I ran into this problem with my 1/4 scale SR batteries eindecker. I deceided to deviate from their design and do it with a full flying elevator. At first I simply did scale outline elevators with a carbon fiber tube for the pivot point, mounted inside the rear af the fuselage. It worked ok except for the fact that every once in a while the plane would start" porpoising ". Throttling back would correct it and it seemed it only happened when I was flying at full throttle( and even then, not all the time) I tried tightening the pull-pull lines but at a certain point the elevators began to bind . The problem persisted. A flame out and hard landing caused the one piece elevator assembly to break and I redesigned it with a two piece elevator inside a tube at the rear of the fuselage. I had to cut away part of the rudder to clear the tube at the rear. I think the porpoising problem was actually caused by positive Ackerman and so I have made my own control horns and hope to have the problem corrected by next week. I think the porpoising was a problem because with full flying elevators when you change the attitude of the control surface one side of the hinge line gets pulled up or down while the other side is forced up or down by air flow. This would probably not become a problem with push rods but with pull-pull both lines must remain taunt otherwise air flow can work against the input. Or maybe the problem only occurs when the elvator is in the nutral position or close to it adn the air flow forces the elevator in the opposite dirrection? anyway I have designed my contol horns so that the attachment point is right on the centerline of the pivot point. Wow, it would have been so much easier to just follow the directions!!