Flying on the Cheap - Wood

"David Melby Cavalier" wrote in message
oups.com...

wrote:
To All:

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 incorrectly marked? 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 AC13 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.

-R.S.Hoover

If Fred plans to build a plane from wood purchased from Home Depot, he
should also build a pine box. He will need it. I have never found
wood acceptable for aircraft at HD or any other such outlet. Why pass
on such bad information?

Since Veeduber's original post, I have taken a long glance at the lumber in
both of the local big boxes, as well as a couple of the local independents,
and I have concluded that the method is reasonable and a good compromise.
Arguments in favor include:
1) the high cost of shipping for small quantities,
2) the ability to buy a little at a time
(think empenage kit, wing kit, etc.),
3) laminating can be used to defeat any remaining tendency of wood to warp
or bow over time,
4) laminated wood better resists splitting, and
5) the relative amount of expertise (a/k/a experience) required to evaluate
small (thin) peices is less than that required for large (thick) peices.

There is no question that more work, including a lot of clamping, is
required to splice and laminate. But the old timers all swear by it and I
have NEVER heard or read anything in opposition from an experienced source!

That does not mean that I either will or will not use wood as the base
material for a composite, only that I kow it to be a sound engineering
material.

Peter

On Thu, 10 Aug 2006 12:49:22 -0400, "Peter Dohm"
wrote:


"David Melby Cavalier" wrote in message
roups.com...

wrote:
To All:



Since Veeduber's original post, I have taken a long glance at the lumber in
both of the local big boxes, as well as a couple of the local independents,
and I have concluded that the method is reasonable and a good compromise.
Arguments in favor include:
1) the high cost of shipping for small quantities,
2) the ability to buy a little at a time
(think empenage kit, wing kit, etc.),
3) laminating can be used to defeat any remaining tendency of wood to warp
or bow over time,
4) laminated wood better resists splitting, and
5) the relative amount of expertise (a/k/a experience) required to evaluate
small (thin) peices is less than that required for large (thick) peices.

There is no question that more work, including a lot of clamping, is
required to splice and laminate. But the old timers all swear by it and I
have NEVER heard or read anything in opposition from an experienced source!

That does not mean that I either will or will not use wood as the base
material for a composite, only that I kow it to be a sound engineering
material.

Peter


cellulose is a macro molecule assembled from sugar.
apart from some grain characteristics that are species related the
main arbiter for wood strength is its density. since most of the wood
you see in commercial sales areas is all at 12 to 15% moisture content
the density of the wood is a reasonable guide as to its strength.
compression, tension and izod tests are actually what you need but the
visual inspection of wood to exclude defects (from the actual length
of the cut piece as veedubber suggests) has served builders well for
all of aviation.

the only other piece of information needed for laminating is an
understanding of whether the lignin binding the cellulose together is
a thermoset plastic or a thermosoftening plastic.
thermosoftening lignins allow for woods that can be steamed and shaped
very easily. thermosetting lignin makes for a wood that should only be
used in straight pieces.

my own wood is sourced via a relative on the other side of the country
from the stocks of a chap who makes bee hive boxes for the apiary
industry. Talk to people and ask around because you can find some
amazing wood sources. One chap I know stumbled on 3 pieces of 50 year
old spruce, that were absolutely straight grained for over 30 ft, out
of a deceased estate. he now has enough for two aeroplanes.

if you look around and weigh woods you will find an amazing array of
suitable woods. I'm writing from Australia but in my local Bunnings (
a clone of your home depot) I can get Alaskan Yellow Cedar that ranges
from useless heavily knotted pieces to pieces with close straight
grain for over 6ft. I can find Australian "Oak" in densities from just
heavier than Spruce to nearly 65lb per cubic ft. I can occasionally
source Oregon Pine (Douglas Fir) that is usually suitable.
Mainly though I use Queensland Hoop Pine in the manner that Veedubber
suggests.

One of the keys to getting a supply of good wood is to be prepared to
buy it when it becomes available. It is a natural product and supply
is variable. There are lots of people out to obtain good woods.

I think my aircraft will have about 8 species of wood in it by the
time I'm finished. I'm using it because Wood structures are
permanently repairable.

Stealth Pilot