Michael,
I'm not sure if you're planning to use a PVC or XPS (Is that expanded
polystyrene?) in place of plywood as used in the fuselage of the KRs,
for example. The plywood is structural in the case of a KR-type fuselage
box. In this type of construction the foam is typically used to give
shape for the overlaid structural glass layers, such as the wing
surfaces. Typically any foam used in aircraft construction like this is
ignored as contributing anything to the strength of the finished
structure. It's just to establish the shape.
That being said, I'm confident you could build a wing using foam ribs,
sheeted with, say, .062" E-glass printed circuit board fiberglass
laminate, and you'd have a structure with strength comparable to a wing
with wood ribs and, say, 3/32" plywood skins. You would need a lot more
ribs than what is found in the KR wing structure though. A different
type of structure would have to be designed, more along the lines of a
wood wing - stick or ply ribs, wood spar, plywood skin. In this case the
foam ribs would be load-bearing structural members. It looks to me like
the ribs in a KR wing are not carrying any load at all. They along with
the foam between them position the skins which are molded over them and
in the process the skins are bonded to the spar caps. You would have a
very tough time trying to curve .093" (3/32") e-glass for wing skins,
and having worked with both I believe 3/32 ply is lighter than .093
glass and stronger than .062 glass but I have not tested to compare. In
very cold temperatures the ply retains its flexibility and e-glass
becomes noticeably brittle.
If you're thinking of glassing over foam attached to the fuselage sides
and bottom, don't. The skin needs to be attached to the truss structure
with an adhesive bond. Use a skin made of all glass if you can't get the
plywood. Don't build with foam between the truss and the skin. No load
can transfer to the skin, the foam will distort and probably begin to
crumble with any shear loading.
Burning PVC does release toxic fumes when burned including dioxin, a
known carcinogen along with others which are dangerous and toxic in the
short term. But if your foam is burning in flight you have other bigger
problems to deal with, unfortunately...
YMMV
Chris K
On 2/1/2021 10:38 AM, Michael via KRnet wrote:
Greetings
At the moment I am still completing the 90% complete, 90% to go stock KR-2
project that I am hoping to finish in the next year to 18 months and then
get airborne.
The question I asked was more of a theoretical one for a future planned
project after this one and because finding some of the materials taken for
granted elsewhere in the world has been a challenge here. I have spoken to
a few local suppliers while running around buying bits & pieces and the
response has been positive and most have stated that PVC or XPS similar
laminates would be stronger and lighter than the equivalent marine-ply
although their experience is in general construction or boat repair and not
aircraft construction. I am trying to find the article where one
manufacturer went as far as to say that their structural XPS or PVC is 24%
lighter for the same comparable strength than plywood.
They are also non-flame spreading (ie will melt and might burn in a fire
but will not propagate a fire) and will not create burning blobs or noxious
fumes like standard polystyrene.
This all said, I am still far from finding a perfect alternative and seeing
some of the other discussions, I am not the only one :)
Regards
שָׁלוֹם עֲלֵיכֶם (Shalom aleichem)
Michael L Griffin
mileg...@gmail.com
Fax : 086 733 0650
Cel : +27 83 462 0462
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On Sun, 31 Jan 2021 at 16:48, Flesner via KRnet <krnet@list.krnet.org>
wrote:
Michael Griffin, have you made any progress on selecting building
material / or made plans to proceed on your project? Any luck
contacting current or past KR builders in the country? Inquiring minds
want to know.
Larry Flesner
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