- May 27, 2019
I always thought that the grain in spars/spar caps should run parallel to the ground too. At Sun n Fun I was looking at a kit aircraft and the spars were running with the grain vertical to the ground and I questioned the representative. He told me that was the correct orientation. On the plans to my first aircraft the cross sectional drawing of the spars showed the spar caps (webbed spars) with the grain running horizontal. I asked two other designers/suppliers of wood kit aircraft via email and neither of them answered my question. I got on a forum and asked the same question and what an argument that started! Then I got on the website of a very popular aerobatic biplane manufacturer and looked at photos of spar tests and many of the spars were laminated of 1/4" thick pieces but some of the tests showed spars laminated with layers of 1/4" with the laminations in the horizontal to ground plane BUT the individual pieces in the laminated spar had the grain lines running vertical. When I tested 3/4" x 3/4" pieces to failure, some solid, some scarfed, some laminated, I tested them in every configuration and found that the pieces made of three 1/4" layers glued in the horizontal but with the grain running vertical to the ground in each stick were right up there as far as strength as the ones that had three layers in the horizontal with grain also in the horizontal. What I don't know is bending strength. I may be wrong but in theory it seems to me that the best would be the later, horizontal laminates of horizontal grain as far as the wings being able to flex and bend. I'm not an aeronautical engineer though so i have to rely on what the pros say. I guess the hardest thing is to figure out just who the real pros are!