Rather than a design issue, I think its more of a setup problem and quality control on ThuderTigers end. Alan Szabo had a post on RR, he was helping a guy that had a woof problem, they tried every thing they could think of to fix the rotor head and it still did it. They finally took the head off of Alans Raptor that didn't woof and put it on the woofing helicopter, well it still woofed. I am convinced that the problem is in the pitch control arm and or the servo used on the collective and how its mounted. Its possible that some pitch control arms are not as rigid as others due to temp variations during the moulding process or any number of variables.