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Drywall Gussets



 
 
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  #1  
Old October 26th 03, 09:51 PM
Veeduber
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Default Drywall Gussets

It's all about strength to weight.

Feathers aren't very strong. But then, birds aren't very heavy.

Fabric is stronger than feathers, except for the quill. Even cotton fabric.
Or resin-coated paper. And wood makes pretty good quill-stuff. Or even grass.
Bamboo is grass.

One of the tricky bits is carrying the load around a corner. Loads concentrate
at corners. As they go around the corner the load often twists, converting
simple bending moment calculations involving compression and tension into
load-paths so complex we're forced to kneel at the alter of Delta Vee and work
them out one prayer at a time.

Ultimately it comes down to the Fastener, the way we attach the vanes of the
feather to the quill and the quill to the wing and the wing to the body of the
bird.

Aluminum alloy scores high for practicality, being as strong as mild steel but
only one-third the weight. To carry the load around the corner you simply bend
the aluminum, trapping the load inside. To transfer the load you bend it
again, poke a hole through it, plug the hole with an aluminum pin and hammer it
tight, the number of pins determined by the load. (Hint: See ‘Riveting 101')

But wood scores highest for practicality because it is universally available
and less expensive than metal or fiberglas or foam or castaway string bikinis.
(ANYTHING can be made to fly.)

To turn a wooden corner we use gussets. And our fastener is usually glue.

All modern glues used in aircraft construction are stronger than the light,
strong softwoods normally used for aircraft construction. Rather than telling
us how many fasteners to use, with wood the load tells us how much surface area
we must slather with glue. This is when we learn that a quarter-inch square is
not a quarter of a square inch but only a sixteenth. With a butt joint only a
sixteenth of an inch square even the strongest glue fails when the load tries
to turn the corner. That's where the gusset comes in because a gusset allows
us to multiply the area of the glue joint by a factor of at least 10. If the
load is very large we add blocks at the corners, increasing the glue area still
further and shortening the path the load must follow as it navigates the turn.
The strongest corners are formed with glue blocks AND gussets, allowing us to
multiply the gluing surface to WHATEVER is required to produce a safe joint.
Of course, that makes them heavier. Such belt & suspenders methods are only
used when know the extra weight is justified by the need for additional
strength.

THE NATURAL ORDER OF THINGS

There is a natural order to the universe, such as the need to sow before you
can reap, and in the universal constants of gavity, motion and so forth. Long
before there were things as science or engineering there were Natural
Philosophers, fellows who studied the natural order of things and tried to
understand them. That's not allowed today. Today, birds fly strictly in
accordance with scientific principles and bumble-bees are forced to walk :-)
But the natural order of things continues to exist. Just as there is a natural
order to the planting of crops or the erection of a house, so too is there a
natural order to building a airplanes.

Plywood is the most commonly used shear-web material found in wooden airplanes.
It is also the most commonly used gusset material. In the natural order of
building wooden airplanes, gussets are made from the residue of plywood left
over from paneling operations, such as building the sides of the fuselage or
making a built-up wing spar.

In the natural order of wooden aircraft construction you begin with a large
plank of suitable wood and cut it to create your spar caps and longerons and
stringers. In this way the largest and longest pieces are created first and
the smallest pieces of wood, typically those used to make ribs, are made from
the residue of the earlier cuttings.

In the natural order of wooden aircraft construction the fabrication of the
ribs is not addressed as a task in isolation. Fabrication of ribs is a minor
event incidental to the construction of the airplane as a whole. During
fabrication of the spars, tail feathers and fuselage, when you find yourself
with a few spare minutes, you make a rib. Or add gussets to one already made.
Or sand a rib. Or varnish it. No matter how many ribs are required, you will
have finished them long before you are ready to assemble the wings and at the
expenditure of no time at all since the effort has been distributed across all
the other chores.

The small sticks used in the typical rib give it an airy, fragile appearance.
In fact, when properly assembled, that fragile looking rib is overly strong by
a factor of two or even three. Which is another way of saying an airy rib
could be airier; that it is over-built and too heavy because of it. But so
long as ribs must be assembled by humans with sausage-sized fingers we must
accept quarter-inch sticks as the smallest practical size for ribs. In effect,
we humans are the limiting factor when it comes to optimized ribs. This is a
reflection of the Practical Factors versus those which are possible.

Frankly, the extra mass is no big deal. The typical light airplane has only
two dozen ribs or so and the difference between optimal and practical is
usually less than a pound even in an airframe that may gross out at half a ton
or more.

The Practical Factors are why the gussets used on most airplane's ribs are
overly thick and far heavier than needed. That's because gussets are free, the
by-product of earlier steps in the construction.

If the builder has plenty of money they may opt for a sheet of ply specifically
for their gussets but common sense usually prevails, especially after they run
the numbers and see that they've just spent forty dollars to save three ounces.
Twenty dollars a pound, we can live with. Two hundred dollars, we can't.

THE UNIVERSAL GUSSET

If you wish to save both weight and money on your gussets stop thinking of
plywood and look elsewhere. Indeed, gussets and corner blocks represent a
crude solution to the problem of carrying a load around a corner. The only
reason we are still sawing out corner blocks and nailing down gussets is
because that's how de Havilland did it in 1916.

Nowadays we have fiberglas. And staplers. And urethane glue.

Need a quick gusset? Saturate some fiberglas with glue and wrap it around the
parts to be gussetted.

Messy, eh?

Try this: start with a pallet of some sort; cardboard or plywood. Lay a piece
of plastic food wrap over the pallet and put your fiberglas on that. Now
saturate it with glue and put the thing in place by handling the plastic wrap.

Not so messy, eh?

Urethane glue expands as it cures so it's customary to install a clamp or apply
some weight to the sandwich until the glue has cured. In many cases you can
leave the cardboard pallet in place and simply staple it down, driving the
staples THROUGH the cardboard. Or put a weight on it. Or sandwich it between
scraps of metal or ply and clamp it with clothes pins.

Fiberglas is too expensive! (I heard someone say.) They're probably thinking
of fiberglas fabric, which is rather dear if ordered from an aircraft supplier.
Local suppliers of fiberglas typically charge about half the amount asked by
aircraft suppliers. (San Diego, CA.) Fiberglas tape is very handy for
gussetting chores since the woven edge keeps it from unraveling. (But beware!
Tapes are typically woven from six to eight ounce fabric; fine for gussets on a
fuselage but much too heavy for those on a rib.)

If you want some lightweight fiberglas you can find it at any lumber yard.
They call it Drywall Joint Tape. It comes in rolls, typically two inches wide
by whatever length they happen to sell. Locally I can buy it in rolls as small
as one hundred feet or as long as the market will bear. Professional drywall
installers use rolls holding 500 feet and more. Cost is usually less than two
cents per foot, dropping to about a penny per foot for the largest,
commercial-grade rolls.

Most look at the eighth-inch mesh of drywall tape and turn up their nose. You
can't make a cowling out of stuff like that nor cover the wings of a KR or
Notsoeze. But it does a fine job at making gussets.

How? By folding it over or layering it until you have sufficient strands to
give you the strength you need.

Glass fiber is stronger than steel. You can prove this for yourself by cutting
a piece of drywall tape about a foot long then peeling off ONE STRAND of the
stuff. Use a surgeon's knot to tie one end to a dowel or other bobbin of
significant radius and the other end to the handle of a bucket. Then add weight
to the bucket until the strand breaks. Now go weigh the bucket. Do that eight
or ten times and average the result, you'll know how strong the stuff is. But
doing it just ONCE should give you a good idea as to its usefulness.

How strong of a gusset do you need? (Be careful here; remember, your ribs were
already twice as strong as needed.) You really don't need the strength of
eighht-inch birch ply for a rib gusset. Nor even that of sixteenth inch in
most cases. We only use those sizes because of the Practical Factors.

Making small ribs, such as for the Practice Wing? Then try two layers of
drywall tape. As a matter of fact, before using this stuff you will have to
learn how, and while you're doing so, go ahead and make up several different
layers of fiberglas.

Remember that mention of the Natural Order of things? There is a natural rule
for gusset strength too. Make a sample T-joint, allow it to cure, then break
it. The sticks should ALWAYS break first. If your drywall gusset tore or came
loose, try it again with an additional layer of fiberglas.

Why glue instead of resin? I think the proper question is, Why NOT glue
instead of resin? We don't need the added strength of epoxy or vinylester
resin; the weakest component in the structure is the WOOD and all modern glues
are stronger than wood. Besides, the glue is right there, ready to go. In
fact, urethane glue appears to be better for this type of thing than does resin
because the glue expands as it cures. One it has cured you trim away any
excess and are left with cellular type of structure that is much lighter than a
solid chunk of resin.

(If / When... Santa arrives with a digital camera, photos of this method will
be posted in the Practice Wing file in the ‘files' archive of the Fly5kFiles
mailing list over on Yahoo.)

- - - - - - - - - - - - - - -

Flying is all about strength to weight. Sez so, right there in all the books.
But in modern-day America flying has become more about MONEY than anything
else. Fiberglas gussets are universally available and inexpensive. They
aren't in any of the books, of course. And never found at those wonderful
seminars. Alas, the guys who are trying to keep grassroots aviation alive in
America often can't afford either the books or the seminars. But they still
fly, usually behind converted car engines and sometimes with a bit of
drywalling on their ribs, not because of all the books or those expensive
seminars but in spite of them.

-R.S.Hoover

PS - If you think the ‘Practice Wing' project is only about wood, it's not.
But the wooden wing comes first because it is the least expensive and its
materials are universally available. Once it's finished I'll show you how to
do the same thing using aluminum but I will assume that you've already
completed the practice wing in wood. This is because of that natural order
business. About 85% of what you need to know to build a wing is COMMON to ALL
wings, the only things that change are the materials and the methods.
  #2  
Old October 26th 03, 11:02 PM
BD5ER
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If you want some lightweight fiberglas you can find it at any lumber yard.
They call it Drywall Joint Tape. It comes in rolls, typically two inches
wide
by whatever length they happen to sell



Getting kind of high tech here aren't we?G

What's wrong with good old fashion paper for the gussets? Was good enough on
my old Aeronca. 65 years young and the gussets are still doing their job.
Lots of suitable paper out there to replace the "fish paper" Aeronca used.
Pink roofing paper - a roll of it should make a few hundred planes worth of
gussets. Or you can use the left over to make model rocket engine cases if you
wish. Nice big ones too, not those wimpy things Estes sells. Water color
paper comes in weights that make good gusset material. Even heavy Kraft paper
works. Grocerys all come in plastic now so you may have to buy it too. And if
you must spend the money for the same stuff that came in the certified Aeronca,
take a look at some of the gasket paper sold in about every auto store.....
  #3  
Old October 27th 03, 04:15 AM
ThJester99
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Mr Hoover, I am ALWAYS amazed and intrigued by your posts. Thank-you for
sharing your knowledge and love of airplane building. I am currently going
through an A&P program, and ill never learn this stuff there. Only through the
sharing of information and thought processes from people like you can we gain
this invaluable knowledge. Thank-you for taking the time and caring enough to
passon what you know and have learned. Keep posting. Ill keep reading and
sacing.
Jesse Matlock
PP-ASEL
A&P student
future airplane builder
  #4  
Old October 27th 03, 04:23 AM
Bernie the Bunion
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ThJester99 wrote:

Mr Hoover, I am ALWAYS amazed and intrigued by your posts.


Me too...... And I'm just an armchair lurker.
  #5  
Old October 27th 03, 07:22 PM
VideoFlyer
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Mr. Hoover has a wealth of knowledge and an amazing ability to tell a story.
I'm a fan!

Dave
  #6  
Old October 27th 03, 10:03 PM
Wright1902Glider
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The first set of built-up ribs used paper and cassien glue for gussetts...
those being the ribs of the Wright 1903 flyer. As best as I can tell, this was
the first time that built-up ribs and a double-surface wing were used. (If
anyone has evidence to the contrary, please let me know.)

One hundred years ago, there were no certified aircraft parts or materials.
Wood came from a lumber yard, and you graded it yourself. Fabric came from the
general store and was more commonly used for making ladies' underwear. And
hardware? Well, you got some steel and made it yourself.

Keep it up R. S. Without guys like you to remind us how to keep inventing, we
may all forget and start flying Cessnas.

S. D. Harry Frey
Wright 1902 glider
hull #7
 




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