> > >Usually a reference point is chosen at the tip of the tractor spinner so

 > all the
 > >moments are of the same sign. The CG position is then the
distance aft from
 > >the tip of the spinner. As I recall this is the way it is taught in PP
 > ground school.

 Another strategy is to use a point in space, far enough ahead of the fire
 wall to account for the longest spinner and lightest engine, say five feet.
 (make a spot on the bottom center of the fire wall to hang a plum bob from).
 With the plane in flying attitude, mark the floor at the tip of the plum bob
 and the reference point for all further calculations is five feet out in
 front of the mark on the floor. The calculations are easy, and the point the
 calculations are based on won't change even with the spinner, prop or engine

 > off.

James Russell wrote:

Hi all:

  The datum being a point in space is hard to measure and the
tip of spinner
changes with the prop extension, mount, etc.  I know this is all
largely personal
preference, YMMV, etc. I know W&B can get rather confusing with positive
and negative moments.

  To (try to) avoid errors I'm doing an Excel spreadsheet to
keep track of my proforma W&B as I build. Just a thought...

 I like this group because you seem accepting of differing
viewpoints w/o
the sometimes personal attacks of other list members. Plus, I'm doing
a engine change on a '86 RX-7 and all this rotary talk is leading me
to think about my choice of a IO-360 for
my Cozy...

Regards,
James

 
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