Paul wrote:

The formula is the sum of all moments both positive and negative divided
by the total weight regardless of what reference point is chosen.

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.

Paul Lamar

vance wrote:

The problem with the reference points chosen by most designers is that
it is an imaginary point in front of the aircraft, and you need some
place to start measuring from. The KIS 2place was set up with rather
conventional practice like that - with zero being the prop flange for
the Limbach engine - but nobody else used a Limbach so the 32.25? at the
firewall became the de facto reference. In designs that I have a hand
in, I campaign for a physical place on the plane ie the aft of the
firewall on the cruiser. On an earlier original design I used the 25%
chord line on the wing (typical center of lift) so the goal was to end
up with small moments for flight comditions. The math works no matter
where you start. - Vance

 
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