Friday, November 24, 2006

VW- More on Sealants

I've used ‘Ultra-copper’ on a lot of engines and liked it, except for the latex base. As a sealant it was better than Permatex since it formed a thinner seal, but as an assembly component I didn’t like the way it would sometimes peel off the case due to an accidental touch. Permatex would smear but there was always some left.

I started using the Loc-tite stuff because it not only formed the thinnest seal I’d ever seen (thinner is better when there’s no gasket), it was at least as tenacious as Permatex; you didn’t have to worry that an accidental touch would force you to clean & recoat the parting line.

But as I recall, the thinner for both Permatex and Ultra-copper was something so potent it made boy-mechanics give birth to two-headed motorcycles. I wouldn’t put Loc-tite on a bagle but otherwise it’s fairly benign; Hypolon has some trichloroethane, the other compounds are mostly silicones.

One note of caution, and the reason I’ve made this a general posting: If you’ve got to fix it and drive on, use Permatex. But if you’re building engines in the privacy of your own bedroom, with lots of time between assembly and test-running . . . at least 8 hours (more is better) . . . then use the newer sealants. They have a required cure time. The only guys I know who don’t like them are the types who never read labels; don’t give them a chance to do their job.

If you think about it, outfits like the Loc-tite Corporations are to be numbered among the Good Guys, white hats and all. They’ll never tell you red cars or faster or waste your time expounding on the virtues of 500 watt stereo systems in a bug. The value of what they sell is obvious. And if you’ve got some sexy sealing problem, they’ll usually offer some free advice on what might work.

VW - Paint Your Engine

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Recent comments make it clear a lot of folks are not aware of the benefits of painting their engine. The basic reason for doing so is preservation. When fitted with a full-flow oil filtration system the VW flat fours can deliver 150,000 miles or more of service before the lower end requires overhaul. Indeed, when fitted with hydraulic cam followers and other modern innovations such as electronic ignition, it’s not uncommon for a properly assembled engine to deliver 100,000 miles of service without requiring any form of repair.

A light coat of flat black paint on the magnesium-alloy crankcase not only protects it from corrosion, it enhances the heat-flow characteristics of the surface. The cast iron cylinders benefit even more, although they are more difficult to paint. The trick is to get the paint right down into the bottom of the fins. To do so calls for the use of a suitable brush, made by cutting off half the bristles from a small (1/2") paint brush; not an artists brush, the regular sort does fine. You must use a brush instead of spray because by the time you’ve sprayed enough paint to reach the bottom, you’ve flooded the upper part of the fins and made an unholy mess. So start with the brush, take your time and give your new jugs at least a day to dry before handling them.

The barrels on high time engines, especially those operated in cold climates where corrosive substances are used for snow removal, are often found to have virtually no fins at all when the engine is torn down for rebuild. Ions of the corrosive material, common rock salt in most cases, attach readily to unprotected cast iron, and once attached are impossible to remove without boiling with a ‘getter.’ This means that once the corrosive ion finds a home on your cast iron cylinders, the corrosive action will continue year round, thanks to water vapor in the air.

Rusty or corroded metal makes a fine heat insulator, as every weldor knows. A few ounces of paint judiciously applied prior to assembling your engine is not only the mark of an experienced mechanic, it is one of those performance-enhancing tricks so simple it is often overlooked. But in the long term it means greater service life and lower operating costs.

Before you succumb to the conventional wisdom that painting is an unnecessary luxury, drop by any airport and examine the engines that inspired the original Volkswagen engineers. Aircraft engines are painted as a matter of course, although such niceties were ignored with the VW in the interest of economy. The original idea was to replace rather than overhaul the engine, and to replace it fairly often -- typically, at something less than 100,000 km (62,000 miles). Alas, this option is no longer practical in todays economic climate.

-Bob Hoover

VW - Flaming Distributors, Batman!




Distributors normally don't run hot.

Even when the engine over-heats, the location of the distributor, up on a relatively cool corner of the crankcase, combines with the heat-flow path and the ratio between the neck of the distributor and its body to cause the distributor to run significantly cooler than the crankcase.

So if you have a hot distributor it's a pretty good sign that something is wrong ...and that 'something' is usually that your engine has been improperly assembled.

Have you got a distributor handy? If you'll examine the neck of the distributor you'll see a hole drilled into the neck just below the groove for the O-ring. A machined trough runs from the hole to the bottom edge of the neck. As you may have guessed, the hole and the trough are there for a purpose, which is to lubricate the shaft of the distributor.

Grab a crankcase and peek down inside the hole where the distributor goes. On the uphill side of the hole -- the side nearest the center-line of the crankcase -- you'll see a small window cast into the side of the hole. When the distributor is properly installed, the trough on the side of the neck of the distributor connects with that window. Of course, that only happens when the engine is properly assembled, since the orientation of the distributor's pinion gear dictates the installed orientation of the distributor. (On the Type I engine, on the pinion gear the slot for the dog-gear on the distributor must be perpendicular to the center-line of the crankcase when the #1 cylinder is at 7.5 degrees before TDC. The smaller segment must be toward the pulley. Note that the alignment is for the static firing point and not TDC.)

Notice that the window opens onto the cast 'shelf' that serves to align the thermal insulator that isolates the fuel pump from the crankcase. The angle at which the distributor is installed and the location of the shelf combine to provide a constant supply of oil for the distributor shaft. Install the distributor incorrectly and you've just shut off the oil to the distributor-shaft bushings. And while the distributor shaft only rotates at half the speed of the crankshaft, it still needs a drop of oil now & then.

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Back in the Good Ol' Days -- whenever that was -- it was pretty rare to run into an improperly installed distributor. Nowadays it's become pretty common. Why? Mostly because incompetent mechanics install the distributor's pinion gear incorrectly. Which leads to plugging in the distributor so that no oil can get to the oiling trough.

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Gotta hot distributor? Then there's a high probability it's been running without adequate lubrication. So fix it. Bring the engine to the firing point on #1, pull the pinion gear (you'll need the special puller) and re-install it correctly, making sure to provide the proper number of shims.

How common is this problem? I wish I could say it was rare but about half of the engines I see have the distributor drive-pinion off by one or more teeth. Which is a good reason to send the fellow on his way, at least here in the Peoples Republic of California. Because as soon as you lay hands on the thing you can be held liable for any future problems. Neat, eh? And you wondered why it's so hard to find a good VW mechanic :-)

Fortunately (for me) you don't have to pull the distributor to see that it is mis-aligned. Installing the distributor's drive-pinion incorrectly is good evidence that whoever assembled the engine doesn't know their ass from their elbow, meaning there's liable to be other, less visible problems, such as mis-aligned bearing shells, wonky valve train geometry and even a mis-aligned cam gear -- all of which they will lay on your doorstep since you were the last person to work on the vehicle.

So you smile, praise their paint job, tell them you're too busy right now and wave them on their way.



> Is it always the same set-up? > I have a '59 with the leads different plugged than my '73...



Yes.

I assume the main purpose of the window is to provide lubrication for the upper end of the pinion gear, which uses the parent metal of the casting as a bearing and must be a nice fit in the bore because of that, something you check prior to assembly when blueprinting the crankcase. (A lot of used crankcases fail this test.) It is the orientation of the drilled hole and machined trough on the neck of the distributor that makes it clear they are meant to align with the window.

I've never measured the width of the window in the wall of the distributor bore (it will vary slightly from one casting to the next) but it's an oval which I think is about 5/8" wide on its major axis. The trough on the distributor comes only to the top of the window, meaning it has to be nicely centered to keep from being obstructed due to the oval shape.

Given that the bore is about 27mm in diameter, and that the pinion gear has 12 teeth, I suppose you could be off by a tooth on either side without obstructing the oil channel... assuming a wide and relatively square window. But any greater mis-alignment will drastically reduce the amount of oil getting to the distributor bushings -- or shut it off entirely.

Fortunately, the alignment of the pinion gear is easy to check: First, find a picture of the correct orientation in the factory service manual. (I wouldn't trust Muir; a lot of the drawings are inaccurate.) Bring the engine to the firing point for #1 and simply pull the distributor. On a bug you can look directly down the bore (on a bus you'll probably need an inspection mirror).

-Bob Hoover

PS -- I've received a couple of interesting messages from folks eager to argue about the orientation of the pinion gear. I'm not.

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>>But this point that Bob brings up is more serious: if the erstwhile type IV > owner installs a spankin'-new SVDA distributor, figures out the wire > re-mapping and congratulates himself on doing right by his ride, he could be > in for a nasty surprise.



I don't consider myself qualified on Type IV's (ie, the '1700,' '1800,' and '2000' engines, to use VW's terminolgy ['Type' actually refers to the chassis.] My post was specific to the '1600' & earlier crankcases (and I'm too lazy to go dig a T4 our from under the bench :-)

But it should be easy enough to check, assuming you're up to your eyeballs in T4 parts. One method is to compare a stock distributor from a T4 to the same thing from a T1. If the groove is in a different location relative to the vacuum can then you may have a problem. Method #2 is examine a T4 crankcase to see if it has a similar window and if it is in the same relative location as on the T1. If the answer(s) is yes, then you've got a problem, since as you've pointed out, the location of the canister on the beetle distributor forces you to install the distributor in a position that will block or reduce the flow of oil to the bushings.

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As a point of interest, the earliest VW distributors were made of cast iron, fitted with replaceable bronze bushings having a spiral oil channel. I don't recall them having an external oiling groove.


> Isn't the timing for cylinder #3 slightly retarded normally, so that one > would run cooler? Is that not accomplished with the distributor? >

Yes. At least, for all carburetted engines starting with the 1965 model year and continuing until the introduction of the external 'dog-house' oil cooler. The #3 lobe of the distributor was ground with 2 degrees of retardation. Because of the 2:1 ratio between the cam and crank that means #3 will be retarded by 4 degrees. (There's a VW Service Bulletin covering the beginning dates & serial numbers. I assume another was issued when they went back to the unmodified cam but I've never seen it.)


If you're unsure of the provenance of your distributor you should have it checked on distributor testing machine. Or you may check it using a stroboscopic timing light and accurate degree-wheel. Simply transfer the timing light's pick-up to the other three spark plug leads, noting the firing point for each.

The main hazard is that if an early distributor is installed incorrectly the fellow is liable to time the engine using the #3 lobe (ie, directly opposite #1 in the firing order). Which means the other three jugs will now be advanced by four degrees. During hot weather, with the engine under a heavy load, 4 degrees of unwanted advance can result in detonation. And has.


> is there a way i can get the distributor drive gear out without having the > special puller?


(First things first: Start by pulling the fuel pump and removing its push-rod. Now you're good to go.)

On an old (ie, worn) engine you can sometimes extract the pinion gear by driving a hardwood dowel into the recess for the compression spring. Of course, if the dowel breaks off in the hole you're pretty well screwed.

I've used a pair of 'reverse-pliers' (ie, squeezing them causes the jaws to open rather than close) which were designed to extract the broken-off pipe from pop-up lawn-sprinklers. They have hardened jaws which grip the pinion gear in a manner similar to the special tool.

However, I've done it a time or two and this is one of those cases where experience counts. Because the pinion's driver-gear is a spiral (ie, the 'brass gear' on the nose of the crankshaft) the pinion and the crankshaft must be rotated as the pinion gear is extracted. Not a bunch... just a tad. There is a certain feel that tells you when it's coming out okay. Plus, the circular inclined plane that drives the fuel pump often builds up a layer of varnish on its outer edge, making it a tight fit in the upper bore. Here again, there's a certain feel that tells you if all is going well or if you should back off, rotate the thing and try again.

Like riding a bicycle, touch-typing or hitting a fast-ball, once you've done it a few times -- once you know you can do it -- it's really pretty simple. And once you know you can do it you will see other ways to get a grip on the pinion gear.

But the main reason for using the regular tool is that once you've removed the pinion gear you are expected to replace it, and that is one hell of a lot harder to do without the proper tools.

-Bob Hoover

AV - VP Accidents & Icing

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The accident figures are similar for other VW-powered designs and whatever the actual accident rate, it's too high. It's also largely preventable, in my opinion, at least with regard to engine-related events.

I've seen my share of fouled plugs and fifth-hand Vertex magnetos with fried points. And even attended a couple of Total Idiot tear-downs where we found no evidence of fuel anywhere in the system and a couple with no oil in the engine. (You really should safety-wire the sump plug.)

The real pissers were the cases where we couldn't find anything wrong. There was fuel in the carb, the ignition system provided a spark at the proper timing and all the controls were intact, at least up to impact. In a couple of cases the engine was still in running condition. If that happened once or twice it would fade into the statistical background but when you personally see a dozen or more cases like that it's a strong indication of a fundamental flaw. But one that leaves no obvious clues.

What you do have is the accounts from the surviving pilots; the classic 'loss of power,' or 'the rpm began to drop'. Track that back to the location of the event, dig out the best records you can find for the weather at that particular time & place and guess what you'll generally find? Conditions ideal for the formation of carb and manifold icing.

Any discussion of icing gets you into a matrix of factors but the basics are pretty simple: The vaporization of gasoline is endothermic -- it absorbs heat. It doesn't matter if you're using a carburetor or the latest gee-whiz slide-valve piece of shit, you've got a mini-refrigerator attached to your engine and if the local atmosphere isn't able to provide enough heat to keep the endotherm above the freezing point any water vapor in the air is going to appear as ice.

With a carburetor you can bootstrap yourself into this situation by reducing the throttle. This leaves just a tiny gap between the throat and the throttle plate (usually of brass) and a lip of ice can appear on the down-stream edge of the throttle plate quick like a bunny, even here in sunny southern California. (The oft-repeated claim that slide-valve gas passers don't ice up because they don't have a butterfly valve (ie, throttle plate) is fallacious. Under the right conditions the whole damn slide can ice up.)

That's why we pull on Carb Heat before we begin our let-down.

But you can also get ice in your manifold, even under full-throttle conditions, assuming you have long intake runners that are not provided with any form of supplemental heating. Lycoming routes their runners through the sump. Continental usta say theirs picked up enough heat from the cylinders. Volkswagen vehicles use the exhaust gases from one jug to heat their manifold. And yes, Virginia, your slide-valve Lake, POSA, Aero-Carb or whatever is just as susceptible to manifold icing as a carburetor. So let's forget the 'ice-free' bullshit.

The key point here is that some of the VW engines which suffered the classic 'loss of power' syndrome had no provision of any form for carb heat and all of the others had carb-heat systems incapable of providing enough heat for worse-case conditions.

How much heat is that? About 90*F over ambient, according to the FAA.

Based on experiments I did here at the shop, a carb-heat stove fed by just one cylinder of a 2180 engine couldn't produce a ninety degree rise when passing enough air for the engine. Reduce the throttle, you reduce the amount of air required but you also reduce the amount of heat available. Catch-22. You need the heat from two jugs, not one.

A related part of the problem is that the typical homebuilder's heat exchange isn't very efficient. I know mine wasn't even though I religiously copied the design advocated by the experts of that era - an old screen-door spring wound around the exhaust stack with a muff made out of a couple of tomato cans. (ie, Pietenpol, Leslie Long, et al). Indeed, after a few months out in the weather you generally got more rust out of the thing than hot air.

Pot-scrubbers worked better than door-springs and didn't rust. (ie, those big stainless steel pot-scrubbers; hardware stores usta carry them back in the paint department; you used them for scrubbing walls & woodwork before applying new paint. And for pots too, I suppose.)

Tapping the waste heat from two cylinders is better than using just one. And you can make a better heat exchanger, too. Assuming you know how to weld and are willing to devote a bit of time to it.

I don't have a pat answer for the carb heat problem although I'm convinced that with converted VW's it's one of the few cases where more is better. My current effort in that direction is using a stud gun to weld a bristle of pins to two sections of carbon steel exhaust pipe which will be mounted side by side inside a stainless steel muff and fed by two cylinders. The idea is to produce the best transfer of heat with the least restriction to air flow and I think the idea will work out... eventually :-)

It's worth mentioning that most of my experiments don't work out :-) Not that they're total failures -- you always learn something -- but they are seldom totally successful. What does work is to keep adding what you've learned from the last experiment to your next one. You'll eventually arrive at a system that meets your needs, although it's never as simple & easy as the original concept. Sorta like life, in that respect :-)

I make my heat boxes out of whatever scrap aluminum is available. Riveted construction. Sized for the Tillotson Model X carb (ie, 1-7/8" inlet). Controls are simple Bowden cables. Drawings & photos are included in the HVX files.

-R.S.Hoover

PS - In conjunction with this thread I was asked why all this hot poop I'm handing out hasn't appeared in their favorite aviation magazine. The simple answer is because they don't want to pay for it.

After spending years (in some cases) to develop a suitable solution, such the carb-heat thingee, to convey that information to others in an understandable fashion may require dozens of drawings, photos and illustrations supported by thousands of words of text because in technical writing (which is what I do for a living) it isn't the simplistic straight-line path that's important, it's explaining what to do when things go awry; identifying the potential problems some distant reader might encounter and providing workable methods of avoiding such mistakes and in some cases, of recovering from them.

Package all that in camera-ready copy, send it off to an aviation magazine and if they express any interest at all, they may offer a $100 for your months of effort. If they offer anything at all. Some believe you should give them material for their magazine... which they then sell for a tidy profit.

Old news, really. It's been thrashed out time and again on other newsgroups. Mentioned here in passing because of a couple of messages from folks who were not aware of it. -- rsh

AV - 'Line' Oil

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Back before Randolph's there was Lyon Paint Co., from somewhere in Ohio (as best I can recall). Lyon was one of the first companies to specialize in AIRCRAFT lacquers and enamels.

Structures fabricated of welded steel tubing were typically given a dose of linseed oil before being sealed by a threaded plug (near the tail), bolted plate (at the front, typically behind the engine mount attachments) or welded plug (all manner of struts; N-strut, cabanes, lift-struts, etc).

Why linseed? Because it is a 'getter' for oxygen; the linseed oil (ie, made by pressing flax seed) plasticizes as it oxidizes (ie, absorbs the oxygen), turning into a thin layer of varnish.

No oxygen means no rust.

So how did plain old-fashioned linseed oil become 'line oil?'

It didn't. It was always LYON oil. Specially refined linseed oil with a neutral pH (ie, neither acidic nor alkaline). Back then, common linseed oil as used for finishing furniture, improving the flowability of oil-based paint and so forth, was never meant to be used on steel and its pH was not a factor when applied to wood.

So Lyon's 'aircraft-certified' linseed oil became the standard for doping the interior of steel tubing. Nowadays, any high quality linseed oil will serve since all are now close to neutral with regard to pH.

Lyon Paint Co. used the head of a lion as their logo. If you'll examine air-race photos from the 1930's you should be able to spot their logo.

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Sealing the interior of welded tube structures assumes the structure can be closed; sealed off from the atmosphere. When that was not the case the interior of the spar, strut or tube was painted, typically with an anti-corrosion paint, diluted about 5:1, applied by flooding (ie, filling and then pouring out) or by 'sponging' -- using several small pieces of sponge tied to a length of marline or rib-lacing cord. The sponges were saturated with paint as they were pulled into the tube and then pulled through.

Sponging was the preferred method since the interior of seamless tubing always has some residue of lubricant used in the forming process. Sponging served to 'brush' the anti-corrosion paint onto the surface. Of course, you couldn't sponge a tube if it had any interior obstructions. Flooding was the preferred method for shorter sections, the paint usually preceded by one or two floods of solvent followed by an air-blast. Such messy little chores often fell upon the shop gopher (which was me, fifty years ago :-)

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The need to protect the interior of the tubing reflects the propensity for mild steel to rust. The standard procedure when repairing or recovering an early fuselage or landing gear was to pull the plugs, usually a socket-head set-screw installed in a weldment, and see if there was any liquid 'line-oil' left inside. Most airframe manufacturers cited how much 'line-oil' was used (usually about a pint) and where to pour it in , after which the fuselage was tilted and rotated, the plug(s) removed and any residue allowed to drain out.

Thanks to a whiff of chromium 4130 is less prone to rusting than 1025 and during WWII many structures did not receive 'line-oil,' although some critical parts such as engine mounts and landing gear yokes were pumped full of dry nitrogen under pressure via a Schraeder valve (think of an industrial-grade tire valve) and fitted with a simple pop-up pressure gauge. Any drop in pressure was good evidence of a cracked weld.


-R.S.Hoover

AV - Oil Temps & Sensor Locations


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In theory, you may install an oil temperature sensor in any location, so long as the sensor is bathed in an active flow of oil. This is to ensure the sensor reflects any change in the oil's temperature as soon as that change begins to take place. As the Pilot-in-Command your main interest is any anomalous change in oil temperature, rather than the temperature itself.

One reason for our lack of interest in precise quantified temperature data is cost. Calibrated mechanical instruments, certified accurate within a given degree of precision are expensive. When precision accuracy is combined with reliability you’re looking at a very expensive piece of goods. Instruments developed for land-based vehicles aren’t especially accurate but are sufficiently responsive for our needs.

The reason we are less interested in the magnitude of the temperature shown on the gauge is because the temperature of the lubricant varies throughout the engine. That is, you may see a wide variation in oil temps from the same engine, depending on where in the temperature is measured. When using low cost instruments, rather than rely on specific numerical readings we insert the sensor into the active oil flow and by reference to other, more critical parts of the engine that may not allow convenient temperature-sensing, we calibrate the meter to our particular engine, dividing the scale of our meter into colored arcs to show the safe operating range.

For example, the oil temperature gauges installed on some VW industrial engines placed the sensor at the inlet to the oil pump and divided the face of the meter into red, yellow and green arcs and provided no numerical information at all. By comparing the VW system against a 400 degree mercury thermometer borrowed from the chem lab at Modesto Junior College, I found the green arc covered (approximately) 170 to 220 degrees on the Fahrenheit scale, followed by a yellow arc extending up to about 250*F and a red arc beyond that. This seemed rather low until I learned that the oil temperature in the valve gallery was typically a hundred degrees higher than that being sensed at the inlet to the oil pump. Clearly, the intent was to warn the operator to reduce the load on the engine when the valve gallery oil temps exceed 350*F.

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The How-To information for installing a temperature sensor at the inlet to the oil pump (ie, VW's factory-preferred method) (*) has been posted to the internet numerous times since 1994 and there are a couple of web sites that offer step-by-step photographs of the procedure.

-R.S.Hoover

(*) The VDO instrument cluster offered starting in 1970 (?) was a dealer-installed option and subject to numerous Service Notes over the years due to their often hilariously incorrect readings)

AV - Varnish

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> > Do you varnish the inside of all the drilled holes and underneath all fittings before assembling wings? <<



Always.

Because a hole exposes the end-grain of the wood it usually receives extra attention, such as blocking the back-side of the hole with your finger and FILLING the hole with diluted varnish, poured from a small can, etc. Wait a few moments then position the can under the hole and remove your finger. (Proper orientation assumed.)

You won't appreciate the need for this until you've removed the fittings from some older wooden structures. Or rather, tried to remove them :-)

Even with cadmium plated AN hardware you'll often find bolts corroded solidly into the wood, fittings deeply etched with rust on their back-side and so forth. The VP's landing gear attachment bolts are especially prone to corrosion due to their location and the depth of wood.

If you want to add a sealant to the shank of the bolt you may find paraffin (white mineral wax) to be a better choice than vanish.


>>Thanks for the reply. I was contemplating using Q-tips to varnish the holes......certainly like your method better.<<


Before Q-tips there were patches.

Traditionally, a 'patch' was piece of cotton fabric about the size of a silver dollar. You made them out of scrap left over from a covering job, or cut them out of tape. For repair work you had doped patches and plain. Fabric-covered aircraft that actually worked for their living were always getting holes poked in them. The typical hole resulted in an L-shaped tear. Small tears, you'd use a curved needle to take a couple of baseball stitches to hold the tear closed then apply a doped patch. (The idea here is that the dope was the same color as the airplane; otherwise you used a clear-doped patch.)

Nothing really new. In Vietnam we used aluminum beer cans and a smear of RTV. (By the Vietnam era most fabric covered control surfaces were Razorback -- fiberglas, rather than cotton.)

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Point is, to varnish a drilling in a wooden structure, if you couldn't flood it with dilute varnish you poked a piece of safety wire through the hole, made a little hook on the end to catch a varnish-soaked patch. Then you used a soda straw, piece of tubing or a pump-can oiler to flood those outta-postion holes, the patch being pulled partly into the hole to plug it... and finally through the hole to 'paint' it.

Some guys used a rib-stitch needle and a triangle of tape; poked a corner of the tape through the eye of the needle, sorta twirled it to make the plug.

And 'tape' means a roll of cotton fabric, two to four inches wide with pinked edges. And pinked edges means.... (this could go on all night)

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Kind of an interesting point in all of this, in that while most assembly and re-covering manuals talk about sealing holes in wood, I can't recall any that told you how to do so. The methods I've described above I learned from my dad, an old time A&P, or from other mechanics.

Also note that all the stuff I've mentioned -- safety wire, fabric tape, patches and so on -- is stuff that would normally be available & near at hand if you were working on airplanes. Working in your garage, covering with dacron, if Q-tips are all you got, then usem.

The important point is to provide a good seal inside every hole through wood. Aircraft wood is twelve to fifteen percent water by weight. Softwoods, such spruce, pine, hemlock or fir... the stuff commonly used in aircraft ...like to absorb moisture and does a good job of transporting it from one end of a stick to another, which is why stored wood usually gets its end-grain sealed with wax, tar or paint.

After the wood is used to build something, the last step in the fabrication is to seal the whole surface of the wood. Once you've sealed the wood with varnish or whatever, its moisture content remains fairly stable and if protected from sunlight, it sort of goes to sleep -- it stops aging, or at least, slows down to the point where the process is not apparent to humans. Periodically, such as when we replace the fabric, we re-new the seal of the wood. This isn't unique to airplanes, it is the natural order of things that applies to anything made of wood.


> I think 50/50 is too thin. 90/10 is more like it. The very experienced fellow who painted and/or varnished everything in and on our house told me this.<<


Houses aren't airplanes :-) Experience derived from house painting or furnature building is of little use when it comes to protecting the structure of a wooden airplane.

Airplanes are largely built of softwood. The first coat of varnish should in fact be little more than thinner. The objective is to seal the wood at the microscopic level, which thinned varnish does perfectly well... if you thin it enough. For spruce, fir and pine a first coat of only 25% varnish to 75% thinner is not unusual (ie, ratio of 1:3)

The first coat is allowed to dry until #120 paper produces only a dry white powder with no clogging at all. The sealed surface is then sanded lightly. 'Scuffed' was the old-fashioned term; some times you heard it described as 'dulled' but either definition leads to misinterpretation unless you've seen the procedure being done. It is basically a light but complete sanding with fairly fine paper, after which the surface is wiped down with a clean tack-rag, frequently turned. The finish coat is usually 75% varnish thinned with 25% thinner. (ie, ratio of 3:1) Nowadays I suppose everyone uses White Mineral Spirts as thinner. When using real spar varnish we used turpentine.

The above procedure is valid for the interior structure of wooden aircraft. For the exterior -- wooden struts, gear-legs and tail skegs, the second coat was given an additional sanding; the final coat was full strength varnish, properly laid-on. Varnished exterior surfaces were frequently inspected and renewed as required. Interior structures were sanded & renewed with each re-cover. (Ed. Note: 'Full Strength' might still mean some degree of dilution with thinner, especially if the varnish were old. The reason here has to do with application rather than penetration, in that the varnish must be thin enough to flow-on in a smooth coat.)

A point many overlook is that with airplanes, the finish is supposed to weigh as little as possible. This dictates methods and procedures that are never used with furniture, gun stocks, marine bright-work and so forth, each of which differs from the others to some degree.

As for application of the final varnish coat with a spray gun, while this is commonly done when refinishing props, struts and large panels of fabric(*) or plywood it is seldom used for the interior structure of wings due to the large number of edges, nooks & crannies, for which a brush generally goes a better job.

-R.S.Hoover

(* - At one time varnish was a common finish for cotton & linen fabric.)
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