Sunday, November 19, 2006

Flying On The Cheap - Wood II

The low cost and wide availability of wood in America makes it a good choice for the budget-minded builder of simple flying machines. The only problem is that when picking the stacks at the local lumber yard you need to know a fair amount about wood in order to select the most suitable pieces. Part of that chore involves visiting the local yards - - or box stores - - fairly often in order to see the new pallets as they're put out for sale.

Today I lucked into some outstanding shelving (!) and a pallet of 1x2 furring strips that included some superior wood. I loaded up a cart with as much as I could afford. Waiting in the line at the register (Home Depot has a long way to go in that department) another homebuilder saw me as he entered the store and stopped to chat.

Although I've given demonstrations about grading lumber, including side-by-side testing in which samples of various woods are compared to Sitka spruce, my habit of using commonly available wood to fabricate real airplanes is often treated with humorous condescension, as it was today.

"Building another airplane?" he said as he started to leave, plus the patronizing smile.

"Same one," I nodded. Actually, same ones, as in plural, except most of this wood was destined for an on-going primary glider project.

He looked at the cart-load of lumber, started to say something witty, finally just walked off with an airy wave. To him it was just a cart-load of box-store lumber. Had he taken a closer look at the furring strips he would have seen they were Western Hemlock, ran about 24 annualr-rings per inch and came from a tree that was at least six feet in diameter(*). The grain had a run-out of less than one inch in eight feet and most of the sticks were almost perfectly cross-grained. That's because they were probably sawn from the cauls produced when log was sawn into a square cant. Back in the old days, the cauls would have gone into the kiln or boiler as fuel but nowadays they use laser-guided computers to figure out the maximum yield from every log and the cauls -- the camber-faced slabs from the sides of the timber -- probably went to an edger instead of the scrap heap and ended up as saleable pieces, including my bundle of furring strips.

The shelving was another situation entirely, being plantation-grown stuff running six rings to the inch near the edge and barely eight toward the middle. The tree was probably about 12" in diameter when it was harvested and probably about 30 years old. But what made these particular pieces of shelving candidates for aviation was the fact each piece on my cart was a center cut.

In producing construction-grade lumber the tree is cut into sections from 12 to 24 feet in length then each section is squared and run back & forth through the saw (or in a really big mill, through a continuous series of saws) and turned into slabs of the desired thickness, as dictated by the market. By comparison, the wood for masts, spars, ladder rails, bannisters and aviation-grade lumber is usually quarter-sawn, an entirely different proposition from plain-sawing.

The point here is that the center-cut of a plain-sawn log has EXACTLY the same grain orientation as if it were quarter-sawn. That means it will dry without warping and its characteristics of strength will be uniform.

So will it be useful in an airplane? Probably. But the wood has the final say in the matter. It's still pretty wet and the shelving needs to be re-sawn to isolate the usable outer sections from the center. Give it a bit of time, turning it occasionally and a fair percentage of it should prove useable. And if not, I'll use some of it to make Smilin' Jack and his friends some little toy airplanes; give'm to them at Christmas; tell them it's for their grandkids.

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The furring strips were $1.15 each and I bought twelve. The shelving was $5.67 each and I bought three pieces. Total was $30.81 at the Home De-pot on San Marcos Blvd. That eats up my 'airplane wood' budget for about two months but 'long about March I'll start prowling the stacks again. Lotta ribs in an SG-38 and every time you take it out you can count on them kids busting a few. But by the time the thing is mostly patches the new set of wings will be ready, along with a new crop of kids.

The Smilin' Jacks don't know what they're missing :-)


(*) - - Trees are round. With a pocket loupe and your 6" machinist's scale you can measure the chord across the arc of the annular rings of a fine-grained scantling with a fair degree of accuracy. With that, you can calculate the diameter of the tree at that point. Do it enough times, just looking at the piece will give you a good idea of its diameter.

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