Wednesday, July 4, 2007

AV - Chugger's Progress - III


The ribs of a fabric covered wing are interesting things, their role in the lore of flight often as confused as the plot of a Russian novel. The ribs themselves are nothing more than sticks of softwood. Sitka Spruce is the preferred material because it offers an excellent ratio of weight to strength but it has become so expensive that most of the people who would like to fly can no longer afford to do so.

The shape of the rib is blocked out in a jig which holds the sticks in place while gussets are secured to the joints with glue and tiny nails. The nails are so small they are usually positioned with forceps or needle-nosed pliers before being driving into place with a small hammer.

This method of construction, using glue and nails to secure some kind of stringer to some form of plywood, dates from the earliest days of aviation and is known to provide light, strong and durable structures. But cults devoted to the particulars of this method have sprang upand their partisanship often obscures the basic goal. The Wood Cult insists the only stuff suitable is Sitka Spruce, the Plywood Cult says only a particular type of plywood can be used and there are similar cults for the nails, how to install them, remove them and so on. And we haven't even gotten to the Glue Cults.

In modern-day America flying is all about money. If you have a lot of it you can fly; if you don't, you can't. Low-cost alternative materials and methods are out there but to determine their suitability for airplane construction you're pretty much forced to emulate the Wright brothers and conduct your own experiments. But even that is discouraged. After all, there's no need to experiment now that we have all those helpful, friendly Cults. Of course, if you can't afford to follow the cultist's advice, why, then you simply can't afford to fly.

I don't agree with that. I'm not an engineer but I do quite a bit of experimenting in which I compare inexpensive materials against cultist-approved stuff. In doing so I've learned there's no such thing as a Bad Experiment. Things often fail in unexpected ways but there is knowledge to be gained from such failures since they tell you 'Don't Go There.' In fact, most of my experiments are failures in that the Cheap Stuff rarely equals the performance of the Expensive Stuff, with the rib shown in the opening photo being a good example. Although it will bear the same weight as an identical rib fabricated from Sitka Spruce and aviation-grade plywood it weighs more than twice as much, a clear Failure according to cultist criteria.

Of course, in a more practical vein, that horribly heavy Cheap Stuff rib, weighing-in at a massive 3.88 ounces cost less than fifty cents, whereas it's more elegant Cultist Approved cousin, at a svelte 1.8 ounces, cost more than $5. Or to look at it another way, the plane would gain three and a quarter pounds while the builder would save over $100.

Oddly enough, when it comes to Flying on the Cheap, cost isn't the only factor. See all those tiny nails? It takes about half an hour to assemble such a rib, regardless of the materials you use. Rich or poor, when building an airplane your time has no value but poor people tend to have less leisure time than rich people. Even if I could come up with inexpensive alternative materials the sixty-eight million or so Americans who earn less than the median $28k per year generally don't have the luxury of enough spare time to build an airplane. That meant methods of saving time as well as money are a valid area for experimentation. As a result of those experiments I have largely replaced aircraft nails and tiny hammers with other fasteners and different methods.

I recently described using a pneumatic brad-driver in a post to this blog that was meant to offer a bit of comic relief about the Evils of Experimentation. ( http://www.justaddgingerp.com/2007/05/av-n666rh.html ) I was surprised by the vehemence of some of the messages it produced.

Didn’t I know that aircraft nails are always 20 gauge? (about .025"). Pneumatically driven brads are always 18 gauge or larger. Didn't I realize that a brad that size would split a cap-strip of Sitka Spruce to flinders?

Fig 1 will give you some idea as to what all the fuss is about. The 20 gauge aircraft nails are above the pocket-ruler. Below it is an assortment of 18 gauge brads, which are rectangular in cross-section and about .035" x .048". That's a fairly massive spike to drive into a quarter-inch cap-strip.

I’ve been experimenting to find out how large the stringer or cap-strip has to be to prevent splitting. But not when driven into $7.95 Sitka Spruce, which is actually rather forgiving when it comes to nails. I was more interested in learning what happens when you use pneumatically-driven fasteners in the far more brash $1.98 Douglas Fir and even lowly .89 cent Western Hemlock ( the stuff Bernard Pietenpol used for his spars ).

The fact the brads are rectangular caused me to wonder what would happen if I took the grain of the wood into account. So I made a test-spar -- just a pair of stringers with a shear web of luan ply -- and then broke it under controlled conditions. (That's the spar mentioned in the earlier article.)

My mention of foamy glue, meaning polyurethane, caused the Glue Cult to point out that it was not certified for use in aircraft and suggested I try T-88, which was ‘stronger.’ The fact T-88 is also uncertified somehow escaped their notice, as did the cost and availability. In fact, most of my experiments are with Weldwood ‘Plastic Resin,’ which does happen to be a certified adhesive. But the point missed by the Glue Cultists is that all modern adhesives have a higher shear strength than any of the softwoods normally used in aircraft. While there are many reasons to use a good epoxy, strength isn’t one of them.

The tricky bit with a water-mix glue such as Plastic Resin is that that it requires a significant amount of pressure due to the water causing the wood to expand. Polyurethane glues require a similar amount of pressure because the glue itself expands whilst curing. When properly clamped and cured each of those adhesives is stronger than the wood and while ‘Plastic Resin’ is far less expensive, polyurethane is more convenient to use, especially when doing spare-time experiments.

Some of my experiments were to determine how many brads are needed to achieve the required amount of pressure, along with what type of brad was best for different combinations of plywood & stringers, and how small the stringer could be before the orientation of the brad was significant.

Even the pneumatic brad-driver caught a bit of hell from the Cultists. Nails were meant to be driven by hand; pneumatic tools were evil incarnate, the airplane would fall apart and famine and plague would surely follow. Clamps are apparently okay. Ditto for wedges. But nobody used pneumatic brad-drivers. (Splits the wood, you know.)

Oddly enough, no one mentioned nailing strips, a standard method when applying plywood skins. Which caused me to wonder if the people who took the time to write were merely expressing Conventional Wisdom – stuff they’d heard was bad, rather than stuff based on their own experience.

Another fact offered by the Cultists was that aircraft nails were always removed. Because of the weight, according to one; because they work out of the wood, according to another. And of course, they always rust.

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Perhaps the most important thing I've learned from my experiments is to ignore the Cultists. I'm sure their interest is well-meant but I'm equally sure it is misguided. The goal is not to build an unsafe airplane but an inexpensive one.

As a point of interest I've found most of the information offered by the cultists, while originally based on a kernel of truth, to be incorrect for most modern-day materials. For example, pneumatically driven brads come in sizes even smaller than aircraft nails and are available in stainless steel as well as bronze. And they are less expensive than aircraft nails. (My grandfather called them cigar box nails.)

Below you'll find a couple of photos of a test-piece assembled from hemlock, luan plywood (ie, doorskins) and 23 gauge pneumatically-driven brads 5/8" long. When driven atop a steel plate the brad neatly bends itself over and locks into the wood in a manner remarkably similar to the copper brads used on a clinker-built hull. On this end of the piece the spacing was rather random but for stringers up to 3/8" wide the glue-line exceeded the strength of the wood when fastened with one brad every two inches.

Chugger's fuselage is little more than a tapered box, plated with plywood forward, covered with fabric aft. The typical structural member is 3/4" square hemlock or fir. To ensure adequate strength I made samples of the various joints used in the fuselage and tested them to destruction. Experiments included plain joints, joints having filler blocks, joints having gussets on one or both sides, and joints having both filler blocks and gussets. The latter type of joint lead to some interesting problems in that I could not get it to fail with the test equipment I had at that time. When I was finally able to load them to the point of failure, the nature of the failure was unlike anything I'd seen before. A number of iterations proved the unusual mode of failure was not related to the test sample, which lead me to digging through books on structural engineering, where I eventually found that mode of failure described. Over all, it has been an interesting is rather slow education.

A basic tenet of flight is the ratio of strength to weight but materials having the optimum ratio, such as Sitka Spruce and aviation-grade plywood, are expensive. When building on the Cheap, using commonly available materials to achieve the required strength causes the resulting structure to comes out heavier than one built of aviation-grade materials. Experiments tell us how far we can go with our weight reduction efforts before the structure becomes unsafe. Experiments also show how the use of non-traditional tools can result in a significant savings of time when assembling a complex structure.

The Chugger will be inexpensive compared to virtually all other designs. And it can be assembled rather quickly. The result may not be very elegant but it will not be unsafe.

-R.S.Hoover
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