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

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

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)

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

wrote:


Is it fair to say that with a particular engine type, that SFC can be
used to estimate HP? (because that's what the equations are doing
seems to me)



Varies somewhat with an engine's efficiency. Most normally aspirated
air cooled engines are in the .42 to .45 range. That's leaned out to
best efficiency remember. Car engines are I believe, around or little
under .4 by using electronic fuel injection and low friction (narrow)
bearings etc. and the more efficient diesels in the mid .3s. IIRC, the
turbocompound radials like the R3360 were in the high .3s, among the
most efficient gas engines built, even if they didn't run like that very
long before something went south.

I think .45 lb/hp/hr will give a valid fuel consumption number, accurate
to a few percent, for pretty much any power setting that can be done at
lean mixture on just about any gasoline air cooled, carburated engine.

John

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...

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

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

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?

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