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Matt Hapgood wrote :"Don said that, for the Mark II tail the CG was moved
FORWARD 3 inches, from 24.5 to 27.3.  Well, this works for me, but doesn't
seem to be consistent
with other information that I have read on the list.  And I would think that
most people would have a rather significant AFT CG problem with these
revised numbers."

The change in the Lancair CG limits with the Mark II tail is not just
related to the longer engine mount that goes with the larger tail.  The
limits for a CG range are defined by a combination of longitudinal static
stability and control authority.  Basically the further aft that the CG is,
the less the longitudinal static stability, until it meets the center of
pressure, at which point you have zero static stability.  The airplane will
still be flyable even when the CG is aft of the CP (i.e. negative
stability), but the workload gets very high, and it is an undesirable
situation.

The forward CG position is less well defined.  The further forward the CG
the greater the static stability, until you reach a point where there is
insufficient control authority to achieve the maneuver required.  Typically
you run out of aft stick at low speed.  The manufacturer will define a CG
limit that allows full control down to the stall, plus an arbitrary margin
for safety.  Control authority is a function of tail volume, defined as the
stabilizer area multipled by the moment arm from the CP of the wing to the
CP of the horizontal stabilizer.  So when we fit the larger Mark II tail, we
increase the tail volume, and we can afford to move the forward CG limit
forward, since we now have sufficient control authority to overcome the
greater static stability.

Generally speaking an aft CG situation is more dangerous than a forward CG.
The improved handling of the Mark II airplane is partly because we can move
the CG to a more stable configuration, and partly because the pilot has more
control power.  If your airplane is heavy, then you really do not want an
aft CG, as the increased airplane inertia degrades the handling.  In your
case you should not have a problem with your CG situation.

Jerry Fisher

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