Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Thu, 24 Jun 2004 03:08:49 -0400
Message-ID: <redirect-224250@logan.com>
From: bob mackey <nospam@pure-flight.com>
Subject: CG and Gross Weight [LNC2]
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103MD is a Lancair 235 with small tail, and an O-320 on a short
engine mount. Fuel is in the nose, and in the outer wing leading
edges.

A few days ago, I took off with a load of just under 1600 lbs,
and with the CG near its aft limit.

           weight  station   moment
plane      1027     23.9    24545
pilot       140     44.0     6160
passenger   160     44.0     7040
nose fuel    60      8.6      516
wing fuel   100     23.0     2300
baggage      80     76.0     6080
hats         10     91.0      910

TOTALS:   =====     =====   =====
Gross Wt   1577     30.15   47551

(CG Limit 24.5-30.3 per manual)

The airplane was weighed just a few weeks ago, so I am certain
of the accuracy of these numbers.

The takeoff roll accelleration was about what I expected for the
weight, but rotation was sluggish. We used most of the 3100' runway.
When the aircraft did rotate, it showed a definite tendency towards
over-rotation. Right away, it felt to me like the CG was too far aft.
Pitch was slightly divergent and had to be constantly corrected to
maintain a stable airspeed. At first I was too busy to raise the
gear, then I chose to leave the landing gear down to keep the CG
from moving any further aft.

As we climbed slowly to 3000 feet, I started the transfer pump to move
fuel from the wings to the nose, and headed for a nearby airport with
a longer, wider runway (5000 feet), as I wasn't sure I could safely land
where I had launched. Along the way, I explained the situation to my
passenger then had him drag my flight bag from the baggage area to his
lap, while I leaned forward over the stick. Once the flightbag (10 lbs)
was moved to the seat, the pitch oscillations completely stopped, and I
could comfortably trim the plane to fly hands-off. The flightbag moved
the CG forward no more than about 0.2". Fuel transfer might have also
contributed a little.

I was fortunate to have had previous experience flying sailplanes
with the CG fairly aft to reduce trim drag and pitch pressures. Without
that experience, a stall just off the end of the runway would have been
all too easy. I am fairly certain that a stall-spin in this configuration
would not have been recoverable by me from any altitude. The recent
discussions on LML also helped me to be immediately aware that we had an
aft-CG condition and to remember that the landing gear retraction might
make it worse. Burning the fuel in the nose tank might have made it much
worse.

Based on this experience, I will be modifying the weight & balance to limit
the maximum aft CG to 29.5" (from firewall front face). The heavy battery
that is now behind the seat will be replaced with a lighter battery
someplace further forward.

Another reason not to increase the gross weight too much is the speed penalty.
At 1300 lbs, this airplane is about 20 kts faster than it is at 1600 lbs.

BTW - the Champion 48103 oil filter fits (barely) without cutting the nut off.

- bob mackey
  flying a 235
  building a 320
  103MD -at- pure-flight.com
  [use this email address - the one at the top is spam bait]