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