spar plywood grain direction

[idamon]

Please explain the best ply direction for wooden box spars. The rear spars I inherited with the project have grain perpendicular to the length of the spar, seems odd. I read that the grain should run parallel with the length of the spar, or 45 for best results. Please clarify. Can I used the rear spars that I have and make the front 45? Picture is of rear spar. <br><br>Thanks

[Thomas Stute]

see my response to your other thread:

Top and bottom members of spar shall have the grain orientated along the spar. Side walls/shear webs shall have a +/- 45° orientation.

[idamon]

So, can I still use the rear spar? Grain is perpendicular to long axis of wing spar. I have read several people do it this way to save money because the added strength at 45 or 90 parallel is just not necessary... I think that is baloney but the spar is already made so I need some good reasons to make a new one...

Thanks

[WLIU]

Rear spars carry a lot less load than main spars. Most rear spars are in place to attach the flaps and ailerons. One clue, on some aluminum airplane's, the rear spar is only bolted to the fuselage using a -6 bolt.

What thickness plywood are the shear webs? What type plywood? I will guess that with both a front and rear shear web, the plywood faces are much stronger than needed.

With that in mind, your rear spar with the plywood oriented as you specify should be fine. But I am assuming that it has vertical members inside where all of the ribs will attach, the aileron bracket bolt holes will be drilled, etc. Yes?

Did I miss your identification of what type of airplane this will go into?

Best of luck,

Wes

[idamon]

Buliding a Taylor monoplane. 2mm thick 5 ply, probably american birch because it is cheapest, lightest, and strongest without penalizing other categories. The rear spar does have vertical members where the ribs attach, but so does the front spar.... Thanks for the advice.

[Clarke Tate]

The spar plywood oriented as your photo will be fine. While adding stiffness the actual additional strength is minimal with the 45 degree plywood. The square laid plywood, such as on your spar, seems to have been normal design practice despite NACA reports from the early30s, such as NACA Report 344 (http://naca.larc.nasa.gov/search.jsp?R=19930091413&hterms=NACA+344&qs=Ntx%3Dmode%20matchallpartial%20%26Ntk%3DAll%26N %3D0%26Ntt%3DNACA%20344), showing the 45 degree usage as preferable. I assume the use of square laid plywood is as much for economic reasons as anything in many designs.