Thursday, November 8, 2007

Chugger's Rib - III

A gentleman has taken me to task for daring to publish anything so stupid as an article about cardboard ribs. Unfortunately, he was unable to offer any cogent reason for my stupidity other than it simply wasn't done. So I will continue to seek answers for the stupid questions I ask myself, for while I've shared this information with you, it was never meant to be anything other than a private journey.


Stick-ribs are held together by gussets. Every joint usually gets two, one per side. The ribs on each end of the wing are often paneled with plywood -- a kind of over-all gusset -- to provide an anchor for the fabric. Those ribs will be discussed down below. But all of Chugger's other ribs are simply sticks & gussets.

Ideally, every gusset would be a different size and shape, reflecting the load at that particular joint. These are usually called polyform gussets and Fig 2 tells you why. But with twenty-eight ribs in the wing and up to forty gussets per rib (ie, twenty per side), if each is a different shape and size it imposes an enormous work-load on the homebuilder, not only in making more than a thousand gussets but in keeping them separate during construction, which for some builders can span several years. So instead of using unique, polyform gussets we come up with a universal gusset, meaning they are all of the same shape and size. Or nearly so :-)

The size of universal gussets is determined by the the joint which experiences the m
aximum load. For the rib shown, maximum strength is required on the joints adjacent to the front spar. But when we make all of the gussets that size they will be larger than needed for all of the other joints. And that means there will be some weight penalty. This penalty can be reduced by using gussets of different sizes but while the weight penalty falls as the number of sizes is increased, the more different sizes you have, the more it will cost to make your gussets, measured in both time and money.

Clearly, this is one of those cases where we need to compromise. Strength of course is never compromised so the remaining factors are weight, cost and time, with our choice of material lurking in the background. After a number of tests I settle on two sizes of gusset as a practical minimum, not counting the wedge-shaped gusset needed for the trailing edge.


Universal gussets may be square, rectangular, triangular or even segments of a circle.
but the choice of shape is usually based on your tools, time and materials, influenced by personal preference. And don't forget that the fuselage uses gussets too. In a popular design that used plywood ribs having lightening holes, the circular cut-outs were divided into halves and quarters and used as gussets on the fuselage.

As you can see from the drawing below I settled on a triangular shape, with the smallest size formed when needed by simply cutting one of the larger gussets in two.


Having amassed a carefully collected pile of packages that once held beer or soft drinks, I used a pair of 10" shears to lop them into pieces acceptable to a lever-type paper cutter. The paper cutter sheared the pieces int
o strips two inches wide and up to fourteen inches long. The strips were stacked two deep and chopped into enough two-inch squares to loosely fill a 2-lb coffee can. I've no idea how many there are but suspect I've more than enough for an airplane. Although I've not yet convinced myself that I want to build an airplane with 'cardboard' wings :-)


In either case, the ribs move through the jig in a stately procession completely aside from other activities and projects. The fourteen individual sticks used for the vertical and diagonal members have been cut in bundles to the required length but have not been given a detailed inspection until now. What I look for -- and discard -- is any stick having an irregular grain.

Fitting the sticks in the jig takes only a few minutes then comes mixing the glue (if required) and positioning the gussets, stapling them down by hand if I'm working after-hours and pneumatically when I'm not. All tolled, the process takes between twenty and thirty minutes and is done without interruption. Then the jig goes back on the rack and things are cleaned up & put away on a handy shelf.

A rib already glued & gusseted must have the staples removed, which takes about fifteen minutes. I've sharpened a scrap of steel for this chore, having found a pneumatically-driven staple embeds itself far deeper into a fiber gusset than a plywood gusset; deeply enough to render useless the staple-remover tools previously used.

When I have a spare moment the edges of gussets that extend beyond the upper camber of the rib are trimmed away using a razor if small or a flapper-disk in an angle-head grinder if large. No effort is made to fair the upper camber at this time; that task will be done using a belt sander, one rib at a time, just prior to assembly. Nor will any effort be made to block the ribs together on a set of false spars and sand them to perfectly-matching uniformity, since the resulting surface of the fabric-covered wing will be determined by the tape & fabric when the cover is stitched to the ribs.

Once a rib has been gusseted on one side, with the staples removed and gussets trimmed, it gets flipped over and sanded. This can take quite a while if using only sandpaper and a block of wood but takes only a couple of minutes with a powered block-sander. Since the wood is fairly soft the sandpaper is fairly fine. Particular attention is paid to the edges of the sticks, looking for any 'wild-grained' sticks missed in previous inspections.

I've used one such rib to experiment with attaching a plywood shear-web to the un-gusseted side. The wing requires at least one such rib per panel; two if the tip does not receive a bow. I used 1/8" luan for the ply and as you can guess, it came out heavy as hell. But hell for stout as well. The tricky bits are laying out the locations of the sticks so as to keep the staples aligned. But it also brought to light the 'handed-ness' of the rib in that you need both a left and a right, dictating the need for a second rib-jig of the opposite 'hand.' Fortunately, this is an easy matter to resolve since you already have assembled ribs to use as a pattern for the new jig. I will cover this in more detail in a future posting.

Once sanded the rib is ready to receive its second set of gussets. This is done with the rib flat on the bench and goes quite quickly. The rib is then put aside to cure.

I have been experimenting with ribs and gussets for a couple of years now. This has been a spare-time activity, recording what I've learned in a jumble of notes. Over that time our daughter has served a tour in Iraq, friends have passed away and a host of other things, problems as well as successes, have come and gone (including our recent brush fires.) The ribs were always there, used to fill spare time that developed when other projects became stalled for want of money, materials or a spare set of hands on the bucking bar.

I think the ribs will eventually find their way into a wing panel that will be covered with something inexpensive and then broken under bags of sand (always an interesting procedure). That should tell me if I've wasted my time with my 'cardboard' ribs :-)

-R.S.Hoover

3 comments:

Anonymous said...

All advances in understanding are preceeded by moments of heresy.

Dusty said...

Some people are not smart enough to know how smart you are! I look forward to each installment and I am not even going to build a plane.

sdustin said...

Bob,

Why go to the trouble of building a wing and breaking the ribs that way? If all you want to know is cardboard gusset strength vs alt materials, why not just build a few sets of samples that are representative of your joints; they are going to fail in tension or in compression since the rib is essentially a truss structure (pinned joints=0 moment). My bet is buckling from compression will give you enough out of plane deflection to delaminate the gussets if the gussets are what fail at all.

Ideally, it would make sense to me that the test program would be to determine whether the gussets give you enough strength at the joints to force the failure away from the connection and into the wood. The lightest gusset that does that optimizes your design, right?

Have you considered doing the statics and calculating based on a unit distributed load and rib geometry what the load is in each member of a rib is? Once you know the location of the maximum tension ad compression loads are you could build a jig and test those members with their respective gussets in the failure mode you expect.

The big concern I would have is in the variability of the material source, not necessarily cardboard vs. wood. I can't imagine that Tecate is running a sigma 6 QC on their 24-pack containers?

If the failure is dramatic enough, in my experience it's sure hard to see what broke first. Then again, it's kind of fun to just pile the weight on and watch what happens.

I'm probably not telling you anything you don't already know, but just in case...

Shaun