those wingtip vortices are swirling masses of air. They can cause engine disruption in jets, and when the control surface of an aircraft flaps, alierons. rudders, mobile canards) hits these, it is normally enough to take the control stick out the hand of the pilot. think of how much pressure that would take....and think about the fact that to override the autopilot it only takes 25 lbs of pressure. Couple that with the buffetting of the vortice itself making each control surface do something different, and your plane is one big expensive missile. Also, if you have that kind of pressure pushing down on one wing, and you have plenty of lift under the other, you are going into an uncontrolled barrell roll. The plane will probably porpoise (nose up, then nose down, then nose up again....etc etc) as the pilot attempts to regain control. This is a bad thing.
here's an example.......when you pass your hand through water....slicing like a karate chop (for lack of a better term), as oposed to an open hand slap, it flows through easily, but the water fills in the place where your hand was from both above and below, and you can almost see it filling in. Now when you get a plane doing that through the air a supersonic speeds, the air does the same thing, but in a much more violent manner. The plane is creating a near vaccuum, and the air is rushing to fill it. those different wind currents will hit one another, and swirl violently about. that is why they are dangerous.
the problem increases ten fold when we look at the inherent instability of a forward swept wing aircraft (x-29, Switchblade), and about 5 fold with variable geometry wing aircraft (f-111, f-14, Switchblade), or with a machine with mobile canards (f-15E, f-16 ATFI).
so far the only way to even combat this problem is through the use of vector nozzled engines and reaction control valves to replace the aforementioned control surfaces. Some of the newer designs on the shelf are using them, the Switchblade, uses reaction control valves to a certain degree, but she isn't in production, more's the pity.
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