a "perfect" sheet of C grain blasa wood

Balsa grading is done based on the orientation of the cells in the balsa . Balsa has two main types of cells, longitudinal and radial. As the tree is standing, the longitudinal cells are vertical, and the radial cells (or rays...) are horizontal, from the center of the tree out. In the picture below, the longitudinal cells appear as dots, and the rays appear as lines.
Grading balsa has been done in the same way for over 60 years. The standard designations of A, B, and C have been use to describe the grain orientation of the cut sheet, and a view of how the sheets are cut from the balsa billet are shown below. (Note that the rectangles with the grain designations would represent the -end- of a balsa sheet...)
A grain is that which has the longitudnial cells running the length of the sheet, and the rays perpendicular to the sheet. A grain is generally used for wing spars and outlines, and is more readily bent around forms for wing tips. Note in the picture below you can see the long darker cells, which are the longitudinal cells. This contributes to the stiffness over the length of the sheet, and why it is suited well for spars. A grain sheets, due to the orientation from the original log, generally have a high degree of uniformity from the sheet, since all of the width of the sheet came from the same growth cycle of the tree. A grain is also very flexible across the width of the sheet, and stiffest along the lenght of the sheet.
C grain has the rays oriented across the width of the sheet, and a "perfect" sheet of C grain should look like this:
Note that you can see the rays traveling virtually the entire width of the sheet, which contributes to the stiffness. C grain is generally used for ribs, motorsticks, and booms, and is very stiff when rolled into tubes. C grain is somewhat like a natural plywood, due to the orientation of the longitudinal and radial cells, and has charistics of plywood in that respect. As a billet of balsa is being cut for C grain, due to the nature of the growth rings, the billet will eventually not be "perfect" C grain. In the picture below, you can see a sheet cut which is typically considered C grain, but is a few degrees from a true C grain sheet.
As the sheets diverge from being cut along the rays, the sheet takes on a more mottled apperance instead of distinct wide lines, and eventually turns into B grain.
"Mottled" C grain and BC grain can many times be used as effectively as "perfect" C grain. In gdjlhqgnvz fact, some actually prefer BC grain for some applications, as when a tube made from it does break, it can be easier to repair. Grading of balsa is obviously not a science, and many sheets could fall into either category, but generally the predominate characteristic of the sheet will be used for the final grading.
In any given billet, you can theoretically orient the billet to get any of the above grains. However, some billets are more suited to one grade or the other, and generally sheets are cut in the orientation that is most efficient and has the highest yield. As you can see in the above picture, a billet oriented with A grain on the top would not need to be rotated during the cutting process much if at all, and all sheets would basically be "perfect" A grain. Balsa stock generally comes in 3"x3"x36" blocks, which then are cut in two, (for the standard 18" sheet length") and then each block is sawn in two, with each billet measuring approximately 1.25"x3"x18".
It is almost always possible to orient a 3"x3" block to get perfect grain orientation, but since low density balsa stock is in fairly short supply, and in the attempt to get the highest balance of yield to quality, some billets are cut "as is" instead of cutting them up to get a perfect billet. In other words, given a block of good 4# balsa, it is better to have two billets than one, i.e. twice as many sheets. This is where the in-between grades of AB, B, and BC generally come from.
During the cutting process, my setup allows me to angle the billet to optimize the grain orientation. However, on each change, the billet has to be squared off before the cut, resulting in waste and less yield. Depending on the particular billet being sawn, this may or may not be desireable. Due to the lower yield when cutting sheets graded as either AP or CP ("perfect" A or C grain) are priced at a slight premium.?
article resource: http://www.balsafactory.com/ .