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In https://aviation.stackexchange.com/a/80365/20394, @GuyInchbald says "Nevertheless, getting the darn thing into production after 60 years of failure was a staggering achievement."

What was so challenging about the Viggen's canards that it took 60 years of failure? After all the very first planes were canards.

And how did Saab solve The Problem on its way to manufacturing the Viggen?

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There is not one problem with the canard layout but a whole rag-bag of problems.

Firstly, it is worth noting that the reason the Wrights built their first Flyers as canards was to make them deliberately unstable. They believed that a plane could not be both stable and controllable; a stable plane would just settle on its course and it would be difficult to change course (This was quite a common belief and was not dispelled for some years). Since they were after controlled powered flight, they went for instability. This they achieved by adding a canard elevator with no extra angle of attack. It was widely recognised that such a flying machine required extreme skill and was too dangerous to take to any speed or height.

When Horatio Barber subsequently introduced his ASL Valkyrie in 1910, he gave his canard that extra angle and stabilised it. It was a safe enough plane by the standards of the day, which is to say no attempt at aerobatics and strictly avoid going near the stall. But it proved a one-off product, with the French style of tail proving much easier to adjust on a prototype and get the production version acceptable.

Many of the design issues arising with a practical canard layout are described in the associated Wikipedia article. If the foreplane carries significant lift, it is likely to stall before the main wing and may disrupt the airflow over the latter. Not only is the main elevator now stalled and useless, but the wing is also losing lift. Such a situation may not be recoverable. On the other hand, if the foreplane does not contribute to lift then the main wing needs to be stable in its own right. A stable wing has other problems, which is why tailless aircraft were also rare. Even if the stall is benign and the foreplane simply drops, pitching the nose down and initiating its own recovery, there is the risk of a phugoid cycle developing in which it bobs up and down with ever-increasing energy until something goes horribly wrong.

Later experimental types such as the FW Ente and MiG Utka were relatively docile, having large enough surfaces with low enough loadings to avoid the worst of the problems. However they were somewhat slow and bumbling. While these experiments might have been developed into practical designs, the necessary research appeared pointless when conventional types were already perfectly adequate.

As engines got more powerful and planes flew higher, faster and further, the potential benefits of the canard led to a re-examination. The combination of good manoeuvrability with a tame stall and high efficiency proved elusive. Types with complex high-lift control canards, such as the Curtiss XP-55 Ascender, sought to avoid these issues but it turned out the canard would still stall under some flight conditions, and when it did so it was lethal.

The canard received new interest in the supersonic era, due to its potential for a smaller wing, higher manoeuvrability and a better CG range than a pure tailless swept or delta wing. But, even with a thin delta wing on something like the North American XB-70 Valkyrie, the core stability and control issues remained. (Concorde would not need much manoeuvrability and got over the CG issue by pumping fuel to and fro, so it would abandon the canard).

Torsten Örnberg at Saab realised that these problems could be overcome by close-coupling the canard to a delta or swept wing, using the canard to positively affect airflow over the main wing, and in particular to create and stabilise lifting vortices at low speeds. From his US patent, filed in 1963:

"This invention relates to aircraft of the delta wing canard configuration and refers more particularly to improvements in aircraft of the type having a thin, sharply swept-back main wing and a secondary wing located ahead of the main wing, which improvements overcome certain heretofore existing problems relating to the stability and controllability of such aircraft."

Part of the reason this worked is because the swept and delta wings are relatively easy to make stable in their own right, and a great deal of work in that area had been carried out during and after World War II. The patent also discusses various problems with other canard arrangements, so I am not going to repeat all that in detail.

What I do suggest is that Örnberg's innovation was a remarkable creative insight into a tangle of issues that had defeated every aeroplane designer since Horatio Barber. Usually, such close-coupling of obstructions in front of the wing causes at least as many problems as it cures, and it appears far from obvious that it would work this time round. I'd go so far as to call it a stroke of genius, and its success after half a century of failure a stunning achievement. Its significance also went well beyond its immediate application to the Viggen, the first canard to enter production since 1910. Suddenly the inhibitions of half a century fell away, other designers began to understand how to think about canards and they started appearing on new combat types and being retrofitted to several derivatives of the Dassault Mirage III. The Viggen inspired Burt Rutan to start a similar revolution in civil aviation. Not bad for a relatively small country which seldom sold its warplanes abroad.

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  • $\begingroup$ Interesting; I've heard canards being sold as safer because they stall first so the nose pitches down and starts the recovery without necessity of pilot intervention, but you mention in most cases the disruption of flow over the main wing meant it did not actually recover at all, so it wasn't really true. $\endgroup$
    – Jan Hudec
    Aug 4 at 18:45
  • $\begingroup$ @JanHudec the real problem seems to be when both main wing and canard stall, leaving one at the mercy of fuselage drag effects around the CG (helps with "conventional" design, hurts with canard design). The (higher AoA) foreplane stalling first is not a problem, as Barber realized in the 1910 improvement, as long as the main wing is unstalled. The plane (due to the more forward CG placement) will pitch down and recover. $\endgroup$ Aug 4 at 21:32
  • $\begingroup$ Amazingly, in those early times, a pitch up stall crash (from very low altitudes) was considered "better" than to pitch down and "nose in". Later they realized only the latter was recoverable, and, from higher altitudes, survivable. Even the great Kelly Johnson was sold on canard designs until the "double stall" issue was uncovered with the Ascender. $\endgroup$ Aug 4 at 21:41
  • $\begingroup$ @JanHudec I don't say that most do, just that in general they may. It was more a case of rather too many designs for comfort failing to recover nicely. $\endgroup$ Aug 5 at 16:28
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Canard designs shouldn't be summarily classified as "failures" without understanding the entire design of the aircraft.

In the age old struggle of maneuverability vs stability they provide a very useful tool in decreasing excessive pitch stability of delta type aircraft.

In the case of the Viggen, they also enhanced high AoA lift for STOL while also fitting into the supersonic shock cone at top speed.

As with the "tiny tail" debates of modern airliner design, there are 2 considerations for lifting surfaces: balance of forces around CG (easy) and post stall behavior of the aircraft (critical).

A stalled aircraft will sink from its flight path. Now previously non-lifting forces also act on the CG. When they are behind the CG, a stabilizing pitch down force is created. Ahead of the CG ... well, you know!

Adding washout to the rear wing of a canard aircraft will help this quandary (while you're at it, why not put some behind the wing!).

Enter the Canadian goose: Branta Canadensis. This long distance flyer is classic canard, with its aerodynamic head contributing to lift. But it's fan shaped tail as at the ready should any directional stability issues arise, other wise it is neatly folded away to reduce drag.

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  • $\begingroup$ I don't think you mean pitch stability. What the canard allows is a long moment arm for the elevator control forces, making them more effective compared to mounting them on the wing trailing edge. However the Viggen did not use its canard for this purpose. $\endgroup$ Aug 4 at 12:01
  • $\begingroup$ A canard, movable or not, will reduce pitch stability, which deltas have a lot of. $\endgroup$ Aug 4 at 13:53
  • $\begingroup$ A slightly off-topic request... do you have any sources of information of the flight characteristics of a Canadian goose? Everything I find in googling is related to the v-formation and I'm more interested in what you describe of the head being a source of lift and the tucking of tail feather in flight to reduce drag. iow, flight characteristics of a single goose, not a flock. $\endgroup$
    – CGCampbell
    Aug 4 at 14:15
  • $\begingroup$ @CGCampbell at 40-50 mph that head will lift (much like the hand out the window). Folding the tail feathers in (which relaxes stability and reduces drag) is also seen in albatross. Pitch can then be controlled merely by nodding the head. The vee formation certainly helps them, and I wonder for a single bird if the head out in front will impart even a tiny bit of extra energy to the wings. These birds do fly a long way. $\endgroup$ Aug 4 at 14:30
  • $\begingroup$ @RobertDiGiovanni You are quite wrong on both counts. It is well known that a canard with a higher incidence increases stability, and the pitch stability of sharp deltas is notoriously nonlinear. Can you source those somewhat unusual claims? $\endgroup$ Aug 4 at 14:47

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