Sunday, November 19, 2006

Flying On The Cheap - Wood

A few years ago I posted an article offering some hints about how to build an inexpensive airplane that was safe and reliable (Flying on the Cheap, October 2001; Google will find it for you). The key point in the article was the use of commonly available mild steel tubing for the fuselage. Of course, that meant you had to weld and nowadays most folks don't, nor do they want to learn. Wood's the thing, according to non-welding folks, citing its use in KR's and Piets and dozens of other airframes, each held as the Perfect Design by that particular person.

The advocates of wooden airframe construction have a valid point, at least here in the States. Because of the rise of the box stores (Home Depot, Lowes, etc), wood is more commonly available than steel tubing and despite what many think, there's plenty of aviation-quality wood lurking inside the knot-hole collection at your local box store. The task of the lo-buck builder is to cut away the non-aviation parts, glue the good stuff together and go flying. Thanks to modern adhesives, the likelihood of a novice builder producing an unsafe glue-joint is small.

To support the builders of Box Store Bombers I shared my woodworking experience in several articles posted to this newsgroup (How to Make Ribs Out of Old Orange Crates [Nov 2002], Wooden Notes [Jan 2006] and several others). Surprisingly, homebuilding at that level is not an especially welcome topic on the rec.aviation.homebuilt newsgroup. This lead to relaying such information in private posts to guys who were interested in actually building something. Like Fred. Which isn't his name, but work with me here.

After considerable thought Fred settled upon a single-place, VW-powered KR-ish design as being the best match for his particular situation. In working toward that goal he didn't find much support, especially from the only organization that claims to speak for grass-roots aviation in America. Thanks to an income of only $25k or thereabouts, to the EAA, Fred and the millions of people like him simply do not exist. But Fred is determined to build and fly his very own airplane, even if he has to use the local box store for most of his materials. Indeed, he doesn't have much choice. The total cost for his box store lumber will be under $100 whereas a kit of aviation-grade spruce would cost about a thousand dollars by the time the freight was paid. The lumber will have to be resawn and spliced but that's the reality of Flying on the Cheap.

Fred is bucking additional headwinds in that he has zero woodworking experience, doesn't own a table saw and has only a limited amount of shop space. But he does have a computer and lots of determination. With a bit of help, he has been making slow but steady progress.

Having finally gained access to a table saw, Fred began prowling the box stores for suitable lumber. (His KR-ish design uses built-up spars, the main spar having caps two inches square, the aft spar about an inch.) His next message verged on panic. Did I know that all of the lumber sold in box stores was marked incorrectly? Some of their one inch wood was only three-quarters of an inch thick! And all of the 2x4's he looked at were only one and a half by three and a half!

I assured Fred that the box stores weren't trying to cheat him, explained about rough versus finished dimensions and pointed him toward some places on the internet where the matter was explained in more detail. His reply sounded a bit discouraged, having realized he won't be able to cut a 2x2 out of any piece of finish-size two-by-something lumber. The only way he's going to get a good 2x2 stick is to start with 1x2's, spliced to get rid of any knots, and glue them together to get his 2x2's. That's the reality of Flying on the Cheap (why does that sound familiar...) but as a matter of fact, except for the splices it isn't that much different than starting with a spruce kit.

I sent him some drawings showing how to slice up a 2x4 to produce spar caps. Because of the knots he'll probably need about eighteen 2x4's, resawn and laminated to produce the required number of spar caps. It is labor intensive but there's surprisingly little waste since the residue of spar cap production yields the intercostals, stringers and tail-feather spars.

Any mention of splicing spars usually sets several heads of hair on fire, even though such splices are an accepted practice, their details fully covered in the old CAM or the new AC43 manuals. In fact, once you've made up a suitable fixture to ensure all of your splices will have the same angle, doing a long-splice in solid wood is no more difficult than doing a short-splice in plywood. (Hint: Solid wood, you want an angle of about 1:15. With plywood, thanks to the orientation of the plys, you can develop full strength with angles between 1:10 for mahogany and 1:12 for birch. As a point of interest, boat builders commonly use 1:8 for either.)

Making up a scarfing fixture tends to drive a lot of homebuilders crazy as they fiddle and tweak, trying to achieve a precise angle of 3.8 degrees for a long-splice. Or 7.1 degrees for a boat-work 1:8. The truth is, the precise angle doesn't really matter. What matters is that all of the pieces be cut to exactly the same angle... and that the splice be strong enough for the task at hand. This degree of accuracy can be achieved using nothing more complicated than fixtures assembled from scrap wood, one for scarfing solid stock, the other for scarfing plywood. In each case the wood gets clamped in the fixture and the same cutter - - a portable circular saw - - is used on both.

(As you might suspect, scarfing actually begins at the box store. When picking the stack for lumber having the proper grain and run-out, you envision the cuts you'll make when resawing. In many cases you can orient the piece so that any knots fall entirely within the pieces you are cutting off, leaving you with a knot-free stick.)

When using box store lumber for airplanes or boats there are a host of details bobbing just beneath the surface, ready to sink the unwary. Most of those details can be resolved with a dose of plain old-fashioned Common Sense, such as keeping your saw-blades sharp, adjusting the rate of your cut according to its depth and so forth. Fortunately the details tend to be fail-safe. And self-educating because of it. Do it wrong, you'll end up with a part that can't be used. But once you get the hang of it, splicing longerons or spar caps is no more complicated than checking the air in your tires.

- -

For more than fifteen years millions of low-income but air-minded Americans waited for Light Sport Aircraft to become a reality. Sadly, that reality is airplanes and flight training which remain too expensive for the average American. As the LSA concept turned into vapor-ware I began receiving more mail from people like Fred who have decided to follow a different drummer. On the whole, I think this is a good idea. Based on more than ten years of such messages, folks like Fred cleave closer to the ideals of grass-roots aviation. These folks have learned more about their engines and airframes than the typical kit-builder and some have acquired a remarkable depth of knowledge in engineering and aeronautics. But I don't think we'll see Fred at Oshkosh - - it's simply gotten too expensive. Indeed, I've a hunch a lot of these fellows will end up flying 'black' - - completely off the books of any Agency or organization. Not because they want to but because it's the only way they can keep the Dream alive.


1 comment:

Anonymous said...

Very good. I just discovered your
website or blog. I love it. I was
needing a tad of motivation and I
got it with your posting.
I noticed that from your first pa-
ragraph that we are of about the same age, and have exactly the same
interest, hobby and passtime.
I was expecting today in the mail
the building plans for F.R.E.D.
wish is a small parasol type of
sport airplane. It is an all wood constrution. They did not arrive
today. Maybe monday..
It may be my last airplane building
project, it as to be enjoyed to it's fullest.
I am looking forward to read your
other pages.. Because of way of
life and also by necessity I simply
enjoy doing things on the cheap.
Over the years good or bad, I did
develop it to an Art form.
Maybe we can share some trade se-
crets some day..
Anyway, solong for now, and I book-
marked your page to come back to
it tonight.
73, de VE3-TDZ
doug.maloney, ak Baldineagle.
yet. Maybe monday..