I am building a wing for a Serenity (formerly Warner) "Spacewalker," which uses a boxed (partial) I-beam spar on the outer wings. My question involves the selection of lumber for the built-up (three layers) spar caps at the top and bottom of the outer spars. The spar is a boxed I-beam (with three laminations of spruce for top caps and three for bottom caps) for the first two feet, and then transitions to no "I" web (but still boxed) for the remainder, with the center cap being 7 ft long and the outer caps being 10 ft long. Like so:
There are stabilizing blocks in the outer sections beyond the "I" web. My question is about the necessity for vertical grain in each of the spar caps. Is it alright if one of these (3/4" thick top to bottom x 3-1/4" wide) is not "vertical" (less than 45 degrees)? To me it is hard to comprehend the benefit of "vertical grain" in each laminated spar cap, when the plywood (outer) shear plates help bear the bending loads and shrinkage is somewhat better controlled by the lamination process. I've looked in the FAA Advisory Circular and in Bingelis' book for the answer but no joy. Does anyone know of an authoritive document on selecting and orienting lumber for built-up (laminated) spars of different types?