Originally Posted by
Self loading bear
The balsa core does not add strength it keeps the outer and inner plywood or fiberglass at a fixed distance.
Same as the web of a H-beam keeps the lower and upper flange apart.
If you would replace the web by a massive fill with balsa it would be lighter but with same strength.
Putting the grain perpendicular to the outer layers makes the core more difficult to compress (dents) and the sheets of balsa can be more easily plied in the mould. And glue can penetrate into the pores reducing possible delamination.
Thanks SLB, that makes sense. Thinking about it later, I wondered whether it was about achieving better glue penetration through the end grain.