Some Useful Conversion Tables
We’ve used it for years, but what is it and where does it come from?
PART 1:
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 BALSA 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.
Strength of balsa wood in comparison to other woods
| Species | Weight Lbs./Cu. Ft. | Stiffness Strength | Bending Strength | Compression Strength |
| 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 |
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. See table below for some general cutting guidelines.
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.
BALSA GRAIN — LEARN HOW TO IDENTIFY ALL THREE GRAIN TYPES
In selecting balsa sheets for use in your model, it is important to consider the way the grain runs through the sheet as well as the weight of the sheet. The grain direction actually controls the rigidity or flexibility of a balsa sheet more than the density does. For example, if the sheet is cut from the log so that the tree’s annular rings run across the thickness of the sheet (A-grain, tangent cut), then the sheet will be fairly flexible edge to edge. In fact, after soaking in water some tangent cut sheets can be completely rolled into a tube shape without splitting. If on the other hand the sheet is cut with the annular rings running through the thickness of the sheet (C-grain, quarter grain), the sheet will be very rigid edge to edge and cannot be bent without splitting. When the grain direction is less clearly defined (B-grain, random cut), the sheet will have most intermediate properties between A and C grain. Naturally, B-grain is the most common and is suitable for most jobs. The point to bear in mind is that whenever you come across pure A-grain or C-grain sheets, learn where to use them to take best advantage of their special characteristics.
A special thanks to the friendly people at Sig, who graciously permitted us to reprint the above text from their website. The original article text can be found on the the Sig website.
Did you know?
The balsa wood business started during World War I, when a substitute for cork life jacket floats was required. The drawback in harvesting balsa is the need to manually harvest each tree as it only grows as individual specimens within fairly thick jungle.
PART 2:
Selection of Balsa Wood for flying Model Construction
Finished balsa wood, like you would use for model aeroplane construction, varies considerably in density and grain. Balsa can be found weighing as little as 4 lb per cubic ft up to 24 lb or more per cubic ft (65 to 360 kg/m3). Commercially available balsa for models will weigh between 6 and 18 lb per cubic ft (95 to 285 kg/m3). Eight to twelve pound balsa (120 to 192 kg/m3) is most plentiful and is considered medium or average weight. The six pound (95 kg/m3) or less is considered “contest grade” owing to good lightness, although durability due to inherent weakness is not a priority and such light examples can be rare or even impossible to obtain. Light grade balsa is nominally 6 to 8 lb per cubic ft (95 to 135 kg/m3). The denser the balsa wood, the stronger and harder it is, so sometimes a trade off can be made by using a more dense material that is thinner, but weighs about the same for a given strength.
Balsa Log conversion to planks
Grain type and direction determine the rigidity or flexibility of a balsa sheet more than density does. The image at right shows how a log is converted into planks with different properties, an A grain sheet, cut from the log so that the tree’s annular rings run across the thickness of the sheet it will be fairly flexible from edge to edge. On the other hand, if it is C grain, cut with the annular rings running parallel to the surface of the sheet, it will be stiff and rigid edge to edge. If the grain direction is less clearly defined as in B grain, the sheet will have properties intermediate between A and C types. B grain is the most abundant and is suitable for most jobs. As B grain is most common, it can be available in surplus quantities quite cheaply, so the model builder may have a surfeit of this type of grain… This can be overcome by making formers and wing ribs from this type of grain and then lightening the items by cutting circular holes, then covering with thin tissue that can be shrunken and doped to give a much stiffer item that is resistant to splitting.
Whenever you come across good examples of A-grain or C-grain sheets, learn where to use them to take best advantage of their special characteristics.
A-Grain also known as a Tangent cut
A-Grain has long fibres that show up as long grain lines against a smooth creamy background and is very flexible across the width of the sheet, it warps easily and readily bends around curves, but lacks stiffness. Use for sheet covering tightly rounded fuselages and curves and fabricated wing leading edges (D-boxing), forming tubes, strong yet flexible spars that can withstand ‘hard landings’, hand launched glider fuselages.
Don’t use for unsupported sheet balsa wing or tail surfaces, ribs, or formers. To make A grain balsa wood more pliable and easier to bend without breaking, soak it overnight in a bucket of water with a small amount of ammonia (or bleach) added. It should be bent or shaped while wet, and then clamped in the correct shape over a former until it is completely dry.
B-Grain also known as Random or Mixed grain cut
Grain lines are shorter than type A, and it feels noticeably stiffer across the sheet. These B grain sheets vary a great deal in properties, often a single sheet will vary from A to C grain across it’s width. This cut of balsa wood is useful for general purposes… Use for flat uncomplicated fuselage sides, trailing edges, wing ribs, formers, planking of soft gradual curves, wing leading edge sheeting (select the softest portion of sheet for the tightest curvature). Generally, try to avoid buying sheets that change grain type dramatically across the surface (which can cause warping). If A or C grain is not available, you can sometimes find a portion of a B grain sheet that will make the part concerned. Occasionally, a variable grain structure may suit a particular part that requires different characteristics in different areas, but such occasions are rare.
Don’t use if type A grain or type C grain will do a significantly better job.
C-Grain also known as a Quarter grain or Quarter sawn
This grain type has shorter, more mottled grain and is the most pleasant in appearance, sometimes looking like fish scales (occasionally the fish scales are of ‘shot silk’ iridescent appearance). It is very stiff across the sheet, is brittle and splits easily (sometimes too easily) longitudinally. This is the most warp resistant type, but it is difficult to sand effectively. Use for sheet balsa flying surfaces, fins, flat fuselage sides, wing ribs, formers, trailing edges.
Don’t use for curved planking, rounded fuselages, rounded tubes, hand launched glider fuselages, or wing spars.
End Grain also known as a Cross grain
There is a forth way of cutting balsa wood which involves cutting thin slices perpendicular to the run of the grain, this has very little application in model aircraft apart from the very largest models that may be a quarter scale.
The cross grained pads are used as fillers between stiff outer sheets to form laminated composite materials that are light and yet stiff.
Model supply outlets will have a large display of balsa sheets, sticks, profiles and blocks that you can choose from if you are going to build a model from scratch. When you select the pieces you wish to purchase you should keep their final use in mind. The lightest grades should be reserved for the lightly stressed model parts… Nose blocks, wingtip blocks, fillets, etc. and the heavier grades kept for important load bearing parts of the structure… Spars, stringers, motor peg mountings and so on. To a large extent, this selection process is already partly done for you by the materials supplier, who will purposely cut the lightest raw balsa into blocks and the heavier grades into sticks and strips. Sheets can be cut from the whole range of possible densities.
When selecting balsa sheets for use in your model, it is important to consider the way the grain runs through the sheet as well as the density of the sheet. The grain direction governs the rigidity or flexibility of a balsa sheet more than the density does. For example, if the sheet is cut from the log so that the tree’s annular rings run across the thickness of the sheet (A-grain, tangent cut), then the sheet will be fairly flexible edge to edge. In fact, after soaking in water some tangent cut sheets can be completely rolled into a tube shape without splitting. If on the other hand the sheet is cut with the annular rings running through the thickness of the sheet (C-grain, quarter grain), the sheet will be very rigid edge to edge and cannot be bent without splitting. When the grain direction is less clearly defined (B-grain, random cut), the sheet will have intermediate properties between A and C types. Naturally, B-grain is the most common and is suitable for most jobs.
Laminating Balsa sheets can change the properties significantly as can laminating balsa with other materials like birch ply or plastic foam. Ply structured from sheets that have the grain direction alternating at right angles for each layer can have two different characteristics depending on whether the part concerned is cut with the long axis along one of the grain lines or cut on the bias. Bias cut parts will be somewhat more flexible and able to resist shock without cracking or splitting.