Saturday, November 17, 2007
Chugger's Rib - IV
Does everyone understand that the strength in the joints of the sticks in a rib is due to the gussets and not the contact between the sticks? If not, you need to put on your thinking cap and do a few experiments until you understand how a load in the sticks behaves when it encounters a joint. You don't need to build a rib to teach yourself what is going on its joints. Use whatever is handiest -- match-sticks, toothpicks or whatever -- to make a basic T-joint and then break it. Make one without a gusset, then one with a gusset. Then try replacing the gusset with a thread or a piece of toothpick.
When you do this you will see that as you put a load onto the leg of the T, one side of the gusset will see that load in tension whilst the other side sees it as compression.
There's a bunch of common-sense assumptions here; that the load is imposed in-line with the axis of the rib and so forth... that I won't bother to go into; the key point I want to get across is that both tension and compression are present.
If you've made a sample joint using thread in place of a gusset, when you broke it you will have seen that while the thread does okay in tension it's worthless when it comes to compression, proving the old saw that you can't push a rope :-)
Cardboard doesn't do very well in compression either, and that's really what this posting is about. In fact, the only reason my cardboard-gusseted rib is able to bear its designed load has more to do with the number of bays in the truss and the spacing of those bays relative to the intended load, in that the load at any given joint is low enough so that it does not cause buckling on the side of the gusset subjected to compression.
This wasn't by accident. As mentioned in a previous post I've been messing with ribs for a couple of years now and you may have noticed that I've not bothered to post the results of my early experiments. Which were pretty awful :-) But even failure is data of a sort and I eventually came up with a rib that should do the job, even though the gussets are nothing but 'cardboard.'
Of course, a four-bay rib is heavier than a three-bay rib, but I assumed at the outset that using low-cost, commonly available materials would impose a weight penalty.
A second purpose of the experiments was to discover just how big that penalty would be, since another design-constraint was that the bird had to be able to fly -- safely -- behind a converted Volkswagen engine. And despite the hype from the hucksters, no matter what the VW's displacement might be, it's maximum sustainable output is only about forty horsepower, about the same as the Continental A40.
So what is that weight penalty? About four-tenths of an ounce per rib; about three-quarters of a pound for the whole wing.
Keep in mind, the normal or unaccelerated load on the wing is less than seven pounds per square foot or about 30 lbs per rib, and about a third of that will appear in the plywood D-cell that makes up the leading edge, leaving about 20 lbs to be dealt with by the remainder of the rib.
(Photos and drawings to follow)