Thursday, October 25, 2007
Chugger's Rib - II
For the last two months I've been making ribs.
The first one was made of Sitka spruce, aviation-grade plywood, T-88 epoxy and pneumatically-driven Arrow JT21 staples, which were then removed. When you include the cost of shipping the materials, each rib will cost between eight and twelve dollars.
For a load of 3.3g the central portion of the rib -- the part between the spars -- must be able to bear at least 88 pounds or 40 kilograms. The rib was tested in the manner described in... (http://bobhooversblog.blogspot.com/2006/11/rib-testing.html) The rib was then laid aside and I began to experiment.
I made ribs out of different varieties of wood including redwood plaster lath and cedar fencing, as well as Western Hemlock and Douglas Fir. The wood was sawn to size with different types of saws -- table saw, band saw, sabre saw, jig saw and even a portable circular saw. The ribs were assembled in the jig previously mentioned (ie, Chugger's Rib) using gussets made of 1/8" doorskins (as described in...
http://bobhooversblog.blogspot.com/2006/11/av-how-to-make-ribs-out-of-old-orange.html) as well as aviation-grade plywood of 1/32" and 1/16" thickness. Some examples got the gussets applied to just one side of the rib, some to both. Then I tried gussets made of paper; some of which was thick, some was thin, some were done single-sided, some were both. I used Weldwood Plastic Resin and Gorilla Glue and Elmer's Ultimate Glue and some epoxy crap from the hardware store that came in tubes and T-88 and even Titebond III, although I didn't make a full rib for every combination of wood, sawyering, gusset-type, gusset-position and glue. I already knew how the glues performed with various woods but the ribs presented some new combinations, such as a cedar rib and mahogany gussets, or a Douglas Fir rib and paper gussets. Sometimes I'd use one glue for the central portion and a different glue for the trailing edge. All tolled, I made about two dozen ribs; almost enough for an airplane.
Then I broke them.
I didn't break them all at once, I broke them serially, usually after allowing them to cure for a week.
ALL of the ribs were strong enough to support the 88 lb load. My basic 'over load' was a 25 lb bag of lead bird-shot. For the first few ribs I loaded 2-pound pigs of melted wheel-weights onto the pallet until I got something to break -- often something on my test rig :-) It was all very precise and scientific and a total waste of time because the only factor of any value was where the failure would initiate rather than how much weight it took to break it. Since I could only discover the where by adding mass, if the rib passed the basic test I simply stepped on the pallet whilst supporting myself on the edge of the work bench, allowing my weight to come onto the pallet until failure occurred.
Except for the Sitka spruce used in my 'control' rib, all of the wood used in this experiment was scrap of one kind or another. The wood used in the rib shown in the illustrations is from a 6' piece of redwood plaster lath, picked up on a job site. Ripped on the band-saw, the 1-1/2" wide lath yielded four pieces of 1/4" square stock; twenty-four linear feet. Each each rib requires about 16' of 1/4" square stock.
The paper gussets were dug out of the garbage. In addition to the paper shown in the illustrations I used paper gasket material, a manila file folder and paper from a box of 'Wheaties.'
As a general rule, paper's strength is proportional to its thickness. However, the paper packaging for bottled beer appears to be stronger than that used for canned beer. I suspect the former is treated with a resin to improve its resistance to water.
Titebond III was used on the rib shown in the illustrations. All of the glues I tried performed equally well in that none of them failed at the glue-line. Urethane glues offered some advantage because of the fillet formed during their expansion. Gorilla Glue expands about twice as much as Elmer's Ultamate Glue and often caused a mess because of it. Titebond III proved to be the handiest as well as the least expensive.
Temperature was not a factor during the experiment, ranging from over eighty degrees to a low of about 65 (Fahrenheit scale). Relative humidity ranged between less than 10% to over 50%.
In some of the photos you can see a whitish dust. That is the residue of the brush fires we've had here in San Diego county. The stuff is so fine that it got into the shop & house. Vacuuming seems to be the only way to get rid of it and even then, it clogs the filter. Sweeping or brushing simply drives much of it back into the air.
Although 'fiber' gussets have been used in the past, I'm a bit wary about trusting my life to materials obtained by Dumpster diving. Low cost (ie, less than fifty cents per rib) and universal availability are the main advantages, followed by ease of fabrication (ie, gussets may be cut with scissors).
When using paper (ie, fiber) gussets you should apply a gusset to each side of the joint.
During testing the rib was not glued to the stubs used to emulate the spars. Failure of the rib usually occurred at the joint immediately adjacent to the spar, typically at about 4.3g (ie, ~115lb). When thicker gussets (ie, beer vs soda pop) were used at those joints allowed the rib withstood >5g. Once I've finished exploring the materials aspect I will glue a test-rib to the fixture to determine the rib's ultimate failure strength.
-25 October 2007
Note -- Why use different saws? Because a lot of you don't have a table saw.
The 12" band saw fitted with a 1/2" 8-tpi blade was the best alternative but was still pretty awful compared to a table saw. If all you have is a portable circular saw then make it into a table saw.
To use a saber saw or jig saw you have to start out with wood that is already 1/4" thick, such as plaster lath. This stuff is pretty shaggy -- they leave it rough to provide a good 'tooth' for the plaster or stucco. You'll need to clean it up with coarse sandpaper or a plane.
-rsh, 26 Oct