Monday, April 20, 2009
On the Wearing Out of Things
Strop my old cut-throat razor as good as I can, that ten-cent throw-away it's laying on will give me a better shave. Oh, maybe not for the first stroke or two. Well-steamed whiskers under a layer of hot lather, there's a velvet touch to that first stroke of the blade. But after that the edge on the throw-away will pull steadily ahead until you feel slightly foolish, bobbing and weaving against your image in the outside mirror while the other campers try not to stare.
Little boy whispers too loudly to his dad: "What's that old man doing?" It's called shaving, son. Yes, even with a beard you need to touch-up the trim-work now & then. So rarely that an old fashioned razor makes good sense. That is, I used to think it did. But since I started adding to the planet's land-fill I'm forced to admit that a throw-away gives me a better shave.
The reason why that is true has a lot in common with why a lot of home-builders are throwing away as much as twenty-five percent of their engine's power. 25%
Impossible! (I heard someone say. Probably the grandson of the fellow who said nothing would ever beat his cut-throat razor.) The reason it is possible is because the fellow has never flown -- nor driven -- a professionally-built engine. And before you start telling me that I have to be wrong I'd like you to take another look at that old straight-razor of mine. Sharpen it to perfection and within half a stroke you can feel it starting to drag. That's all it takes to turn the edge. You can restore it by a few judicious strokes on your finest strop but you will have to repeat the process, half a stroke then strop... half a stroke then strop... as your shave progresses. By comparison the machine-sharpened blade in that inexpensive throw-away is good for about one and a half shaves before it even begins to dull.
There's really nothing new in all that. Cutting tools dull in use, especially so when cutting metal. For example, when cutting the bores for the crankshaft and camshaft, they will start with multi-point cutting bar that creates a hole on the high side of the allowed tolerance. After cutting a certain number of crankcases they will gauge the bores, then cut a few more, gauge it again and keep repeating the process of cutting and gauging until the inspector tells the machinist it's time to replace his cutting tools.
In other words, there isn't a single perfect size but a range of sizes that grow steadily smaller as the machinist produces crankcases. This difference in size represents the tolerance for that particular machining operation. (You will find all of the tolerances listed in the factory workshop manual.)
Machining the bores for the two shafts generates a third dimension and its related tolerance. That's the distance between the center-line of the bores. Since the two shafts are coupled together by a pair of gears you will also find a range of acceptable gear sizes, each having its own tolerance.
Volkswagen used nine different sizes of gears. To properly assemble a VW engine your first step is to determine the fit of the valve gear. Which begs the question: What size valve gear does your engine use?
When you assemble an engine using the wrong size of valve gear two things are immediately evident. One is that the steel cam driver gear will commence to chew up the aluminum driven gear, generating lots of swarf to block your filter and contaminate your bearings. The other thing is that the engine will not produce as much power as if would if fitted with the proper gearing. But worst of all is the culmination of those two errors. Not only is the bad to begin with, it will become steadily worse in use.
Engines are incapable of healing themselves. Start with a bad lower end and things can only get worse. But engines are capable of humor. Ask someone what size cam gear their engine uses and the odds are they won't be able to tell you, even though they have just spent half an hour expounding upon their experience as a builder of VW engines. And while they can't tell you what size gear they used -- many will even argue that only one size is available -- they are equally sure that, whatever size was needed is exactly the size they've used :-)
The reason I've mentioned all this has to do with the following scenario: Someone has just spent a sinful amount of money to assemble a 2180cc VW engine. But having done so, when pitted against a supposedly identical airframe and power-plant they have discovered their engine is not quite as powerful as they expected. Indeed, a few have had the unfortunate experience of having a similar airplane fitted with a smaller engine out-climb them. And go faster. And burn less fuel.
Obviously, something is seriously wrong. He's paid all that lovely money and done everything the various experts have told him to do but he's in much the same situation as the fellow with an old straight-razor who discovers he can get a better shave from an imported throw-away that costs only a dime.
Another aspect of this scenario is purely personal and may be an error on my part. As best I can recall, I used to be asked what's wrong with my engine three or four times a year. But since being diagnosed with cancer (June, 2008) it seems that question is popping up more frequently than before. (I can imagine what you're thinking. Believe me, pard, I've been thinking the same thing :-) But win, lose or draw, I think all we can do is play the cards we're dealt. So, whether the question has been coming up more frequently... or even if I'm just thinking it has, it seems worth addressing, which is the purpose of this article.
So how do you help someone diagnose an engine problem via email? It's a bit harder than it may appear because while you must address the engine as a system -- as something that must work together as an harmonious whole -- when the components are produced by a chaotic collection of manufacturers, you must begin at the component level and work your way up. Each component is blueprinted, then components are assembled into sub-assemblies and -- finally -- you can assemble the sub-assemblies into that harmonious whole.
The reason this difficult is not the blueprinting nor the assembling into sub-assemblies. Those tasks, while time consuming, are quite straight-forward. The difficulty arises from what you must do when a part or sub-assembly FAILS to meet its required specifications.
In a factory, any part or sub-assembly that fails to meet spec is simply laid aside to be re-worked, substituting parts until you achieve the desired goal. But the homebuilder is usually working with only ONE ENGINE'S-WORTH OF PARTS. If they are an experienced automotive machinist they may elect to re-manufacture the supposedly 'new' part. Or they can try to get the supply to ship them a replacement. Or whatever. The truth is, the MOST DIFFICULT part of assembling an engine today is simply finding one engine's-worth of parts.
Many of the fellows needing help have removed and dismantled their engine, looking for the Smoking Gun that is preventing them from flying as fast or climbing as quickly as those Other Guys.
But when the engine is still on the airplane, and has flown enough hours to establish a history -- say, a hundred hours or so -- you can leave it alone and assume it has been properly assembled. And yes, it is a risky assumption; a double-edged blade that can cut you both ways. Such as the fellow who casually asks: "You keep mentioning Compression Ratio... What is that, exactly?" Or the fellow who thinks a 300 fpm Rate of Climb is okay... in an airplane that normally climbs at about a thousand feet per minute.
What you're running into here is the fact that in many cases the person seeking advice is unable to evaluate good performance from bad performance because they have only one airplane's-worth of experience. Or the fellow who's engine has more than an eighth of an inch of end-play. But it starts at the first flip of the prop and never gives any problems... So why should he change things?
(Yeah, I'm smiling too. But I'm also serious. I get a lot of questions you really would not believe.)
So the root of the Decision Tree is bifurcated into Component Level Questions and Hundred Hour Questions. For the purpose of this article I'm going to start with the Hundred Hour Questions and do so by starting with the Lower End -- crankshaft, camshaft, con-rods, pistons & jugs, cam gearing, valve timing and so forth: the foundation of the engine.
Ninty-nine times out of a hundred the search ends at the cam gear. There is simply no reason to continue when we lack the basis for setting the valve-train geometry. And when you can't set your valve train geometry the probability of giving away as much as 25% of your power is very real indeed. As is the likelihood that you would not be aware of it. In fact, there is no accepted standard of success when it comes to the conversion of Volkswagen engine for flight. The fellow has assembled an engine and the thing actually ran! Even more amazing, it produced enough power to fly the plane! Clearly, we are dealing with the epitome of success. To have someone come along and imply that things could be even better...
The only down-side to the supposition that any engine which runs and produces enough power to fly the plane is a success is the specter of catastrophic failure. One of the most compelling reasons for the use of an Otto Cycle engine is that it provides a number of easily recognized precursors well in advance of any mechanical failure. But that is only valid when the engine was properly assembled to begin with.