Original Posted By: "Douwe Blumberg"
A few days back, the subject of wood cabanes came up again. I rememberedthat Clif Dawson had sent me some facts and figures to back up, Why it's OKto use Wood for Cabanes:"Spruce has a tensile strength of 6700 lb/square inch.For the sake of argument let's say a strut is 1" X 3 1/2". Withstreamlining, the area should be 2/3 of the square area, or 2.3 square inch.There are four struts or 5.2 square inches. That's 34840 lbs (yes I know thefront ones take the majority of the load) .Dividing on the assumption ofequal load on a 1200 lb AC we get 29 g! How much less if properlycalculated? 25 g? 20 g? Is this adequate?There's a bolt at each end. The strength here is based on how much forcerequired to pull a plug of wood out by the bolt. That plug has two faces,the square area of which is the width of the strut times the distance fromthe end to the bolt. If the strut is 1" thick and the bolt is 1" from theend then you have two faces each 1" square or two square inches. The "shearparallel to the grain" is 1120 lb per square inch. So we have a strengthhere of 2240 lb. Four struts so that's a total of 8960 lb. That's still over7 g. If we added another such bolt we have 14 g capacity. Taking intoaccount the higher front strut stress I'd bet we still have at least 10 g toplay with.And this is for plain, solid spruce, no plywood, no laminated straps orembedded tubing or anything else to complicate matters. You can add a little more strength by using Western Hemlock or Doug Fir butnot much. "Gary Boothe Cool, Ca. Pietenpol WW Corvair Conversion, mounted Tail done, Fuselage on gear (18 ribs down.) ________________________________________________________________________________