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#11
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VW Reality
Birds aren't engines. See...
http://people.bath.ac.uk/ccsshb/12cyl/ ....note the references to the thermal efficiency and specific fuel consumption of smaller engines. -R.S.Hoover |
#12
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VW Reality
On Thu, 31 Jan 2008 11:28:57 -0800 (PST), "
wrote: another aspect of the VW reality that has been alluded to by other posters, who gave details of the symptoms but seemed unaware of the causes, is destructive harmonic vibration in the crankshaft. in the middle of the crankcase is a web that supports the centre bearing. above that in the middle is a gap where the case and this centre piece have a hollow part. in the usual VW car engine the harmonic charactersitics of the crankshaft are such that the destructive vibration occurs a few thousand revs above the max possible rpm so the engine never experiences a problem. in aircraft installations with large heavy propellors attached to the shaft the harmonic range occurs at a lower RPM. particularly with large 3 bladed ground adjustable composite props the destructive harmonic range comes down to about 3,200rpm. most VW's would run at around those revs so the problem becomes an unavoidable one. the destructive harmonics are indetectable in the cockpit above the normal engine vibrations but in the engine's interior those harmonics deliver hammer like blows to the crank web. the hollow area above the centre of the internal web gets a fair shaking and develops a fine hairline fatigue crack. this crack leaks copious amounts of oil and of course there is no way of welding oily electron so the case becomes junk. the problem can be avoided completely by using lightweight fixed two bladed wooden props. so again there is an upper limit to how much power you can extract from a VW. a 1600 will be a long lived engine. an 1835 pretty good. check out how little cylinder wall remains in a 2100 and you'll probably stck to the more modest displacements. get the installation right though and the vw will pull the aircraft around for decades. english Druine Turbulents demonstrate this. Stealth Pilot |
#13
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VW Reality
wrote in message
... Birds aren't engines. See... The same concepts apply. Unfortunately, I was applying the wrong concepts. I mis-understood the term 'thermal efficiency'. http://people.bath.ac.uk/ccsshb/12cyl/ That's one nasty engine! ...note the references to the thermal efficiency and specific fuel consumption of smaller engines. AH HA. -R.S.Hoover Thank for the extra help for the slow. - Steve (Another one not left behind) |
#14
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VW Reality
On Feb 1, 5:29 am, Stealth Pilot
wrote: another aspect of the VW reality that has been alluded to by other posters, who gave details of the symptoms but seemed unaware of the causes, is destructive harmonic vibration in the crankshaft. - - - - - - - - - - - - - - - - Perhaps not. The VW crankshaft is of the 'boxer' design, where the connecting rod throws are located in the same plane and symmetrically about the center main bearing. With the cylinders arranged as opposed pairs this allows the use of a camshaft having only four lobes. By about 1914 both Porsche and Reimspiess had assembled engines of this configuration, which was copied by several American engine builders. By the late 1930's it was understood that the boxer design had a number of short-comings, one being accelerated wear of the cam, since each lobe was required to actuate a pair of opposed valves, but also a curious pattern of wear that appeared in the center main bearing. Fortunately, these things were relatively easy to fix... unless Hitler and the Nazi Party happened to be your business partner. The bottom line is that the Volkswagen engine retained these inherent flaws whereas the Continental and Lycoming did not. Advances in metallurgy - specifically, the method of gas-nitriding cast iron developed by Krupp - eventually resolved the cam-wear problem but pounding-out of the center main bearing web turned out to be a flaw inherent with the boxer geometry. Indeed, it was not fully understood until the mid-1950's when the problem was finally defined as phlugoid motion generated by the geometry of the VW crankshaft. The best description of the problem - and to understand the cure - is to do a side-by-side comparison of a VW and a Subaru crankshaft. - - - - - - - - - - - - - - - - - - - - - - - - - - - in the middle of the crankcase is a web that supports the centre bearing. above that in the middle is a gap where the case and this centre piece have a hollow part... ....the destructive harmonics are indetectable in the cockpit above the normal engine vibrations but in the engine's interior those harmonics deliver hammer like blows to the crank web. the hollow area above the centre of the internal web gets a fair shaking and develops a fine hairline fatigue crack. this crack leaks copious amounts of oil and of course there is no way of welding oily electron so the case becomes junk. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - I'm afraid I can't quite follow that. I've uploaded some pix of the center main bearing web to my blog... http://bobhooversblog.blogspot.com/ .....that may help you to see why I don't understand your description of where your crack occurred. Other than cracks resulting from catastrophic failures such as a thrown rod, the only cracks I've seen in the vicinity of the center main bearing web have been the product of shoddy machine work, either due to improper clearencing or using cutting tools having the wrong radius. Another factor is where - and how - the propeller is attached to the crankshaft, specifically with regard to the THRUST BEARING. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .....particularly with large 3 bladed ground adjustable composite props the destructive harmonic range comes down to about 3,200rpm. most VW's would run at around those revs so the problem becomes an unavoidable one. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Again, perhaps not. The only reason for running the engine at that rpm is because most flying Volkswagens are fitted with hot-rod cams. If you retain the stock cam and simply retard it by four degrees you will move the torque-peak to about 1800rpm. By then adjusting the displacement and compression ratio in keeping with the thermal limitations of the cylinder heads, you will have a sturdy, reliable, DURABLE engine capable of delivering in excess of 75hp @ 2750rpm for take-off and about 40hp for cruise. But such an engine is a far cry from the dune-buggy mentality on which most flying VW's are based. As for your mention of cylinder wall thickness, apparently as a limitation to power output, the wall of a 94mm barrel is actually thicker than that of the typical 92mm barrel. But the real problem isn't the thickness of the wall but the competence of the SEAL between the wall and the cylinder head. Proper assembly and the use of fully annealed copper seals makes this a non-issue, assuming the compression ratio is within reason. I'm sorry your engine suffered a crack but I've a hunch you'll find a lot more reasons for it before you work your way down to something as exotic as 'crankshaft harmonics,' although this may be only a difference of terms. -R.S.Hoover |
#15
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VW Reality
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#16
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VW Reality
wrote in message ...
******************************************* "...the great little VW conversion with a 2.0:1 reduction producing 103 HP." --------------------------------------------------------- To All: In the context of an engine converted for flight the figures above are wildly fallacious. At best, they represent a 'dyno blip,' at worst they may be an out-right lie. Here's why: The maximum SUSTAINABLE power available from ANY air-cooled engine is determined by the engine's ability to cool itself -- to couple its waste-heat to the atmosphere. And with a carburetted, spark-ignited, gasoline-fueled engine there is a LOT of waste heat to be managed since such engines are no more than 25% efficient when it comes to converting the heat of combustion into torque at the crankshaft. That means that for every horsepower measured at the crank you must generate at least four horsepower's-worth of heat in combustion. These basic rules of thermodynamics are made even worse by two additional factors, the first being 'Economy of Scale' in that smaller engines are LESS thermally efficient than larger engines, and the basic definition of Standard Day conditions -- 59.9 degrees on the Fahrenheit scale and an atmospheric pressure of 29.92 inches of mercury -- a fairly cool day. With those laws of physics as preamble the next factor worthy of note is the physical dimensions of the Volkswagen cylinder head and the fact that ALL VW heads have the same exterior dimensions. This is because they must fit under the stock VW engine shrouding. No manufacturer of VW heads, either stock or after-market, offers a head having more fin area. Indeed, most after-market heads have LESS, due either to thickening of the combustion chamber wall or even eliminating one of the eight fins -- and in a few cases they have done both. All -- ALL -- Volkswagen heads in common use today are derived from the heads developed for the 1300cc engine; their external physical dimensions remained exactly the same for the later 1500 and 1600 engines. The output of the 1300 engine was approximately 40hp and could SUSTAIN that level of output indefinitely under Standard Day conditions. This engine was bored-out to 83mm to produce the 1500 engine, then over-bored to 85.5mm to create the 1600 (actual displacement 1584cc), the maximum output of which was 57bhp for carburetted models, achieved in the 1971 model year. But that level of output could only be sustained for a bit less than FOUR MINUTES, until the cylinder head temperature exceeded safe levels, again under Standard Day conditions. So what's this 'safe level' of CHT? About 450 degrees on the Fahrenheit scale. This reflects the fact that VW heads are made of CAST aluminum (as opposed to a forging) and the fact aluminum is a 'white short' metal, meaning it becomes frangible when its temperature enters the 'plastic' range. A characteristic of white-short metals is that when heated they fracture like a cube of sugar when subjected to stress. The floor of the frangible range is a bit higher for a forging -- about 550F according to Pratt-Whitney -- but can be as low as 400F in a casting, depending upon the alloy. A common thread used to impress technologically naive buyers is tales of driving a Volkswagen bug or bus for hours on end with the throttle wide open. The fact the engine was was probably producing LESS THAN TWENTY HORSEPOWER goes unsaid. This involves the Horsepower Myth and generally leaves a large black question mark hanging over the heads of those without an engineering background but it needs to be touched upon since ignorance can be as deadly as a machine gun when it comes to aviation. The Horsepower Myth was create by James Watt in order to sell his modified Newcommen steam engine to mine owners. To do so he added the element of TIME to the power equation and from that day to this the general public has been comfortable with the idea that 'horsepower' represents a given quanta of energy... which it does... but only within a defined unit of TIME. And from that day to this, that arithmetical loophole has been used by those eager to prey upon technologically naive consumers. Indeed, in the early days of aviation those predations cost so many lives that government agencies had to step in, requiring the manufacturers of aircraft engines to justify their claims of power and durability. ------------------------------------------------------ All of which tends to leave the average homebuilder with more questions than answers. Fortunately, the ENGINES themselves are incapable of lying, especially when it comes to FUEL CONSUMPTION. The Specific Fuel Consumption (SFC) of all -- ALL -- air-cooled, gasoline-fueled, normally aspirated Otto-cycle engines is clumped near the 0.5 mark, meaning it takes about 0.5 POUNDS of 'gasoline' (*) per HOUR to produce ONE HORSEPOWER'S worth of torque at the crankshaft. For aviation gasoline that works out to about 12bhp per gallon per hour. For a 103hp engine that works out to 8.58 gph. -------------------------------------------- (*) -- Thanks to additives and dilutants (such as alcohol) gasoline intended for automobiles has LESS potential energy. --------------------------------------------- So when someone tries to sell you their Whiz-Bang 103hp VW engine, simply ask about its fuel consumption. If they give you an honest answer, such as 'nine gallons per hour' your next question should be 'For what TBO?' (And if they try to feed you the usual '3gph' bull****, simply walk away.) The truth is, by simply spinning an engine faster you can claim an impressive amount of 'horsepower' -- up to 1500bhp for some 'VW' powered dragsters (but with a TBO measured in MINUTES). Some years ago turbosupercharged VW engines were all the rage... until people learned they needed a valve job about every ten hours, no big deal if you're only SELLING such engines -- but of some importance to the folks who actually FLY them :-) -R.S.Hoover We made 11.89 in a 1/4 mile with our 1800cc 'dragster' Ghia, quad carbs and total run time of minutes before complete teardown and rebuild. Fun, but would never go in a plane. We did not have a dyno but it sure ran nice! The 1800cc did better than the 2180... |
#17
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VW Reality
Bob Aeropower here in Perth was an aero engine conversion business run by Sig Muninger and his dad as an adjunct to a healthy VW car overhaul business. ....probably about 10 years ago. The problem caused a string of failures in aeropower conversions. identifying and solving the problem cost Sig over $100,000. The problem and the work involved in identifying it's cause were explained to me by Sig himself as we stood beside the test rig. what can I say? Your mileage may vary. just dont stick a heavy prop on your conversion. Stealth Pilot On Fri, 1 Feb 2008 13:10:58 -0800 (PST), " wrote: On Feb 1, 5:29 am, Stealth Pilot wrote: another aspect of the VW reality that has been alluded to by other posters, who gave details of the symptoms but seemed unaware of the causes, is destructive harmonic vibration in the crankshaft. - - - - - - - - - - - - - - - - Perhaps not. The VW crankshaft is of the 'boxer' design, where the connecting rod throws are located in the same plane and symmetrically about the center main bearing. With the cylinders arranged as opposed pairs this allows the use of a camshaft having only four lobes. By about 1914 both Porsche and Reimspiess had assembled engines of this configuration, which was copied by several American engine builders. By the late 1930's it was understood that the boxer design had a number of short-comings, one being accelerated wear of the cam, since each lobe was required to actuate a pair of opposed valves, but also a curious pattern of wear that appeared in the center main bearing. Fortunately, these things were relatively easy to fix... unless Hitler and the Nazi Party happened to be your business partner. The bottom line is that the Volkswagen engine retained these inherent flaws whereas the Continental and Lycoming did not. Advances in metallurgy - specifically, the method of gas-nitriding cast iron developed by Krupp - eventually resolved the cam-wear problem but pounding-out of the center main bearing web turned out to be a flaw inherent with the boxer geometry. Indeed, it was not fully understood until the mid-1950's when the problem was finally defined as phlugoid motion generated by the geometry of the VW crankshaft. The best description of the problem - and to understand the cure - is to do a side-by-side comparison of a VW and a Subaru crankshaft. - - - - - - - - - - - - - - - - - - - - - - - - - - - in the middle of the crankcase is a web that supports the centre bearing. above that in the middle is a gap where the case and this centre piece have a hollow part... ...the destructive harmonics are indetectable in the cockpit above the normal engine vibrations but in the engine's interior those harmonics deliver hammer like blows to the crank web. the hollow area above the centre of the internal web gets a fair shaking and develops a fine hairline fatigue crack. this crack leaks copious amounts of oil and of course there is no way of welding oily electron so the case becomes junk. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - I'm afraid I can't quite follow that. I've uploaded some pix of the center main bearing web to my blog... http://bobhooversblog.blogspot.com/ ....that may help you to see why I don't understand your description of where your crack occurred. Other than cracks resulting from catastrophic failures such as a thrown rod, the only cracks I've seen in the vicinity of the center main bearing web have been the product of shoddy machine work, either due to improper clearencing or using cutting tools having the wrong radius. Another factor is where - and how - the propeller is attached to the crankshaft, specifically with regard to the THRUST BEARING. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ....particularly with large 3 bladed ground adjustable composite props the destructive harmonic range comes down to about 3,200rpm. most VW's would run at around those revs so the problem becomes an unavoidable one. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Again, perhaps not. The only reason for running the engine at that rpm is because most flying Volkswagens are fitted with hot-rod cams. If you retain the stock cam and simply retard it by four degrees you will move the torque-peak to about 1800rpm. By then adjusting the displacement and compression ratio in keeping with the thermal limitations of the cylinder heads, you will have a sturdy, reliable, DURABLE engine capable of delivering in excess of 75hp @ 2750rpm for take-off and about 40hp for cruise. But such an engine is a far cry from the dune-buggy mentality on which most flying VW's are based. As for your mention of cylinder wall thickness, apparently as a limitation to power output, the wall of a 94mm barrel is actually thicker than that of the typical 92mm barrel. But the real problem isn't the thickness of the wall but the competence of the SEAL between the wall and the cylinder head. Proper assembly and the use of fully annealed copper seals makes this a non-issue, assuming the compression ratio is within reason. I'm sorry your engine suffered a crack but I've a hunch you'll find a lot more reasons for it before you work your way down to something as exotic as 'crankshaft harmonics,' although this may be only a difference of terms. -R.S.Hoover |
#18
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VW Reality
On Thu, 31 Jan 2008 11:28:57 -0800 (PST), "
wrote: Bob I admire your work so I'm going to irritate you with an argument. I wish someone would post a concise guide to a style of VW conversion that is durable and simple. have never seen one that had the tests of time behind it. 1600 or 1835cc for preference. as to the hollow part... in your blog page top photo look at the left side rusty stud. look just to the left of that stud and you'll see an area where the metal that the stud screws into doesnt extend to the case outer side. that is the hollow part I was referring to. bad terminology?? the cracks that occurred in Sig's cases were in the centre of the case top just to one side of centre. it's the common crack area I believe. btw to continue your rib baking experiment you should read up the Molt Taylor, Jerry Holcombe experiments on TPG (Taylor Paper Glass). they used a medium thickness kraft paper. seems like the stuff that some paper gaskets are made of. Btw commercial Laminex is/was just paper sheets and epoxy.the top paper sheet is printed with the pattern. keep up the good work. enjoy your posts. Stealth Pilot |
#19
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VW Reality
Stealth Pilot wrote:
Bob Aeropower here in Perth was an aero engine conversion business run by Sig Muninger and his dad as an adjunct to a healthy VW car overhaul business. ....probably about 10 years ago. The problem caused a string of failures in aeropower conversions. identifying and solving the problem cost Sig over $100,000. The problem and the work involved in identifying it's cause were explained to me by Sig himself as we stood beside the test rig. what can I say? Your mileage may vary. just dont stick a heavy prop on your conversion. Stealth Pilot Was he trying to use metal props? Or cast cranks? |
#20
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VW Reality
On Sat, 02 Feb 2008 12:56:55 -0800, cavalamb himself
wrote: Stealth Pilot wrote: Bob Aeropower here in Perth was an aero engine conversion business run by Sig Muninger and his dad as an adjunct to a healthy VW car overhaul business. ....probably about 10 years ago. The problem caused a string of failures in aeropower conversions. identifying and solving the problem cost Sig over $100,000. The problem and the work involved in identifying it's cause were explained to me by Sig himself as we stood beside the test rig. what can I say? Your mileage may vary. just dont stick a heavy prop on your conversion. Stealth Pilot Was he trying to use metal props? Or cast cranks? neither. Stealth Pilot |
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