Control Line Aerobatics - 2008/01

IT’S TIME to build! I trust that those of you who are going to follow
along with this project have obtained plans for the Hole Shot or for one
of the other flapless models I mentioned last time.
I like to start with the fuselage. The Hole Shot has a 3/8-inch-thick
profile fuselage. It should be cut from 3/8 medium to medium-firm
balsa.
I use the pin-punch method to transfer shapes from plans to balsa. Ilay the plans over the balsa stock and hold it in place securely. Then I
use a straight pin to punch holes through the plans and into the balsa, at
roughly 1/4-inch intervals around the perimeter of the piece I want to
duplicate.
When I have punched the entire outline, I remove the plans and
connect the pinholes with a fine ball-point pen. It’s important not to
forget to pin-punch around the wing and tail openings and the motormount
slots.
I use a scroll saw to cut the fuselage blank. If you are going to be a
serious builder you should purchase a good scroll saw right away if
you don’t have one. It’s an investment in building quality.
The scroll saw should have a quick-release mechanism to allow the
blade to be fed through starter holes in places where you have to cut
out a portion of the interior of a part. Such a place on the Hole Shot
fuselage would be the wing hole.
I’ll add to this list of necessary tools and describe how to use them
as we proceed. Hey, that’s the whole purpose of this series after all!
The hard 3/8-inch square maple motor mounts are next to be
measured and cut. I drill lightening holes in my mounts well aft of the
area where the engine will be positioned.
This is a good place to discuss weight consciousness. The way to
achieve light models is to look at each part you intend to glue into or
onto the model and ask yourself if it is too heavy and too strong for the
job it has to do.
The path to lightweight building is simple. Be conscious of grain—
not gram or ounce! There are 15.43 grains in a gram. If you leave an
extra gram in here or there 28.35 times, you’ve added an ounce.
I know; that sounds ridiculous, but those who think and build with
this thought in mind always seem to produce outstandingly light
models. Lightweight building is a frame of mind!
Use your scroll saw to cut shallow notches the width of the blade in
the maple mounts on the edge where they will contact the fuselage
blank. These notches should only be approximately 1/32 inch deep.
They will allow for more gluing surface and yield a much stronger
bond.Install the maple mounts using a mix of
slow-cure epoxy and microballoons. Slowcure
epoxy will work its way deeper into the
wood fibers on either side of the bond before it
hardens. I’ve seen components on airplanes
built with fast-cure epoxies come apart after a
season or two.
The microballoons are also important.
Epoxy is just a mix of resin and hardener.
There’s not much in there to actually form a
solid mechanical bond. By adding a small
amount of fiberglass microballoons to the mix,
you get a much stronger grip between epoxied
pieces and it will not break down easily in
time. This is especially important in regions
where there is high stress or vibration, such as
in the motor-mount area.
Be sure to wipe any excess epoxy/
microballoon mixture from the fuselage side
surface. We want a flat and clean surface on
which to glue the doublers.
Once the epoxy on the motor mounts has
cured, cut and install the 1/16 birch-plywood
fuselage doublers in the nose area. You can
use the fuselage blank as a template to lay out
the doublers. Cut them out slightly oversize
using your new scroll saw and attach them to
the fuselage using the slow-cure epoxy/
microballoon mix.
Weight this sandwich of parts down flat
against a clean, hard, flat surface. Make sure
nothing has shifted before you go off to let the
epoxy cure.
Once the epoxy has cured, carve and sand
the edges of the doublers flush with the
fuselage blank. Plywood can be tough to carveand sand, and this is where the proper tool
and technique must be applied.
I’ll stop here and discuss more tools. I’ll
be doing a lot of that in this series.
Carving and sanding are two of the most
used and useful skills in building. You must
learn as much as you can about how to
control a knife and a sanding block. These
two tools will make or break your model in
regards to how perfect it looks when it’s
finished.
The ability to safely and skillfully control
a knife is imperative if you want to learn to
build well. There are many different types of
knife handles and knife blades, and they all
have specific applications.
The most common modeling knife blade
is the ubiquitous #11. It is fairly narrow and
tapers sharply to a fine point. This blade is
sharp and can cut easily through most
densities of balsa, and it can even be used to
whittle thin plywood. It is usually mounted
in an aluminum handle that is approximately
1/4-5/16 inch in diameter.
You can usually find handles in hobby
shops, to accept #11 blades, that have rubber
or polyurethane coatings, and these are much
easier to grip. Many of these handles also
feature hexagon-shaped end pieces that are
designed to keep the knife from rolling off
the table and finding your big toe. (Ouch!)
You usually hold the #11 blade handle
like a pen might be held for cutting and grip
it firmly in the palm of your hand for
whittling. I’ve found that hobby-grade #11
blades vary greatly in quality and the ability
to hold an edge. You want to purchase highgrade
blades.
I buy these blades in boxes of 100 at a
time and change them often. I’ve tried
sharpening them with some of the specialty
sharpening devices but have found that once
the stock edge is dulled, the blade will not
hold a resharpened edge very well. Nothing
will ruin a part quicker than a dull knife
blade!
Another useful blade is the whittler type
(commonly called a #26). It fits into a
somewhat larger-diameter handle—usually
approximately 1/2 inch—and features a long,
flat surface with a cutting edge on one side.
This blade is useful for carving blocks and
tapering long sheets of balsa.
There are many different types of sanding
blocks. I strongly recommend the type that
requires you to attach the sandpaper to the
block’s surface with some sort of contact
cement.
The sanding blocks that have soft pads
between the block and the sandpaper are
useless for our purposes. You want the
sandpaper to lay firm against the block, and
contact-cementing it to the block’s surface is
the only way I know to achieve this.
I advise against wrapping a loose piece of
sandpaper around a block and holding it in
place with hand pressure. The paper can slip
and bunch up, ruining the sanded surface.
When you use the contact cement to attach
the sandpaper to the block, make certain that
the block surface and the backside of the
sandpaper are clean and smooth. After
attaching the sandpaper, press the block,
sandpaper face down, against a flat table and
press hard. You want the sandpaper to lay flat
against the block.
While you are pressing down, roll the
block over on its edges a few times. This will
make the sandpaper fold around the block a bit
and prevent the corners and edges of the
sandpaper from digging into the surface you
are sanding.
There are many very nice commercially
available sanding blocks that fit the criteria I
have described. Great Planes has an extensive
line of Easy-Touch extruded-aluminum
sanding blocks to which sandpaper can be
attached with contact cement.
I suggest getting several Easy-Touch
blocks in several lengths. The blocks range in
size from 11 to 44 inches. I use the 11-inch
variety most of the time, but I find the long
blocks to be extremely useful in sanding the
surface of the sheeting on a foam-core wing.
You should purchase enough sanding
blocks that you have several grits of sandpaper
available at any given time. I like a full range
of grits between 180 and 320 for general
building purposes.
After you have worn out the sandpaper, lay
the block in a shallow pan filled with a small
amount of mineral spirits. (Do this outside!)
The sandpaper will delaminate fairly quickly,
and then the block can be wiped clean and
resurfaced. Keep your sanding blocks fresh
and sharp by changing the sandpaper often!
Perma-Grit makes another useful sanding
block. This company has a wide range of
sanding and cutting tools, and although they
are fairly expensive they last virtually forever!
Tiny bits of tungsten are welded onto a
metal plate, and that forms the sanding
surface. Perma-Grit has a line of sanding
blocks that are extruded rectangles with
different grits of tungsten on each side.
The advantage of the Perma-Grit sanding
block is that it can be set on its side with the
sanding surface 90° to your building board.
This allows you to square up pieces almost
perfectly with little effort.
After you have sanded an area—especially
where two parts meet—run your finger over
the seam. If you can feel that seam, you will
see it after the model is finished.
Sanding an area where two parts of
dissimilar densities are joined is even more of
a challenge because the softer material will
sand more easily and quickly than the harder
material will. Ridges, bumps, and uneven
curves are often the result.
You will need to learn how to control your
sanding blocks to work the harder material
against the soft without removing too much of
the latter. For this reason you should never
sand without using a block of some sort.
Uneven hand pressure on the back of a piece
of otherwise unsupported sandpaper will yield
ripples and low spots.
The subject of sanding blocks and knives
and the techniques required to master themcompletely would take many pages to cover.
In a future issue there will be a
comprehensive article about cutting and
sanding as part of a series about model
building.
Back to the fuselage. Carve most of the
excess 1/16 plywood from the doublers using
the #11 blade knife. Try to get within 1/32
inch of the balsa fuselage core piece with the
carving process. Finish by sanding the edge
flush against a Perma-Grit sanding bar laid
on its side or carefully sand the edge with a
normal block, checking often to ensure that
you are not angling the edge.
Sand the entire edge of the balsa fuselage
so it is smooth and properly contoured from a
profile view.
Before the top and bottom edges of the
fuselage can be rounded, you must scribe a
centerline on the edges. I suggest that you do
this with a #2 pencil. If you use a ball-point
pen for this job, be sure to sand off all the
ink. Any ink left on the balsa after sanding
could bleed through and ruin an otherwise
perfect paint job later.
Sand the edges of the balsa fuselage core
to a pleasing round shape. Do not round off
the area where the plywood doublers are
attached. You can break that edge slightly
with a sanding block, but it should not be
fully rounded.
At the rear end of the plywood doublers
there will be a drop-off to the surface of the
balsa fuselage core piece. You can add balsa
“ramps” to fair this step in smoothly if you
like.
Decide which type of landing gear you
are going to use and drill the appropriate
holes in the doublers in the correct spots. The
Hole Shot can be built with normal twowheel
landing gear or with a single strut for a
one-wheel type. The one-wheel gear is
lighter and simpler, but the choice is yours.
Wow, we’re out of space already and all
we’ve done is make the fuselage! The idea
behind this series is to learn to build using
proper techniques, tools, and materials. It will
take awhile to complete this project, but by
then you should be armed with impressive
building skills and have a shop full of great
tools and know how to use them. We are on
our way.
I can only submit three photos for use in
this column each time. I hope the written
descriptions of the building process and the
other things I cover will be clear enough.
I am taking many pictures as I proceed
with this project and will gladly E-mail a set
of low-resolution photo files to anyone who
requests them as we go. Simply send a
request to me at the E-mail address in the
headline of this column.
Til next time, Fly Stunt! MA
Sources:
Great Planes Model Manufacturing
(217) 398-3630
www.greatplanes.com