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Another post here triggers this question;

Since I was a kid I have read endless articles about how canards are more efficient because both wing-like surfaces are lifting, whereas a tail surface is adding weight.

Yet the most efficient aircraft, gliders, almost never use this layout. A few have, Solitare for instance, but obviously this claim is oversold given the complete dominance of conventional layouts.

So what’s the story here? Is it that they are more efficient but some other effect makes them unsuited for most roles? Or is it simply not true in practice?

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No. It's not true in practice.

  1. While the conventional layout puts the whole tail into the downwash of the wing, a canard will add downwash to the center wing and upwash to the outer wing. This means that while the tail of a conventional layout sees less angle of attack variation than the wing (which is good to prevent tailplane stalls), the up- and downwash variations with varying angle of attack will spoil the lateral lift distribution on the main wing of a canard airplane and increase its induced drag.
  2. Since the canard surface needs to fly at a higher lift coefficient than the wing for natural stability, it will have less margin for control. It is no coincidence that successful canard designs are unstable. Also, this means the big wing surface cannot be fully used for lift, which is wasteful and increases take-off and landing speeds.

More on this can be found here.

Since I was a kid I have read endless articles about how canards are more efficient …

The reason for this is simple. It is much more exciting for authors to claim they know better than all the experts than to humbly admit that the state of the art is really the best under the circumstances. The incentive to laud canards above all else is much too enticing, and human psychology tends to prefer the sources which confirm a preconceived opinion. In the end the authors really think they have found a better solution and become evangelists for a better world, especially when their own understanding of the matter is limited.

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Well for example for the JA-37 the canards do not produce a lot of lift. They do however produce a vortex that passes over the main wings which in turn makes the main wings lift greater. That also allows for higher stall AoA point and shorter take-off distance. They do increase drag and weight though, so there is downsides to having them.

On more modern aircraft where canards can be rotated they can also double as speed brakes and probably increase pitch moment too making the aircraft more agile.

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On the whole a canard does not lead to greater efficiency. However its aerodymanics are complex and little about it is obvious. Pretty much any statement can be - and often is - contested.

Yes a canard will be used to lift the nose during takeoff, helping the main wing instead of adding to its load. Since liftoff is the highest-loaded yet slowest-moving moment in the flight, this does genuinely help wing loading and therefore size and weight, making the airframe overall more efficient.

But in a basic arrangement, for a plane to be stable the forward surface must be loaded more heavily than the rear. On a canard the main wing is therefore loaded below its maximum capacity, which means that it must be oversize and much or all of the above saving is lost. On a tailless type, the wing must similarly be made overlarge so that the rear surfaces can act as the tail (through either reverse camber or, if swept, tip washout). On the other hand of course, both canard and tailless avoid the need for a long empennage hanging out the back.

Fitting a high-lift canard so that the wing can work harder after all leads to safety issues such as the wing stalling and dropping while the canard still holds the nose up, and suchlike. It has generally proved too unsafe to put into production.

The canard's downwash will also tend to mess up the oncoming airflow for the wing, and that mess changes under different flight conditions such as speed and angle of attack, so the wing must have a good safety margin to tolerate such disturbances. That means more inefficiency.

However canards can provide performance efficiencies at high angles of attack by helping to manage the airflow over the main wing, in say a dogfight or when taking off and landing.

In most situations a canard works out more trouble than it is worth, either inefficient or dangerous. But there are exceptions.

The first two successful man-powered aircraft, the Gossamer Albatross which won the Kramer prize and the Gossamer Condor which crossed the English Channel, were both canards. Interference with the wing was much reduced by the low Reynolds number of the canard at its very slow cruising speed. It was also set physically lower, so that the wing passed over rather than through its downwash. So just because the canard is not worth it for sailplanes does not mean that it is inherently inefficient for all low-speed applications.

Several later variants and local modifications of the classic Dassault Mirage III tailless delta fighter were noticeably improved by adding canards. Although less efficient in cruise, at high AoA the canard improved the airflow over the wing to generate extra lift which outweighed the extra drag, improving its efficiency under such conditions. This made it more efficient in a fight for example, with the extra lift allowing the plane to hold altitude better during a tight turn.

Ultimately, if canards were less efficient when and where it really matters, they would not have been chosen for so many 4G/5G combat aircraft, such as the Eurofighter Typhoon, Dassault Rafale and Saab Gripen.

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  • $\begingroup$ Human-powered airplanes fly at only one speed and altitude, so the interference between canard and wing can be taken into account. Not so for all other types of airplanes (OK, except maybe for HALEs which also fly at only one speed), where the changes in angle of attack will vary interference and often spoil efficiency. $\endgroup$ – Peter Kämpf Jan 17 at 20:33
  • $\begingroup$ @PeterKämpf On the Condor and Albatross, things got more complicated in a turn, as the canard was canted sideways to act as a fore rudder, and this changed its flow pattern differentially with respect to each wing. The Albatross was also subject to significant gusting and accompanying dynamic flow effects during parts of its flight. Nothing with canards is ever simple. $\endgroup$ – Guy Inchbald Jan 18 at 12:15
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Yes, but it is hard to get the right ones which will have minimal impact on the areas of the main wings. One advantage for using canards (not planes with three surfaces like the Sukhoi Su-37) is that the tyres aren't pushing down into the runway on takeoff like with conventional aircraft, which have to produce downwards force at the back to lift the nose. They also have more agility pointing the nose during manoeuvring. However, they do reduce the downward field of view of the pilot.

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