At 08:52 AM 7/1/2004, you wrote: > > thickness # plys parallel perpendicular > > 0.125" 3 15.17 5.544 > >... but after looking at these numbers, I think I just started an argument >with myself. I had to wonder what's wrong with vertical, since that would >be "parallel to the grain", and by far the strongest. The point of these >faces is to keep the spar caps from separating (the verticals would take >handle the buckling). given the 3x strength difference from vertical to >horizontal, I'd have to vote for vertical grain, like the plans show. > >So I went to talk to my stress analyst buddy, a guy who does aircraft and >space vehicle stress analysis every day for a living, and then goes home and >designs his own aircraft just for kicks. He's knows this stuff inside and >out. He says those webs are shear webs, and therefore should be at a 45 >degree angle if you want the ultimate strength of the spar system. (like a >truss). He says the verticals are there to break up the plywood into >manageable shear panels. He votes for 45 degrees to the spar, and after >that vertical, but horizontal doesn't look so good to me OR him...
There is no question that diagonal is the best. Quoting from ANC-18 ... "Although square-laid plywood has been used extensively as shear webs in the past, the present trend is to use diagonal plywood because it is the more efficient shear carrying material. It is desirable to lay all diagonal plywood ... so that the face grain is at right angles to the direction of possible shear buckles. ... (It) is much stiffer in bending in the direction of the face grain and offers greater resistance to buckling if laid with the face grain across the buckles." Tony Bingelis talks about this in one of his articles on spar design. I can't find it in any of the books, so it must have been in the series that he wrote in the 80's when he was building a Falco. I tried to find the article reference with a search of the Sport Aviation articles on EAAs web site but it was not running. I will try later. The shear stress is parallel to the spar caps since one is trying to get longer and the other is trying to get shorter. That is the definition of shear. The shear strength of plywood is higher when it is parallel to the face grain compared to the perpendicular. ANC-18 is out of print but I do own an original copy of it, as well as ANC-5 and ANC-19. I offer copies for a little bit over my cost. Look at the Airparts link below and do NOT request it from me over the KR network. Talk to me directly. Don Reid - donreid "at" erols.com Bumpass, Va Visit my web sites at: AeroFoil, a 2-D Airfoil Design And Analysis Computer Program: http://www.eaa231.org/AeroFoil/index.htm KR2XL construction: http://users.erols.com/donreid/kr_page.htm Aviation Surplus: http://users.erols.com/donreid/Airparts.htm EAA Chapter 231: http://eaa231.org Ultralights: http://usua250.org VA EAA State Fly-in: http://vaeaa.org
