Saturday, November 18, 2006

The Pee-Chay Catastrophe

Fellow called me last night. The sky was falling, Martian's were attacking New Jersey and his daughter was getting married to a registered Democrat. He didn't say none of that but you could hear it in his voice - total panic.

Did I have any Spruce, he asked.

Yes I do, as a matter of fact.

Real honest to gawd aircraft certified Spruce? He asked

Sure. Whatcha need? I ast him.

Would I, could I please oh pretty please sell him just a little teenie tiny bit?

Hell, I'd give him some, if he'd calm down and tell me what he needed.

It's for a corner block, he sez. He's scratch-building one of Roger Mann's designs and he's got the fuselage side all laid out and the parts cut and the glue's already starting to foam and he forgot to cut a piece of corner blocking and doesn't have any spruce left and he can't buy any more until the second mortgage goes through and his Dream Machine will be ruined if he can't find some Spruce right now!

When I stopped laughing I told him, So use a hunka fir. Or a piece of pine.

Long silence. Then, "You... you... HEATHEN!" And he hangs up the phone.

In this fellow's defense I gotta say he's been getting monthly injections of coffee and donuts at the local EAA chapter so he tends to be a bit irrational.

The corner block over which the fellow was so het-up is an isosceles triangle about two inches high and maybe one and three-eighths across the base. It is three-quarters of an inch thick. That's about one cubic inch of wood, folks.

A cubic foot of aircraft spruce weighs about 28 pounds so a cubic inch weighs around a quarter of an ounce. Make the part out of pine it'll weigh about the same, make it out of fir it'll weigh maybe three tenths of an ounce... mebbe nine grams.

So the newbie was getting all het-up over .05 ounces. Not half an ounce, five-hundredths of an ounce. Two grams. He could save ten times that much by blowing his nose before each flight.

Yeah, okay - weight is important. But so's reality. I happen to know this particular newbie plans to include a number of ‘improvements' in his airplane such as full IFR panel... when he doesn't even hold a PPL. Odds are, his airframe will come out about thirty pounds heavier than the designed empty weight, not only from his ‘improvements' but from the fact he doesn't know any of the thousand tricks that allows you to build ‘light.' Building light is a habit but one most homebuilders never acquire since their life-time accomplishment is usually limited to one airframe.

As for substituting fir for spruce, a corner block is a means of distributing what would otherwise be a concentrated load by increasing the gluing surface between the load bearing members. It helps the load ‘turn the corner,' reducing the stress on the junction between the longeron and the connecting member, be it vertical or diagonal. Since the load is concentrated in the outer-most fiber of the material you could drill at the centroid of the corner block and it wouldn't even notice. Fir happens to be stronger than spruce. So long as the corner block has the same height & base a small lightening hole at the centroid would not cause any loss of strength. By the same token, a slightly larger corner block of pine - with a small lightening hole - would give equal strength because of its increased gluing area, but a larger block might change the stiffness factor of that portion of the fuselage (although it would not in this particular case).

The real message in this posting is the fact that while Sitka spruce - Picea sitchensis - is some really lovely wood, that's all it is... wood. Some of its characteristics make it the optimum choice for use in aircraft but its present price and availability has made it grossly impractical for use in most newbie's puddle-jumpers. In fact, wood that is perfectly suitable for the typical light airplane can be found in almost any lumberyard.

And right about there everyone jumps up with their hair on fire and starts chanting "Heathen!" as they dance around the donut machine. (Some Chapters make their own. They call them ‘Homebuilt Donuts.')

Before you whip out your bastinado didja know that back in the Day when the government usta certify wood for aircraft there are ‘aircraft' certification specs for just about any wood you can name? Douglas fir, Western white pine, hemlock, sycamore, cypress... (The USDA got out of certifying aviation-grade lumber in the 1950's. What passes for a 'certification' today is just the retailer telling you the stuff appears to meet the old government specs.)

Spruce is beautiful stuff to work with but there are thousands of board-feet of perfectly good lumber from other species of trees lurking in your local Home Depot or Lowes. The trick is knowing how to tell good lumber from bad. And since you're here, I might as well tellya....

Start by defining wood in quantified terms. It's density will tell you how much it weighs and all else being equal, lighter wood is better than heavier wood. Experience has shown that the most suitable characteristics are found in wood having a certain minimum of growth rings per inch, with twelve being a fair norm. Experience has also shown that wood having a straight, uniform grain is better than wood which does not. The orientation of the grain relative to the particular stick, spar or stringer is also significant. Diagonal grain is only useful when the piece is to be cut up into triangular corner blocking. In all other cases you want the grain to be either flat or vertical to within ten degrees or thereabouts. For aircraft use we try to find wood with a minimum grain run-out of one inch in fifteen. The strength-to-weight ratio for wood reflects its density, which reflects its water content and varies slightly from one type of wood to the next but the optimum strength-to-weight ratio for all softwoods clusters near a moisture content of fifteen percent. The strength of wood is usually defined with three values, one reflecting is compressive strength, the second its strength in tension and the third having to do with its bending or shearing strength.

You can determine grain count, orientation and run-out by visual inspection. Density is determined by calculating the volume of the piece in question then weighing it. Determination of moisture content is done by baking sample pieces and weighing them periodically. When they stop losing weight you may assume they as dry as they can get. Comparison of their dry weight to their original weight will tell you the approximate moisture content.

To determine the strength of a piece of wood, you break it under controlled conditions. One method is to fasten a test piece sixteen inches long and one quarter inch on a side over a fulcrum having a radius of two inches so that fourteen inches of the stick extends outward. The inner end is fastened to the frame of the test jig. A fitting is attached to the stick two inches from the end and weights are suspended from the fitting until the stick breaks. (If you prowl the archives of the Forest Products Laboratory you'll find a Technical Report describing this testing procedure and others, including some pictures of the machines they used.)

All of which means less than nothing to the guy trying to build an airplane on a budget because he doesn't have the background that will allow him to interpret such data in terms of lumberyard pine and fir.

The truth is, 99% of wannabee homebuilders have never had their hands on a hunka aircraft spruce. Any talk of substituting locally available materials is wasted simply because the average homebuilder has no means of comparing the materials that are locally available to those of certified aircraft quality. Oh, he can measure the run-out and count the growth rings and it doesn't take a rocket scientist to recognized a barky edge, knot or pitch pocket. But any mention of tensile strength or bending stress is akin to chanting Eye of Newt and Toe of Frog because even though he's literate enough to follow a set of airplane plans, the average homebuilder is no more familiar with structural analysis than he is with the fourth act of Macbeth. (Now there's a Pop Quiz for all you airplane builders :-)

But we can fix all that. Call up Aircraft Spruce and order a grab-bag of spruce cut-offs. It will cost you about twenty-five dollars. When it arrives, study it. Count the rings per inch. Measure the run-out. Lookit the grain orientation. Now go down to Home Depot or your local lumber yard and start looking for the same stuff. When you find some, cut yourself some sample sticks and break them. THEN DO THE SAME WITH SAMPLES OF CERTIFIED SPRUCE.

Now you've got a standard of comparison.

In most cases your Home Depot pine won't test out as strong as certified spruce. So try it with fir. Douglas fir will typically test out stronger than spruce for the same size stick but will weigh a little more and is usually harder to work with, since it likes to split.

With that data in hand you'll be able to make an informed decision. Fortunately, there's not much thinking involved in corner blocking. If you make - and then break - some sample blocked corners comparing spruce to pine or fir you'll need laboratory-grade instruments to detect any significant difference in strength.

Ribs, stringers & longerons are a different matter.

If you want a real confidence builder, make up a couple of ribs outta certified spruce, rig a jig and test them to destruction. Then do the same using pine or fir. Or both. But be prepared for sticker shock.

Stick-rib the size for a Cub (i.e., 63 inch chord) you need mebbe ten foot of stick stock per rib and a Cub's wing (i.e., about 35 foot of span) needs about two dozen ribs, plus your ailerons and whatever. Smaller wing is no guarantee you'll use less. Lots of smaller wings use closer rib spacing, meaning more ribs, so that even if you need only eight foot of stock per rib you might end up using the same amount of stock for the smaller wing.

If you use certified stock you're probably looking at fifty to seventy-five dollars for a bundle of quarter-inch sticks. Buying wood from Home Depot and ripping it yourself, you're probably looking at seven to eight dollars.

That savings (ie, more than 90%) also applies to your longerons, although it's lot harder to find suitable stock of sufficient length.

Yeah, I know. Sacrilege! The hills are alive with the sound of newbies crashing into them when their sugar pine ribs turn to flinders.

I doubt it. Back when the world was young and so was I, my dad showed me how to build what he called ‘stick-ribs.' (*) He cut up a 2x4 for the sticks, showed me how to mix the glue (five .45 cal shells of powder to two shells of water. Weldwood ‘Plastic Resin,' aircraft certified from the local hardware store), then turned me loose to ‘make ribs.' I made three. The first took me all day and was an awful thing. The second one took only an afternoon and came out pretty good. The third took me mebbe an hour and was a thing of beauty. After the three ribs had cured for a few days my dad rigged a testing jig and we loaded the second rib with cans of bird shot, fishing sinkers and even hunks of scrap iron until the thing broke and spilled BB's all over the floor.

The 2x4 was probably pine or hemlock; I know it wasn't Douglas fir. It took better than three hundred pounds (!) to fail a rib built by a nine year old kid with zero experience. That's about six g's for the typical light airplane at 1200 pounds gross. Truth is, if I hadn't had so much trouble with those damn teenie-tiny nails, the thing probably would have taken another hundred pounds.

(* - He also showed me how to build routed ribs, metal ribs and riveted ribs, with several variations of each. No telling what you'll find inside a wing, he said. But if it's broke they'll expect you to know how to fix it.)


About six hundred years ago in a little ravine on an island in the Alexander Archipelago, a pair of seedlings sprouted. In 1998 they got cut down. One of them was a Sitka spruce, eight feet through the butt and a hundred sixty feet tall. The other was a Western hemlock, six feet through the butt and a hundred thirty feet tall. Both had grain as straight as a die. Both averaged sixteen to twenty annual rings to the inch. The spruce ended up as certified for use in aircraft construction and you can buy a piece of it, one by six by eight feet for about ninety dollars. The other tree got its heartwood turned into molding and window trim, while most of the sap wood became 2x4's and ended up at Home Depot selling for a dollar thirty-nine each.


So what's all this mean in real terms? Set of ribs for a big wing? Made outta 5/16" fir instead of 1/4" spruce? Maybe three pounds gained and forty dollars saved. No big deal. But the real meaning is more significant in terms of the survival of grass roots aviation because those seemingly insignificant economies add-up to safe, reliable airplanes that any youngster can afford.

If you can afford ninety dollars for a one-by-six that's been anointed with an inspector's stamp, go for it. Quality spruce is lovely stuff to work with and it'll save you having to learn anything. Personally, I plan to go on being a heathen, picking the stacks for the occasional lost soul, hoping to convert them to the true religion of Aviation.


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