Control Line Aerobatics - 2009/05

OUR PROJECT Hole Shot model is nearing the point where it
will be ready to be assembled into one piece. Last time we
installed the control system and glued the wing halves together.
The assembly should be dry by now.
The next step in foam-wing construction is to reinforce the
center-section wing joint. This is necessary because of the bending
loads that this model is likely to encounter. We installed a joiner
spar that connects the full-depth spars that were installed in each
wing half, but experience has shown that even more support is
required.
There are several options for doing this; the one you choose
depends on the type of model you are building (profile or built-up)
and the type of finish you intend to apply (paint or heat-shrink
film). I have decided to paint the fuselage and tail section of my
Hole Shot and use heat-shrink film on the wing panels.
I will paint out onto the wing on either side of the fuselage a
couple of inches and then cover the outer portions of the panels
with the film. This allows me to apply fillets to the wing-tofuselage
joint, which will yield a much stronger and better-looking
model.
The blend point where the painted area and the film-covered
areas meet will be covered with a chordwise accent stripe. I’ll
discuss the center-joint reinforcement for use with this finishing
method later.
If you completely painted the model, the center-joint
reinforcement would be a simple matter of covering the joint twice
while applying the 0.2 carbon mat to the entire wing. I explained
the method of applying the 0.2 carbon mat in the May 2008
column.
The idea is to cover half of the wing and let the 0.2 carbon mat
go across the center joint and stop a couple inches out on the
opposite wing panel. After letting the modeling dope dry awhile,
sand the edge of the carbon.
Cover the other half and, again, overlap the center joint and let
the carbon extend a couple inches out onto the first panel that you
covered. In this manner, the center joint would be double-covered
with the carbon mat.
It is important not to sand the carbon that is covering the center
joint. The 0.2 mat will not be very “thick,” but it will be extremely
strong.
It is also vital to make sure that the carbon mat is completely
saturated with the modeling dope. If it is not fully wetted, the joint
will not be strong enough when the dope dries.
I have reinforced the center-sections of several fairly large
built-up models using this technique, and they have proven to be
extremely rigid and strong. The beauty of this method is that there
are no cloth reinforcement edges to blend into the balsa wing
sheeting before painting.
If you opt to use the finishing method I described at the
beginning, the carbon mat route could still be used but is not
suggested. Instead, I recommend that you reinforce the center joint
with a strip of 2-ounce-per-square-yard fiberglass cloth.
This fiberglass-cloth strip should be roughly 3-4 inches wide
and long enough to go from the wing’s LE to TE in one piece. You
will have to punch a hole in the fiberglass at the appropriate point
to allow it to go over the bellcrank mount post and lay smoothly
against the wing’s
surface.
Mix a half
ounce of epoxy
resin. (I use and
recommend ZPoxy
Finishing
Resin, which is
The Hole Shot starts looking like a whole airplane
[[email protected]]
Control Line Aerobatics Bob Hunt
Also included in this column:
• Wing-joint reinforcement
• Are you a CL enthusiast?
The spackle is sanded almost completely off of the fiberglass.
What remains effectively fills the weave. The cloth edge is now
blended perfectly into the balsa wing-sheeting surface.
Regular vinyl spackling is used to fill the fiberglass weave and blend
the edge of the fiberglass into the balsa wing sheeting.
Fiberglass cloth center-section reinforcement is applied using ZPoxy
Finishing Resin. A heat gun makes epoxy resin flow more
freely and allows it to fully saturate the cloth.
May 2009 129
05sig5.QXD 3/24/09 2:01 PM Page 129
available from Super Glue Corporation.)
Using an acid brush, begin applying the
resin to the fiberglass cloth. Move from
front to back and try to use as little epoxy as
possible while still fully saturating the cloth.
After the epoxy has been applied, use a
modeling heat gun (the type that is used to
apply heat-shrink film) to lightly heat the
fiberglassed area. The warmth will help
make the epoxy flow and fully saturate the
cloth.
You may have to use the acid brush to
remove excess epoxy resin after this. It is
heavy, and only enough should be used to
fully wet the fiberglass.
Let the excess fiberglass extend just
beyond the wing LE and TE; do not try to
trim it while it is wet. When the resin has
fully cured (overnight is recommended
when working with it), use a sanding block
or heavy-duty emery board to sand against
the edge of the cured fiberglass/epoxy strip.
The sandpaper will cut off the fiberglass
cleanly.
Repeat the preceding process to apply
the fiberglass reinforcing strip to the
centerline on the bottom of the wing. The
edges of those strips will be unsightly at this
point! They need to be blended into the
wing’s surface.
Do that by liberally smearing DAP
DryDex spackling (obtainable at Lowe’s,
The Home Depot, or your local hardware
store) over the fiberglass strips and out onto
the wing on either side of the fiberglass strip
by at least a half inch. This is a messy job,
and the idea is to use way more spackle than
you think you’ll need to fill the weave and
achieve the desired blend.
After the spackle has dried, start sanding
it off using a foam-backed sanding block or
the heavy-duty emery board. (I use these
emery boards for many modeling sanding
chores. They are available from See Temp: a
model airplane building-supply company
that usually has a booth at major hobby
consumer shows. I purchase the boards in
sleeves of 50 at a time.)
The idea is to fill the cloth so that no
weave shows after finishing and to make the
edge of the cloth blend gradually into the
wing’s surface. Be extra careful not to “dish”
the balsa on the wing while sanding the
spackling.
If you think you might have a problem
doing that, lay a strip or two of masking tape
chordwise on the wing, just outboard of the
area that you will be sanding. This way, you
will be able to learn quickly how to control
the sanding block or emery board so that it
will not be a problem.
There will be a slight visible “ramp” from
the edge of the fiberglass to the surface of
the wing after this process is completed, but
it will be so minor that it will not be
unsightly. If you use the carbon mat method
I described earlier, this “ramp” will be much
less visible, if detectable at all.
In the next column I will cover installing
the upper and lower bellcrank mounts to the
outside of the wing and installing the wing
into the fuselage. I will also outline a
foolproof method of aligning the wing and
stabilizer centerlines. This will ensure a
great-flying aircraft!
If you have even a passing interest in flying
CL Precision Aerobatics (Stunt), consider
joining the Precision Aerobatics Model
Pilots Association (PAMPA): the AMArecognized
SIG for CL Aerobatics.
Membership benefits include a
subscription to Stunt News, which is a
bimonthly, 98-page newsletter (magazine,
really) that is chock full of how-to articles,
contest reports, history articles, construction
features, regular columns about all aspects
of CL Stunt flying, and much more!
PAMPA features three membership
options at different prices. One offers you
the option of receiving Stunt News on a CD,
in color, via mail. Another gives you the
option of registering on the PAMPA Web
site and downloading the newsletter in
color. Or you can opt to receive the
newsletter in printed form through the mail.
To join PAMPA, drop an e-mail to
David Gardner (the secretary-treasurer); see
the “Sources” list for his address. You can
also join by downloading the membership
form online at the PAMPA Web site. Either
way, it is the best way to keep up with and
learn about CL Stunt flying.
Till next time, fly Stunt. MA
Sources:
Dap Products
(800) 543-3840
www.dap.com
See Temp
www.seetemp.com
Super Glue Corporation
(800) 538-3091
www.zap.supergluecorp.com
PAMPA:
[email protected]
www.control-line.org