LETTERS TO THE EDITOR
WITH REGARD TO THE CHEVELOCITY article (April issue), I have some
comments regarding some composite techniques that the builders
mentioned on page 44.
As an engineer at Scaled Composites and the builder of two
composite aircraft, I believe that the advice to add Cabosil to
whatever resin the builders used is contrary to normal wet (hand)
layup practices. Adding any filler, thickener, or additive to the
laminating epoxy prior to wetting out the fiberglass cloth is not
recommended. This could possibly weaken the structure.
Also, stippling the layup with a stiff brush usually leads
to pushing more air into the layup rather than removing it.
Squeegeeing is a far more effective and quicker technique.
A faster and higher quality technique involves wetting out
multiple layers of cloth on a flat table over thin plastic sheeting.
This can be squeegeed to an optimum glass/epoxy weight ratio
while removing almost all excess epoxy while not introducing
any air by pushing a brush into the layup. The layup can be cut
to shape and applied onto the slightly wet-out substrate. After
squeegeeing to remove any trapped air bubbles and excess epoxy,
you’re done. Peel-ply if desired to minimize later finishing work
and/or edge lift.
The fear of having resin run out of a layup, even the thinnest
epoxy in a warm environment on a vertical surface, is generally
unfounded. Resin will only run out of a layup if there is far too
much resin in the layup to start with.
Next, the idea that adding plies in arbitrary areas to increase
strength due to increased speed is also almost always unfounded.
Without a stress analysis, the chances that the added structure will
have a useful effect rather than merely adding weight is very small.
While in this particular case, the builders have had good luck
with the techniques they’ve used, I believe that it would not be
wise for others to attempt to follow these techniques.
Marc J. Zeitlin, EAA 475937
We shared Marc’s letter with the builders of the CheVelocity, and
here’s their response. We share their exchange for the benefit of other
REGARDING THE ADDITION OF Cabosil, the resin manufacturer
recommends a 1 to 3 percent by weight Cabosil addition to prevent
resin drain out in coarse BID vertical layup applications. We added
Cabosil at the rate of only 0.5 percent by weight to attain the desired
result when needed.
Dan Horvath, EAA 528609, and Bill Mulrooney, EAA 486658
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