The following picture is reproduced here with the permission of rcisfunrem of RunRyder.
One of the problems on the Trex is that of tightening down the flybar control arm grub screw sufficiently that the control arm is locked to the flybar.
Unfortunately in many cases this has led to the screw stripping as it is trying to make a good 'bite' onto the rounded flybar rod.
If you strip the flybar control arm thread then either using a slightly larger grub screw or using thick CA in the thread can resolve it.

The picture below shows how to overcome this problem. It is a simple matter of grinding the flybar such that it has a little 'flat' on it for the grub screw to tighten down upon. The grub screw itself should also have a tiny bit of grounding performed on it to put a flat on it's end.
This little modification allows for the grub screw to not need as much tightening down. The 'flats' should be ground with the flybar paddles fitted as this allows the flybar to sit on a flat surface with the area to be ground facing directly upwards, it also stops the flybar from rotating whilst you grind it.
Before refitting the grub screws it is necessary to screw them in and out of the flybar control arms just to make sure the threads are tapered correctly.
Lastly don't forget to locktite the grub screws. Thanks to YSR Rider for some of the instructions reproduced here.