Canard (aeronautics)

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]

Some early aeroplanes such as the Brazilian Santos-Dumont 14-bis and French Canard Voisin had tail-first configuration which were seen by observers to resemble a flying duck — hence the name.

General characteristics

Unlike a conventional tailplane, a canard surface is trimmed to increase lift as speed increases. This equates to a negative coefficient for trim drag.^[4].

Canard classes

Canard designs fall into two main classes: the lifting-canard and the control-canard.^[5]

Other classes include the close-coupled type and active vibration damping.

Canard (yellow) on a Mirage IIIS

Rutan Long-EZ, with lifting-canard ahead of the cockpit.

A deflected control-canard on an RAF Typhoon F2

Lifting-canard

The first airplane to fly, the Wright Flyer, was a lifting-canard. In this configuration, the weight of the aircraft is shared between the main wing and the canard wing. 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.^[6]

For example, a lifting-canard generates an upload, in contrast to a conventional aft-tail which typically generates a download 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 downloaded conventional tail, so the difference in overall induced drag is actually not obvious, and depends on the details of the configuration.^[6][7]

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.^[6][7]

In any case, pitch stability requires that the lift generated by the canard wing is significant, so in order to minimise induced drag on the canard, it is usually of higher aspect ratio and greater airfoil camber than a control-canard.^[5] To achieve stability, the change in lift coefficient with angle of attack should be less than that for the main plane.^[8]

One way in which this can be achieved is to use the same aerofoil for both planes, but to rig the canard at a higher angle of incidence. This tends to increase drag induced by the foreplane, which may be given a high aspect ratio in order to limit drag.^[8]

With a lifting-canard, the main wing must be located further aft of the center of gravity range than with a conventional aft tail, and this increases the pitching moment caused by trailing-edge flaps. Aircraft with lifting canards cannot readily be designed with sophisticated trailing-edge flaps.^[5]

Control-canard

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. A control-canard mostly operates at zero angle of attack. Combat aircraft of canard configuration typically have a control-canard. In combat aircraft, the canard is usually driven by a computerized flight control system.^[5]

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 pitchup.^[5]

Control canards have poor stealth characteristics, because they present large, angular surfaces that can reflect RADAR signals.^[6] The Eurofighter Typhoon uses software control of its canards in order to reduce its radar cross section.

Close-coupled canard

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.^[9]

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 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.

Active vibration damping

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.

Examples of canard aircraft

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.

Lifting-canard types

Control-canard types

Close-coupled canard types

Active vibration damping types

Concept aircraft

Lifting-canard types

• Lockheed L-133

Gallery

The first powered airplane, the Wright Flyer, used dual, vertically-stacked canards	Eurofighter Typhoon of the Royal Air Force displaying at the Farnborough Airshow, 2006	Dassault Rafale, in service with the French Navy (Marine Nationale) and the French Air Force (Armée de l'Air)	Canards visible on a JAS 39 Gripen at the Farnborough Airshow
Grumman X-29, an experimental aircraft for forward swept wing research	The Rockwell-MBB X-31 Enhanced Fighter Maneuverability Demonstrator Aircraft	Canards (just behind the flight deck) on the XB-70 Valkyrie experimental bomber aircraft	Closeup of a Piaggio P180 Avanti's canards
The Beechcraft Starship Executive Transport	A Pterodactyl Ascender II+2 showing its canard control surface	Saab 37 Viggen of the Swedish Air Force	Miles Libellula (1941)

References

• Daniel P. Raymer (1989). Aircraft Design: A Conceptual Approach. American Institute of Aeronautics and Astronautics, Inc., Washington, DC. ISBN 0-930403-51-7. 
• R Wilkinson (2001). Aircraft Structures and Systems (2nd edition ed.). MechAero Publishing. 

Notes

See also

• Leading edge extension
• Stabilator
• Strake#Aircraft
• Tailplane
• Tandem wing

External links

• Desktop Aero - A Summary of Canard Advantages and Disadvantages