Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sun, 07 Jul 2002 14:42:08 -0400
Message-ID: <redirect-1620735@logan.com>
From: Guy Buchanan <bnn@abac.com>
Subject: lml Web Archive
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Hello all,
	I am a retired composites designer who lurks about, helping out where I
can. Gary is right, you can't substitute one material, whether carbon,
glass, or kevlar, for another without analysis. You can easily weaken
structure by adding stronger material, or replacing a weaker material with
a stronger material. Yes it involves stress concentrations, but also load
paths and the like. I would even hesitate, in the world of aircraft, to
replace an all glass design with all carbon without serious study. (Another
problem with mixing materials is strictly aesthetic, and that is the
variation in shrinkage which shows up over time unless your wet laminates
get post-cured. After a while all your wet laminations will show through
the paint, requiring re-fairing and paint.)
	The issue of the difficulty of laminating glass has already been discussed
but I will add to it. The Glasair folk use something called 7781 for their
glass laminating fabric. It is a very tight weave which gives superior
properties in high quality, (read: vacuum bag or autoclave,) laminates.
Unfortunately it is almost impossible to wet out and pull bubbles through.
Therefore, if the Glasair designers were using optimum 7781 properties in
their design the average builder couldn't help but build a sub-standard
aircraft, from a strength point of view. Hopefully that wasn't the case.
And I don't know if Lancair uses a similarly difficult fabric. As has
already been stated, a preferable material would be a 5 or 8 harness satin,
which has good properties, drapes well, and wets well. BUT DON'T SWITCH
WITHOUT LANCAIR APPROVAL.

Guy Buchanan
Buchanan & Newcomer