Does the plane need full trim travel or is it usually a matter of a
small correction?
Would it work to reduce the speed mechanically by using an idler arm
that pivoted at one end, had the servo attached at the other, and the trim tab
link at an intermediate point? That would slow the movement and increase thrust
at the cost of reduced travel. Looking at my IV-P elevator it seems there might
be room enough to do something. Not in the aileron though. -Bill
Wade
Sent: Sunday, May 29, 2011 4:47 PM
Subject: [LML] Re: Change the speed of the ROC servo for
trim
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