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From: "Field, Peter B" <Peter.Field@MW.Boeing.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Subject: RE: lancair.list V1 #72
Date: Mon, 16 Aug 1999 17:11:24 -0500
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>>
Marv:
     Greg Nelson's tie down CG test sounds like a good short cut, but I
think it's a lot of work and insufficiently accurate.  Nothing beats a good
set of initial calculations which are based on the standard CG determination
methods (using sensitive scales), not to mention the fact that the FAA
requires every pilot to know where his CG is for the flight he is about to
make.  Making a graph of permissible weights and CG locations really isn't
hard and by laminating the graph you'll have something that will stand
repeated usage (like before every flight).  A couple of technical points to
think about are: 1) The airplane when airborne doesn't rotate about the CG,
it rotates around the aerodynamic center of rotation which we hope is aft of
the CG.  One can move the CG, but not the center of rotation.  2)  The
landing gear are somewhere aft of the CG and a lot lower and not a good
substitute fulcrum for your aero center of rotation.  The point is that aft
CGs can be really tricky on take off when the center of rotation moves from
the MLG axles to the aerodynamic center of rotation (PIO city).  It is
really important to know when you're at or even near the aft limit (lots of
people and baggage) not only for gross weight considerations but for
handling qualities.  Conversely, you might get airborne comfortably with a
very forward CG, but be in for a real surprise when you burn down during the
flight and run out of trim on landing (can't flare).  Many people have
killed themselves and their passengers flying airplanes out of CG limits
when they thought they were within a % or two.  Anyway, who wants to be
putting batteries on the tail never mind the fact that you'll have to put
weights in the left seat because you'll be out of the airplane heaving
weights around.  There's a lot more to CG location and handling qualities
which I'll be glad to explain directly to anyone who wants to know.


Cheers, 
Pete


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