Hi,

a major consideration in choosing the orientation (in any wood construction) is the way wood changes shape with changes in moisture content. Most of the dimensional change is around the circumference of the annual rings - they lengthen and shorten and effectively the ring radius changes. If the rings are tangential to the long face of a board then it will cup in one direction or the other as the moisture content changes and the ring radius changes. That also changes the board width by a significant amount. If the rings are tangential to the short edge then the long edge stays flat and the thickness varies but only by a small amount (the same % of course).

In a box spar, if the top and bottom boom's depth were to be changing in relation to the spar web depth then they are stressing the glue bond between their edges and the ply. If the rings are perpendicular to the glue joint then there is insignificant movement in the glue line. The aim therefore is to have the rings at 90 degrees to the ply face, but of course we have to be realistic and use the best wood we have which will usually be a bit of a compromise.

Grain orientation does affect bending stiffness, try bending a square section and you will see that it bends easier in one direction.

I would recommend that you look up ANC-18 (Design of Wood Aircraft Structures) and ANC-19 (Wood Aircraft Inspection and Fabrication) both may be downloaded from here: http://www.lonesomebuzzards.com/cgi-...?m-1352247687/ They contain very clear explanations about the requirements for (and advantages of) laminating spars.

regards,

Colin