Which is a better material for building our model airplanes, foam or balsa?
Foam is a man-made product. There are no foam trees or foam mines to be found anywhere. So don’t bother looking.
There are many different types of foam. To say “foam” is like saying “wood” or “metal”. For a useful comparison, you need to be specific.
My favorite type, and the one that I’m going to compare to balsa, is called extruded polystyrene foam or XPS foam. Polystyrene is one of the most commonly used plastics and is made from petroleum.
Don’t confuse it with its close relative, expanded polystyrene (EPS). EPS is less expensive, but much weaker.
XPS is sold under many different names. Depron, Cellfoam 88, and Styrofoam are names you may have heard. Since it is a completely man-made product, it can be produced in an infinite variety of densities and strengths.
Balsa wood is considered a hardwood not because of its strength but because of the type of tree that it grows in. In fact, balsa is the softest commercial hardwood.
95% of commercial balsa comes from Ecuador, most of it cultivated specifically for commercial production. The wood is so light because the green tree contains large quantities of water. After an extended kiln dying process of two weeks, the wood is ready to be used.
Balsa is about four times heavier than XPS foam. Most of the other woods that we use are about twice as heavy as balsa.
The modulus of elasticity of XPS is about the same as that of balsa. This is a measure of how much a material resists bending and twisting while still springing back to its original shape. In other words, how stiff is it?
Here’s another way of looking at it. A stiff material requires a high load to temporarily deform it. A strong material requires a high load to permanently deform it.
When the lighter weight (density) of XPS foam is taken into account, it handily wins the stiffness contest against balsa. XPS is about four times stiffer, by weight, than balsa. In fact, when its very low density is taken into account, XPS has great stiffness, better than most other model airplane building materials that we use. As we will see later, this is a key point.
Cost and Quality
Foam wins hands down when it comes to cost, availability, uniformity, and overall quality.
You can buy foam sheets in any size with almost any density/strength trade-off desired. It is relatively inexpensive and very consistent in quality. When you buy a piece of foam from a manufacturer, you can count on it being exactly the same as the last piece you bought from them. There is no time or money wasted sorting through and picking just the right piece to use for a given project.
On the other hand, no two pieces of balsa wood are exactly alike. It comes in a wide range of densities. It also has a grain that foam lacks. Taking the grain into consideration complicates designs. Compared to foam, balsa is relatively expensive and harder to find in local stores. The quality can be very hard to predict. It is common to buy a sheet of balsa that turns out to be unsuitable for the intended project.
The Achilles heel of foam is its strength. When it comes to both tension and compression, balsa is about fifty times stronger. Even after we take into account the much lighter weight of foam, balsa is still about ten times stronger than XPS foam when measured by weight.
When compared by unit weight, most woods are remarkably similar to each other. Their strength and stiffness is almost exactly the same. Of course, when it comes to low weight balsa is king.
If we include composite materials in our comparison, the hands-down winner for both strength and stiffness is carbon fiber. Foam and balsa do not even come close, even if we measure by weight. It is not cheap, though, and there are potential health issues.
Stiff Is Difficult
Foam has an interesting combination of properties. It is low in cost and very low in weight. It also has an excellent stiffness to weight ratio.
When looked at by weight, balsa has pretty ho-hum properties. Nothing special when compared to wood from other trees. The real value of balsa is a density in between foams and other woods.
But here’s the clincher: it is harder making a model airplane structure stiff enough than it is making it strong enough. Put another way, you are more likely to run into problems in a model airplane design from lack of stiffness than from lack of strength. For that reason alone, foam trumps balsa as a generally more useful model airplane building material.
The biggest challenge I faced in my ModiFly model airplane design was making the fuselage stiff enough. I have run into similar issues with other model airplane designs I have done. Lack of strength is much easier to fix.
Don’t get rid of your balsa pile just yet. Using wood to add strength to a critical part of a model airplane design will not be going out of style any time soon. With its unique properties, balsa will also continue to be with us for a long time.
I have designed model airplanes using every material in the model airplane designer’s toolbox. They all have a right place and time to be put into use. The key to being a good designer is knowing when those right times are, which only comes from experience and study.