S-Tec 50 Autopilot

[RSalinasP@aol.com]

I have been reading off & on about problems with the S-Tec 50 two-axis
autopilot and opinions from different guys on how to fix such problems.
My personal experience was that both, the aileron and the elevator servos,
were over controlling and producing oscillating movement which could go from a
very uncomfortable back and forth (right left) swaying on the aileron axis to
a severe and uncontrollable up down oscillation on the elevator axis.
After talking to the guys at S-Tec they told me to change the value of the
resistors that control the "gain" on each of these servos. And so I did, by
obtaining a trim pot and wiring it to the appropriate place with an extension
wire so that I could "trim the pot" as needed in flight until a nice smooth
level flight was obtained. After the flight I removed the trim pot and
measured the resistance value and then proceeded to install a resistor closest
to that value.
Needless to say, that this worked fairly well on the roll servo but not so
well on the elevator servo because as has been mentioned before (Martin
Hadley?), the CG is critical in the amount of gain that the servo requires for
it to perform.
A couple of years later I decided to fix my autopilot so that it will track my
Garnin 100 GPS  (in fact never tracked the VOR either) and in doing so was
able to gather new information from the S-Tec service guys as to what "Gain"
means to the servo unit. This turned out to be the "real fix".
Let me try to explain, to begin with, when I originally installed the
autopilot I built extensions to the servos so that I could increase the travel
of the arm and thus avoid the danger of the arm going over center and still
having full travel on the bellcrank. What happened as a consequence is that
the servo is putting out a given amount of movement and this movement is being
amplified by the longer arm and as a result the control surface (aileron or
elevator) move too much and therefore the darn thing reacts and does the
opposite and so you have oscillation.
The fix was simple (at least in my case), I went back to the original resistor
values and got rid of the servo extensions and "VOALA" it performs beautifully
in both axis but it still won't track my GPS/VOR so I use my DG heading bug.

Hope this is of help to those involved

Ricardo Salinas - Classic Std. RG Velocity N199RS :-)