| Canard | |
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| Canards (blue) on the Saab Viggen |
In aeronautics, canard (French for "duck") is an airframe configuration of fixed-wing aircraft in which the forward surface is smaller than the rearward, the former being known as the "canard", while the latter is the main wing. In contrast a conventional aircraft has a small horizontal stabilizer behind the main wing.[1][2][3]
The early aircraft Santos-Dumont 14-bis of 1906, reminded[4] the French public of a flying duck with its shape - no tail and small control surfaces in the front. Thus the term became part of the English language.
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Unlike a conventional tailplane, in order to achieve longitudinal stability a canard surface is trimmed to increase lift as speed increases. This equates to a negative coefficient for trim drag.[5]
A canard design tends to be less controllable than a conventional design because ailerons on the main wing may be subject to turbulence from the canards that varies widely at different angle of attack, leading to conditions of deep stall. If the ailerons were located on the canards, the lever arm would be too short due to the narrow span, and also the twisting motion would be too far forwards of the center of mass.[6]
Canards have poor stealth characteristics because they present large, angular surfaces that tend to reflect radar signals.[7][8] The Eurofighter Typhoon uses software control of its canards in order to reduce its radar cross section.
Canard designs fall into two main classes: the lifting-canard and the control-canard.[9]
Other classes include the close-coupled type and active vibration damping.
In this configuration, the weight of the aircraft is shared between the main wing and the canard wing. It may be described as an extreme conventional configuration with the following features:
The pros and cons of the canard versus conventional configurations are numerous and complex, and it is impossible to say which is superior without considering a specific design application.[7]
For example, a lifting-canard generates an upload, in contrast to a conventional aft-tail which typically generates a downward lift force that must be counteracted by extra lift on the main wing, which may appear to unambiguously favor the canard. However, the downwash interaction between the two surfaces is unfavorable for the canard, and favorable for the conventional tail, so the difference in overall induced drag is actually not obvious, and depends on the details of the configuration.[7][11]
Another example is that the upward canard lift appears to increase the overall lift capability of the configuration. However, pitch stability flight safety requirements dictate that the canard must stall before the main wing, so the main wing can never reach its maximum lift capability. Hence, the main wing must then be larger than on the conventional configuration, which increases its weight and profile drag. Again, the relative merit depends on the details of the configuration and cannot be generalized.[7][11]
In any case, pitch stability requires that the lift slope of the canard wing is lower than the lift slope of the main wing [9] : to achieve stability, the change in lift coefficient with angle of attack should be less than that for the main plane.[12]. The first powered airplane to fly, the Wright Flyer, a lifting-canard, was pitch unstable. Following Wright Flyers had some ballast to the nose.
The most common way in which pitch stability can be achieved is to increase 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[12]. A canard airfoil has commonly a greater airfoil camber than the wing.
With a lifting-canard, the main wing must be located further aft of the center of gravity than with a conventional aft tail, and this increases the nose-pitching moment caused by the deflection of trailing-edge flaps. Highly loaded canards do not have sufficient extra lift to balance this moment, so lifting-canard aircraft cannot readily be designed with powerful trailing-edge flaps.[9]
In the later control-canard, most of the weight of the aircraft is carried by the main wing and the canard wing 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. Combat aircraft of canard configuration typically have a control-canard. In combat aircraft, the canard is usually driven by a computerized flight control system.[9]
One benefit obtainable from a control-canard is 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 main wing can be optimized without having to guard against pitch-up.[9]
In the close-coupled canard, the foreplane is located just above and forward of the main wing. At high angles of attack the canard surface directs airflow downwards over the wing, reducing turbulence which results in reduced drag and increased lift.[13]
The canard foreplane may be fixed as on the IAI Kfir, or have landing flaps as on the Saab Viggen, or it may be moveable and also act as a control-canard during normal flight as on the Dassault Rafale.
A close-coupled canard is very useful for a supersonic delta wing design which gains lift in both transonic flight (such as for supercruise) and also in low speed flight (such as take offs and landings).[6]
A moustache is a small, high aspect ratio foreplane of close-coupled configuration. The surface is typically retractable at high speed and is deployed only for low-speed flight. First seen on the Dassault Milan, and later on the Tupolev Tu-144.
A large aircraft flying fast at low altitude can experience significant aerodynamic buffeting, leading to crew fatigue and reduced airframe life. Aircraft such as the B-1 Lancer incorporate small canard surfaces as part of an active vibration damping system that reduces these adverse effects.
Some aircraft that have employed this configuration are listed below. A few types are listed twice, for example where the foreplane acts as a control-canard during normal flight and as a close-coupled type at high angles of attack.
Early canards
Experimental canards
General aviation and homebuilt canards
Executive canards
Military canards
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