There were two rudders that fluttered on lancairs that I saw. One at merced during a high speed pass and one at santa paula testing for the osh 500. I had the first large tail 320 and the shake test done on my plane revealed a Ed speed tail bending mode that could be fixed by adding plys top and bottom.  there was discussion from lancair that there annals didnt show this mode. however this mode was right at cruise speed and since the weight penalty was only 5 lbs I added the plys
dave