From: "Scott Dahlgren" <dahlgren@itsnet.com>
To: "___Lancair list" <lancair.list@olsusa.com>
Subject: fiberglass and carbon fiber
Date: Thu, 24 Sep 1998 10:34:29 -0600
Message-ID: <000b01bde7d9$336c58a0$275b29c0@scottdah>
Importance: Normal
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on carbon fiber on the longerons:

It is tempting to mix carbon and glass due to carbons very high specific
strength. however this should never be done in a structural application,
especially on the longerons. here is the reason.

a longeron beefed up with carbon, especially uni-directional, will fail at
much lower loads than a longeron with no carbon fiber added. this is because
the uni-carbon lay-up, has a very high strain modulus compared to
bi-directional glass lay-up(5-10 x!!). this now puts all of the load on the
carbon 5-10 times as fast(therefore 5-10x as hard) as on the glass on a hard
landing. the glass in proximity to the carbon sees no-load (it can not flex,
so it adds no strength) and the carbon is seeing the whole load and more
load to boot. here is an example of what happens to your longeron on a hard
landing or high g maneuver:

take 10 lb green sapling 8 feet long and drop it 5' onto a fence. Both ends
of the sapling will flex down, absorbing most of the load.  Now take a 10 lb
steel bar 8' long and drop it on the fence from the same height and it will
dent the fence and bend the bar (uni-carbon is often 1.5 to 2 x as stiff as
steel).  The load on the steel, and the fence (or the longeron and landing
gear) is far higher because they can not flex to absorb the load.

Mixing strains, like you have in one small area, is something a monocote
(the loads supported through the whole body) aircraft is definitely not
designed for because the flexing is necessary for the frame to develop
strength.

a far worse problem is that somewhere in the lay-up between the engine and
the longeron you must transition from glass to carbon fiber. those
transition areas are HUGE stress risers, and will almost surely fail with
time (I have never seen them NOT fail with steel in my line of work).

imagine sewing a small patch of steel cloth onto the center of a cotton
sheet. if you pull the cotton sheet hard, you can visualize how the cotton
will tear all around the steel patch, and at a much lower load than with no
patch. this is because the material under the patch could not flex with the
adjoining material.

As a fellow EAA'er I highly recommend sanding off all the carbon fiber and
adding a few extra layers of glass, if you want more strength. transition at
least 6" between each extra layer you decide to add, just as they do on
spars, to keep the glass layers from becoming stress risers (it can be just
as bad to add too much glass).

Fiber glass is a very good safety material.  it has very high impact and
flexing abilities.  the reason glass planes keep flying is that the glass
flexes under most impact/impulse loads and the full load never develops. it
makes up for lower fatigue life.

Hope this helps

Scott Dahlgren