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.
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Newsgroups: rec.aviation.homebuilt
From: Veeduber
Date: Sun, Oct 26 2003 1:51 pm
Subject: Drywall Gussets
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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.

Messy, eh?

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.

-R.S.Hoover
-26 October 2003

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.

CONCLUSIONS

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.

-R.S.Hoover
-25 October 2007
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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

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Brush Fires

We've had some. A lot of you asked how we were doing, others wanted to know why a California brush fire was such a big deal compared to their locally grown variety. I've posted a collective response on RAH and RAMVA.

Being a native Californian we're probably better prepared than most California families, although the main emphasis of our preparedness has been toward earthquakes. We keep the Important Stuff in one drawer of a filing cabinet and each of our vehicles carries a Bug-Out Kit that includes water, sturdy shoes, toilet paper and so on. I've also got a Medical Bag and that's what I want to talk about in this article.

Designed primarily for medical emergencies resulting from an earthquake, the medical bag focused on treatment of lacerations and fractures. But after breathing brush-fire smoke for the last week I've added a few items.

Saline nasal spray
Gauze face masks
Bronchial dilator
Expectorant
Oxygen

The saline nasal spray is commonly available. In a pinch you can make your own or even use plain distilled water. By preventing your nasal passages from drying out your body can filter out a lot of the crap suspended in the smoke before it gets to your lungs. And there is a lot of it -- your nose will run like a river of tar.

The face masks are more effective than a T-shirt or bandanna, although either will serve to keep out the particles of soot and charcoal. But only an industrial-grade mask can keep out the particles that clot your nose and end up in your lungs. And they don't make industrial-grade masks for children.

Smoke from a brush fire contains all manner of chemical contaminants, many of which trigger allergic reactions that can cause the air-ways in your lungs and bronchial tubes to swell. Bronchial dilators are medications that reverse the swelling. Albuterol is one such drug. It comes in an inhaler. You'll need a prescription for it, unless you buy it from an off-shore source.

An expectorant is anything that makes your lungs juicy and causes you to cough. (Breathe enough smoke, besides making you vomit, you'll be hacking up lungers that look like raisins.) For a first-aid kit the stuff you buy over the counter, such as Mucinex, should work well enough.

Oxygen is just that. The same thing you use in your plane. I've got an E-pax kit that includes a D-sized bottle of oxygen (about as big around as a thermos but two thermos-bottles in length), a regulator and two oxygen masks. Once you buy the bottle you can get it refilled at the airport ($) or at most welding supply houses. (Hint: Many welding supply companies are also the local source of USP-grade oxygen.)

If we don't learn from our experiences we're doomed to repeat them. This last week has been hell and the nearest fire was ten miles from the house. Even so, the smoke just about drove us out. Maybe this message will save a few of you from having to learn this lesson at first-hand.

-Bob Hoover

PS -- I'm depending on your common sense here. Read the labels. Learn to adjust the oxygen regulator.

AV - Chugger's Tail

Back in May I mentioned I was using Pete Bower's Fly Baby as the basis for Chugger's tail. At that time (ie, 16 May 2007) I uploaded the drawings for the rudder and vertical stabilizer box-spars, which use 1/8" plywood shear-webs and 3/8" square cap-strips (as opposed to the 1/2" square cap-strips used in the original design). Chugger's tail is smaller in both height and span but I've tried to retain the elegant curves Pete used.

To ensure the four shear-webs would be identical, after rough-cutting them 1/8" door-skins I tacked them together on their center-lines and sanded them to final size as a single 1/2" thick slab. Since door-skins have an inner and outer face, when stacking them I made sure the rough-sawn blanks were oriented with the inner faces against each other.

The horizontal stabilizers use a built-up C-section for both their spars and diagonals whereas the elevators use box spars. As you can see in the drawings the diagonals are of a different size than the spars and are best handled as a matched pair. The six shear-webs for the spars are identical in taper but the elevator shear-webs are shorter. I treated these as matched sets of three, cutting the elevator shear-webs to length after sanding the stack to the proper taper.

Design by Concensus

Back when the world was young and I still had hair the Navy hired a gaggle of eggheads to contribute to the design of what eventually became the Spruance-class of destroyers (i.e., DD-963 class). At that time I was the Leading Chief of the computer shop for Pac Fleet's cruiser-destroyer force. I was told to give the eggheads access to anything they wanted in the way of maintenance and repair data, which I did with a cheery aye-aye, sir.

Marvelous stuff, watching those eggheads at work, doing their computerized statistical analysis of equipment failures, tracking everything back to the manufacturer on one hand and the Navy schools on the other.

The product of their work was a list of recommended equipment to go into the new ships; only the best stuff as determined by its failure rate, required maintenance man-hours, mean time to repair and so forth.

Which was all bullshit, unfortunately.

At that time (early 1970's) ComCruDesPac had about 137 ships. The analysis covered such things as electric motors, pumps, air compressors, ammo hoists and so forth, the ancillary systems that are the glue of a modern-day warship. (The hull design and the turbine powerplants were determined by other groups.) The objective of the study was to determine the best of that equipment and on the surface, their methods of analysis appeared valid. But in providing them with data I noticed that while all destroyers had high-pressure air compressors (for example) some of them had never failed. (Not many... four, I think.) Same thing for the other components. All of the ships used a certain type of gear-head motors but a few ships had never reported any problems with them. Which brings up a point worthy of mention.

Even though built to the same plan, vessels within a given class are not identical. The ships are built at different yards and while their specs were identical their equipment came from a variety of manufacturers. In the case of electric motors for example, while most of the ships used motors from General Electric or Westinghouse a few of them had motors from manufacturers I'd never heard of. The key point here is that some ships had never reported any form of failure for certain pieces of equipment.

The bottom line is that the study failed to consider the possibility that some equipment had never failed. Their final report identified only equipment that had failed, giving high marks for designs and manufacturers that failed the least often.

Which completely ignored the Really Good Stuff.

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So what's all that got to do with airplanes? Quite a bit, when it comes to home-builts.

A fairly common thread on various aviation-related mailing lists and newsgroups is someone polling the subscribers in hopes of determining the ‘best' ...whatever. The best way to paint a spam can; the best brand of tire; the best vacuum pump and so on. Which gets down-right scary at times. (One such poll decided that the ‘best' aluminum was 6061 :-)

Polls and surveys, and the methods of statistical analysis that supports them, are valid tools. But only when your sample is an accurate reflection of the population being polled. Ask a room-full of pre-schoolers to define a balanced diet, don't be surprised if the answer is graham crackers and milk. In a similar vein, wood makes the best fuselage (according to builders of Pietenpol ‘Air Campers'), welding is easy (according to experienced weldors) and flying is inexpensive (according to people earning $100k p/a or more). In the case of the New Ship Design Study Group they failed to include the entire population of ancillary equipment, inadvertently limiting their investigation to equipment having a history of failure. (They were aware of the others but deemed them ‘statistically insignificant.’)

SO WHAT'S THE POINT?

The Internet offers unprecedented access to information but does not provide any means of determining if that information is valid. Indeed, within the field of home-built aviation only a small percentage - - probably less than five percent - - of the available information is valid and even then, only in a particular case. The remainder is either skewed by commercial interest or is a reflection of ‘conventional wisdom,' wherein the poster is simply parroting something they have heard.

Common sense has become remarkably uncommon stuff in modern-day America. Given the risk inherent in rising above the ground on wings I believe the wiser course is to treat all information on the internet as invalid until you can test it yourself. Fortunately, with a technical subject such as aviation the required tests are fundamental and well defined. For the homebuilder, especially those lacking an engineering background, the tricky bit is devising methods of applying such tests to their particular situation.

-R.S.Hoover

PS - - So what happened with regard to selecting failure-prone equipment? I've no idea. By the time the first of the new class slid down the ways I'd been retired for a number of years. But it's interesting to note that several of the Spruance-class have been scraped after barely twenty years service. (Navy ships are designed for a minimum service life of thirty years.)

I identified the Really Good Stuff aboard our own ships and submitted a report on the matter, producing a minor controversy with regard to maintenance. Sailors know what I'm talking about and it really doesn't apply to anyone else.

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(The above was originally posted to RAH in 2004. Recent posts to this blog [Chugger's Rib, 4 July 2007 and Chuggers Progress - III on 3 Sept 2007] have generated quite a bit of mail from folks who are upset by my failure to use Sitka Spruce, aviation-grade plywood and T-88 glue. The whole purpose of the Chugger files is to explore the use of less expensive materials that are commonly available. This isn't really a new idea. During World War II aviation was forced to use other woods, different glues and so on. History shows the planes (and gliders) flew just as well. Unfortunately, there isn't a lot of quantified data for those alternative materials. )

AV - Chugger's Rib

Labor Day, 2007.

Weather has been hot. A few miles north of us a mountain lion was seen drinking from a swimming pool and the coyotes are staying close to the few creeks that still have water. To hot to work in the shop, even with both fans going. Even the breezeway is pretty warm.

Too hot for welding and too sweaty for working on the fittings, I turned my attention to Chugger's wing. I have tentatively settled on the 4415 airfoil and needed some ribs for testing. I converted the 4415 coordinates into a rib drawing of the required chord, laid a few lines across it for alignment and printed it out. You'll find it in the Wing folder in the Chuggers Group, along with a pattern for the nose rib.

I didn't have a suitable piece of 3/4" plywood for the rib jig but did have some particle board that was wide enough. I don't like to use particle board for jigs because it's nothing more than thick paper and warps like a bitch but I glued a couple of stringers across the bottom and after the glue had cured, soaked it good with dilute varnish. That was a couple of days ago.

The several sheets that made up the pattern were trimmed along one edge using a straight-edge and razor. The jig board was given a coat of un-thinned varnish and each page of the pattern was painted with varnish on its back-side. The pages were then stuck to the varnished jig-board and aligned. Bubbles were chased to the edge of the sheet with my thumb and the whole thing was left to dry. But if you've never used this method, don't. It happens to be a quick & dirty method but varnish isn't a very good adhesive when applied to typing paper. It will hold the paper in position long enough to install the bits & pieces that will hold the rib's sticks in place. In doing so it will also fasten the pattern to the jig board. The whole thing will then get a coat of Deft Satin Finish Wax, which I understand is no longer available in the USA due to the tree-huggers. (The advantage of a wax finish is that nothing sticks to it.)

At this stage the thing looks like hell but it should work okay and only took a few minutes, if you don't count the clean-up :-)

I'm going to try using an Ison-type wing with wooden drag/anti-drag struts instead of wires or rods. The red hatch-mark is where the diagonal struts will pass through the rib. I'm also going to try building the ailerons in situ following the lead of Leonard Mulholland. I've not yet decided how to do the leading edge. I'd like to use 1/16" (1.5mm) birch ply but it is fairly expensive. Unfortunately the less expensive foam & fiberglas alternative, of which I've already built several samples, is about 3X heavier than the plywood.

I'm still tinkering with the leading edge but it looks as if I'm going to have to bite the bullet and run up to Corona (ie, Aircraft Spruce) for a couple of sheets of 1/16" ply.

After posting an article about building stick ribs in which I used 1/8" doorskin gussets attached with 1/4" aircraft nails and Weldwood 'Plastic Resin' glue I got several messages from people who found it impossible to use such small nails, having found they couldn't hold them with their fingers. The secret is to not hold them at all but to use the magnetic end of your tack hammer to pick them up and drive them into place. Unfortunately, that takes a bit of practice and since most of you are first-time builders I'll try using staples and/or pneumatically-driven 23 ga. wire brads.

The wing span will be a tad more than 28 feet, dictated by the available work-space (ie, about 15'). Chord is 56" so the wing's area will be approximately 125 square feet for a gross weight of 850 lbs, giving a 1-g loading of about 7 lbs per square foot. At 3.3-g that's about 22 lbs. With a rib spacing of 12" that's about 100 lbs per rib. Given the lift distribution of the NACA 4415 at its maximum angle of attack that means the portion of the rib between the spars will see about 80 lbs, the trailing edge will see almost no load at all and the remainder will be concentrated near the leading edge. One reason for cobbling-up a rib jig at this stage is that I want make and then break a few ribs to ensure they'll be strong enough.

According to the classic design formulas as published by Raoul J. Hoffmann (and others) in the 1930's, the aileron should be about 40% of the semi-span in length and 20% of the chord in width. As with the leading edge structure, this is another area I'm still tinkering with. If everything works out I'll post the required patterns in the Chuggers file archive.

-R.S.Hoover

PS -- Be sure to read Chugger's Progress - III posted on 4 July 2007. This post (ie, Chugger's Rib) produced a couple of comments that made it pretty clear their authors were not aware of what has gone before.

AV - Crankcase Basics - II

Before wasting any time here, go read:
http://bobhooversblog.blogspot.com/2007/05/av-crankcase-basics.html

...especially the part where I say:
you will need nuts and washers and bolts to fasten the case studs and parting-line. Here again, there are kits available but most are the shoddiest stuff imaginable and price is no guarantee of quality. The nuts and washers may have a wash of zinc plating, good for at least a week’s exposure to the weather. Or they may not. And you can toss the ‘exhaust nuts.’ They are copper plated steel. (The good stuff is bronze.) Before you can use any of this crap on an engine you must provide it with some form of corrosion protection. If you don’t, not only with the nuts rust to the studs, you’ll see galvanic corrosion between the washers and the crankcase that will eventually cause the fastener to loosen.

Although any after-market VW retailer will be delighted to sell you that shoddiest stuff imaginable, the hands-down winner is J.C.Whitney because they usually charge more than most.

Back when I had hair I wrote an article ('Cows') explaining why it was a good idea to not buy VW parts from J.C.Whitney. Fig 1 offers a nice example of why this is still true. In the picture you can see the supposed 140-piece contents of JCW's catalog #xxx380749. (The 'xxx' is the catalog prefix which changes from minute to minute but the basic number stays the same.) A fellow chugger paid $15.99 plus shipping for what you see in the photo only to discover that most of the fasteners were unusable or not needed. (As of 8/23/07 the price is $17.99 making it even worse.)

Here's why: See those sixteen M10 nuts? (Lookit Fig 1A) Didja notice the M8 head stays illustrated in the 'Crankcase Basics' article? The M10 nuts & washers are for a pre-1971 crankcase, which you are not using if you're building your engine an a Universal Replacement Crankcase. And if you are starting out with a used crankcase then the odds are you already have a perfectly suitable collection of M10 nuts & washers.

Indeed, other than the six M12 nuts shown in Fig 1B everything else shown in Fig 1 is available from the local Borg for significantly less than JCW prices. But even then, the JCW parts are not the sort of stuff you want to use when assembling a good engine.

A point often overlooked by the shade-tree types is that several VW fasteners are also oil seals. The washer and in some cases, the nut, must be prepared and installed in such a manner as to prevent oil from leaking out around the fastener. The best example of this is the four lower head stays on each side of the engine that are terminated inside the valve galleries but this rule also applies to the six M12 nuts, the pair of M8 studs adjacent to the #1 cam bearing and the two M8's that support the #4 Main Bearing.

Fig 3 shows the type of M12 nut used on later-model Volkswagen engines. The red ring is an elastomeric seal that bears against the heavy washer which itself is bedded in Permatex or other non-hardening sealant, another of those 'unimportant' details casually disregarded by non-professional engine builders, most of whom insist that it's normal for the VW engine to leak like a bitch. Being a stock VW part, the nuts are commonly available but their price varies wildly from an honest thirty-five cents or so to more than a dollar from the typical Screw-the-Newbie suppliers (who always seem to run the biggest ads :-)

I'll cover the proper application of fastener sealants at the appropriate time. Or you can dig it out of the VW factory service manuals.

But the most regrettable failing of such hardware kits is their failure to provide real exhaust nuts. What you get is a regular steel nut with a wash of copper plating, guaranteed to last for at least thirty minutes before welding itself to the exhaust stud. What you want is a bronze or brass nut, installed upon a bronze, brass or copper washer with a lavish application of anti-seize compound. If you have a small lathe these are easy to make from bar stock but they are also available from the Usual Suspects.

Fig 4 shows a baggie of brass exhaust nuts sized to accept a 12mm wrench, allowing them to be used on the lower exhaust studs without interference when using a custom-built exhaust manifold as is common with aircraft engines.

To me, an engine is a forever kinda thing. There is no Magic Bullet. The reliability of any machine is nothing more than a reflection of paying the keenest possible attention to the smallest details of its assembly. Using the correct fasteners is a big part of that.

-R.S.Hoover