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.
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