Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Sun, 14 Sep 2003 23:03:25 -0400
Message-ID: <redirect-2579543@logan.com>
From: "Charles R. Patton" <charles.r.patton@ieee.org>
Reply-To:  charles.r.patton@ieee.org
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Subject: Re: [LML] Re: Gross Weight & Balance of IV-P, Other considerations
 for Lancairs
References: <list-2579244@logan.com>
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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