| Jim, I had that issue. Lancair avionics put a 200k pot in series in the original installation back in 2004. I have a 28v airplane driving the 12v Ray Allen servo. You might have something different? I thought it never worked because 98% of the pot travel results in the pitch trim being fully off. But I found that there is a very small range where there is speed control so that's where I'm at now (about 25% of the original speed). It takes a few seconds to trim out pitch now--before it was the "bump". The autopilot behaves much differently now without huge bumps as it captures an altitude (Tru-Trak).
If I get the chance later, I will probably swap out the 200k for a fixed value and I'm guessing it should be something in the order of a few ohms. It will be important for you to test any lower voltage setting at altitude and have the ability to recover from a bad setting. If you drive the trim at a lower voltage than rated it can work on the ground but might not be sufficient to work in the flight levels leaving you with no electric pitch trim. Best way to start is time the full up to full down time trim speed and work from there. I went from 6 seconds to about 25 seconds.
Some day I'll add the pitot switch to further numb the trim speed at cruise and wick it up a bit for approach. Ultimately, a PWM setup is best and then there are no altitude concerns.
Paul Legacy
On 2011-05-29, at 8:32 AM, Jim Nordin wrote: I know this has been discussed before and I’ve given a cursory look on the archives. But is there a simple way of adjusting / changing the speed of the servo to slow it down some for pitch trim in a 4PT? The pitch change is too fast at speed where just a small bump has too much effect.
Jim
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