Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sat, 01 Mar 2003 19:50:25 -0500
Message-ID: <redirect-2041095@logan.com>
From: "Mike Hutchins" <mhutchins@attglobal.net>
Subject: Gear Doors
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Bill Hogarty

I agree that your idea for a torque arm makes good sense.

Consider, however, that the real strength is derived not from the tube
itself, but from the form it creates over which you place your layups.
Maximum strength is achieved by maximizing the distance between the
load-bearing skins. Strength increases in proportion to the distance between
the skins raised to the 4th power, all else being equal. Thus, doubling the
thickness of your torque arm would result in a theoretical 16-fold increase
in stiffness.

Using a real world example of a composite sandwich, increasing the form
thickness from 3/8" to 3/4", will result in a 7-fold increase in stiffness,
without additional layups ("Composite Basics" by Andrew C. Marshall).

So, go grab some foam and make some tall hat sections in the desired shape
of your torque arm (of course, they can't be so tall as to interfere with
gear retraction). Make sure you get a good, stiff bond between the foam and
the existing surfaces of your gear doors. This can be achieved by coating
the surfaces of the foam with a very thin layer of thick micro, and then,
while the thin micro layer is still uncured, scuff and prep the locations on
the gear doors and paint a thin layer of resin on the surface of the gear
door. Place your foam form pieces in position and put your bid layups over
the top of the foam. Since you are essentially creating a shear web, the
fiber orientation should probably be +/- 45 degrees.

If there are any composite engineers out there, please chime in and correct
me.

Good luck Bill, I hope you get back in the air soon. Please post some
pictures from your finished door mods so we can all learn from your
experience.

Best Regards,
Mike Hutchins