Balsa - Standard Model Plane Construction Material
Balsa is light, strong, affordable, easy to work and readily available
Once this essential concept is grasped it is easy to understand why balsa wood is such a popular model airplane building material. Balsa is readily available and has an exceptionally high strength-to-weight ratio. Balsa can be cut, sanded and shaped with everyday hand tools. Balsa absorbs shock and vibration well, and accepts a wide variety of glues and paints.
Even full scale fighter aircraft, such as the World War II de Havilland Mosquito were made from balsa wood to conserve scarce metal resources. It is little wonder that so many types of model planes use balsa as their primary construction material.
Balsa trees are native to Central and South America. Ecuador produces over 95 percent of the world’s supply of commercial balsa.
Slightly over half of Ecuador’s balsa production comes from plantations with up to 1,000 trees per hectare (compared to around 2 to 3 trees in this same area in nature). The trees grow rapidly, gaining heights of up to 90 feet and a diameter of 12 to 45 inches within ten years. The speed of growth is the main reason for the lightness of the wood.
Light weight secret
The secret to balsa’s light weight has to do with its cellular structure. Only around 40 percent of balsa is solid material. The remaining internal portion is water.
Balsa wood harvested from nature contains five times as much water by weight when compared to the actual wood materials. Most hardwoods contain very small portions of water as part of their makeup. Green balsa wood undergoes a rigorous two week kiln drying process to lower this excess water level to around 6 percent.
Finished balsa wood weights can vary for a variety of reasons. These include the age and size of the tree, location of the trunk cut and the kiln drying process.
Commercial balsa wood used for model airplane construction typically weighs between 6 and 18 pounds per cubic foot. The most common weights available are between 8 to 12 pounds per cubic foot. Six pounds or less is considered contest grade, but is challenging to come across.
Interestingly enough, balsa is not the world’s lightest wood. There are three to four wood varieties that weigh less. However, any wood lighter than balsa is exceptionally weak and completely unsuitable for model airplane construction.
Where to get balsa
With the proliferation of foam construction methods and ready to fly RC models, the selection of balsa at your local hobby shop may be limited. There are a wide range of mail order companies that can meet all your balsa needs. I have had great success with the folks at Balsa USA for my modeling requirements.
Because the range of balsa wood densities can vary so much, be sure to specify what types of balsa wood you want. Use the lightest grades possible for sections of your model that do not bear weight or require strength, such as nose cowls, fill-in and shaped wing tips. Select a heavier cut of wood for areas requiring strength such as wing spars, fuselage formers and stringers.
For smaller radio control airplanes weight is an especially important consideration. When creating an original design keep the aircraft structure as simple as possible. Less structure equals a lighter weight model.
There are three types of balsa grain. A-Grain contains long grain lines, is flexible across the sheet and readily bends. A-Grain is ideal for sheeting fuselages and covering wing leading edges. As the A-Grain can easily warp, do not use this grade of balsa for solid sheet wings or tail surfaces.
B-Grain shares qualities of both A and C graded balsa. B-Grain feels a bit stiffer across the sheet, and can be used for general purpose model airplane construction. The best use of B-Grain is for wing ribs, fuselage formers and planking.
C-Grain sheet balsa has a characteristic mottled appearance. C-Grain is warp-resistant, does not bend easily, and when properly used can build the lightest and strongest model plane.
For simple designs like the Yard Ace or Finch, consider buying a greater number of common balsa stick and sheet sizes (1/16” square, 3/16” square, 1/32” and 1/16” sheet, etc.) than required. Carefully select the proper density and strength needed from your wood supply for that aircraft section. Keep the remaining wood parts in your wood bin.
Balsa structure and electric flight
Electric RC motors produce no vibration. As a result, the airplane fuselage or wing structure needed to support the electric power plant is much less robust that than required for gas engines. The lightness inherent with any electric model plane translates to a less robust wood structure (both balsa and plywood) for everything from landing gear attachment plates to wing mounts.
RC pilots are very fortunate to have balsa wood readily available as a versatile and affordable building material. Balsa is easy to work with, durable and can be incorporated as a major part of any aircraft’s structure. The radio control model airplane hobby literally would not be possible without this remarkable material.
Author: Gordon McKay