fiberglass mesh in foam wings

[Rick Kent]

 I believe Bob Violett uses fiberglass cloth and carbon fiber under the wing
skins as part of the layup in his made-in-the-mold wings for his jet kits.
Of course, with those jets, one can ill afford a wing failure in 200+ MPH 
zillion-G vertical pullup.

Rick 
-------Original Message-------
 
From: discussion at nsrca.org
Date: Tuesday, December 21, 2004 12:27:41 PM
To: discussion at nsrca.org
Subject: Re: fiberglass mesh in foam wings
 
As I recall:
I believe Dave Guerin originated this process. Examination of crash damaged
and failed wings revealed that carbon fiber will eventually peel a thin
layer of foam loose since the foam flexes and the carbon is rigid. In time,
the carbon is just along for the ride. The fiber glass window screening with
its loose & wavy weave has a little give to it. I have not observed any
delamination of the screening in spite of noting that different rolls of
screening seem to be harder to wet with epoxy. 
 
If you are building a wing with a thin airfoil some kind of spar-like
structure is cheap insurance. I have had wing failures with the screening in
place that held together well. They were caused by off - field landings.
 
Originally I think Dave carried the screening all the way to the tip cap and
lapped them over the edge. Experience indicated this was not really
necessary, the epoxy works well as an adhesive in this application. I think
the whole process originated with the very thin Desafio wings but that is
another story. It really is not necessary to carry the screen to the tips
but it is easier to do a neat job this way. If you put a piece of sand paper
a little wider than the screening on a piece of aluminum angle it will only
take a few swipes to recess the excess screen into the foam. Don't mess with
the sheeting, it is thin enough! In my shop I use the same sanding bar that
I use to true sheeting. 
 
I doubt thicker wings benefit from this treatment.
 
Someone (up Chicago way I think) puts glass between the foam & sheeting.
Sounds like a great idea but if I did it it would be heavy.
 
I have seen Prophecy wings that were glassed after sheeting and painted. I
expect they were a little heavy but they felt like they were indestructible.
 
Remember, this is as I recall, and YMMV!
 
John Ferrell    
http://DixieNC.US

----- Original Message ----- 
From: Ed Alt 
To: discussion at nsrca.org 
Sent: Tuesday, December 21, 2004 7:39 AM
Subject: fiberglass mesh in foam wings


I'm getting ready to sheet a pair of Temptation wings and have questions
about using the fiberglass screen mesh reinforcement strips they call for. 
First, how necessary is it to even use any?  I can see where it obviously
adds strength and stiffness in those sections directly under the mesh, but
at the same time, it will also create a raised-up section of sheeting
directly adjacent to the mesh strip.  It seems possible that there would be
no bonding of sheeting to foam for about 1/4" to 1/2" wide all along those
seems, at least not if a thin layer of epoxy is used.  It also seems that at
the point where the sheeting does begin to bond with the foam again, it
would be subject to shearing forces that might tend to pop the foam beads
out over time due to bending loads on the panel.  Maybe not? I have no
experience with this method, so it's all speculation.  It seems that a
fairly heavy application of epoxy would be needed to bond foam to mesh, mesh
to sheeting.  Is it possible that the same or better strength result could
be accomplished with a slightly heavier wood selection and still be as light
or lighter than adding mesh and extra glue?
 
Asuming the mesh is really necessary:  The 2nd question is about the
adhesive to use if the mesh is used.  I normally use either Pro Bond or
Elmers polyurethane glue, which does a terrific job of bonding several bead
layers deep into the core.  I think it probably stiffens the structure alot
as a result.  However, it's not clear to me if this would be a good adhesive
with the mesh applied.  In theory, it would do a good job of filling that
gap I'm worried about along the mesh edges.  Has anyone tried this and are
you happy with the results?
 
The last question is about how far to run the aft reinforcement strip.  Do
you really need to run it all the way to the tip?  At some point, you really
get diminishing returns as you get further out in the panel. Sorry for so
many questions!
 
Ed
 
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