Hello the balsa composite sandwich crew.
I encountered a company out of Colorado which seems to offer a range of balsa types and densities. It’s called Specialized Balsa, LLC.–specializedbalsa.com It seems they can weigh out different planks and grains and organize your order accordingly. This was interesting to me because it’s my impression that timber selection in different parts of the board is significant. Along those lines, I did a little internet research trying to better understand balsa properties and flex return in timber.
Everyone here has been so helpful and generous with information. I hope this might help get some minds ticking.
From SIG site on balsa wood:
The secret to balsa wood’s lightness can only be seen with a microscope. The cells are big and very thin walled, so that the ratio of solid matter to open space is as small as possible. Most woods have gobs of heavy, plastic-like cement, called lignin, holding the cells together. In balsa, lignin is at a minimum. Only about 40% of the volume of a piece of balsa is solid substance. To give a balsa tree the strength it needs to stand in the jungle, nature pumps each balsa cell full of water until they become rigid - like a car tire full of air. Green balsa wood typically contains five times as much water by weight as it has actual wood substance, compared to most hardwoods which contain very little water in relation to wood substance. Green balsa wood must therefore be carefully kiln dried to remove most of the water before it can be sold. Kiln drying is a tedious two week process that carefully removes the excess water until the moisture content is only 6%. Kiln drying also kills any bacteria, fungi, and insects that may have been in the raw balsa wood…
Finished balsa wood, like you find in model airplane kits, varies widely in weight. Balsa is occasionally found weighing as little as 4 lbs. per cu. ft. On the other hand, you can also find balsa which will weigh 24 lbs or more per cu. ft. However, the general run of commercial balsa for model airplanes will weigh between 6 and 18 pounds per cu. ft. Eight to twelve pound balsa is considered medium or average weight, and is the most plentiful. Six pound or less is considered “contest grade”, which is very rare and sometimes even impossible to obtain…
Most hobby shops have a large rack of balsa sheets, sticks, and blocks that you can choose from if you are going to build a model airplane from scratch. Undoubtably, because of the nature of balsa, the actual weight of each piece of wood of the same size can vary slightly. When you select the pieces you want to buy you should keep their final use in mind. Logically one should select the lightest grades for the lightly stressed model parts (nose blocks, wingtip blocks, fill-ins, etc.) and the heavier grades for important load bearing parts of the structure (spars, fuselage stringers, etc.). To a large extent, this selection is already partly done for you. Here at SIG, we purosely cut up our lightest raw balsa into blocks, and our hardest raw balsa into sticks. Sheets are cut in the entire wide range of density…
In selecting balsa sheets for use in your model, it is important to consider the way the grain runs through the sheet as well as the weight of the sheet. The grain direction actually controls the rigidity or flexibility of a balsa sheet more than the density does. For example, if the sheet is cut from the log so that the tree’s annular rings run across the thickness of the sheet (A-grain, tangent cut), then the sheet will be fairly flexible edge to edge. In fact, after soaking in water some tangent cut sheets can be completely rolled into a tube shape without splitting. If on the other hand the sheet is cut with the annular rings running through the thickness of the sheet (C-grain, quarter grain), the sheet will be very rigid edge to edge and cannot be bent without splitting. When the grain direction is less clearly defined (B-grain, random cut), the sheet will have most intermediate properties between A and C grain. Naturally, B-grain is the most common and is suitable for most jobs. The point to bear in mind is that whenever you come across pure A-grain or C-grain sheets, learn where to use them to take best advantage of their special characteristics.
A-GRAIN sheet balsa has long fibers that show up as long grain lines. It is very flexible across the sheet and bends around curves easily. Also warps easily. Sometimes called “tangent cut.” DO use for sheet covering rounded fuselages and wing leading edges, planking fuselages, forming tubes, strong flexible spars, HL glider fuselages. DON’T use for sheet balsa wings or tail surfaces, flat fuselage sides, ribs, or formers.
B-GRAIN sheet balsa has some of the qualities of both type A and type C. Grain lines are shorter than type A, and it feels stiffer across the sheet. It is a general puropse sheet and can be used for many jobs. Sometimes called “random cut.” DO use for flat fuselage sides, trailing edges, wing ribs, formers, planking gradual curves, wing leading edge sheeting. DON’T use where type A or type C will do a significantly better job.
C-GRAIN sheet balsa has a beautiful mottled appearance. It is very stiff across the sheet and spits easily. But when used properly, it helps to build the lightest, strongest models. Most warp resistant type. Sometimes called “quarter grain.” DO use for sheet balsa wings and tails, flat fuselage sides, wing ribs, formers, trailing edges. Best type for HL glider wings and tails. DON’T use for curved planking, rounded fuselages, round tubes, HL glider fuselages, or wing spars.
I also found this interesting on a site about wooden English longbows:
In contrast, the English cut their bowstaves then carved them such that the heart wood towards the centre of the tree limb ended up on the face of the bow while the sap wood immediately under the bark ended up on the back of the bow. This took advantage of the natural characteristics of the tree itself because heartwood tends to be stronger in compression while sapwood tends to be stronger in tension.
This taken as a whole suggests to me that it would be most advantagous to use extra lite balsa on the bottom skin if possible and strategically place extra dense wood in critical deck applications. I realize this is not a new thought, but it does create a new question in my mind:
What density or variation of densities would be most appropriate for balsa rail fabrication? Does anyone have experience or experiments suggesting an answer?
