ABCs of Balsa, The

A closer look at modeling's mainstay

ALTHOUGH LITE-PLY, foam, plastics and composite materials like carbon fiber are all used in model airplane construction, the leading building material-balsa-is the same as it was 75 years ago. Because balsa is a staple in model airplanes, it is essential that a modeler know something about this wonderful wood.

Despite the care taken by manufacturers, occasionally substandard wood finds its way into kit boxes. If you're knowledgeable about balsa and its characteristics, for only a few cents, you can replace these pieces with wood from your dealer's stock and thereby avoid trouble and frustration.

Most of the world's supply of balsa, considered a hardwood in the lumber business, comes from plantations and the wilds of Central and South America. It takes seven years for a tree to grow to a size suitable for harvest. When cut, the trees must be dried to remove their very high moisture content before the balsa can be processed for shipment.

The demand for balsa is high! Shipping, construction and aircraft industries as well as the military's demand for balsa is in the tens of millions of board feet each year. This demand requires high production, which keeps the price stable. In today's world, balsa is a bargain! If the exporters of balsa depended solely on the model trade, the cost would approach that of a precious metal!

To understand the differences in balsa sheet, look at a crosssection of a balsa block. The concentric ridges that get larger the farther they are from the tree's center are called "annual rings." Look closely, and you will see streaks that radiate outward from the center like the spokes of a wheel. These are the medullary rays. These rays, and the angle at which a sheet of wood is cut to them, give the sheet its grain characteristics.

There are three basic types of grain in balsa: A, B and C. The actual grain always runs lengthwise down the sheet, and the grain types are based on the sheet's relationship to the medullary rays when it was sawed off the log. A-grain is cut with the rays perpendicular, or nearly 90 degrees, to the face of the sheet, and it is very flexible. It is excellent for sheeting curved surfaces and can be soaked in water and wrapped into a tube. B-grain is cut so that the rays intersect the face of the sheet from 40 to 70 degrees and is the most common. It is a general-purpose wood that can be used for most applications. C-grain, or quarter-grain, as it is sometimes called, is cut with the rays perpendicular to the edge of the sheet. C-grain has a mottled, almost iridescent appearance, and it is very stiff across the face of the sheet. It is used where rigidity is important, as in ribs and formers. Look at the cross-section of the balsa block: A-grain sheets would be cut along the left side, B-grain along the top and C-grain along the bottom. As you can see, there are very few true A and C cuts from each log; but the 10- to 20degree variance makes little difference in application.

Along with grain types, the weight of the balsa from where the sheet was cut is stated in pounds. Light or indoor wood is in the four- to six-pound range, medium is six to eight pounds, and 10 and over is heavy or hard. Balsa is also available in weights of 15 pounds or more, but spruce or pine works better for model work in that weight.

Balsa is really a wonderful wood! Carving it with a knife is almost sublime. The softer grades respond gently to a sharp blade, yet in the harder grades, the wood can fight back like a cornered cat. Pay attention to the balsa that's provided in your next kit; the best way to learn about balsa is to use it.

Copyright Air Age Publishing May 2000
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