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Jim,
I have given some thought to the yaw problem you so elegantly describe.
There's one thing that could be looked at closely ... the possibility the
vertical stab/rudder shape are acting like an airfoil to create horizontal
lift (yaw).  

The process could be started by one side of the vertical having more airfoil
shape than the other side, thereby producing enough lift to overcome the
streamlining airflow.  As the streamlining airflow overcomes the lift, the
correcting force starts a recovery until the yaw cycle is complete.  

Additionally, if the rudder is out of balance, it could enhance the airfoil
shape.  It does this by creating more camber effect, thus, providing more
lift to one side, and then to the other side, as the cycle completes.  The
rudder acts much like a flap on a wing and creates lift.  Of course, if it's
out of balance this "flap" would have a mind of it's own and would "hunt"
for it's center position and, of course, never find it.

If the left and right sides of the vertical ARE symmetrical, then the
vertical may not be pointing straight down the road.  

I'd look at the airfoil symmetry, rigging of the vertical and the balance of
the rudder.

Jerry Grimmonpre'   LNC2
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