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Scott Krueger wrote:
My heavy airplane was more responsive in pitch with the heavy load and with
the CG, although within range, moved towards the rear. Just a gentle
oscillation until the weight was reduced. The autopilot just couldn't find where to
set the elevator.
Pushing the CG aft is equivalent to turning up the gain in the pitch axis of the autopilot. If the gain was optimum at normal CG’s, then you would be on the path to unstable control stability regimes.
Remember that if the CG is aft enough, you actually have positive feedback in the control, i.e., it isn’t even stable for hand control, you’re trying to keep the plane from pitching itself to the stops. As you approach that control cross-over point, you require less and less control input for a given pitch change. To the autopilot, that is just another way of saying the control system gain looks like it is increasing -- less output required for more authority. Depending where the system stability lies when you started, it will enter an oscillatory regime.
Regards,
Charles R. Patton
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