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
Balsa is strongest when the grain
runs the length of the wood.
Sanding with the grain produces a
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 itís 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 itíll tend to split very quickly.
Stage 1: Getting the wood
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
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 youíll 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
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 practicallyóthe
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
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 isnít 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 youíve 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
Even if youíre 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, youíll notice that it tends to
spring back a little bit, that is okay, itís normal.
You can now glue your balsa to the airframe.