General Notes on Composites
This section summarises some
general information, materials, techniques, lessons learned, etc. regarding the composites work
I've done on this airplane.
Note that the primary structures of the RV-7A are all-metal, and the
composite parts on the airplane are considered not structurally
critical. As such, when it comes to the composite work, Van's
specifies materials and techniques only vaguely if at all. This is
mostly left up to the builder.
Materials
Epoxy
I chose the West
Systems product line for the following reasons:
- Good performance. Not necessarily the very best, but more than
adequate for this application.
- Reasonable cost (compared to more exotic higher-end epoxies).
- Wide availability. Spruce, etc. with no hazmat shipping
charges. Also, local boating suppliers.
- Wide use in the RV community, which means a track record and wide
knowledge base.
I'm using the 105 resin with 206 "slow" hardener. This
gives good working time at ~70°F ambient temperature. The West
Systems 300 Mini Pump Set is used to accurately dispense the proper 5:1 ratio without
necessitating the use of a scale.
Structural filler
Good old
fashioned flocked cotton, i.e. "flox". Cheap, available,
and does the job.
Non-structural
"fairing" filler
Bought some West
Systems 410 Microlight filler. Essentially, it's a microbaloons
type product. Haven't used it yet.
Pigment
TAP Plastics Premium Pigment,
available from Spruce. This alters the color of epoxy with no
significant effect on its other properties. It takes an incredibly minute amount of this stuff to turn
an ounce of epoxy very dark gray. Very effective in darkening
flox. Less effective in darkening fiberglass layups because of how
thin they are -- they remain fairly transparent, like a tinted
window.
Fiberglass cloth
Bought some Rutan RA7725 8.8
oz/yd² bid plain-weave cloth
(54L x 48W threads per inch), available from Spruce. It was
recommended by other builders, and is a "typical" general
purpose bid cloth. Experimenting with it some, I wasn't terribly
pleased. It tends to fray easily (yes, I cut on a 45° bias).
Also, although it has a somewhat limited range of stretch, within its
range it distorts quite readily -- too readily -- making it difficult to
handle.
On the advice of some different builders, I then bought some Rutan RA7781
8.92 oz/yd² crow foot weave cloth (60 x 54 threads per inch), also
available from Spruce. I like this one a lot better than
RA7725. It does not have nearly the same tendency to fray. It
also has a greater range of stretch (as would be expected from a crow
foot), and yet does not tend to spontaneously distort as much, and
therefore is actually easier to handle. And as an added bonus, it
has a much flatter weave texture, which should make it easier to achieve a
finish-quality surface where needed.
Note: I don't know the relative strengths of these two cloths, but since
I'm using them in more-or-less non-structural applications, I'm going to
wave my hands and declare that either cloth will be plenty strong enough.
Carbon fiber
No. Carbon fiber
promotes corrosion in aluminum that it comes in contact with. Some
RV builders have unfortunately found this out the hard way. So why
do some still use carbon fiber? It is stiffer, stronger, and lighter than
fiberglass, and simply has more "cool factor". There are
ways to deal with the corrosion issue, as well as other problems that
carbon fiber presents (more difficult to work with, propensity for hiding
voids, more expensive). But the bottom line is
that carbon fiber's advantages over fiberglass simply do not outweigh its
disadvantages for its application in an RV. In an all-composite
aircraft like a Lancair, that's a different matter.
Peel-ply
Using dacron tape as a
peel-ply. Available from Spruce in various widths.
References
