cmyr wrote:

As I recall , the horizontal grain orientation is preferred because
the adhesive has access to every layer of the wood in the 2 sparcaps
when joined to the front/rear ply webs. With vertical oriented caps,
only the front/rear wood layers would carry all the loads to the ply
webs when assembled.


Though this seems like a sensible choice, I thought I would look in CAM
18 (repair procedures) which is now all but 50 years old, an FAA
publication long out of print, except through the EAA. It has useful
guidance on the repair of wood spars. It makes the point that glued
scarf joints should run parallel to the general direction of the grain
(sec 18.30-2(b)(2) page 6.

If that rule were applied to the web/cap interface, it seems to advocate
a vertical grain in the cap.
Diagrams of preferred joints also seem to show grain as diagonal in caps
and solid spars..... It advocates face grain in the ply webs of box
spars running in the long direction of the spar.

Brian W