You're right, that one looks significantly different than what I've seen, but again I have only a little experience with this type.You sure about that? Take another look at the 5th picture in the initial post. The rudder looks substantially taller than what your beautiful drawing shows.
The height of this overhang above the tailboom tube would sure give it a lot of torque on the tailboom. So if this upper portion of the rudder was starting to swing back and forth, it would sure as heck put a cyclical or reciprocating torsional vibration into the tailboom tube... I think Bob's on the right track here.that coupled the bending and torsional deflections of the rudder.
This was my initial thought as well but I was initially reluctant to speculate about this because they said that the tab fell off and yet the flutter continued. However, since they have now confirmed that the tab did not fall off, I think this would be the likely cause. However, there may be other contributing factors. The redesign of the control horn may have resulted in a horn that was less stiff and a reduction in the torsional frequency of the rudder. Also changes in the rudder shape may have had an effect on frequency.Taped-on trim tabs do work on RV-x's, but don't forget that they are applied to a metal surface, which has underlying structure to stiffen it. Taping a trim tab to a fabric control surface *could* be the equivalent of adding one to a flag. Even if the tab was over a longitudinal stiffener, it could still pull the fabric loose from the stiffener which would allow the tab to flutter constrained in only one direction by the stiffener. I know that it's an 'after' pic, but if I'm seeing the image of the added tab correctly, it looks as if the tab doesn't extend forward past the trailing edge tube of the rudder at all. If that was the case, the tab would have almost nothing to keep it from moving.
These are my thoughts as well. An odd bounce on the grass started an oscillation that would have damped itself out on a stock airframe, but the taller rudder and other modifications created a situation where the initial oscillation didn't damp, but instead amplified. That taller rudder has a lot more ability to twist the boom than the stock rudder, and that might have been the real underlying issue. Just a guess, though.Or perhaps something happened during the touch and go as the tailwheel bounced over the grass. Especially if parts were modified, like the taller rudder, that doesn't necessarily point to a fleet wide problem as many examples of this model have been flying safely for many years, though of course it should be looked at.
In 3&4, the horizontals are folded up, completely hiding the fin and rudder. The blue stripes appear to be on the *bottom* of the horizontal stabilizer; not the side of the rudder.