Balsa wood was a special thing to me as a kid. To me, it represented the essence of model airplanes and model rockets. At the time - the 1960s and 70s - plastic and foam as model components were considered a sign of cheapness, low quality, amateurishness. It was like having "Made in Japan" stamped on it. Now, of course, it's a different world where Japan is renowned for some of the highest quality electronics and cars and the plastic and foam ARFs represent some of the highest-performing aircraft at the flying field. I have owned a few of those foamies, but still, at least for my tastes, nothing beats the look, feel and aroma of balsa. Somehow the tell-tale surface texture of foam, even with a nice paint job, ruins the authenticity of an otherwise beautifully factory-finished scale F4-U Corsair or P-38 Lightning. Sorry, that's just the way it is. Sig Manufacturing was, in the aforementioned era, one of only a couple major balsa suppliers (Midwest being the other). The Sig catalog had a really nice primer on balsa with a treatment of history, tree harvesting and production, and technical information like density of balsa sheets, sticks, and blocks. That information from an early 1970s catalog is reproduced here. I also have a page on balsa wood density and weight, and a reproduction of an article on balsa tree foresting and harvesting by Sig Manufacturing from one of their 1970s era catalogs.
For people who like to know everything!
Courtesy of International Balsa Corp.
What Makes Balsa So Unique?
Balsa is the softest and lightest commercial wood in the world. It weighs only 4 to 18 lbs. per .cubic ft., averaging about 9 lbs. per cu. ft. But pound for pound, it is stronger in some respects than pine, hickory or oak - it is one of the strongest woods in the world for its weight, particularly in tension. It is resilient - with high compressibility and recovery. It is buoyant - 1 cu. ft. will carry over 50 lbs. of water. Its high insulation value is equal to cork. Balsa's high electrostatic qualities make it an insulator against electricity. Balsa absorbs shock and vibration well. It is inert - no fumes, dust or reaction. It is stable and will not warp readily. It is durable - withstanding heat, cold, dampness long storage and weathering. Balsa is easily workable and can be processed with the same tools and techniques as ordinary woods. Balsa can be processed to simulate many other materials and takes these finishes and processes well.
The "Friendly" Wood
Most people like wood anyway. It has such a friendly feel. But Balsa has this "friendliness" to a higher degree ... it is pliant to the will of man, so light, so soft, so easily worked into so many things. Lay a piece of Balsa before a man, woman, boy or girl, and they can't resist picking it up, hefting it, feeling its smoothness, pressing their finger into it to test its submissiveness. With Balsa every person is an artisan, an artist. Balsa Wood is used so extensively in building models of all kinds because the hobbyist has found that it will give him a lighter, stronger job, and greater flexibility in its construction and performance, than with any other material.
The Perfect Nurse
In this age of science, when man is always trying to improve on Nature, it is unusual to find a substance where Nature has outdone man. People ask "What process do you use to make Balsa so light?" There is no process. Balsa is just naturally light. Balsa grows only in the humid rain forests of South and Central America. Actually Nature did not design Balsa as a crop tree. While it needs a lot of moisture, it grows only in well-drained sites. When the young seedlings of the forest giants start to grow on the forest floor, they need a nurse tree to shade them and keep them from drying "out in the hot tropical sun. That nurse tree must grow fast, with a spreading crown and large leaves to provide shade for the babies.
Yet there can't be too many leaves because they would shut out air and sunlight from their charges. While the nurse tree must grow fast, it can't put too much beef into its body. The accent is on Volume, not on strength. And it must not live too long .... just long enough to see the seedlings through to where they can take care of themselves. The Balsa tree goes all out to live up to the characteristics of a perfect nurse.
Speed vs. Strength
What happened to the wood in the Balsa tree when it sacrificed substance for speed of growth? Under the microscope we find the basic parts the same as in all woods - Nature is conservative in a way, and when She finds a good, basic design, She adapts it in various ways, so that She obtains a variety of forms. She begins with a basic material, cellulose, which weighs 97 lbs. per cubic foot. She molds it into various shapes - tubes, cells, long, tapering fibers. So far the parts are the same for all the different kinds of wood, and if She were to stop there, there would be only one kind of wood. But now She diversifies - She makes one part large, the other small, She thickens the cell wall on some, slims them down on others. She leaves a big opening here, a small opening there. She bunches some together, others She spreads out. Here and there She adds a pinch of salts, some crystals, pumps in some resin or tannin, and by arranging the same few elements in hundreds of different ways, She comes up with thousands of different species.
In Balsa, Nature has cut every possible corner to do a fast, efficient job. Everything that would build up weight has been eliminated. The cell walls have been slimmed way down. The spaces inside the cells have been enlarged, so that the ratio between solid matter and space, is as small as possible. Most woods have gobs of heavy, plasticlike cement, called lignin, holding the cells together. In Balsa, lignin is at a minimum. The result is a wood which may weigh anywhere from 4 pounds per cubic foot, to 12 pounds per cubic foot. (As the tree grows older, passes its prime and begins slowing down in growth, it may develop wood as high as 18 lbs. per cubic foot.) Compare this with one of the lightest, native woods, Spruce, which weighs 23 lbs. per cubic foot. But, you will ask, "Hasn't Nature gone too far and robbed the tree of all its strength?"
Nature has many tricks up its wide sleeve. It has given the Balsa tree a tough, stringy bark, which unlike many trees does not flake off but builds up as the tree grows. She pumps the Balsa cells full of water until they become rigid - like an automobile tire full of air. (The green Balsa Wood may contain as much as five times as much water by weight as wood substance. Compare this with green, native woods, which may contain only three-quarters the weight of water as against the weight of wood.) But the most important to us is that She takes the Balsa cells and by twisting them about each other, arranging them with consummate skill, she gives the wood itself an inherent strength.
Romance of Logging
There is no such thing as woods of pure Balsa trees. They grow singly, or in very small, widely scattered groups. It is a matter of finding one tree here, another tree 100 yards over the hill. Because of the way the individual Balsa trees are scattered throughout the jungles, it is not possible to use many of the established logging procedures used in this country. Machinery requires a more or less concentrated stock of logs. It also requires people who are skilled in its use and maintenance. To the surprise of our efficiency experts, we have found that the best way to log Balsa is to go back to the methods of Paul Bunyan. Chop them down with an axe - no saw; haul them to the river by ox team; float them down the river to the mill in rafts.
This procedure cannot help but bring back the old, romantic days of logging about which so much has been written in the United States. You have the same hairy-chested logger, maybe a little shorter than his counterpart in song and story, but still rough and tough and hard as nails. You have the same ox skinner with his long, bull whip and his marvelous mastery of profanity in three languages. You have the river rats floating the rafts down to the mill and whom, for shear guts day after day, I challenge you to match anywhere in the world. These are the men whose muscle and blood and sweat make your model airplane.
Hurry Up, Amigo
Because of their vulnerability to attack by insects and fungi, it is very important that the Balsa logs be taken out of the woods and delivered to the sawmill just as quickly as possible. Unlike the logging of other timbers, you cannot cut down a great number of logs and stack them for transportation while awaiting a propitious time. They must be taken out of the woods immediately, preferably the same day, and dragged to the river. They must then be floated to the mill without delay. The best logs are received at the mill within ten days to two weeks after they have been cut.
At the sawmill we don't waste any time with them either. The rafts are broken up immediately arid the logs are dragged up a long ramp to the head saw, where they are squared. From the head saw they go to a gang saw, which converts them into boards.
Importance of Kiln Drying
The next step is one of the most important in the process of manufacturing Balsa boards. It is kiln drying, by means of which the moisture is removed from the wood. Without kiln drying Balsa would not have its characteristic lightness, nor its strength. Kiln drying kills bacteria, fungi, and insects; because the wood is dry it prevents them from returning. Balsa that is not kiln dried, or improperly kiln dried, bears very little resemblance to the finished product which most of us know. But kiln drying is a rather fine art. For the sake of economy, the wood should be dried as quickly as possible. but if it is dried too quickly, it may be irreparably damaged - slits appear in the wood, and the boards twist and warp. You may have a condition where the outside may be dry and the inside damp - this is knows as case hardening. When a case hardening board is cut, it will twist like a cork screw.
Drying is accomplished by the application of heat through steam heat exchangers, and through the regulation of the amount of humidity. If the heat is held at too low a point, it may increase fungus attack instead of killing it. If the heat is too high, it will kill the natural elasticity of the wood. If the humidity is too low, the wood will dry out too fast, and will result in all the drying defects mentioned before. If the humidity is held too high, the wood will not dry. The dry kiln operator then has to walk a tight rope between the two extremes, manipulating his valves and controls so that the fastest, most efficient drying is accomplished, with a minimum of damage. This is done by placing the Balsa lumber, stacked in layers, on cars with spacers between the layers, to permit the passage of air. The cars are placed in the kilns, which are long and tunnel-like insulated compartments where they are subjected to controlled heat, humidity and air circulation.
After spending from ten days to two weeks in the kilns, the lumber is removed, cooled gradually and is then reworked to remove defects; it is planed to thickness and is then packed in bales for shipment via ocean steamer. On arrival in this country, it is sold to other manufacturers, who cut the wood into the many useful things, for which Balsa is so well suited.
Balsa (or balsawood, whichever you prefer to call it), has been the standard material for model airframe construction since it first became available commercially in suitable cut sizes some thirty-five years ago in this country and even earlier in America. Although, botanically at least, balsa is only about the fourth or fifth lightest wood in the world it is the first of all the woods which combine strength with lightness. On a strength/weight basis, in fact, balsa compares favorably with most other woods - even oak (see Table I). This is one of the main reasons why it is so suitable for aeromodeling, where strength is required for minimum weight. Many other materials which are as light as, or lighter than balsa, also fall down on this question of combining strength with lightness and cannot be used in small sections - expanded polystyrene, for example.
The other great advantage of balsa is the ease with which it can be cut or carved, and joined with quick drying cement. Having a fairly open structure, balsa cement impregnates and adheres strongly to balsa with the result that properly made glued joints are as strong or stronger than the wood itself. With balsa readily available in a wide range of sheet, strip and block sizes, very few tools are required for working balsa either in solid form or for the assembly of built-up frames, etc.
At the same time, however, there are disadvantages. The balsa tree is very fast growing, reaching a height of 15 feet of more within a year and growing to between 60 and 90 feet within the next six to ten years. After that time the tree begins to deteriorate and rot. As a result both the density and quality of the lumber obtained by felling balsa trees can vary enormously.
SIG - The Most Famous Name in Balsa!
Reprinted from Aero-Modeler
The actual density of balsa can vary from as little as 4 lb./cu. ft. to as much as 24 lb./cu. ft. (which is about the same as obeche). Practically all the commercial balsa available, however, falls within the range of 6 to 16 lb./cu. ft. with the overall average tending to run about 9-10 lb./cu. ft. The strength properties of balsa vary directly with density - the heavier the wood the stronger it is.
Balsa is normally graded by density, although, the actual descriptions are largely arbitrary and not always identical between different suppliers, or different model designers specifying grades to be used. The most general commercial classification is "light", "medium" and "hard", as under -
The more expert modelers adopt a wide. range of grading, typically as under -
Logically one selects the lightest grades for the lightly stressed parts (e.g. block wing tips, sheet fill ins, etc.)and the heavier grades for spars and longerons. Even here, however, practice can differ. Some modellers prefer to use very hard balsa for longerons and spars and keep weight to minimum by reducing the actual size of the sections used. Others prefer to use a lighter grade and compensate for strength by using larger section.
Note: the strength/weight ratio of 10 lb. cu. ft. Balsa is rated at 100 and the strength/weight ratios of other woods calculated accordingly. Thus a figure of less than 100 shows a performance inferior to 10 lb balsa on a strength/weight basis; and a figure of over 100 a superior performance
Both systems have there advantages and disadvantages. The use of hard grades and small sections actually gives the best overall strength to weight ratio (see Table I). On the other hand, the smaller size spars may be more difficult to handle for building and also lack local stiffness. Using larger sizes normally gives greater stiffness and local strength, although it is also easy to add excess weight as well unless the grade is carefully selected. Also if too light a stock is chosen in the interests of saving weight, the resulting structure may be weak. For most purposes, however, Table II can be used as a guide for balsa grade selection.
In practice, grade selection can only be made with reference to actual weights of individual sheets or strip lengths. To save having to work out density in each case, consult Tables III, IV and V.
A tip to remember here is that as far as grading is concerned, suppliers of cut balsa tend to favor selection of the harder grades for the smaller sizes (or thickness) of strip and sheet as being easier to handle. Thus one is more likely to find mostly "hard" grade in 1/16 in. sq. for example, and more medium to soft in 3/8 in. sq. Similarly, the proportion of "soft" is likely to be higher in 3/8 in. and 1/4 in. sheet than in 1/32 in. or 1/16 in. sheet.
In point of fact "cut" is more important than appearance in the case of sheet stock since this largely controls the rigidity, or strength of the sheet. And cut depends on the way the original lumber is cut from the log and then finally machined - see Fig 1. If the "cut" is such that the annular rings effectively run across the thickness of the sheet (tangent cut) the sheet will be fairly flexible, edge to edge. It, on the other hand, the cut is such that the annular rings run across the thickness of the sheet (radial or quarter-grain sawn), the sheet will be rigid. It will also be appreciated that a piece of lumber cut from either section A or section C of the log can have a final cut for turning into sheet which is either tangent or quarter grain, depending on which face the cut is made from. If the section of lumber is "random cut" the grain direction is less clearly defined and irrespective of the direction of final cut the sheets will have intermediate properties between "tangent cut" and "quarter-grain".
It is difficult to distinguish between random cut and tangent cut by appearance, or even by simple bending tests, but quarter grain shows up quite clearly by the speckled appearance of the surface. True quarter-grain sheet, in fact, would be too stiff to bend to even moderate curvatures without splitting - and quite impossible to roll into a tube shape as can be done with carefully selected light density tangent cut stock.
Again "cut" recommendations are summarized in the form of a table for convenience of reference (Table VI). The point to bear in mind is that "cut" and "grade" are interrelated, so that the best choice of balsa for a particular job is based on both. This applies particularly in the case of sheet balsa parts. The stiffness or otherwise of spars is best judged by actual test - e.g. to obtain a pair of matched spars, select two of equal weight and appearance .
In this manner, and in the latter case in particular this may mean examining and testing a considerable number of individual strip lengths in order to arrive at a set of four more or less identical pieces. Many experienced modellers, in fact, prefer to cut longerons and spars from sheet stock in order to achieve complete matching. In general, however, this is only advantageous where relatively small sizes are involved - e.g. longerons not greater than 5/32 in. sq. section and spars not more than 1/8 in. thick. Cut longerons and spars in thicker sheet generally suffer from inaccuracies due to the difficulty of making long accurate "square" cuts in thicker sheets.
In cutting a set of matched longerons, the sheet should be marked before cutting (e.g. with a ball pen) so that the final lengths are identified end for end and used the same way round for a complete match - Fig. 2. The same applies to cutting parallel spars. In the case of tapered spars however, the best match is obtained by cutting the two spars about a common "bottom" line rather than an angled cut separating the two - see. Fig. 3.
A modeling blade with a fine taper is usually best for cutting fine sheet. The same blade may also be used for parting off longeron and spacer sections up to 1/8 in. square; although some modelers find it easier to work with a less tapered blade - Fig. 4. The squarer blade is also better for cutting thicker sheet as the fine tapered blade can more readily run off "square". The razor saw comes into its own for cutting larger sections.
The best direction of cut is normally obvious, but some specific recommendations are. summarized in Fig. 5, together with explanatory notes. The main thing is to avoid stressing balsa sheets across the grain (in which direction it is weakest), so cross grain cuts should always be made from the edge inwards, rather than outwards towards an edge. Also, when cutting at an acute angle to the grain, make the direction of cut so that the grain will tend to pull the blade away from rather than into the component shape.
Resources for Balsa Wood on the Airplanes and Rockets website:
Posted November 23, 2013