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Quite frequently in building
with balsawood we need to bend balsa into a curved surface. For curves with
fairly large radii, this can be done without any problem. When it comes to
convincing balsa to bend around complex, varying, and tight curves (such as
tail planes or wingtips), balsa has to be assisted into making these curves
without crimping or snapping. The reason why we choose to bend balsa around
such curves is for a couple of reasons:
Strength: Balsa is strongest when the grain runs the length of the
wood.
Finish: Sanding with the grain produces a smoother surface.
Economy: It's cheaper to make a wingtip out of a strip of balsa than
to use up a larger sheet of balsa and having to discard the bulk of it.
The available methods of getting balsa to bend more can be broken down into
sections: laminating, one-sided moisture/heat, chemicals, long soak. With
all bending operations it’s suggested that you start out with the most
flexible piece of balsa that you can obtain, typically this is referred to
as A-grain balsa. Do not attempt to use C/quarter-grain balsa as it’ll tend
to split very quickly.
Stage 1: Getting the wood flexible
Laminating: The process of using laminating to make balsa curve
around corners is based on the principle that a thinner sheet of balsa can
be curved at a tighter radius. The radius of curvature limit varies between
materials, but essentially it represents a percentage of compression (or
tension), caused by the difference in curve radii between the inner and
outer limits of the balsa. Thinner balsa will be able to be bent tighter
before the same critical difference of curvature occurs. Using the
laminating process can be a fairly tedious one, but it does produce an
appealing (to some) visual appearance. Laminating produces the strongest,
but also heaviest, resulting form.
One-side moisture/heat: If you take a sheet or strip of balsa and
dampen one side you’ll see that in a few seconds that the balsa starts to
curve away from the dampened side. Conversely, if you apply a hot iron to
the sheet of balsa, the balsa will curve toward the heated side. The reason
why this occurs in both cases is because of a difference in moisture content
in the balsa wood cells. The more moisture in the cell, the more it expands.
In the damp application, the damp side of the balsa expands causing the
sheet to curve away. With the iron application, the moisture is driven out
of the balsa cells on that side to contract and causing the balsa to curl
in.
Chemicals: Sometimes you really need to get a piece of balsa around
things are already too thin for laminating practically—the solution can
sometimes be to chemically adjust balsa to bend. Clouded ammonia (water with
ammonia in it) or Windex will make balsa especially flexible. The action by
which this occurs is the breaking down of balsa cell walls. Interestingly
some people have reported that using vinegar also works, the key appears to
be to soak the material in a non-neutral pH substance. For clouded ammonia,
use a 50/50 mix with water. Caution: use this mix in a well-ventilated area.
Ammonia can suffocate you. If you would rather not take the potential risk,
consider using the long-soak method. Long soak: If using chemicals such as
ammonia or vinegar isn’t your idea of a pleasant experience, you can soak
the balsa in hot/warm water for an hour or more (depending on the
thickness). The heat is useful to accelerate the absorption of the water
into the cell structure.
Stage 2: Setting the shape
Once you’ve made your balsa flexible, you can commence to shape it to your
needs. For simple curves, such as cylinders, cones and such, you can simply
apply the wood to the formers or suitable shape holder (having a good
selection of tins, tubes, and rods help here) and tape/hold the balsa to the
required shape and allow to dry. Even if you’re using the framework itself
to form the curve, do not attempt to glue the balsa at this stage. Wet balsa
and glue do not work together. Wait until the balsa is completely dry. Be
forewarned that this sometimes can take a day or two in the cold weather.
When you remove the balsa from its former shape holder, you’ll notice that
it tends to spring back a little bit, that is okay, it’s normal. You can now
glue your balsa to the airframe. |
