The stock roll trim system has a tendency to slip. Some have found that putting some belt dressing will improve things, but I have found the springs that are supplied are too weak, too (can’t put stronger ones due to slippage issues). So, I thought I’d take a crack at building a better roll trim. Since the basic problem is slipping of the rope on the roll trim servo capstan, I replaced the rope and capstan with a timing belt and pulley system. I bought both from McMaster. I replaced the idler pulley on the pilot side with a home-made wider pulley to support the 1/4″ timing belt. Other than that, its basically the same system. The extra grip of this system allowed me to put stronger springs (I’m not sure the exact spring rate). I used electrical crimp fittings to attach a ring terminal to the end of the timing belt to interface to the spring. You’ll notice in the pictures that I also moved the attach point lower for the springs on the bell crank. I drilled holes in the large washer to attach the springs at the same point that the aileron cables are attached. That gives the springs a better lever to act on the bell crank.
With this system, I can now adjust the trim and the control stick will actually move back/forth. The timing belt will slip on the pulley, and is easily overcome with some hand pressure on the stick. First flight indicated it works very well.
