Balsa wood for RC model planes

For almost 30 years, modelers and us have been pronouncing the word balsa with almost sacred reverence, treating this wood as the most precious and saving every little bit of waste. It is therefore not surprising that now, when after a long time balsa is beginning to be abundant in stores again, they approach its use as if with shyness and awe. And yet, balsa is the most basic material for model makers, just like flour for bakers or bricks for masons. And just as the same flour can be used to bake both good and bad bread, and bricks can be used to build a massive or flimsy house, so one must be able to work with balsa. Don't let the Igra team get mad at me for proving my claim by pointing to their balsa kits: disregarding the basic principles of working with balsa often leads to the fact that the built models are very different from each other. One modeler is lucky enough to have individual parts (by chance) made of the right balsa in his kit. He is satisfied with the performance of the model and cannot praise it enough. The other, on the other hand, will receive a kit for the same money, from which the assembled model will break on the first flight, because the choice of balsa turned out to be (by chance) quite the opposite. It is most often the case that the fuselage is made of the softest piece, while the wings, made of heavy and hard balsa, would probably withstand being run over by a steam roller.

Therefore, if you want to build with balsa with success, illusile know its peculiarities, know how to sort it and process it in a suitable way.

Table I compares the properties of balsa and common types of wood in our country; expresses the relationship between strength properties (stiffness, flexibility, compressive strength) and weight. The default value (100) takes the properties of medium-hard balsa. Figures lower than 100 therefore express worse characteristics and vice versa.

Wood Stiffness Flexibility Compressive strength*
Soft balsa 90 78 84
medium 100 100 100
hard 112 115 136
Spruce 82 93 103
American basswood 100 111 111
White pine (Pinus strobus) 87 101 113
"Pseudotsuga" 80 97 114
Oak 62 90 96

*) We do not know how the individual data (stiffness - strength - flexibility in compression) were measured and they are not even established technical concepts from which it would follow; however, the table will serve for comparison.

An invaluable advantage of balsa is that it can be easily processed and joined. With the right method of gluing, we can even achieve greater strength in the joint than in balsa wood itself. A few tools are enough for processing, especially if we already have balsa in semi-finished products - boards of various sizes.

However, when working with balsa, it is also necessary to know its disadvantages. The balsa tree grows very quickly and after reaching a certain age begins to rot just as quickly. Therefore, the quality of balsa wood is very different, so we can get boards of widely different quality from one beam. All this must be kept in mind especially when choosing balsa. Otherwise, it may happen that we make, for example, wing ribs from apparently identical boards, but part of the ribs will be of a different quality and the wing will collapse.

The specific weight of balsa is (about) 60-350 kg/m3. It can be roughly said that the lighter the balsa, the less strong it is and vice versa. Most foreign companies therefore classify balsa for model makers (at least preliminarily) into soft, medium and hard (soft, medium, hard). In professional magazines, however, it is classified much more carefully, into 8-10 types.

Differences in balsa strength and weight can be partially compensated for in model construction. If we have lighter balsa, we have to increase the dimensions of all parts, and on the contrary, with harder balsa, we reduce the weight by using smaller dimensions (cross-sections of beams, hull longitudinals, thickness of ribs, etc.). Both methods have advantages and disadvantages. By using smaller dimensions and harder balsa, we usually achieve greater strength, but the material is more difficult to process, regardless of the fact that, for example, the longitudinal members of the hull bend under the tension of the coated paper between the partitions. However, when choosing larger dimensions, one must proceed very carefully so that the weight does not increase disproportionately. If too soft balsa is used, there is a danger of the entire structure collapsing, especially after the cover is removed.

In Table II, we therefore provide at least a rough overview of the types of balsa recommended for individual parts of the model.

A type of balsa Use
Special — indoor models
Light: — wings (full) of throwing gliders
— various fillings or termination of the supporting surfaces of free models
— semi-solid or hollow hulls of loose models
— coating of leading parts or even entire wings and tail surfaces
— all-balsa wings of small models
— fillings of the fuselage and wings and tail surfaces
— propellers for small rubber models
Medium light: — hoods, covers, cabins
— semi-solid or hollow hulls of tethered and RC models
— outflow and leading blades of smaller and medium models
— all-balsa wings and elevators of larger models
— lattice constructions of planked hulls
Medium: — partitions for structural hulls
— inlet and outlet rails of larger models
— full all-balsa wings and tail surfaces of tethered models
— ribs
— hull stringers for small and medium models
— all-balsa propellers for Wakefield models
Hard: — wing spars
— fuselage stringers of larger models or stringers of small dimensions
— center ribs of smaller models
Very hard: — main beams of large and tethered models
— templates

In practice, the type of balsa is determined by comparing the dimensions and weight of individual planks, estimating by color (basically, lighter balsa is lighter and better quality, while darker balsa is heavier). A widely used method is scratching with a nail, which with some practice will allow a fairly accurate estimate. In this context, it is good to remember that harder balsa is easier to cut into thinner boards, and therefore, when choosing, it is easier to find harder balsa in these dimensions.

Balsa beams must be leveled before cutting so that at least two sides are straight and perpendicular to each other. Otherwise, the unevenness will be copied when cutting and the boards will be twisted and crooked. Thrift is therefore out of place here, it could take revenge by devaluing all the boards cut in this way.

It also depends a lot on the method of cutting. The prevailing opinion is that balsa is cut on a band saw with the smallest cut (i.e. waste). However, this is questionable because the cut after the band saw is mostly unclean (hairy) and uneven with the traces of each round of the band. Therefore, the board still needs to be sanded, which something will fall out of the saved section.
When sanding, a lot of unpleasant balsa dust is produced, regardless of the fact that it is difficult to maintain the same thickness of the board, even if we have sandpaper on a flat, hard surface. You can't overlook the fact (and this is especially important for indoor models) that when sanding sc balsa compresses at the same time and thus increases its specific weight. In addition, the resulting pressure damages the internal structure of the wood, which loses its strength and creates internal tension.

From my own experience, I recommend cutting balsa with a small circular saw with the smallest possible cut or, even better, with a milling machine, which gives a good cut surface that does not need to be processed further.

balsa cuts

When choosing and cutting wood for individual parts of the models, the so-called "wood cutting" is important. According to the orientation of the years, we distinguish three cuts of wood. Figure 1 shows them well. They are radial section A, tangential section В and random section C.

Radially cut boards (A - fig. 1, 2) are suitable for making ribs (with the exception of indoor models), drain rails and hull covering. It is not suitable for coating curved surfaces and for pouring into pipes.

Boards cut tangentially (B - fig. 1, 3) are usually used for: covering curved surfaces; shell rolled hulls; fuselages and fuselage rear sections of room models; ribs of indoor models, rigid covering of wings and elevators (whole or parts). They are most easily recognized by the fact that years form "mirror surfaces" on the surface (Fig. 3).

Randomly cut boards (according to C -fig. 1, 4) fit most other parts of the model. However, it is necessary to try each board, because their years have different inclinations. They are also the most difficult to recognize, as they form a transition from cut A to almost cut B.

It is difficult at all to recognize the type of cut at first glance at the cutting board. Sometimes it is difficult to distinguish cut A from cut C. However, a plank of cut A cannot be bent transversely without cracking, and it cannot be twisted like a plank of cut B.
When designing the model and choosing the building material, all the above aspects must be taken into account. The specific weight of the balsa as well as the type of cut of the planks are basic and yet largely neglected points of view when building a model. Although the choice of balsa will probably be limited for some time, we recommend working with balsa in this way, at least within the given possibilities.

Everything that has been said refers to the period when the modeler is just preparing to build the model, when he constructs, calculates and selects the material. In the next article, you will learn about processing balsa when building your own model.

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