Balsa Facts
Sig Manufacturing Co,
Inc has been one of the Radio Control industry leaders since 1951, manufacturing
an extensive line of radio control, control line and free flight model aircraft
kits as well as numerous accessories and a complete assortment of balsa for the
modeler who enjoys scratch building. The nice people at Sig have graciously
permitted us to reprint the following article from their catalog, which will
tell you all about balsa.
Interesting Facts about Balsa Wood. . .
Model
airplanes are no different than any other type of flying machine, large or small
- THE LIGHTER IT IS BUILT, THE BETTER IT WILL FLY! With that in mind, it is easy
to understand why balsa wood has been the standard material for model airplane
construction since it first became readily available in the U.S. in the late
1920s. Its outstanding strength-to-weight ratio enables hobbyists to construct
durable models that fly in a totally realistic manner. Balsa also absorbs shock
and vibration well and can be easily cut, shaped, and glued with simple hand
tools.
Where does Balsa Wood come from?
Balsa trees grow naturally in
the humid rain forests of Central and South America. Its natural range extends
south from Guatemala, through Central America, to the north and west coast of
South America as far as Bolivia. However, the small country of Ecquador on the
western coast of South America, is the primary source of model aircraft grade
blasa in the world. Blasa needs a warm climate with plenty of rainfall and good
drainage. For that reason, the best stands of balsa usually appear on the high
ground between tropical rivers. Ecquador has the ideal geography and climate for
growing balsa trees. The scientific name for balsa wood is ochroma lagopus. The
word balsa itself is Spanish meaning raft, in reference to its excellent
floatation qualities. In Ecquador it is known as Boya, meaning buoy.
How does Balsa Wood grow?
There is no such thing as entire
forests of balsa trees. They grow singly or in very small, widely scattered
groups in the jungle. For hundreds of years, balsa was actually considered a
weed tree. They reproduce by growing hundreds of long seed pods, which
eventually open up and, with the help of the wind, scatter thousands of new
seeds over a large area of the jungle. Each seed is airborne on its own small
wisp of down, similar to the way dandelion seeds spread. The seeds eventually
fall to the ground and are covered by the litter of the jungle. There they lay
and accumulate until one day there is an opening in the jungle canopy large
enough for the sun's rays to strike the jungle floor and start the seeds
growing. Wherever there is an opening, made either by a farmer or by another
tree dying, balsa will spring up as thick as grass. A farmer is often hard put
to keep his food plot clear of balsa. As the new balsa trees grow, the strongest
will become predominate and the weaker trees will die. By the time they are
mature, there may be only one or two basa trees to an acre of jungle.
How long does it take a Balsa tree to grow?
Balsa trees grow very
rapidly (like all pesky weeds). Six months after germination, the tree is about
1-1/2 inches in diameter and 10 - 12 feet tall. In 6 to 10 years the tree is
ready for cutting, having reached a height of 60 to 90 feet tall and a diameter
of 12 to 45 inches. If left to continue growing, the new wood being grown on the
outside layers becomes very hard and the tree begins to rot in the center.
Unharvested, a balsa tree may grow to a diameter of 6 feet or more, but very
little usable lumber can be obtained from a tree of this size. The basla leaf is
similar in shape to a grape leaf, only a lot bigger. When the tree is young,
these leaves measure a much as four feet across. They become progressivly
smaller as the tree grows older, until they are about 8 - 10 inches across.
Balsa is one of the few trees in the jungle which has a simple leaf shape. This
fact alone makes the balsa tree stand out in the jungle.
The perfect nurse!
Nature evidently designed the balsa tree to be
a "nurse tree" which would protect the slower-growing species of trees from the
scorching jungle sun during their critical early years. For instance, in an area
of the jungle that has been ravaged by a tropical storm or other natural
disaster, the balsa trees will quickly sprout and begin to shoot up to
impressive heights in a very short time. Their fast growth, and the extra large
leaves they have in their early years, provide shade to the young seedlings of
the slower-growing forest giants. By the time the seedlings are established
enough to take care of themselves, the balsa tree is beginning to die.
Undoubtably, the balsa tree's rapid growth, fast spreading crown of first very
large and gradually smaller leaves, and it's relatively short life span were
intended to make it the "perfect nurse" in the jungle ecosystem.
How are Balsa trees harvested?
While nature intended the balsa
tree to be a short lived nursemaid, mankind eventually discovered that it was an
extremely useful resource. The real start of the basa business was during World
War I, when the allies were in need of a plentiful substitute for cork. The only
drawback to using balsa was, and still is, the back breaking work that is
necessary to get it out of the jungle. Beacause of the way the individual balsa
trees are scattered throughtout the jungles, it has never been possible to use
mass production logging procedures and equipment. The best way to log balsa
trees is to go back to the methods of Paul Bunyan -- chop them down with an axe,
haul them to the nearest river by ox team, tie them together into rafts, and
then float the rafts of balsa logs down the river to the saw mill.
The
logging team usually consists of two native Ecquadorians, each armed with a
broad Spanish axe, a machete, and a long pole sharpened like a chisel on one end
for removing the bark from the downed trees. Because of the hilly terrain, an ox
team may only be able to drag two logs to the river per day. At the saw mill the
raw balsa is first rough cut into large boards, the carefully kiln dried, and
finally packed into bales for shipment to the U.S. via ocean freighter. Final
cutting and finishing of our model aircraft balsa is done right here at the SIG
factory. As a result of the balsa tree's fast growth cycle, both the quality and
lightness of the lumber obtained from a balsa tree can vary enormously depending
upon the tree's age at the time of cutting.
Why is Balsa Wood so light?
The secret to balsa wood's lightness
can only be seen with a microscope. The cells are big and very thin walled, so
that the ratio of solid matter to open space is as small as possible. Most woods
have gobs of heavy, plastic-like cement, called lignin, holding the cells
together. In balsa, lignin is at a minimum. Only about 40% of the volume of a
piece of balsa is solid substance. To give a balsa tree the strength it needs to
stand in the jungle, nature pumps each balsa cell full of water until they
become rigid - like a car tire full of air. Green balsa wood typically contains
five times as much water by weight as it has actual wood substance, compared to
most hardwoods which contain very little water in relation to wood substance.
Green balsa wood must therefore be carefully kiln dried to remove most of the
water before it can be sold. Kiln drying is a tedious two week process that
carefully removes the excess water until the moisture content is only 6%. Kiln
drying also kills any bacteria, fungi, and insects that may have been in the raw
balsa wood.
How light is kiln dried Basla Wood?
Finished balsa wood, like you
find in model airplane kits, varies widely in weight. Balsa is occasionally
found weighing as little as 4 lbs. per cu. ft. On the other hand, you can also
find balsa which will weigh 24 lbs or more per cu. ft. However, the general run
of commercial balsa for model airplanes will weigh between 6 and 18 pounds per
cu. ft. Eight to twelve pound balsa is considered medium or average weight, and
is the most plentiful. Six pound or less is considered "contest grade", which is
very rare and sometimes even impossible to obtain.
Is Balsa the lightest wood in the world?
No! Most people are
surprised to hear that botanically, balsa wood is only about the third or fourth
lightest wood in the world. However, all the woods which are lighter than balsa
are terribly weak and unsuitable for any practical use. The very lightest
varieties don't really resemble wood at all, as we commonly think of it, but are
more like a tree-like vegetable that grows in rings, similar in texture to an
onion. It is not until balsa is reached that there is any sign of real strength
combined with lightness. In fact, balsa wood is often considered the strongest
wood for its weight in the world. Pound for pound it is stronger in some
respects than pine, hickory, or even oak. Refer to chart below for
comparison.
Strength of Balsa Compared to Other Woods
Species Weight Stiffness Bending Compression
Balsa 8 72 70 75
Balsa 10 100 100 100
Balsa 14 156 161 149
Spruce 28 230 260 289
Yellow Pine 28 222 277 288
Douglas Fir 30 241 291 341
Hickory 50 379 638 514
Oak 48 295 430 366
Basswood 26 261 288 288
Black Walnut 37 301 506 512
About the Chart: Weight shown is in pounds per cubic foot. The
strength of balsa varies in direct relation to its density or weight — the
heavier the wood, the stronger it is. The above chart was designed with 10
lb/ft³ balsa as the median. In other words, balsa at 10 lb/ft³ has been tested
and given a value of 100. The other woods were then tested in the same way and
given a figure that is numerically in proportion. By comparing the relative
strength figures in the chart, it will be seen that balsa is as strong or
stronger, pound for pound, than most of the species shown.
Selecting Balsa for Model Building
Most hobby shops have a large
rack of balsa sheets, sticks, and blocks that you can choose from if you are
going to build a model airplane from scratch. Undoubtably, because of the nature
of balsa, the actual weight of each piece of wood of the same size can vary
slightly. When you select the pieces you want to buy you should keep their final
use in mind. Logically one should select the lightest grades for the lightly
stressed model parts (nose blocks, wingtip blocks, fill-ins, etc.) and the
heavier grades for important load bearing parts of the structure (spars,
fuselage stringers, etc.). To a large extent, this selection is already partly
done for you. Here at SIG, we purosely cut up our lightest raw balsa into
blocks, and our hardest raw balsa into sticks. Sheets are cut in the entire wide
range of density.
To give you an idea how much common sizes of balsa can
vary in weight depending upon the density of raw stock it was cut from, the
following formula has been developed. From it, you can calculate the weight of
any sheet, block, or stick of balsa as long as you know the density of the
stock. Typical densities range from 6 lbs/ft³ to 16 lbs/ft³.
Common Modeler's Tools for Cutting and Shaping Balsa Wood
Balsa
is a very "friendly" wood to work with — so light, so soft, so easily worked
into so many things. You don't need heavy-duty power saws and sanders like you
would if working with a hardwood. In fact, even with an extensive power shop at
their disposal, the professional model builders here at the SIG factory find
that they still rely primarily on 4 or 5 simple hand tools for the majority of
their work. If you are just starting out in the model airplane hobby, here are
the tools that they recommend you get:
X-ACTO No. 1 knife with No. 11 blade for general cutting
X-ACTO No. 2 knife with No. 26 blade for carving
Razor saw for cutting thick sizes of wood
Razor plane for shaping
A knife or razor blade will work well for cutting
balsa sheets and sticks up to 3/16". Always keep replacement blades on hand —
blades do wear our and a dull blade can make it impossible to do a good job.
You Will Also Need Sanding Blocks
In addition to the cutting
tools, you will need an assortment of different size sanding blocks. These are
indispenable tools for model construction. You can buy ready-made sanding blocks
or make your own. The most often used general-purpose sanding block in our model
shop is made simply by wrapping a full 9" x 11" sheet of sandpaper around a 3/4"
x 3" x 11" hardwood or plywood block. Use three screws along one edge to hold
the overlapped ends of the sandpaper in place. Use 80 grit garnet sandpaper on
the block during general construction. Another handy sanding block to have can
be made by gluing 80 grit garnet sandpaper onto a 24" or 36" long piece of
aluminum channel stock. Most hardware stores carry a rack of aluminum in various
sizes and shapes. This long sanding block is very helpful for shaping leading
and trailing edges, and other large pieces, accurately. Last but not least, glue
sandpaper onto different sizes of scrap plywood sticks and round hardwood
dowels. These are handy for working in tight places and for careful shaping
where a big sanding block is too hard to control.
Identifying Balsa
Grain — Continued on
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