Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Fri, 26 Sep 2003 19:35:18 -0400
Message-ID: <redirect-2605479@logan.com>
From: "Jerry Fisher" <jerryfisher@charter.net>
Subject: Gain a couple of knots
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Larry,

I hate to argue, but think that you are using a common fallacy about CG
effects.

The trim drag from your elevator being slightly nose up is a function of the
up load (actually reduced down load) required to achieve balanced flight.
This is not a result of a simple moment and balance about a fixed point as
you are assuming.  When you move the CG aft by putting fuel in the rear
fuselage, you are moving the CG of the aircraft closer to the center of
pressure (CP) of the wing.  That reduces the nose down moment effect of
aircraft weight because the moment arm of that weight is the CG to CP
distance.  This distance is the static margin.  The result is that the
horizontal stab has to produce a proportionally smaller compensating down
load, which it achieves through a smaller elevator deflection; ideally you
could move the elevator in line with the horizontal stab to minimize trim
drag. Hence the effect of a rearward shift in CG is the opposite of what you
have assumed.

The longitudinal static stability of the aircraft is a direct function of
the static margin.  If you were to move the CG until it is at the CP, then
there would be zero static stability, and in still air the tail would not
have to produce any load, up or down.  However it would be a bear to fly,
and would require continuous correction in real conditions.   A further
rearward weight shift would make the aircraft statically unstable, which is
what some fighters (F-16, F-22, Eurofighter) and some airliners do (Boeing
777 if I remember correctly).  The fighters do it for greater agility and
reduced drag, the airliners for greater range.  They use fly-by-wire to give
artificial stability.  You can fly an unstable airplane without FBW but it
is hard work (the British F-4 in some conditions is burnt into my memory).

By the way one reason for the reduced drag of these aircraft is that they
can use a smaller horizontal stab, because the control authority, measured
by tail volume (horizontal stab area times tail moment arm about the CP),
can be reduced in proportion to the reduced tail load.  This is why the
earlier Lancairs with the small tail were sometimes flown with rear CGs, to
give sufficient control authority.

Having regurgitated all that theory (sorry I lost control!), I would be very
cautious about moving weight into the tail.  It is very easy to overcontrol
an airplane with a rear CG, which can easily result in a severe PIO, an
overstress or an accelerated stall and departure, especially at high speed
or on the approach.  And our Lancairs are fairly sensitive by any standards.
I frankly do not think that the minor drag reduction justifies the risks.
Incidentally I am personally convinced that the reason your 360 is faster
than some others is that you have built a glass fibre manifold around your
engine which smooths the cooling air, and reduces cooling drag.  Cooling
drag is usually underestimated as a proportion of total airplane drag.

Jerry Fisher