132 MODEL AVIATION
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Radio Control Soaring Darwin Barrie
How to build a composite fuselage
Everything on the fuselage has been sanded to its final form. You can see the location of
the formers to support the tailboom.
The author’s fuselage has been rough-cut to shape and the boom has been taped in place.
The foam at the tail is long to assist in shaping.
A top view of the fuselage’s final shape. It is 1.25 inches at its widest point.
I RECEIVED POSITIVE feedback on
my column about the preparation for
fiberglass. Many who are new to
composite building were unaware of
preparation needed to finish a fiberglass
fuselage.
This month I am going to take that
topic a bit further and discuss the process
to build your own composite fuselage.
This will be a two-part article discussing
the new hybrid Two-Meter Jerry
Robertson and I are building.
Noted designer, builder, and pilot Jerry
Robertson and I were discussing building
a new world-beater Two-Meter design.
We agreed that the fuselage would be a
pod-and-boom style, which provides for a
light and strong fuselage that is easy to
build. A nose cone would be
incorporated.
The plan was to use the carbon-fiber
sock method for simplicity, strength, and
weight. I will provide details of that later.
This process is great for the homebuilder
to use to make a “one-off”
fuselage and requires no male/female
molds or specialized tools. The first task
was to acquire a tailboom.
Jerry felt that the longer the tail
moment, the smaller the tail surfaces
could be. This would ease construction
and keep the weight down. I acquired an
Allegro Two-Meter tailboom. It was 33.5
inches and perfect.
We laid out the nose moment of the
fuselage based on several factors. One
goal was to make the fuselage crosssection
as small as possible. We also
wanted the nose moment to be long
enough to comfortably fit all the radio
gear and avoid having to add useless nose
weight.
Jerry chose the basic planform of his
successful Sisu design, which is kitted by
Northeast Sailplane Products (www.nes
ail.com). It has a nose that is unique in
appearance and sweeps downward.
One of Jerry’s concerns was the
location of the servos and the ability to
get as straight a line as possible for the
pushrods down the tailboom. The Sisu
style makes this task a bit simpler.
I laid out a more conventional-design
fuselage without the down sweep. The
pushrods lined up nicely and the fuselage
was easier to construct. The nose moment
from the LE to the nose is 10.75 inches.
The first thought was to do a pedestaltype
wing mounting. This became
unfeasible with this type of construction.
Blue or pink foam is necessary to
make the fuselage. Either one is easy to
cut, carve, and sand. I used pink foam
because it is what I had available.
The profile view was drawn and cut
with a band saw. The boom would be inset
into the fuselage 3 inches from the end of
the pod portion. I made the foam slightly
longer here to aid in shaping.
I laid out and cut the top view. Once I
was satisfied with the overall dimensions
and symmetry, I block-sanded the corners
into the desired radii. I used the “TLAR”—
That Looks About Right—method for this.
The boom location was measured, and
the pod was cut to final length. I cut the
fuselage in two sections where the
tailboom would be mounted. I used the
“sectioned” pieces as a template to make
two plywood bulkheads. The boom would
be inserted into the bulkheads.
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I drilled the bulkheads and then
carefully fit them to the boom at the proper
location, taking into account the taper of
the boom. I drilled the pieces of foam to
accept this assembly.
I prepped the boom for epoxy by
roughing the outer surface with sandpaper
and then cleaning it with alcohol. The
entire assembly was glued to the pod with
five-minute epoxy. Care was taken to
ensure that everything was in proper
alignment.
I have built several fuselages using this
method, and one thing I have learned is
that I can mold in the pushrod housings
before using the carbon sock. This greatly
eases construction later and provides for
secure pushrods in the fuselage. In this
case I used .050 carbon pushrods.
The process involves placing the servos
on the profile view to determine the
location and proper height of the servo
arms. Then a straight line is scribed on the
foam for the full-length location of the
pushrod. Since the fuselage pod tapers into
the boom, I had to avoid making
significant bends that would cause
binding.
At this point I had already built the
stabilizer and knew the exit location on the
top of the boom. The rudder pushrod
would simply exit the end of the boom.
I used a Dremel tool with a router
attachment to route a channel for the
pushrod housing. Approximately 4 inches
in front of the opening for the boom, the
channel was tapered inward to provide a
smooth transition into the boom. A nice
transition was made through trial and error
that created no binding.
An inch of the front of each housing
was taped off so it would not adhere to the
fuselage sides during the layup process
and keep epoxy from getting into the
tubes. The pushrods were inserted to
prevent any crushing or kinking of the
housings.
After getting everything lined up, the
housings were epoxied into the foam
channels. Toward the tail where the taper
occurs I mixed microballoons and epoxy to
fill the void between the fuselage sides
where the foam was cut deeper.
The fuselage was ready for the layup
process to begin. I reinforced several areas
with carbon-tow material and fiberglass
cloth. A strip of tow was epoxied on each
side and the top and bottom. I laid 1-inch
fiberglass cloth on the bottom full length
and doubled this in the area of the
towhook, wing saddle, and forward
fuselage area.
After curing I sanded this smooth
without cutting into the fiberglass weave.
All layup was done in one process using
West System epoxy, thinned slightly with
alcohol.
The following items are necessary to do
the layup of the fuselage. The carbon sock,
part C9361, is available from CST—The
Composites Store (www.cstsales.com). It
is the 2-inch version. At first the carbon
looks heavy. When completed and sanded,
it is extremely thin but extremely strong.
You will need West System or similar
epoxy and microballoons. I like the West
System 410 Microlight filler. You will also
need 16-gauge or similar wire and a 1-
gallon can, jug, or bottle with a handle.
A place for the assembly to hang is
required. I used a doorway where I could
keep the door open all night to the outside.
This gives you a clear area to work around
when you do the fiberglass work on the
fuselage.
I’ll complete the fuselage and discuss the
wings of the project next month. I’ll wrap
up this month with the construction of the
tail feathers.
The stabilizer is an all-flying type,
mounted to the boom with a “V” mount. I
used a cross-section of the Sisu C
stabilizer and made two light-plywood
templates. The spar is 3/16-inch-diameter
aluminum tube from K&S, which is
available at most hobby shops.
I cut 16 1/16-inch rectangular pieces for
the ribs. These and the plywood templates
May 2007 133
A view of the stabilizer. The spar is 3/16-inch OD tubing from K&S. The dimensions
include a span of 16.25 inches and a chord of 3.75 inches.
The vertical fin and rudder. The assembly
was made as one piece, and the rudder will
be cut from the fin.
were tack-glued together. The TE was slit
on the chord centerline for the carbon TE
piece.
A length of the aluminum tubing was
cut to 4 inches and one end was
sharpened. This piece was chucked in the
drill press. The ribs and templates were
drilled with this piece for the spar. The
balsa was sanded to match the templates.
The LE is a piece of 3/16 square balsa.
The location of the ribs was determined,
and the LE was notched on the band saw.
The ribs were separated and the
aluminum tube spar was marked with the
LE as the template. The ribs, spar, and TE
were put together and alignment was
checked.
After proper alignment was achieved,
the assembly was glued with thin
cyanoacrylate on the LE and TE. The spar
was secured with thick cyanoacrylate.
The assembly was sanded to its final
form, and the mounting platform was
drilled for the V mount.
This type of assembly is light and
strong, with great torsional stability. The
weight before covering is 8 grams.
The rudder is built in a similar
manner. The photos illustrate the design
and overall method of construction. The
rudder and fin weight before covering is 6
grams. The end of the boom is slotted to
accept the vertical fin. The rudder
pushrod will exit the end of the boom.
The carbon-sock method of
construction can be used for a variety of
other modeling projects. I have made
wheel pants for powered airplanes and
even a small cowl. The sock material is
also available in fiberglass. MA
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