More on Testing

To All:

Or perhaps to that part of All interested in testing by busting things
up... And to those who got their knickers in a kink over the best
orientation of the laminae in a laminated spar CAP.

Here goes:

Go down to the Big Box Store that has forced your local lumber yard
out of business and track down PLASTER LATH.

The entertainment value in itself is worth the trip to the store,
because while they DO carrry it (probability 0.9) the odds of anyone
knowing where it is will be somewhere between small and none.

Plaster Lath is strips of CLEAR Redwood (meaning it's not supposed to
have a knots) ROUGH SAWN 1/4" thick by 1-1/4" wide in lengths of six
AND eight feet.

Here's a hint: It's going to be in the WOOD department, probably
standing up on end. Which isn't the proper way to store it but we're
lucky they even CARRY it... assuming they do. But I've found it at
the local Lowe's and two of the three 'local' Home Depot's ('local'
meaning within ten miles of my home).

When you find it, sort through it to find the 20 (twenty!) best pieces
in 6 (SIX) foot lengths. Grain is going to run all over the place but
that wasn't what this test was about. Then go find a sheet of
Doorskin plywood OR a sheet of 1/8" Luan plywood (if they don't have
doorskins, which most of them won't carry.. and will try to tell you
doorskins are the same thing as 1/8" Luan, which it ain't. Doorskins
are three by seven feet, probably made of Luan, but with WATERPROOF
glue.

Now go buy about two quarts of your favorite glue. At this point you
will realize why a lotta builders still use Weldwood's Plastic Resin..
because two-quarts-worth is going to run about HALF as much as
Tightbond III or any of the epoxies.

What's it gonna cost ya? Figure about $30 for the plaster lath, $15
for the plywood and $10 for the glue (if you're using 'Plastic
Resin'... up to $100 if you're using some SuperUltraSpecialStrong
Epoxy). The joke here is that ALL of the common adhesives used in
aircraft (or boat) work are stronger than the wood.

Finally, go find a TRUCK TIRE shop. You want a USED tube, as in one
that has suffered a BLOW OUT. It's not going to be repaired to hold
air. Make that point clear to the guy; you want it to make
woodworking clamps, not something for the kids to take to the beach
(they turn the inlet pipe around, put it on the OUTSIDE diameter of
the tube so it won't count you. And charge accordingly.) Take it home
and cut it into as many 3/4" wide stripes, as long as you can make
them.

Other handy stuff to have on-hand: A couple of 2" wide bristle
brushes that are cheap enough for you to throw away rather than
clean. You're going to CUT OFF their bristles to about HALF their
length. These are for painting in the glue. Shortening the bristles
makes them stiff enough to use with GLUE, which is thick as hell and
can't be spread with a regular brush.

Portable sander? Handy if you've got one. The redwood is going to be
ROUGH-SAWN. Ideally, you want it about the same smoothness as the
plywood.

Some throw-away vinyl gloves to wear when you're gluing-up stuff.

Okay, here's the Project:

Make 4 (Four) booms, 6 feet long by 1-1/4" square. Use them to make
2 (Two) Box-type beams, 6" deep by 1-1/2" wide by 72" long. (use some
scrap lumber to make some intercostals 3-1/2" long by 1-1/4" wide.)

To make booms you want to sand the lath to get rid of the 'hair' then
wipe them down with a DAMP (not WET) CLOTH. If you get them WET
you'll have to lay them aside until they are dry. Wiping them with a
damp cloth is how you remove the sanding debris.

You should have a cardboard box filled with black rubber bands.
Ideally, some will be as much as six feet long.

To prepare the laminations you'll need a pair of saw horses to which
you've stapled some waxed paper. Using a modified paint brush and
about ONE PINT of glue, apply glue to ONE SIDE of TWO pieces of
redwood. You will want to paint BOTH SIDES of THREE pieces of redwood
lath. You may start assembling the booms now,

The general rule is to apply glue to BOTH SURFACES. The booms are
made of FIVE pieces of lath so you want three of them buttered on both
sides, two of them buttered only on one side. As soon as you can,
start stacking them up.

When you have a stack of five, you can start 'clamping' them using the
rubber bands you've created our of the truck tire's tube. You do this
by over-lapping the first turn starting at the MIDDLE working toward
one end, giving the rubber a modest amount of 'stretch' as you go,
winding itfairly TIGHT. When you get to the end you make a loop from
a scrap of rubber, over-lap it then cut the free end ofthe piece
you're working with.

When you've finished 'winding' one end, flip the boom around and
commence winding the other end.

You really do want to use the 'rubber band' as your clamp. Why?
Because in most cases you will be using a WATER-BASED adhesive. That
means the glued structure is going to SHRINK as it CURES, which means
a regular clamp would simply come loose... but a rubber band 'clamp'
-- or a spring clamp -- will not. You may have some difficulty
keeping things aligned as you wind-on your rubber-band clamp -- that
goes with the territory -- use a rubber mallet to tap things back into
alignment as you wind toward the end.

Make four booms. Using the four booms, make two box-type spars, one
having the laminations horizontal, the other having them vertical.
You may use pneumatically-driven fasteners to fabricate the plywood to
the booms and intercostals. Be sure to drill a vent hole through each
of the intercostals. See Mark Langford's web site for additional
information.

You MAY be able to preserve any excess glue overnight by sealing it
air-tight and storing it in the fridge. (But don't count on it.) Be
sure to make THREE test blocks using the same batch of glue and method
of clamping.

Once you have fabricated your test spar you can start thinking about
the best way to test it. Or you may wish to modify the design,
allowing for deeper intercostals on the ends.

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After gluing, curing takes about a week. Then you must remove any
squeeze-out, typically by using a 4-1/2" or 7" sanding disk. When
pneumatically driven fasteners are used, as in applying the shear
webs, with Plastic Resin or Titebond III you simply keep a damp cloth
handy, wiping away any squeeze-out as it occurs.

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It MAY be of interest to go ahead and laminate an OUTER SPAR as well,
using the 8' pieces of lath to create them for a TAPERED wing similar
to Ken Rands design, except for fully boxed spars on both fore & aft.
The Box Stores may also provide the necessary hardware. SAE specs for
the mild steel used for framing stakes and the like is rather good.
Of course, you pay an enormous weight penalty. But with that in mind
you would have about 100 sq.ft. of lifting surface. If you stuck with
a Single Place design (ie, higher risk from using non-certified
materials, it would not be ethical, in my opinion, to design a two-
place, since the second seat would often be filled by someone who may
not be able to appreciate the risks) you could consider a gross weight
on the order of 800 lbs.

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But as for testing the six foot spar section, the main problem appears
to be how to get a grip on the thing. Personally, I suspect there
would be little if ANY difference in their load-carrying capacity,
regardless of the orientation of the laminae. But I DO think it would
be of interest to see HOW each sample fails.

-R.S.Hoover

Veeduber wrote:
...
Plaster Lath is strips of CLEAR Redwood (meaning it's not supposed to
have a knots) ROUGH SAWN 1/4" thick by 1-1/4" wide in lengths of six
AND eight feet.
... find the 20 (twenty!) best pieces
in 6 (SIX) foot lengths. Grain is going to run all over the place but
that wasn't what this test was about. Then go find a sheet of
Doorskin plywood OR a sheet of 1/8" Luan plywood ....
Finally, go find a TRUCK TIRE shop. You want a USED tube
...cut it into as many 3/4" wide stripes, as long as you can make
them.

....
Make 4 (Four) booms, 6 feet long by 1-1/4" square. Use them to make
2 (Two) Box-type beams, 6" deep by 1-1/2" wide by 72" long. (use some
scrap lumber to make some intercostals 3-1/2" long by 1-1/4" wide.)....


-R.S.Hoover


I built something like this several years ago as a "strong beam";
something to sit on the sidewalls of the house in the attic and secure
the ceiling beams of the living room that had been enlarged by knocking
down a ground floor wall I believe. I was unhappy with a crack in the
ceiling plaster, and decided to put a stop to it with a strong beam.
It was made from two 18 ft spruce two by fours (not aircraft quality,
needless to say) and enough ply to make a 4 X 12 box beam. I drilled
though it over each rafter and placed a threaded rod bolted to a saddle
cross bolted to each rafter.
Was that beam ever stiff! I couldn't detect the deflection when I stood
on the middle (not that I was trying very hard either)

Brian W
p.s. I liked the rubber spiral clamp idea!

On Jul 17, 6:29*pm, brian whatcott wrote:
Brian W
p.s. I liked the rubber spiral clamp idea!
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You'd better... cuz I stole it from Vernon Payne, who pointed out that
EVERYTHING shrinks as it cures... if it's got water in it. An' if
you're using a CLAMP then it's going to come LOOSE... if you don't use
a rubber band. Anyway, it works jus' swell and is worth mentioning.

-R.S.Hoover