REFLECTOR: Fiberglass stiffness

[Ken C. Baker]

Brian,

This happened to us at the factory awhile ago. One of our normal suppliers
offered us some "7725 bi-directional cloth from a new manufacturer in Korea"
at a price that seemed too good to be true. We bought a small roll with the
understanding that if we didn't like it, we could return it. As it turns out
- the price *was* too good to be true. We couldn't work with this stuff to
save our lives. Too stiff - wouldn't saturate properly.

When ordering fiberglass - you do order by weight or by weave style,
unfortunately it gets a little bit more complex when it comes to the
finishes that are applied to the fiberglass. Most glass fibers are treated
with starches during the looming  process that allow them to take the rough
handling associated with weaving. It is a critical part of the manufacturing
process for aerospace grade fiberglass to be cleaned after the weaving to
remove the starches, and for a surfacing finish like Volan A to be applied.
There are different finishes for different applications and resins used.
Volan A, in my experience, seems to be the most common for compatibility
with epoxy based laminating systems.

Occasionally you will see major fiberglass distributors offering
"loom-state" goods at heavily discounted prices. Loom-state is code for "no
surface finish - use at your own risk." I have seen some of these cloths
(especially carbon fiber) being resold (particularly on eBay) by less than
scrupulous sellers promising "aerospace quality" goods. These cloths are
fine if you want to make cosmetic trim pieces for a motor-cycle or something
like that - but my own advice is to *never* use fiberglass purchased on eBay
for structural applications. It is really important to know where your
fiberglass came from, and to know that there is a paperwork trail and
testing data that backs it up. Buy from a trusted source.

When ordering glass online, I would also recommend against purchasing glass
which is described only by its weight and a "course" or "fine" weave
description. Ordering by the weave style "7725," "7715," or "CDB-200" is a
much more accurate way of ensuring that you're getting the right kind of
glass. The lay-up schedules that are called for in the Velocity construction
manual were calculated with a specific weave and weight of fiber-glass in
mind. 

I would recommend laminating over the hard-points for the trim actuator.
There's a fair amount of force that's applied during flight.

Best,

-Ken Baker
kenb at velocityaircraft.com



-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Brian Michalk
Sent: Monday, August 18, 2008 11:25 AM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Fiberglass stiffness

Yesterday we attempted to do some work with the canard trim hardpoints where
the actuator mounts.

The fiberglass we bought from somewhere was coarse BID, but we could not
convince it to lay down over the corners.  I compared it to some fine bid,
and it's much stiffer than the fine BID.  The BID that came with the kit was
nice and floppy, so it would have made the corners just fine.

How would I know what to order?  As far as I know, one orders by the weave
(fine, coarse, etc.), and the cloth weight.  I haven't seen anything for the
stiffness of the cloth which would directly affect workability.

We gave up, and left the wood shim and AL hardpoint to cure as is.  We used
flox for our adhesive.  Do we really need to cover a wood shim with 2x bid
as called for in the manual? 

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