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Canard (aeronautics)

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For other uses, see canard.
Canards (blue) on the Saab Viggen

In aeronautics, canard (French for duck) is an airframe configuration of fixed-wing aircraft in which the tailplane is ahead of the main lifting surfaces, rather than behind them as in conventional aircraft, or when there is an additional small set of wings in front of the main lifting surface. The earliest models, such as the Wright Flyer, the world's first airplane, and the Santos-Dumont 14-bis, were seen by observers to resemble a flying duck — hence the name.

The term "canard" has also come to mean any horizontal airfoil mounted in front of the main wing, whether moving or not.

Canard aircraft characteristics

File:Mirage III MG 1490 canard.jpg
Canard (yellow) on an IAI Kfir

Advantages

  • The fuselage in a canard design is supported in two places, as the plane has two wings. Because of the added support the fuselage does not have to be as strong, which means it can be lighter. However this saving in weight is minimal when compared to the extra weight needed to strengthen and enlarge the wing.
  • Because the canard generates upward lift, it reduces the lift required from the main wing. This is in contrast to the conventional configuration where a horizontal tailplane generates downward lift. Because the main wing is not required to generate extra lift to overcome the downward lift on a horizontal tailplane, the canard configuration allows reduced lift-induced drag of the wing, lowering the overall drag of the aircraft.
  • The canard is designed to stall prior to the main wing. Once the canard stalls, the nose tends to pitch down, thus reducing the angle of attack on the main wing. However, that is not to say that the main wing cannot stall. A vertical gust that causes a sufficiently high angle of attack on the main wing will cause both canard and main wing to stall.

Disadvantages

  • The wing root operates in the downwash from the canard surface, which reduces its efficiency although the effect of the downwash does not cause as large of a problem as the tailplane would experience in a conventional set-up.
  • The wing tips operate in the upwash from the canard surface which increases the angle of attack on the tips and promotes premature separation of the air flowing over the wing tip. This premature separation at one tip or the other would promote wing-drop at the approach to the stall, leading to a spin. This must be avoided by precautions in the design of the wing, and may require extra weight in the wing structure outboard of the wing root.
  • Because the canard must be designed to stall before the main wing, the main wing never stalls and so never achieves its maximum lift coefficient. This may require a larger wing to provide extra wing area in order for the airplane to achieve the desired takeoff and landing distance performance.
  • It is often difficult to apply flaps to the wing in a canard design. Deploying flaps causes a large nose-down pitching moment, but in a conventional aeroplane this effect is considerably reduced by the increased downwash on the tailplane which produces a restoring nose-up pitching moment. With a canard design there is no tailplane to alleviate this effect. The Beechcraft Starship attempted to overcome this problem with a swing-wing canard surface which swept forwards to counteract the effect of deploying flaps, but many canard designs have no flaps at all.
  • In order to achieve longitudinal stability, most canard designs feature a small canard surface operating at a high lift coefficient (CL), while the main wing, although much larger, operates at a much smaller CL and never achieves its full lift potential. Because the maximum lift potential of the wing is typically unavailable, and flaps are absent or difficult to use, takeoff and landing distances and speeds are often higher than for similar conventional aircraft.

Examples of canard aircraft

Aircraft that have successfully employed this configuration include:

A Delta IV class submarine with canards on the sail

Submarines also use canards to control "flight" through water without noisy variation in ballast. Large surface vessels use retractable canards to stabilize in rough seas or extreme maneuvers.

Wikimedia Commons has media related to Category:Canard Aircraft.

References

See also

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