Canard (aeronautics)

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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

Canard foreplanes act in a similar way to conventional tailplane and elevators, but due to swap in position about the centre of gravity control surface actions have the opposite effect.

Advantages

• A canard arrangement produces more lift than a conventional set-up when total lift produced is considered. During manoeuvres the canard control surfaces mirror those of the main wing, adding to the lift to climb and decreasing the lift to descend. This means that the aircraft can move tighter and faster than with a conventional set-up.
• Because the canard generates upward lift, unlike with a tail plane which produces downward or negative lift, there is a reduction in the lift required from the main wing to overcome the weight of the aircraft. This results in a reduction in lift-induced drag. Hence, the overall drag and lift requirements of the aircraft are reduced.
• The canard is sometimes designed to stall prior to the main wing. This means that once the canard stalls, the nose tends to pitch down, thus reducing the angle of attack of the main wing and reducing the likelihood of that stalling. 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 the canard and the main wing to stall.
• In a propeller aircraft, a canard normally uses a pusher configuration. This reduces fuselage drag, because the fuselage is not operating in the increased flow induced by the propeller.

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 usually, 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 (C_L), while the main wing, although much larger, operates at a much smaller C_L 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.
• In the case of a pusher propeller, the propeller operates in the wake of the canard, fuselage, wing and landing gear. Also, the propeller diameter is often smaller than optimum, because of ground clearance considerations at take-off. A smaller propeller operating in a large wake will result in reduced propulsive efficiency.

Although some of the advantages and disadvantages above apply to all situations, a few of the disadvantages can be and have been used in the design of high-performance military aircraft, where aerodynamic instability can allow for a significant improvement in the maneuverability of the aircraft, such as with fly-by-wire control systems. In the civil aviation industry the disadvantages are seen to far outweigh the advantages, and with the notable exception of a range of light aircraft produced by Burt Rutan, few canard-design civilian aircraft have been successful.

Examples of canard aircraft

Aircraft that have successfully employed this configuration include:

Gallery

References

• Aircraft Structures and Systems (Second Edition): R Wilkinson: MechAero Publishing(2001)

See also

• Tandem wing
• Stabilator
• Tailplane

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Last updated on Sunday July 13, 2008 at 10:42:47 PDT (GMT -0700)
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