No. 233:
Balsa Wood

Today, we build with balsa wood. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Model airplanes were the great dream-factory of my early years. Those wonderfully tough frameworks of feather-light balsa wood captured my mind. Slender balsa stringers, carefully bent about thin sections of balsa, gave the plane its shape. That frame was covered with a drum-tight skin of impregnated tissue paper. The result was a half-ounce wing or body -- powerfully strong and beautifully streamlined. The strength-to-weight ratio of one of those gossamer airplanes was astonishing.

South American Indians have long used balsa boats on the great Brazilian rivers, but modern boat and airplane makers were slow to adopt it. It's very soft -- you can easily crush it. You can score it with your thumbnail. But it's almost impossible to break it by pulling on it, and it's shockproof.

You can see more clearly how to use balsa when you know how a structural I-beam works. That's a steel beam with a cross section that looks like the capital letter I. The flat parts on the top and bottom of an I-beam carry the load. All the middle part of the beam has to do is to separate the top and bottom. It doesn't have to be very strong. So the trick is to use balsa to make the middle of a sandwich with a tough skin outside -- rather like we used to make model airplanes.

During WW-II, airplane builders used this kind of construction in a light attack bomber called the Mosquito. It was viciously effective in Europe. When enemy shells passed through it, they didn't start cracks or even weaken the structure very much. The Mosquito's worst enemies were micro-organisms in the South Pacific that ate the casein glue holding it together.

But that was easy to remedy, because the cell-structure of balsa wood makes it very easy to glue. Today, large, light, high-speed boats are being made of one-inch balsa with a hardwood veneer. Their hulls are tough, resilient, and light. They're very good under the wave-pounding inflicted on a high-speed boat.

The important outgrowth of this is a new class of materials called composites. They work the way those balsa boats and airplanes do. A composite is made of strong fibers embedded in a plastic matrix. By spacing the fibers out with plastic, engineers can create light materials stronger than steel. The first non-stop round-the-world airplane was made of composites, and so too will be the next generation of aerospace equipment.

It has to be more than coincidence that these new materials were given us by the generation whose minds were most engaged in model-airplane building -- whose minds were touched by the tactile experience of building with that remarkable wood, balsa.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Rhodes, O.T., 'Fragile' Balsa. Mechanical Engineering, February 1989, pp. 48-54.

This episode has been greatly revised as Episode 2297.