Friday, June 29, 2007

AV - Midnight Turning



It’s twenty-three minutes past midnight, Thursday the 28th of June, 2007. (Better make that Friday the 29th.) I’m just out of the shower, got my pipe going, scotch & water in hand. My wife has already gone to bed, probably mad at me for losing track of time out in the shop. Long list of mail awaiting answers, mostly from kids who’ve just bumped heads with Reality. (‘My bus is on fire!!!! Should I put it out or what?’) But I’m too tired tonight.

I like making things but the enjoyment comes mostly from figuring out how to make them. I made my first coaxially mounted dynamo back in the early ‘70's using parts out of a Honda motorcycle. I’ve since made quite a few of them using different rotors, home-made rectifiers and other variations, figuring out how to mount them on either end of the crankshaft. In fact, if you’ll compare the drawing above to the one I posted a couple of years ago you’ll see that the dimensions have changed slightly. And even the drawing above isn’t carved in stone. Tonight I saw a way to save some time that will appear in the next one I make.

Time doesn’t count when you’re figuring out how to do something. Hours vanish in the blink of an eye. But making a copy of something you’ve already figured out is pretty boring, which is why you start thinking about ways to do the job faster.

The dynamo hub starts out as a three pound billet of aluminum, that lump on the right in Figure 1. It goes into the chuck and the work-face is cleaned up so you can poke a one-inch hole in the middle using a drill. Then you start shaving down the outer diameter, creating what will become the shaft of the hub. What will become the flange is being gripped by the chuck. The 12-inch lathe has a three-horse motor and can take a fair-sized bite but it takes time and you have to keep your attention focused on the job.

Once you’ve got the OD down to where you want it you swap tools, shift gears and cut a the oil-slinger thread into the OD. Then you change to a boring bar and open up the ID to match the nose of a Volkswagen crankshaft, using a broken crank as a gauge. At that stage the thing looks a bit like an aluminum toad-stool.

Now it comes out of the chuck, gets flipped end-for-end. With the shaft now gripped by the chuck you gotta dial the thing in. Since Permanent Magnet dynamos don’t seem to be very sensitive to alignment I’m happy with .001 or less.

I pick-off the overall length using another gauge and face-off the flange, leaving a little lip to index with the rotor. Then it’s back to the boring bar, opening up the forward face to accept a socket for the pulley-hub bolt. Which pretty much finishes the job, except for a few details.

From the lathe, the dynamo hub goes to the arbor press where I use a 6mm broach to cut the keyway. (No broach? Then rig your boring bar as a scraper and rack the carriage back & forth, adjusting the cross-slide to get the required depth.) Then it goes over to the milling machine or drill press where I’ve set up a rotary table, already centered and fitted with a spud to match the ID of the hub. Cranking the knobs of the rotary table through seventy-two degrees at a time, I drill the pilot holes for the five bolts that will secure the dynamo’s rotor to the hub. (If you don’t have a rotary table use the rotor itself as a guide to spot the location of the holes.) After drilling, the holes are tapped 1/4-28.

In Figure 2 you can see how much metal is removed when opening up the ID. In fact, the three pound billet has been reduced to a scant five ounces (leaving you with two and a half pounds of swarf to deal with :-)

There’s a few million minor details I haven’t mentioned, and depending on your tools & experience there’s dozens of different ways to do the job. So long as it fits the crankshaft and the rotor, and spins true around the stator coils, it will work. But I’ll tell you pard, after you’ve made a few, shaving that thing down gets damned boring.

I’ve been trying to find someone to make the hubs and mebbe laser-cut the mounting plate, leaving you to simply rivet the thing together and bolt it to the engine. But today I heard from the last (of three) local CNC shops and the lowest price was nearly three hundred bucks just for the hub... and that was in lots of ten.

I think that’s too much.

The whole idea here was to come up with a method of generating electricity that was inherently more reliable than a belt-driven system. If you can do that, you toss the magneto and use a lighter, less expensive more reliable electronic ignition system. Millions of motorcycles have confirmed the validity of this approach. But the system was also supposed to be more cost-effective than anything presently available. Having to pay nearly $300 just for the hub blows the idea right out of the water... unless everyone makes their own hubs.

Somebody drank my drink. And my pipe’s gone out.

-R.S.Hoover

Tuesday, June 26, 2007

Crank Basics - II




(Be sure to read Crank Basics - I

http://www.ttbird.net/2007/06/crank-basics-i.html )


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

As you can see in the drawing, the journal for the #3 main bearing is adjacent to the flange for the throw of the #2 connecting rod. Since the #3 main bearing is a full-circle bearing rather than a pair of shells, it is trapped on the crankshaft by the cam’s driver-gear and the distributor’s scroll gear, both of which are a shrink-fit. Pulling the gears off the crankshaft is a basic maintenance chore. The procedure is shown in the following photos.

I’ll be using a portable screw-type puller specifically designed for this task, the first step of which is to remove the pulley, dynamo rotor or whatever happens to be on the pulley hub. The oil slinger and #4 main bearing are then slid off the nose of the crankshaft. You’ll need a pair of special pliers to remove the snap-ring.

The #3 bearing should be free to slide back & forth on its journal. Doing so will reveal a gap about a tenth of an inch wide between the bearing and the cam gear. I you use an hydraulic press, the press-plate must fit into this gap. See the previously posted drawing for dimensions.

Whatever method of removal you elect to use, you most protect the nose of the crankshaft. The usual method is to simply thread a pulley-bolt full depth so the head of the bolt contacts the crankshaft. Pulley-bolts come in a wide variety of lengths and head designs. You want to use the short type so that the stress will be transferred directly to the crankshaft and not to the threads.

With a suitable pulley-bolt in place, fit the puller into the gap under the cam gear and pull the gear using a suitable wrench.

When the gears are seriously tight you may have trouble holding the crankshaft whilst turning the puller. If you have a large vise with padded jaws (ie, lead or copper pads) you may find it handier to hold the crankshaft in the vise. If you plan on building more than a few engines you will probably make up a fixture to support the crankshaft. If you make it sturdily enough it will serve to hold the crankshaft while pulling the gears.

The first thing I do after removing the cam gear is to apply grease to its bore. This surface is usually free of oil. If left unprotected it’s liable to have developed a haze of rust by the time you are ready to re-assemble the crankshaft.

-R.S.Hoover

AV - Found in a Barn



More of a shed, actually. On top of a cabinet. Under a carton of magazines tossed up there about twenty years ago, now glued into a solid lump of water damaged paper.

It was a pad of fabric, at first unrecognizable as such. Like the magazines, it was glued together and I tossed it into the growing pile of trash. Later, loading the trash into garbage bags, a layer of the fabric came free revealing a brittle chunk of masking tape and the figure ‘15' that triggered a flood of memories.

It was the summer of 1968. I’d recently returned from Vietnam and was helping a friend recover his Cub. We were using Grade ‘A’ cotton because he’d been given a whole roll of the stuff. We were working in one of the old wooden hangars at Brown Field, under a bit of pressure because the space was borrowed, as were most of our tools.

He’d never done any fabric-work and my skills were stale at best but we got the job done, right through the final coats of Cub Yellow, a glistening black lightning bolt and twelve-inch hull number.

There was some fabric left over and Witt gave it to me. I rolled it around the tube and hauled it home. A couple of years later I took it to the local EAA swap-meet, tagged with the length: ‘15'. But no one wanted fifteen feet of Grade ‘A’ cotton and I hauled hit home again.

I forget how the tube got crushed, bent in the middle. I unrolled the fabric, folded it up and... put it somewhere.

The Cub is long gone, off to where ever old Cubs go. So too is my friend, his death in the mid-west discovered by accident long after the fact, too late for cards or condolences, remembered only as a laughing, smiling fellow Chief Petty Officer with whom I’d shared a couple of tours of duty. And a passion for flying.

I stood there with the hunk of fabric in my hand, the garbage bag waiting. But you can’t throw something like that away.

I washed the fabric carefully in cold water with just a dash of soap. It was pretty bad. The mildew had eaten holes and imparted a leopard-like pattern of stains. But enough of it remains to make a shirt or two; something good enough to wear while working around airplanes. And remembering old friends.

-R.S.Hoover

Sunday, June 24, 2007

AV - Found in a Drawer



The smartest thing about a riveted fuel tank is that they usually weigh less than a welded or soldered tank. Welded tanks usually use some fairly thick stock; .032 is about as low as you can go without blowing holes with your tigger. Lotsa weldors won’t even consider the job unless the tank is .050. Then too, all of your welded fittings have a pretty good flange that you melt into a nice-looking row-of-dimes when you marry it to the matching flanged hole in the tank.

Riveted tank, you can use damn near anything at all. Most guys stick to .020 because that’s what they’re using for wing skins or whatever, but I’ve used .016 and even house siding. Strictly home-built of course but it hasn’t leaked yet and that was back when Jeeter was a pup.

If there’s any trick to making a riveted fuel tank it’s in using a suitable sealant. Back in the Day they used a special neoprene tape and two rows of close-pitched rivets. But by the start of WWII they’d developed seam sealants that really worked, reducing the chore to a single row of rivets. Nowadays we’ve got polysulfide sealants that form a bond strong enough to be used as an adhesive - - you could drill-out the rivets and the thing would still hold together and not leak. (I’ve written about this before... http://www.ttbird.net/2006/12/av-yo-fuel-tank.html)

A riveted tank calls for riveted fittings and that’s where some folks come to grief. To support your filler-neck and outlet plumbing you need a fairly substantial flange and the easy way to make them is on a lathe. A little lathe will do but not everyone has one. That’s where your local EAA chapter can come in handy. Or your buds out at the airport. Ask around, someone will eventually point you toward a lathe you can play with, mebbe even one that comes with a machinist attached.

Personally, I hate that sorta thing because I’m poor and most homebuilders aren’t. They've got enough money to hire someone to do most of their chores whereas I don't even have enough time to do my own. But I know some guys even poorer than me and when they need something turned, I try to help them out.

(What’s ‘poor’? I’m glad you asked. According to the Bureau of Labor Statistics there’s about two hundred and thirty-three million Americans working for wages as of 2006. Line them up according to how much each of them makes and the guy standing in the middle of the line is getting about $28,000 per year. Which means half our work-force - - 116,000,000 people are earning less than $28k per annum. Like me. And a couple other guys I know are earning a lot less.)

Saturday one of these lo-buck builders dials the Secret Number then comes by the shop with his new, riveted fuel tank, complete except for the flanges. I was impressed. He has cast a flange to accept a filler-neck he was planning to glue in place with sealant when I snatched it out of his hand and threaded the thing. (Not a perfect casting but usable.) Whist doing so he told me that besides Show & Tell he’d really come by to see if I could make him a flange for the outlet, something to support the finger-strainer. He didn't want to trust his melted-down beer-cans for the outlet fitting.

Major case of Deja Vu.

Every fuel tank has some sort of strainer. For many years the standard was a finger-strainer made of bronze screen soldered to a brass bushing threaded 3/8-NPT on the OD and 1/4-NPT on the ID. The 3/8-NPT was the standard thread for the fuel tanks on light airplanes, allowing the finger-strainer to be screwed into place. The quarter-inch pipe thread on the ID would accept a small brass shut-off valve or a fitting for hose or tubing.

Finger strainers have largely been replaced by a lighter, all-aluminum set-up in which the strainer is a section of quarter-inch aluminum tubing, mashed flat on one end, slit several times with a hobby-saw, flared on the other end and installed with regular AN tube fittings. This is not only a lighter arrangement than the old finger strainer, it performs better since the kerf of a hobby-saw is typically about .028" wide whereas the bronze mesh would allow particles as large as a sixteenth of an inch.

Still, finger strainers are a standard item, they don't go bad on the shelf, and so inexpensive (a couple of bucks each) that most mechanics keep a few on hand.

A couple of years ago I needed a finger stainer, reached in the drawer and came up dry. I thought I had a couple somewhere but they’d gotten away from me.

Not a problem. I called Flo Irwin at ASS Co. and asked her to send me half a dozen. A pregnant silence oozed out of the phone then Flo asked, did I know what they cost nowadays. No, I didn’t but it couldn’t be more than a coupla bucks, right?

Wrong.

After she told me what they were going for - - and after I got my heart beating again, she said they were only being made by one supplier, hence the horrendous jump in price. So I ordered just one.

Next time I needed a finger-strainer I made my own. In fact, I made about a dozen of the things. Tossed them in a drawer. Forgot about them. And of course, economics being what it is, once the price of finger-strainers started being listed on the Big Board a buncha people started making them and the price came back down to a reasonable level. (I think it’s now about $5. [June 2007] )

Back when I’d made the finger-strainers I’d also made up a batch of fuel tank flanges threaded 3/8-NPT for outlets and smaller jobbies threaded 1/4-NPT for vents and the like. So after threading the filler-neck I dug around in the drawers, picked out a nice flange and an ugly finger-strainer and asked if he wanted fries with that.

Man really can fly, with a bit of help from his friends.

-R.S.Hoover

Tuesday, June 19, 2007

AV - Prop Hubs

It started with this.




At the end of World War II Type 82 Kubelwagens littered the European landscape. In many cases the vehicle was undamaged, abandoned when it ran out of gas.

The Type 82 came in two flavors, those with the original 985cc (70x64mm) engine producing 23hp and the later version fitted with the E-type 1131cc (75x64mm) engine developed in 1943. Given a choice between an early or late Type 82 most folks opted for the later version, whose larger engine produced a neck-snapping 25hp instead of a paltry 23. (Actually, both performed about the same.) It was the 1131cc engine that went into the post-war Volkswagen, remaining in service until the 1956 model year, when the 75mm barrels were bored-out to 77mm, upping the displacement to 1192cc; the '1200' engine. ( Volkswagen engines are designated by a number approximating their displacement whereas chassis are identified by type number. Referring to an upright VW engine as a ‘Type I’ is something of a joke since the Type I (meaning the sedan or bug) used six different engines over the years.)

With the small, light-weight 985cc engine free to anyone willing to pull it out of a defunct Type 82, it wasn’t long before someone decided to bolt it to an airplane, which they could do without using an engine mount thanks to the transmission flange cast onto the front of the crankcase. (With Volkswagens, orientation is always relative to the vehicle.)

Of course, if you bolted the airplane to the flywheel-end of the engine that meant you’d have to attach the propeller to the pulley hub, violating a basic tenet of using a car’s engine in an airplane, which was to put the prop on the beefier flywheel end of the crankshaft. But since they were only looking at 23hp they figured it was no big deal. And it wasn’t.

Before getting into the details it may help if you know a bit of history, such as the fact the Kubelwagen started out as the KDF Wagen, which started out (in 1933) as the NSU Type 32. After Hitler came to power in 1933 the Nazi Party took a strong interest in Professor Ferdinand Porsche’s dream of a People's Car. Over the next five years the Nazi party subsidized it’s design to the point where they largely took over the project, which finally came to fruition in 1938 as the KDF (Strength-thru-Joy) Wagen. Even as the cornerstone of the new Volkswagen factory was being laid the Wehrmacht issued orders to develop a military version of the KDF Wagen, which became the Type 62, the precursor of the Type 82. (As a point of interest, after the basic design was accepted by the Nazis, the factory test drivers were replaced by 200 army personnel who drove the fleet of prototypes day and night until each had accumulated over 50,000 miles.)

The point of all this is that the Volkswagen didn’t just suddenly appear. It was the product of a long, expensive R&D program in which every aspect of the vehicle was carefully studied and engineered, not only as a civilian vehicle but as a war machine as well.

Figure 1 shows the nose of the Volkswagen’s crankshaft. The pulley-hub was designed to transmit about 7hp via a vee belt to the dynamo and cooling fan. To ensure it would never fail at that level of output, it was designed about 5x stronger than necessary, a fairly common practice when dealing with castings. Because that’s what the original crankshaft was. The choice of a cast-iron crankshaft was driven by the mandated design-goal of keeping the price of the KDF Wagen at or below 1000 Marks. Unfortunately, during extensive road testing the cast crankshafts proved too fragile for the task and late in 1936 they were replaced by forgings, although the dimensions remained the same in order to accommodate existing tooling.

The forged crankshaft was mild steel, the DIN equivalent of SAE 1045. Although significantly better than the cast-iron originals it was designed for economy of production, lacking counterweighted flanges and other features commonly found on crankshafts even then. Also note the internal M20x1.5 threads. Metric threads have a sharp 60 degree peak & valley whereas NC, NF and Whitworth threads are rounded. During development the design proved a bit too flexible. Even the fairly light load of the belt-driven blower and dynamo produced a bending-moment sufficient to precipitate the formation of cracks between the sharp threads and the keyways. Volkswagen resolved the problem by installing a 4th Main Bearing immediately adjacent to the pulley-hub. The design has remained substantially the same to this day. (The air cooled VW engine is still being manufactured in Mexico.)

Figure 2 shows the location of the new #4 bearing needed to off-set the bending moment imposed by the asymmetric load of the belt-driven blower & dynamo. The small size of this bearing, only 40mm as opposed to the 55mm diameter of the three real main bearings, makes its ancillary role evident. Now let’s go fly one... and see what happens :-)

I don’t know who put the first prop-hub on the fan pulley but I know how they did it. It was made from a piece of mild steel and probably looked pretty much like Figure 3. There are several variations on this theme. For example, many of the very first prop hubs did not include the reverse thread that serves as an Archimedes’ Pump to keep the oil from inside the engine. Instead, they installed the oil seal from the inner bearing of a Kubalwagen’s front wheel, which happens to have the same diameter as the pulley hub. Some hubs extended farther than others; some where threaded for prop bolts but most expected the prop to be secured with nuts & bolts. Some had a propeller guide-ring as deep as half an inch; others had no guide-ring at all.

Doesn’t really matter. Tapered and bolted to the little 985cc engine, they all flew, after a fashion. Which wasn’t anything new since Volkswagen engines had already flown. In 1937 the Horton brothers were allowed to install a VW engine in one of their flying wings. But in doing so they followed the accepted convention, taking power from the clutch-end of the crankshaft.

The tapered hub worked well enough with the 23hp engine but problems arose when that type of hub was used on larger engines. The bigger the engine, the more the torque and the more critical became the fit of the tapered hub to the crankshaft. Tapered hubs remain available and in use today (2007) but are seldom installed on engines larger than the 1600. Even then, they have a history of breaking off.




You had to be a pretty good machinist to make the tapered pulley-hub precisely match the taper of your prop hub, which was accomplished by lapping the two together. A lot of folks thought there should be an easier way. Which lead to the Shrink-fit Hub, as shown in Figure 4.

To achieve a shrink-fit the interior diameter (ID) of the hub is made smaller than the outside diameter (OD) of the crankshaft. The hub is then heated until its ID has expanded enough to fit over the crankshaft. As the hub cools, it locks itself in place.

According to Machinery's Handbook (an accepted standard), “The intensity of the grip and its resistance to slippage depends mainly upon the thickness of the hub.” (14th Edition, pg 1055). Formulae are provided to calculate the required difference between OD and ID. Unfortunately, the presence of the keyway and the threads of the Archimedes Screw (if present) limits the amount of grip we can produce before causing the metal to crack along the keyway. I’ll get back to this down below but for now you should know that some shrink-fits are not as strong as others. For example, with the hub shown in Figure 4 the strength of the grip will be determined by the depth of metal between the lowest portion of the Woodruff keyway and the lowest portion of the oil-slinger thread, which is .124" – a scant eighth of an inch. That is, when you subtract the depth of the keyway and the threads the effective wall-thickness becomes a mere eighth of an inch.

To get a stronger grip you need a thicker wall, such as the one shown in Figure 5. But a thicker wall on your prop hub dictates the need to machine a larger opening in the nose of the crankcase, a daunting chore for anyone lacking an engine lathe.

Unfortunately, even when you do all of that, you still run the risk of having the prop fall off. A propeller generates some rather massive bending stresses in the nose of the crankshaft. This is because the gyroscopic effect of propeller resists any change in the attitude of the aircraft. And while we as pilots are aware of such changes when they are large, small changes occur constantly during flight. Accumulate enough small stresses, you won’t have to worry about the big ones... because your prop will already have broken off :-)

Figure 7 shows how the crack propagates. It typically begins in the root of the internal threads and connects to the lower corner of the Woodruff keyway. It then follows the corner of the keyway until it intersects the machined groove at the base of the #4 bearing’s journal. In the 1950's I had the unique experience of suffering two such fractures within a 24 month period. The first was a minor event; more of an inconvenience than an accident. The second was a bit more exciting and put an end to my flying for a couple of years. When I finally returned to the air it was behind an engine wearing the propeller on the clutch-end of the crankshaft, a method I’ll describe in closing.

Back then, the experts insisted a tapered hub was the only way to go; if the prop broke off then I must of done something wrong. And the same experts said exactly the same thing the second time it happened. But other folks had also suffered broken cranks. And some of them were wizard machinists. While poor workmanship may have been a factor in some cases it was clearly not true for all.

One very popular solution to the breakage problem is shown in Figure 8. The Woodruff keyway is welded closed, the annular groove is welded full and the three-degree taper is continued right across the journal of the #4 bearing. The internal threads are usually honed away so as to eliminate them as a stress-riser and the bore is re-threaded more deeply into the thicker section of the crankshaft under the cam gear. To index the hub a shallow keyway, similar to that found on the early Continental crankshafts is machined to accept a section of square key-stock. A matching groove is broached in the tapered ID of the hub.

This procedure also applies to the heavy-walled shrink-fit hub.

This does not do away with the gyroscopic loads nor bending moments induced in the crankshaft but the resulting stress is now distributed over a significantly greater area so that the per-unit stress are typically below the level needed to initiate cracking. And if the crack can’t get started, it can’t do any harm. But by eliminating the #4 bearing we have effectively made the crankcase into a bearing and the outer surface of the prop hub into a journal, a role neither was designed to fulfill. This is illustrated in Figure 9.



Running in the parent metal of the crankcase, our longer prop hub will very quickly oval it out due to a lack of lubrication. And without the bearing-support of the crankcase we discover our longer prop hub now serves to amplify the bending stresses that are appearing in the machined groove at the base of the #3 Main Bearing. Our cam’s gear is also developing a very weird wear pattern. It is only a matter of time before we eat the cam gear or suffer another broken crankshaft, this time adjacent to the #3 bearing instead of #4.

The fix for the fix is shown in Figure 10. The prop hub – long-tapered or thick-wall shrunk – is fitted with a sleeve-type bearing. The nose of the crankcase is opened up to support the sleeve and the OD of the prop hub is polished to serve as a journal. There are several variations to this method, most involving how the sleeve receives it’s lubrication. Functionally, all are pretty much the same although I’m a bit surprised that no one has adopted the lubrication arrangement developed by Bob Huggins which was the best of the bunch, in my opinion. But then, my opinion doesn’t count for much. For more than thirty years I’ve been putting the propeller on the wrong end of the crankshaft, filtering my oil and doing any number of things that are all wrong, according to the experts :-)

Figure 11 shows how I make a crankshaft flange out of an old flywheel. The crankcase requires no modification nor is there much machining to speak of. The flange requires a spool to position the prop far enough forward to clear the #3 exhaust stack but a spool is a simple turning; easy to make.

Several years ago while cleaning out a cabinet in the shop I came across samples of all the prop hubs I’ve made and flown behind over the years. I laid them out on a bench and took some snap-shots. When the pictures came back I found somebody with a scanner and eventually posted them to the internet. I think I sent to the FlyVW Group, which used to be on eScribe back then. Later, Yahoo bought eScibe and despite promises to preserve the existing archives, erased them. Since that time several people have asked me to re-post the pictures but I haven’t any idea in the world where they’ve gotten to. Which also goes for the collection of hubs & stuff. I know I’ve given some of the items away, and I used one of the stainless steel hubs on wind turbine. The following photos will give you some idea of what I’ve been talking about. Some are pretty tatty, discovered outside under a bench or forgotten in the back of a drawer.

Image C is a thick-wall, shrink-type hub. This was one of five I made about 1970. The barrel of the hub is 4130, the flange is mild steel. The two are pressed together then welded on both surfaces, heat-treated, then machined. They never came loose but they had a habit of causing the nose of the crankshaft to snap off. Image D shows you the other side. The thing in the background is a basic spool, anodized some silly color. (I was just a kid back then :-)











Image E will give you some idea of the difference between a regular shrink-type hub and the thick-walled variety. The regular one is turned from a billet of 4130. (Expensive!) Image F offers a comparison between a regular fan-pulley hub and the regular (ie, thin-walled) shrink-type hub. Although they don’t look much alike, Image G shows that they have exactly the same hub diameter, which is a tad less than two inches and fits neatly into the nose of a stock crankcase. Installed on a large-displacement engine the thin-walled shrink-fit hubs tended to loose their grip over time. Machine them for a tighter fit (ie, more shrinkage differential) and they would crack along the keyway.




Image H shows what a flywheel looks like from the front when it’s cut-down to serve as a flange. Image J is a view of the back. The holes are threaded because the spool is usually bolted to the flange and pretty much stays there for the life of the engine, whereas the propeller uses regular nuts and prop-bolts to attach to the business-end of the spool. This is one of the first I ever made, probably about 1965. The lack of the O-ring groove sez it’s for a forty-horse crank. Image K shows how it fits the spool.

A lot of folks think there’s a streamlining problem when you put the fan on the clutch-end of the engine. In fact, the VW’s tranny flange is barely thirteen inches in diameter. With a four inch spool and a spinner ten inches in diameter, the tranny flange is completely submerged in the streamline between the spinner and the firewall. (But of course, that can’t be right :-)



With that as preamble lemme give you a glimpse of the future. Image L is a flywheel flange & extension spool from Great Plains Aircraft Supply Company. For folks who can’t get along without a starter, the flex-plate & ring-gear attaches to those extra holes.






The last image is kinda sad. I made my stuff on whatever tooling I had available. I was in the Navy back then, often had to go begging to get some lathe-time. As you can see from the stuff I made, I’m not a very good machinist. (Still learning, though :-) Steve’s stuff is flat-out beautiful. Produced on state of the art CNC machines, marvelously accurate and painfully precise, they are as much a work of art as a piece of machinery. And to own one all you gotta do is give him money. (Seems like cheating :-)

But the real question you gotta ask yourself is why people are still putting the prop on the pulley hub. We're no longer salvaging free parts from Kubalwagens, we're spending thousands of dollars to build 140cid engines based on after-market VW components. All the converted auto engines you can think of - Model A, Corvair, Subaru - bolt the prop to the clutch-end of the crankshaft and go flying. So how did we get trapped inside this box that says Volkswagens have to mount the prop on the pulley hub?

-R.S.Hoover
(June 2007)

Saturday, June 16, 2007

Full-flow Oil Filtration


There’s still lots of people who simply Don’t Get It when it comes to full-flow oil filtration. They like to point to older aircraft engines such as the A-65 - - or even the O-200 - - and insist that changing the oil every 25 hours is all you need do to obtain maximum service.

Unfortunately, they are comparing apples to oranges. The O-200 was designed as an aircraft engine with more than adequate fin-area to deal with the three-horsepower’s-worth of waste heat that is generated for every horsepower’s worth of torque that appears in the crankshaft.

Nominal output of the Volkswagen engine is about 25bhp - - and only about half of that makes it to the rear wheels. (Start with the miles-per-gallon, work backwards.) Short bursts of acceleration, as when passing or merging with traffic, are dealt with by using the OIL as a heat-sink. That is why the VW’s oil pump is about twice as large as the pump from an O-200. Not only is it larger, it typically operates at nearly twice the speed of the O-200's pump. Bottom line is that the VW’s pump moves more than three gallons of oil per minute even though it only needs about six ounces for lubrication. The excess flow is for cooling.

The VW sump holds 2.5 liters of oil. 85 fluid ounces. Barely two and a half quarts. In 25 engine-hours it will re-circulate the sump’s oil nearly seven thousand times. Dirty oil, getting dirtier on each pass.

An aircraft engine will typically hold six to eight quarts of oil, more than twice as much as a VW. And pumps it at less than half the rate. Wanna figure out how many times the sump’s oil gets recirculated in 25 hours? (Go ahead, I’ll be over there copping a smoke.)

Apples to oranges.

The VW engine needs a full-flow oil filter. Installing one will literally double the life of your engine.

Volkswagen adopted full-flow oil filtration with the introduction of the Type IV engine. To retro-fit that feature to earlier VW engines you simply block the output of the pump and re-direct it through a modified pump cover-plate, ideally one having a pressure relief valve such as the Berg unit shown in the photos. After passing through the filter - - and the oil cooler, should you care to plumb one in - - the oil is returned to the engine’s main oil gallery.

This retro-fit became a standard feature of high-output VW’s in the early 1960's and has been depicted numerous times in manuals, magazine articles and even here on the internet. (Bill Fisher devoted a couple of pages to it in his famous ‘How to Hotrod Volkswagen Engines,’ published in 1970.) Indeed, it is such old news - - and such a necessary mod - - that there’s simply no justification for it to not be included on every flying Volkswagen.

-R.S.Hoover

(Ed.Note: Within minutes of posting an eagle-eyed reader asked: Is that another pump-cover being used as a vent?

Yes. It is an $8 bubble-packed item from a local VW after-market retailer. Vapor separation is accomplished by stuffing the chamber below the plate with a stainless steel pot-scrubber. The vent-line is plumbed to the carb-heat box. Since this engine will not use a fuel pump, that opening will used as the oil-filler.)

Thursday, June 14, 2007

AV - Reliable Ignition


Thanks to space-age electronics the ignition system on most modern-day automobiles can deliver 100,000 miles of service - - about 2,000 engine-hours - - and do so without any maintenance at all. Such ignition systems are superior to magnetos in every way except one: they require a source of electrical energy. Solve that problem and you can enjoy the benefits of a reliable, zero-maintenance ignition system for a scant fraction of the price of the typical magneto.

Fortunately, solving that particular problem is rather easy since a fellow named Henry Ford showed us how to do it. He installed magnets on the rim of a flywheel and surrounded the flywheel with a series of coils. So long as the engine was spinning you would have electrical power. That was in 1897, by the way.

Nowadays we have rare earth magnets the size of a dime that are a million times more powerful than the magnets Henry used on his Model T. Which means we can make a 40A dynamo smaller than a pie pan and weighing less than five pounds. Install that on the crankshaft of a VW converted for flight and you’ve solved the power problem. Now you can enjoy the benefits of a reliable, zero-maintenance ignition system for a scant fraction of the price of the typical magneto. (Why does that sound so familiar?)

Here’s how I did it.

PREPPING THE CRANKCASE.

The first task was to design a method of holding the stator coils perpendicular to the axis of the crankshaft. This is dead simple if you want to install the dynamo on the flywheel end of the crankshaft but a bit more difficult if you want to drive it from the pulley hub. One reason for the difficulty is that all VW crankcases are not idential in this area, so the first step was to get rid of any casting flash or as-cast (ie, un-machined) surfaces that might prevent the stator plate from mounting correctly. (The drawings show how to install the dynamo on either end of the engine.)

ATTACHING THE STATOR PLATE

I won’t get into the design of the stator plate nor drive hub here. There are too many drawings and pictures for a blog. Basically, you simply print-out the patterns on an accurate printer, glue them to aluminum of suitable thickness, cut them out and file the edges to split the line. Vertical spacing is automatically accommodated by using aluminum plate of the proper thickness. To keep assembly simple I used rivets, which you may hand-set if you wish.

You’ll need a lathe for the hub but perhaps we can talk someone in making the things. I’ll post the drawings and photos over on the Chuggers Group when time allows.

The stator plate is held in place by fasteners using threaded bores already existing on the engine; basically you simply bolt it on.

INSTALLING THE STATOR

Once the stator plate is attached to the crankcase you simply bolt the stator to it. The stator and rotor are available from Great Plains Aircraft Supply Co. but may also be found at motorcycle junkyards and after-market motorcycle suppliers.



INSTALLING THE ROTOR DRIVE HUB

The drive hub is turned from a 2" thick slice of 4" dia. 6061-T6 bar stock. I've made a couple from tooling plate but it doesn't machine very well. I've even melted down some old pistons and cast one. Which taught me how much I didn't know about casting :-)

This is about the tenth iteration of the design and is sized so it can be turned on one of those 7x10 hobby-lathes... if you have the determination :-)

The drive hub is a light press fit onto the nose of the crankshaft, indexed by the Woodruff key. (Making the keyway is one of the Tricky Bits. It may be filed but I bit the bullet and made up a 6mm broach.) This is a relatively fragile part; if you drop it, it will bend. I used a dial indicator to check the run-out and found about +/- .0015, which is pretty good. I’ve no idea what the maximum allowable might be but clearly, little or no wobble is best.

The hub is attached with the stock VW pulley bolt and warpy washer using thread-locker then torqued to 30 ft.lbs.

ATTACHING THE ROTOR

Be careful here. The six magnets inside of the rotor have enough pull to chop off the end of your finger if you get it in the wrong place. There’s a bit of slop in the bolting holes to allow for alignment. The bolts are commercial stainless steel 1/4-28, half an inch long having drilled heads. They are installed with thread-locker, a regular thickness AN960 washer, an external tooth star washer, torqued to 5 ft.lbs. and should be safety-wired in pairs.

The part shown is as-received from Steve at Great Plains. In a real installation it would be given a light coat of flat black paint on which the engine's timing marks would be stenciled with white paint. TDC, static firing point and maximum advance point are also marked with etchings in case the painted marks gets removed. The stencil is included with the drawings but how to make the thing is up to you. (I used the silk-screen process; a lot of work for such a tiny stencil.)

IT SHOULD LOOK LIKE THIS

As shown, the unit weighed 4-3/4 lbs, not including the pulley-bolt & washer.

I’ve now done a few of these, making hubs and stator plates of different dimensions to suit different engine-mount configurations. If you are careful with your dimensions there won’t be any alignment problems. If there are, you’ve a bit of slop in both the stator and the rotor. If you use shims to align the rotor whilst bolting-up, be sure they are brass, wood or cardboard. If you use steel you’ll be in for a surprise :-) (The unit shown is a hangar queen, a mish-mash of parts used solely for illustration.)

Depending on who you talk to this dynamo is good for up to 40 amps. And perhaps it is. But at an rpm where your prop is most efficient you’ll probably see only ten to twelve amps. Which isn’t bad; at that level of output it should run cool and last just about forever.

Ten amps at a nominal twelve volts is 120 watts. Given its small size the thing is probably about 50% efficient meaning it will take about 240 watts of mechanical energy to give you 120 watts of electrical energy. The difference will appear as heat.

Two hundred and forty watts is about a third of a horsepower, which you should include in your overhead or pumping losses. Unlike a belt-driven alternator, the coaxial arrangement provides a symmetrical load of a magnitude sufficiently small so that bearing wear is not an issue.

The rectifier/regulator should be mounted on the firewall and provided with a dry air-blast for cooling. This is the only part prone to failure and that usually happens because of excessive heating. Ensure a good ground.

There are a lot of details I've not bothered to cover here, such as when using a fuel pump (not a problem) or using the fuel pump's opening as your oil filler (ditto). Some of the details will be included with the file of drawings & photos, whenever I can find time to upload them. The purpose of this post is to explain how to provide a reliable source of electrical energy so you can run a modern, inexpensive ignition system... along with your stereo :-)

-R.S.Hoover
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