An excellent post by Indranil Roy on bharat rakshak explaining the basics of aeronautics that even a person like me who has studied maths till 10th and physics till 12th can understand.
As one increases the angle of attack of a wing, the lift and the drag increase almost monotonically (simplification) as shown by two parameters called coefficient of lift and coefficient of drag. But only to a limit (called the critical angle of a wing), at which the airflow separates (completely) from the top of the wing and wing stalls, i.e. lift falls drastically and drag increases exponentially. So there is a limit to how much lift one can generate and hence a limit to how fast a plane can turn.
Now when a fighter comes in to turn, it has a lot of initial speed, i.e. kinetic energy. So it can use its maximum lift capability to get into the tightest turn. But remember, drag also increases drastically. None of the fighters have enough thrust to overcome this level of thrust. So its energy bleeds off, i.e. its speeds falls down. This period of the turn is called the instantaneous turn and the turn rate is called instantaneous turn rate. But this can't continue forever (generally lasts a few seconds). If the speed of the aircraft continues to fall, then at some point it will go below its stall speed, i.e. it will fall to the ground like a stone. This can be overcome by losing potential energy of the plane, but in doing so you lose height i.e. become the cannon-fodder of your opponent. So to maintain a horizontal turn, the pilot has to lower the drag, i.e. decrease the lift aka lower his AoA. By how much? Till the point that the drag is balanced by the thrust of the engine. At this point, the rate of turn is decreased to the sustained rate of turn. So, the instantaneous turn rate is determined by the maximum lift generating ability of the aircraft and the propensity to reach this state as soon as possible. On the other hand, the sustained turn-rate is determined by the drag and the thrust.
The delta wing is excellent instantaneous turn-rates, because it generates large amounts of lift and naturally wants to reach higher AoA. This is because the leading edge of delta wings readily creates large vortices which add to the lift (called vortex lift). But obviously, this fast rate of turn also bleeds off the energy very fast. Beyond this point, the drag of the delta wing (at low altitudes) and slow hard turns is more than the classical wings. Therefore for maintaining the same rate of turn, a delta wing plane at low altitudes needs to have a better thrust to weight ratio.
So, we have learnt that various wings have various advantages and disadvantages. But aircraft designers have to take care of the whole flight envelop. So what should they do? They chose a design point, and make the plane excellent at that envelop around that design-point. Next they try to add things to mitigate the disadvantages at the off-design points. For example, most planes with conventional wings use LERXes with sharp edges and high sweep which generate vortices like the leading edge of a delta wing. The F-16 uses the slats and flaps as part of wings to increase its area (decrease wing-loading) etc. For delta wings, there are two primary methods:1) use a close coupled-canard and 2) use variable extended slats. A close-couple canard works by shedding a vortex which combines with the vortex of the leading edge of the delta-wing and energizes it (the exact aerodynamics is really long and winded to explain in a forum post like this). The variably-extended slats like in (Mirage 2000 and LCA) also work in the similar way. Going from the wing-root to the wing-tip, the vortices from the inner slat (at lower AoA) energizes the vortices from the outer slat (at higher AoA). I will not go into the details here because it depends on a lot of parameters like the sweep of the wing, the AoA, the sharpness of the edge etc.
In case of LCA, the wing and lift is not at all the problem. The co-efficient of lift increases monotonically till about 35 degrees of AoA. The same goes for Co-efficient of drag. The L/D ratio is one of best too by virtue of its very low wing loading. This gives it excellent ITR and roll rates. You now know that LCA would most probably be limited to 26 degree AoA, because at this AoA it generates enough lift to give it extremely good turn-rates. There is unconfirmed reports by Sjha, that it might even be taken to 28 degrees. The designers (as Maity sahab pointed out) went for a compound delta wing, where the outer part of the wing (with larger sweep) provides the capability for excellent ITR, whereas the inside of the wing helps with STR (kind of like 2-design points). The kink in the delta (can be imagined as the innermost slat with no extension) generates a vortex which energizes the vortex from the innermost actual slat, very similar to the what a fixed canard would have done. The question is how can we make the STR of LCA better. One way is to increase the thrust (brute-force), and the second way is to make it more efficient, i.e. increase the Lift-to-drag ratio. Designers of LCA Mk2 are going for both. The late Commodore says, the thrust could be increased on Mk1 (because intakes don't let the Mk1 obtained maximum thrust) and that in itself is enough.
Okay now, we come to your question. Why did Viggen have a canard in spite of having a compound a delta like Tejas. I have already answered part of that question, because it did not have independent slats like the Mirage 2000 and Tejas (though it tries to do something similar with a dogtooth). The other part of the answer lies in the plane. Remember, what I told you about the design-point. The design-point of that plane was for STOL performance. For that the plane needs to be able to turn its nose up at low speeds. But then the Viggen's elevons were attached to the end of its wings (aka with a short lever arm). So it needed secondary help (aka the canards which had flaps to provide positive lift). Another part of the answer was the Viggen's airframe itself. The Viggen was a pioneer being one of the first fighter planes to use a turbofan engine. However, given the technology of the engines then, the airframe had become very fat and bulky. If they had gone for a traditional wing, the plane would have become really fat, i.e. too much increased in wave-drag (which affects cruise-speed, range and top-speed). So, Viggen's designers (kind of) broke the wing into two wings. The smaller of the two doubles up as a canard. This kind of canard which shares the burden of generating lift along with the main wing is called a high-loaded canard. Of-course it produces other advantages (and also disadvantages).
Anyway, there are many ways of building a plane to do the same thing. Even within canards there are various ways. The canards that you see EF is a lightly-loaded canard aka a control canard, i.e. it does not produce any lift. The canard on Gripen and Rafale are mostly control canards, but can transition to a loaded canard when required (this is only possible with a FBW). For example, one can see Rafale's canards change roles while taking off from an aircraft carrier. It transitions from a control canard to a highly loaded canard soon after the plane leaves the flight deck. Also there is no silver bullet to building a plane which works exceptionally well across the entire (extremely wide) flight envelop of a modern fighter. Otherwise, all planes would have been the same. For example, three of the best designs from an aerodynamic perspective IMHO are the F-16, Mig-29 and the Rafale. These three have the cleanest airframes which use almost all the possible advantages that can be extracted out of airflows around an airplane. The Rafale is probably the best of the lot because it learned a lot from the other two. It uses the boundary layer over the wing (which the other two don't) and a complex interplay between the canard and the wing-body blend. It places its wing at the exact height where the leading edge can be extended with an extremely sharp LERX by the side of the air intake (not present in the initial prototypes). Anyways, none of these 3 look alike. Therefore, ex. AVM Matheswaran's comments can only be put down as bias. Tejas Mk1's short-comings are not a want of canard. Anybody who knows aerodynamics will tell you that easily, as has been evidenced by numerous studies. Believe it or not!
In fact I would recommend reading LCA thread from page 81 onwards
Not a pertinent question.
Isn't the JF17 a stable design?!
Negative static stability
The LCA has everything the JF-17 has but does the Jf-17 have everything the LCA does?
technology of the 21st century.
