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From: Gary Casey <glcasey@adelphia.net>
Subject: Re:  Down anywhere !!  spinning


>  Doesn't the DG's ball always indicate slip/yaw away from the  
> direction of
> rotation?
>  So, step on the ball, we were taught, and it works, stopping the  
> turn.
>
Previously from Tom:

Not in an actual spin, where one wing is stalled, no.  If the  
instrument is on the left side of the panel the ball will deflect to  
the left in a spin, no matter which direction you are spinning.  If  
the instrument is on the right the ball will always deflect to the  
right.

Tom Gourley

I don't understand this last comment from Tom, although I have heard  
it from other sources.  At first glance it implies that the center of  
rotation in a spin is exactly through the center of the instrument  
panel, which would seem very unlikely.  If it were true the down wing  
would have to be moving backward.  On further thought, though, the  
plane is tilted at a significant angle, tending to make the ball go  
to the low side, toward the direction of the spin.  But if one  
assumes that the low (stalled) wing has a AOA of maybe 25 degrees or  
so it must be moving forward - hence the center of rotation has to be  
off the low wingtip, quite a distance from the aircraft centerline.   
In that case the centrifugal (centripetal for you purists) forces  
would push the ball to the outside of the rotation.  The two forces  
then conflict with each other, so what does the ball do?  I'll bet  
nothing reliable.  Yaw rate is then the only useful indicator,  
available directly from the turn coordinator(more correctly called  
yaw rate indicator), or from the heading indicator.

Gary Casey