Expert: Paul Soderman - 3/16/2008

Question
It does appear to me that pound for pound, horsepower for horsepower, that the canard designs somehow quite significantly beat more conventional airplane designs.  It seems to be true whether the canard and comparable plane are tandem seating, side by side seating, two passenger, four passenger, and whether the traditional craft is tricycle or tail dragger.  There appears to be some secret here beyond the usual explanation of the canard taking some of the lift load requirement from the main wing, and the fact that perhaps the canard is better in that it is uplifting rather than downpressuring as some would suggest is the case in a traditional design.  Is there some large, frequently overlooked explanation for the canards overall outperformance over traditional designs?  Thank you.

Answer
Kim
The canard has the advantage over the tail mounted stabilizer/elevator in aircraft maneuverability, but does not work as well with a flapped wing.  Hence, you see them on many military planes and few commercial planes.  Perhaps Wikipedia summarizes the pros and cons best as described here:

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.[citation needed]

   * 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. This reduction in the required lift generation by the wing to over come the weight of the aircraft a reduction in lift-induced drag by the wing. As well as removal of the negative lift generated by the tailplane and the associated lift-induced drag. Overall drag and lift requirements of the aircraft is reduced.[citation needed]

   * 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. 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.[citation needed]

[edit] 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.[citation needed]

   * 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.[citation needed]

   * 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.[citation needed]

   * 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.[citation needed]

   * 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.[citation needed]

   * In the case of an 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 rotation. A smaller propeller operating in a large wake will result in reduced propulsive efficiency.[citation needed]


Although some of the advantages and disadvantages above apply to all situation a few of the disadvantages can be, and have been used in the design of high performance military aircraft were aerodynamic instability can allow for a large improvement in the maneuverability of the aircraft.[citation needed]

Though in the civil aviation industry the disadvantages are seen to far outweigh the advantages and few canard design civil aircraft have been successful though with exception of a range of light aircraft produced by Burt Rutan.[citation needed]

[edit]

Paul