Saturday, November 3, 2007
Keeping Up With The Past
After posting 'Chugger's Rib-II' several people suggested I try making the gussets out of drywall tape, having read of someone using that method. Someone else suggested I use regular fiberglas fabric.
Date: Sun, Oct 26 2003 1:51 pm
Subject: 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; so does 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 gravity, motion and so forth. Long before there were such 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 treated 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.
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 eighth-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. Once 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.)
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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.
-26 October 2003