The "datum" is not something that's calculated, its a "concrete" point
that will remain the same between flights and over the decades, whether
or not you change spinners, for example. You could also call it the
"origin".
The math is easier if you choose a point at one extreme or the other,
and since the engine is a "heavy" player in weight and balance, most
folks choose something like the front or rear of the prop hub or spinner
backplate. This also gives you a convenient item that you can hang a
tape measure from, or push the end of the tape up to, and a surface that
you can stick a straight edge to to measure to things like wing leading
edges.
Or (if the plane is leveled into flying position) a plumb bob can also
be dropped from the prop hub face to the ground, and also from leading
edges to the ground, and with a chalk line, measure between the two.
The plumb bob method is more accurate than the tape measure method, as
you are measuring straight line along the floor, which are parallel to
each other. Likewise measure from the datum to the tailwheel, for
example. Having a builder's square handy is helpful too.
See http://www.n56ml.com/wb/index.html for more details on how to do a
weight and balance, including the weight and balance sheet for my KR2S,
N56ML. There are plenty of other examples out on the internet, many are
probably easier than the one I used. Find one where the items to be
entered are made clear, and don't change any fields that are not meant
to be changed (formulas).
Mark Langford
m...@n56ml.com
http://www.n56ml.com
Huntsville, AL
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