Originally Posted by
speedracerntrixie
I would like to know how you came up with this? I have experienced this a few times. What I believe happens is that during a rolling circle or a flat turn while hard over on the rudder as the airplane is basically in a skid the buffeting airflow off the stab makes the rudder oscillate. having both cables taught solves the issue. This is well documented, try looking up CARF Extra 330 tail wag. It happened so badly to one pilot in the 2002 TOC that he actually lost his rudder. True, it's not common but it can happen. The best prevention is to set up the pull pull correctly from the get go.
How did I come up with this? Easy, about 20 years of using pull-pull on mostly 1/4 scale models but also on many smaller ones. Simple physics will confirm this. The only way that the surface could move further in the direction the tight line is pulling it is to have some external force trying to further push the surface (an impossibility unless you are in a tail slide at which point you would not have any control anyway). I'd suggest that the problems you reported on were more probably caused by flutter due to structure imperfections rather than the pull-pull system. Such things as bad hinges, flexable surfaces, etc.