Original Posted By: danhelsper(at)aol.com
Lorenzo;As a practical matter, I suggest that you use the value of "fiber stress at theelastic limit" for your spars rather than the modulus of rupture. As it is,the old forest product labs data that was published in 1941 for the propertiesof various woods for aircraft structures has an explanatory note with those valuesthat says there is a significantly wide variation between test specimensof the same wood, same moisture content, taken from the same tree or differenttrees. They used an average value, but did your wood come from the averagetree? And are the trees that are being cut today the same as the ones in theforest in the first half of the 20th century, when most of the wood design bookswere written and the research on wood for airplanes conducted?With that said, understand that the fiber stress at the elastic limit is presentedin the tables for a load duration of only 3 seconds. Considering that thenormal category for G loading is +3.8G and that that loading can occur in a coordinated75 degree bank, 3 seconds is probably a good safe thing to design forand "elastic limit" means that there is no permanent deformation of the sparas a result. Let's not fly rigorous high-G maneuvers in our Piets, eh?Modulus of rupture, on the other hand, means if the wood sees it you're going torebuild a permanently broken wing after you land, or it's going to break offin the air and you crash. When the spar experiences this kind of load, it'sirreversibly damaged. This has never happened to a Piet in the air (in the records,anyway)- so we don't want to spoil the safety record and design to themodulus of rupture just to get our name in the history books. Let's use the elasticlimit. Practically speaking, that's 6200 psi for spruce and 8000 psi forDouglas fir.Gene; point well taken. My analysis will be for 1" solid and routed spars, bothspruce and fir. The 3/4" spar will be the exception.Doug; another point well taken, and understood. Evans and others suggest a loadfraction on the main spar of 70% of the total loading on the wing section, atmaximum lift. That's the value I'm using in my analysis. The rear spar carriesless than half of the total loading on the section, but that doesn't meanwe don't need it!Patricia: what a beautiful piece of woodwork! Thanks for sharing the photo ofyour wing. I am always mesmerized by the intricate geometry of wood-framed wings.--------Oscar ZunigaMedford, ORAir Camper NX41CC "Scout"A75 powerRead this topic online here:http://forums.matronics.com/viewtopic.p ... ___Subject: Pietenpol-List: Larry Keitel
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