealth characteristics because they present large, angular surfaces that tend to reflectradar signals forwards.^[12]^[30] Canards have nevertheless been incorporated on several proposed stealth aircraft. Northrop's proposal for the Naval Advanced Tactical Fighter (ATF), termed NATF-23, incorporated canard on a stealthy airframe.^[31]^[32] Lockheed Martin employed canards on a stealth airframe in the Joint Advanced Strike Technology (JAST) program.^[33]^[34] McDonnell Douglas and NASA's stealthy X-36 featured the use of canards.^[35] The Eurofighter Typhoon uses software control of its canards in order to reduce its effective radar cross section.^[36]^[37]

Variable geometry[edit]

A moustache is a small, high aspect ratio foreplane which is deployed only for low-speed flight in order to improve handling at high angles of attack such as during takeoff and landing. It is retractable at high speed in order to avoid the Wave drag penalty of a canard design. First seen on the Dassault Milan, and later on the Tupolev Tu-144. NASA has investigated the use of a one-piece slewedequivalent called the conformably stowable canard, where as the surface is stowed one side sweeps backwards and the other forwards.^[38]

The Beechcraft Starship had a variable sweep canard surface. The sweep is varied to trim out the pitching effect cause by the wing flaps when deployed.^[39]

Ride control[edit]

B-1B Lancer front small fin surfaces

The Rockwell B-1 Lancer shows small front fin surfaces as part of an active vibration damping system that reduces significant aerodynamic buffeting during high-speed, low altitude flight. This buffeting is a leading cause of crew fatigue and reduced airframe life. As placed in front of the plane, these surfaces are described as "canard vanes" ^[40] or "canard fins".^[41]