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.

-R.S.Hoover

(Photos and drawings to follow)

VW -- More on Air Filters

(written in 2002)

I've just finished reading the thread on air filters. If you haven't read it, you probably should. Marvelous stuff. Laughed my ass off.

Yes, there is a standard for automotive air filters. SAE J726 (or maybe 826... there's more than one).

Read the specs? Okay, here's pop quiz: Other than the Beta test (ie, ratio of particles that make it through a filter as compared to the number of particles which go into a filter), have you ever wondered how they arrive at those "efficiency" figures? Come on, think about it. No filter is 100% perfect for all sizes of particulate contaminant. So what are they comparing things to? What is the nearest thing they have to a perfect air filter?

Would you believe it's a multi-stage kinetic type filter, usually having a water-bath catchment... covered by a thin film of silicon or mineral oil to reduce evaporation?

Although we're talking laboratories here, the principle is the same as the oil bath air cleaner on your veedub. Not small, though; you can walk inside some of them. And except for HVAC applications they're almost always used with pre- and post-filtering, including electrostatic precipitators. (Anyone who has ever worked at a chip plant knows about this stuff. Even old sailors, like me because the government has its own chip plant, inside the fence at Ft. Geo. Meade, home of the world's longest indoor shooting gallery... and one of the cleanest 'clean rooms' on the planet.)

But the subject of comparison also applies to the air filters for cars and the point a lot of folks missed is the fact treated paper filters (ie, a non-kinetic labyrinth-type filter) are compared to an oil bath filter (which typically scores about 99.9%).

And those marvelous K&N filters? Last time I checked, they're compared to paper filters. (Go on; lookit up; K&N has a web site and is justifiably proud of their product, which works at least as well as a paper filter, so long as it receives the proper periodic maintenance and liberal doses of K&N's magic elixir.)

All of which means that clunky, old fashioned oil bath air filter on your bug or bus is still a hands-down winner... for anyone who cares to do a bit of reading :-)

As for all that 'flow-rate' stuff... Anyone care to take a WAG at the flow-rate for your 1600cc engine, roaring along at 3500 rpm? No? Howzabout a 2550cc big-bore stroker turning five grand? (Hint: Be prepared to be underwhelmed.) But bigger is always better, right? So more flow has gotta be good, right? Horseshit. The fact a particular filter can flow more air doesn't mean your engine can use it. Those counter-top ‘comparison’ units are just a sales ploy; a kind of IQ test for the technologically naive.

Read the specs. Flow is relative to the pressure differential and restriction. The bottom line is that your oil bath air cleaner does just fine, folks. Always has and always will, so long as you give it the maintenance it requires. For stock engines an oil bath air cleaner has no problem providing more air than the engine can use. (If you think about it for a minute you'll understand why that has to be so.) In fact, the stock oil bath air cleaner works just fine for engines up to about 2200 cc so long as they don't spin over 5000 rpm, a fact you can work out for yourself with nothing more exotic than a #2 yeller pencil and basic arithmetic. Working it out will also give some idea how much you'll have to shorten the service interval when you use an air filter designed for a small engine on a big one. And the service interval of any air filter is a critical factor in determining its practicality.

Indeed, the service interval is one of those things the tea-cozy crowd doesn’t like to talk about. As with any labyrinth-type filter, their effectiveness is a function of surface area. Under harsh conditions, where you might need to service an oil-bath air cleaner as often as once a day, you may need to wash & re-oil a gauze & window-screen air filter every few minutes... which is why you don’t see such filters used for real-world applications.

Don't take my word for any of this. After all, nobody is right all of the time :-) Think for yourself. Go look it up. Work it out. It's your ride and it's entirely up to you what kind of filter you use. You're the Mechanic-in-Charge, not me.

In fact, lemme repeat that: You are the Mechanic-in-Charge. Making the right choices is up to you. All I've done here is express an opinion, albeit one based on experience. In effect, I've described a particular path. The decision to explore it -- or not -- is yours.

The only reason I've bothered to say anything at all is because automotive engineering is not subject to the democratic process. Having a big mouth doesn't automatically make you a good singer, nor a loud voice a good speaker (although it helps :-) The fact someone posts a thousand messages a month does not mean the messages contain anything worth reading. Apply that logic to the fact all your buds are using gauze & tea strainers for air cleaners and you'll see it only means its a popular idea, not necessarily a good one. The task of determining the merit of something is up to you. And I'm not just taking cars here. Every person is capable of error. That fact extends to every organization as well because organizations are nothing more than collections of people. And if you think an individual can screw up, our personal faults pale by comparison to the outright evil of which groups are capable. (And yes, you can look this up too if you wish. Although since 9-11 I've got a hunch most folks can figure it out for themselves.)

The wiser course, in my opinion, is to always think for yourself, even if doing so sets you apart from the majority. Because when push comes to shove, it's your life. And your ride. Not theirs.

-Bob Hoover

PS -- Oil bath air cleaners fell into disuse for passenger cars because of economic factors, environmental issues and the shear size of the damn things.

The size issue had to do with their height. To work properly an oil bath air filter must have a certain minimum height. If you had a 'tall' engine yet wanted a low hood line you had to move the oil bath air filter over to the side, as Volkswagen did with the Karmann-Ghia, Hudson with their dual-carbed 'Hornet' and others. But doing so increases the parts count and complexity of the system and such things carry an inherent increase in maintenance cost. Which leads to...

The key economic factor in the demise of oil bath air cleaners in cars had to do with the time needed to properly maintain them. I can't remember the Flat Rate Manual for servicing an oil bath air cleaner but it was on the order of 0.5 m/hr. No big deal when making 'good money' meant a dollar an hour. But nowadays spending half an hour to clean an air filter would be economic suicide for a dealer paying his line mechanics $37.50 an hour plus full medical, dental and so forth. (What's it take to change a treated paper air filter? Maybe a minute... if you've never done it before. Less, for an experienced mechanic :-)

The environmental issues with regard to oil bath air filters wasn't so much the oil you poured out of the thing. Hell, recyclers will pay you for the stuff. What really sets a tree-hugger's hair on fire is seeing that drum of sandy sludge -- the scrapings out of the bottom of the oil pan -- accumulate over in the corner of the lubrication bay. A high percentage of the particulate matter found in the air above most roadways is toxic. And you've just concentrated 55 gallons of the stuff (and usually hauled it off to the local dump). The solvent-filled parts washer you ran it through was another E-ticket ride for tree-huggers. Not only did it generate its own quotient of sludge, in normal use it pumped a few pounds of hydrocarbon aerosols into the air every day. By the time you've shifted to a water-based high pressure detergent washer, or added a vapor recovery system to your solvent tank and negotiated a HazMat contractor to collect & dispose of the sludge, servicing an oil bath air cleaner has become a very pricey proposition.

There's plenty of justification for doing away with oil bath air cleaners. But their ability to clean the air wasn't one of them. Indeed, such filters remain a standard method for many industrial applications, as a bit of research will show. - rsh

VP -- Overheating

You'll find the original version of this thread on the Usenet Newsgroup devoted to air cooled Volkswagens. It began with a complaint of overheating by someone who done a lot of bolt-on modifications to their engine. They asked the Newsgroup if adding an after-market oil cooler would solve the problem. The general opinion was that it would not, since the engine had been heavily modified even through it still had the stock displacement, which is where we pick up the tale...



>Given that the items are external, I claim it as stock.

----------------------------------------------

Dear D.....,

You've managed to miss the point.

Normally, only about 15% of the engine's waste heat will appear in the oil. If the engine is fitted with the dog-house type oil cooler and all other cooling system components are in place and functional, the oil cooling system provides about 120% of worse-case capacity.

The point you've missed is that elevated oil temperatures are a symptom rather than a problem. Normally, elevated oil temperature simply means the engine is being operated outside of its designed envelope -- the load is too heavy for the speed or the speed is too high for the ambient air temp, or both. And yes, on a hot day this can occur with just you on-board. The cure is to simply take your foot out. Alas, on a southern California freeway that can earn you a ticket for obstructing traffic, which is why a lot of VW bus owners have learned to fly by night :-)

The root problem is whatever is causing those elevated oil temperatures -- and that could be anything from low tire pressure to a loose nut on the steering wheel :-)

Ever had the measles? Your body temp can hit 105. Dunking you in ice water is a sure-fire treatment of the symptom -- it will bring your temperature down. Of course, the shock usually killed the patient :-)

Audit your cooling system. Every part is critical. Check to see that the deflector plate is installed on the underside of the heads. If running after-market dual-port tin the odds are the gull-wing deflector above the manifold hole is missing. Check your spark plug seals -- any loss of air pressure guarantees a loss of air flow.

Check your fan belt. And the diameter of your pulley. (Those wunnerful 'power pulleys' guarantee the engine will run hot.) Reach around behind the blower housing and feel every blade in the fan to insure you haven't picked up leaves or other debris.

Check your oil pressure. Elevated oil temp is one symptom of a worn-out oil pump.

Check your brakes, tire pressure and alignment. Low tires, draggy brakes and improper alignment all demand more work from the engine. The car may be doing 65 but the engine could be doing 90.

Ditto for the clutch & tranny. When was the last time you replaced your tranny lube? (Recommended interval is 2 yrs or 24,000 miles.)

What kind of shape is your engine in? Yeah, I know -- perfect :-) But a lot of 'perfect' engines show up here at the shop with complaints of over-heating. It's not uncommon to find excessive blow-by, indicating worn rings or valves. And excessive blow-by dumps a huge amount of excess heat into the oil. After-market carbs are one the main reasons for accelerated wear of pistons & valves, mostly because of their inadeqaute air cleaners. It only takes about a teaspoon of dust to trash an engine and anyone who has cleaned an oil-bath air filter knows the engine sucks in ten times that much between oil changes. The other failing of after-market carbs is running too rich a mixture, either because of inadequate manifold heating or running a mechanical advance distributor.

----------------------------------------------------

Conventional Wisdom as espoused by the magazines and after-market retailers say that if your oil is running hot you gotta spend some more money by adding one of their sooper-geewhiz oil coolers, with a fan yet, and mebbe one of those ohsokewl thermostatic valves that are guaranteed to leak. And lotsa hose. And fittings. And brackets. And other neat stuff.

Now the oil stays so cool it's kewl. Of course, the root problem -- whatever it is -- is still there, ticking like a time bomb. And when it goes off those wunnerful folks will have some more really kewl fixes you can buy. In fact, the experts have an answer for everything! The only question is, can you afford it?

---------------------------------------------

Hot oil is a symptom. What you need to do is deal with the problem that produced that symptom. Adding an oil cooler is like trying to cure cancer with aspirin; the pain may go away but you're still gonna die.

-Bob Hoover



...then John writes:
>what I've found
>is that most air-cooled VW engines are filthy on their exterior.

(ie, as a major contributor to overheating)

------------------------------------------------

Dear John (and the Group),

Good point.

A friend recently asked me to look at the air-cooled engine on a paving machine (!). I don't know anything about paving machines. Nor those nifty two-cylinder Honda engines. But he'd gone through two engines in two years and it was pretty obvious the Honda guy just wanted to keep selling him new engines at about two grand a pop.

The engine's fins were covered with fur.

Or what looked like fur. It was dust, glued to the aluminum fins with hydraulic fluid.

Actually, it didn't look all that bad... you could brush it off, which they did to the inlet grill every morning. But it would come right back because there was lots of oily vapor getting sucked in by the cooling fan and the machine spends it's life in a dusty environment.

It reminded me of a 40 horse with a leaky oil cooler -- the famous Volkswagen Toaster :-)

The really funny part was the reaction when I pointed out the layer of dust (which makes a dandy insulator). My friend doubted that such a thin layer of crud could be enough to over-heat the engine. But it was easy enough to rig a duct to insure it sucked in only outside air... and without the oily vapor, the aluminum fins stayed clean... and the thing didn't overheat. (Truth is, I think the thing originally came with such a duct... that someone had removed to make it easier to get at the hydrualics.)

Liv & lurn -)

-Bob Hoover

PS -- My personal favorite was the doctor's Porsche and the engine filled with leaves from a Chinese Elm that shaded his reserved parking space. Even when you showed him the cylinders packed solid with leaves he kept insisting they would blow right on through; that SOMEONE must of packed them in there on purpose. Makes you wonder about his diagnostic skills, eh? :-)


Then someone asked:

> What do >mean with the gullwing tins?

------------------------------------------------

Look inside the stock DP cylinder-head tin-ware. There is a gull-wing shaped deflector that directs the blower's output toward the exhaust valves.

Making sure the deflector is a fairly tight fit against the fins immediately adjacent to the two upper-middle studs is a standard procedure when mantling a DP engine. Without a well fitted deflector the cut-out for the DP manifold end-castings is little more than a hole through which your cooling air can escape.

Your OP readings look okay. As a test, change to straight 30W and compare the results. No difference is good, anything else is a clue that needs to be checked out.

It would probably be a good idea to rig some instruments to check the stoichemistry of your mixture. Same for your ignition timing and compression ratio. The fact an engine runs does not mean it is running well.

There IS a reason for this sort of thing. Odds are, it will turn out to be something you've assumed to be okay... and therefore did not check.

-Bob Hoover



Then came several questions, the upshot of which made it pretty clear the person was expecting to find one single problem as the cause of their overheating

-------------------------------------------------

Your engine is a SYSTEM. All of its parts interact. Problems seldom have a specific single cause but are the accretion of numerous small problems, usually things deemed unimportant by the 'experts.'

The deflector plates on the underside of the heads serve to maintain adequate air pressure in the plenum space ABOVE the heads; the gull-wing deflectors direct the air to where it is needed most; the thermostatically controlled flaps are designed to align with the central fin on the cylinder head so that the densest slug of air from the blower is directed to the hottest parts of the heads... all 'unimportant' details. The failure of any one of them may go unnoticed until you get a really hot day, or climbing a grade with a heavy load. Then you will have more waste heat than the engine can managed.

You're looking for a smoking gun -- a single cause of all your problems. Odds are, you aren't going to find it. What you are finding are a host of small 'unimportant' problems, the combination of which have lead to an episode of overheating. Now the camel has his nose inside the tent because excessive heat carries a legacy of future problems; it's a genie that can't be stuffed back in its bottle.

Most people simply do not believe that such close attention to detail is necessary. Indeed, the archives of this Group contain dozens of messages from idiots saying exactly that when someone having more experience tries to explain why their failure to devote any attention to those 'unimportant' details is the reason thier ride is a piece of shit.

"All my buds say..."

"I've driven for ten years without..."

"Nobody does it that way!"

-------------------------------------------------

The good dope is in the manuals. The distillation of thousands of man-years of engineering excellence backed up by more than twenty million engine's-worth of experience. Build a few engines yourself and perhaps you can add to that sum-store of knowledge but most of what you read & hear about Volkswagens is from an idiot with one engine's-worth of experience expressing his expert opinion, or an after-market retailer preying upon your ignorance.

-Bob Hoover

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 maximum 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 into 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

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.
----------------------------------------------------------

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

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.)

- - - - - - - - - - - - - - -

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
-----------------------------------------------------------------

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

----------------------------------------------------------------

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.