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Posted for André Beusch <andre.beusch@bluewin.ch>:
Hi,
I solved a similar problem by changing the pitch computer gain of the S-Tec
50 in my Glasair Super II.
There is a resistor (R57) that can be changed. I increase the value from 18
kOhms to 39 kOhms, and the altitude "hunting" is now gone.
I received this information from S-Tec engineering.
I used a multi-turn 100 kOhms potentiometer connected to wires that were
soldered to the R57 holes to find the best value by adjusting it in flight.
I soldered a fixed 39 K resistor afterwards. This procedure require some
electronics knowledge.
Possibly, R57 is different for an autopilot configured for a Lancair.
Use a shielded cable for the potentiometr, to make sure you don't catch any
interferences that may make the experiment useless.
The hunting problem was more severe at aft CGs, so make sure you also test at
aft CGs.
The pitch computer board is a double sided circuit board (traces on both
side), I think the only double sided board.
I'm not sure if remember well, it must be the first or the second from the
accelerometer side. It is more complex than the other boards.
I f you put the board on the table, component side up, connector towards you,
R 57 is right at the lower short end of the second chip from the top left (a
CD 4029). The silk screen is difficult to read, I think R 57 is written under
the resistor.
The S-Tec 50 autopilot is analog, it has no microprocessor. There is an
accelerometer in the box. It is on the side, a mass suspended on a thin metal
sheet moves inside a coil.
What is also important for a proper pitch stability, is that there is no
slack in the servo command. For example, if you have cables, make sure the
tension is high enough.
Hope this helps - Andre
edechazal@comcast.net wrote:
Scott - Wow! Sounds like you've got the same symptoms I've got. Started happening to me this last April (Sun spot activity?) although that was the hard pitch commands. The porpoising has been sporadic since first flight 4 years ago (300 hours on the plane).
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