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