Tuning the autopilot by sound

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    On a recent x-country against a stiff headwind at 14,000 feet (much higher above) I found an interesting technique to trim the plane on AP that I would like to share. I have a 421C with the 800B autopilot.

    Starting point:
    Level flight, AP on
    1) Adjust rudder trim to keep the ball centered
    2) Inspect roll trim indicator – it should be flat or very close to flat.
    3) Inspect ailerons – they should be very close to level with the wing trailing edge.
    4) Now, listen for the high pitched whine of the AP roll servo in the headsets and adjust the AP roll trim very slightly until you are in a “null” where no whine is heard.
    5) This is the optimal balance point for roll. If there is any whine what you are hearing is the AP holding torque on the ailerons to counter the mis positioned manual roll trim – at the null you are minimizing drag.
    6) You can further have fun with this by adjusting rudder trim, to see if you can true out to a perfect ball, wings level, aileron level with wings and no AP servo activity. Most planes will have a small amount of mis-rigging so there might not be a perfect point.
    7) The other upside is you get an audio clue when the AP starts rolling. Once you start listening, it’s very distinctive.
    8) Another tip – on the ground get the rudder in perfect alignment and make a mark of where this is on the panel trim knob. It should be very close to the center dot. If your plane is slightly mis rigged it is probably faster to have the rudder in perfect position and live with the offset in ailerons.

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