ORIGINAL: CrateCruncher
ORIGINAL: Rodney
This is an old argument that always gathers much comment. If you check out your Strength and Materials handbook, you will see that the stresses in any web is at a 45 degree angle to the span so it really makes no difference in whether the grain is vertical or horizontal in the webbing, both are equally strong in shear loads. Now, if you are worried about crushing (say the cat or dog steps on your wing) then vertical grain is the strongest. If you want maximum shear strength, you need the grain at 45 Degrees to the span but; if you go inverted the shear stresses reverse by 90 degrees so now that same 45 degree is the weakest direction. That is why the specifications for shear webbing on homebuilt planes usually calls out plywood with the grain at a 45 degree angle to the span to give maximum shear strength for both positive and negative G loads. In our models, don't worry, either spanwise grain or vertical grain is equally effective.
You sound
very confident like you've done a lot of analysis in this area but I'm a skeptic. I'd like to see your work to be properly convinced. Using your knowledge of typical RC flight loads and beams in pure bending I'd like to see proof that maximum shear stress occurs at 45 degrees. As I'm sure you already know, there are several methods for showing stress transformation so just use the method your most comfortable with.
You are correct in always asking for proof. Since I am not a registered mechanical engineer, I referenced any mechanical engineering handbook. Just look up cantilevered beams (that is what a wing spar/web combination is) and accept the standard engineering proof, no need for any of us as individuals to try to prove the point when it has been done by the experts. You can also check the government requirements as set up with the FAA and/or other safety organizations that are directly involved with such specifications involving home built aircraft. If you do a search on the forums, I'm sure you will find reams of discussion both pro and con relative to grain orientation which you can sift to your satisfaction. Personally, I tend to believe the reference and engineering books.