The flankers that do have canards are not truly canard configuration aircraft, for starters they have a tailplane!
I would imagine that the carrier approach speed is somewhat less than that for a conventional airfield approach. If so, the canards would be required to provide adequate pitch control, not necessarily more lift, since the aircraft could have significant lift due to thrust vector(and the modified high lift surfaces). We know that the cg of the flanker is rather far aft(somewhere just around the main landing gear position when empty), as are the tailplanes, reducing the moment arm over which the tailplanes can act (a primary design concern is the surface area and moment arm of the horizontal stabs.). This reduces the effectiveness of the stabilisers. Therefore minimum control speeds will be defined so that adequate pitch control in the landing flight regime is possible.
We also know that lift was a concern in the carrier born version as the flaps are slotted instead of plain and the ailerons also droop.
An important point to note about canards is that depending on where they are placed you could ruin the lift over the wing section immediately behind the canard(mainly due to a reduction in effective angle of attack of the wing through downwash effects.). This makes them inherently more complex to design with,and retrofit to existing types, when compared to tailplanes.
The good thing here, which was why this version probably has canards is that you can significantly increase the pitch RATE of the aircraft, as you now have a couple (canard and tailplane) at work. This helps at low speed with control and stability but at higher speeds it means the aircraft will be able to turn tighter for a given set of deflections, with lower drag, maintaining more speed, and reducing the chance of spins and stalls (root stalls before tip in this configuration).
My take on the vortices:
as is correctly stated the vortices created by the LEX on the f18 cyclically loaded the fins, in doing so an extra fatigue system is setup at the base of the fin, directly on the bolts that fasten it to the fuselage, since the bolts will be stronger in the axial direction and not the radial direction(shear), it becomes easier to fatigue a bolt/pin as you tend to apply pressure, moving from its end to its side. this is probably why the fins have stiffening brackets at their base. In order to reduce this fatiguing action the strakes were placed on the LEX. What they do is deflect the whole vortex outside or inside of the fin and ensures that the vortex does not play on the fins, or at least as little as possible. the strakes are placed probably by trial and error with wind tunnel work (I have seen some video of this somewhere on the net).
With regard to the vortices on the fins of the flanker types, I would be more worried about the vortices created by the nose chines when the aircraft is at some angle of attack, as it is these sharp edges that create them, as the same fatiguing may occur(depending on the width and length of the vortex created). small vortices will be set up on the canards at the nose running from root to tip. they will run off the end of the canard and will probably be consumed by the same vortex on the main wing. a vortex will also be set up at the notch between the fuselage and the root of the canard at large canard surfacedeflections however, since a vortex depends on size of object producing it and also the speed of the airflow, again the vortex will probably be consumed by the vortex being created by the fuselage chine, before it ever reaches the tailplane. as these conditions will only occur at large pitch rates and speeds.
Hope this helps a bit!!
coanda
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