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REFLECTOR: Kevlar Vs. Carbon fiber vs. E-glass

David Dotty,

> Kevlar is very strong in tension and very weak in compression.
> Kevlar is also good for scraping, and is often put along canoe keels
> for that reason (scraping on rocks and river banks). Might be great
> along the lower edges of the winglets or the belly of a retract.

Thank-you David....so true.  When ever people speak of Kevlar in
structuralapplications a big red flag pops up in my head.

Armada fibers (Kevlar) have five times the tensile strength of stainless
steel but less than half the compression strength of fiberglass.   This
is compounded by the fact that the fibers have to be "scrubbed" rough
before weaving so they will get some bond to the matrix.  They still
delam easy especially with secondary bonds.  Hand layups are so resin
rich they are not much lighter than glass anyway.  For the same reason
hand layups in carbon don't save weight but they do impart rigidity were
needed.  Cutting and sanding Kevlar just produces fuss which can only be
rid of by high speed sanding with fresh 400 grit.

If you want to save weight here are a few tips:

-minimize the use of micro glass filler...are you using it were foam or
micro ballon is OK?
-Tape glass should not be oversized or sloppy.    Try pre-wets on
plastic both layers at once in a big rectangle then cut into neat
strips, lay in place, besure and peel out plastic then lay peel ply down
-new Velocity kits have many high tech flanged parts.  These are bonded
in with adhesive which is much stronger than laminating resin.  Tape
glassing over the outside of these flanges after bonding is a waste.  If
you can reach the inside a very narrow tape will increase the strength a
little but in most cases is not needed considering the bond strength of
these adhesives.
-West systems 410 micro lite filler and Urethane primers are much
lighter and easyer for finishing than glass bubbles and epoxy primers.
-most Velocities are over weight and the customers needed more resin to
complete the kit..I wonder why?  Dan built one that weighed 1100 lbs.!

Kevlar is lighter and cheaper than carbon and more rigid than glass.
The ultra light cowling on my own V-300 is a tri-hybid.  First a fine
glass was laid in the mold then a coarse Kevlar which was trimmed short
of all the edges.  Then a double coarse carbon was taper laid around all
the edges for stiffening and attachment.  The Kevlar/glass combo when
co-laminated under pressure is actually lighter than carbon and cheaper
too.  It is weaker in compression but in this application what I like is
that it won't rip apart for how thin it is.

Kevlar comes in #49 and #29 one is for structures with normal flat weave
or knit and the other is a tight canvas like weave for ballistics.  The
latter may be handy in the fuselage and on the fuel strakes depending on
were you live and/or if you are in the wrong business.  Fortunately the
Velocity is the wrong design for STOL and stealth so I'm not worried
about any of our customers in that respect.

Alan Shaw