From what I can know tell, this pot adjust the drifting of the gyro at neutral stick position. When this pot is moved a little to the left or the right, The movement of the servo arm drifts to the left or right. This drifting is what was causing me problems and why I had to use subtrim in the radio to stop the drifting either way. But as we all know, using subtrim changes up your endpoints as you add in higher percentages. Know that I fine tuned the gyro with this pot, to elemenate drift from center, I can know set everything up correctly in rate mode, and then switch to HH and lock everything in.
can't get the servo arm to be at 90 degrees on the tail servo as the splines are just wrong if I take it off and move it over a notch either way it’s too much the other way
First, what radio are you using and what menu in the radio do you have activated to adjust gain?
Second, You want to set up rate mode by setting the gain to 45% on either end if your using ATV's to adjust gain. If you using a gyro menu like on the DX7 then you want to use a number higher than 50 for HH mode and a number lower than 50 for Rate mode. I would suggest starting with 65 for one position and 25 for the other. Control should be on a switch like the gear switch so you can hit it to toggle from one mode to the other.
By doing this you might know be able to find a spline that puts the arm very close to 90 degrees. Know its time to make sure your in Rate mode and hover the heli and use mechanical devices to get the tail to hold straight without any drift. You will increase or decrease the length of the pushrod to do this, not the trim in the radio.
When you can hover in rate mode and the tail stays relatively still, you can go to HH mode by hitting the switch and she should continue to hold that same position.