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In a message dated 6/18/2001 6:10:35 PM Pacific Daylight Time,
aaronmay82@lycos.com writes:

<< I am just beginning to build my
 know-how when it comes to all things kitplanes, and have stressed that by
 preluding most posts with the classic "open for scrutiny" line. >>

Aaron:

Here's another bit of info which I thought of after I replied yesterday.

A recent Hartzell vibration survey revealed an interesting situation. There
is a certain Hartzell propeller which is certified on a Lycoming IO-360-A3B6D
for a specific aircraft. This engine is rated at 200HP, and is equipped with
8.7:1 compression ratio pistons (LW-10207-S) and 6th and 8th order torsional
counterweights. In that configuration, the propeller blade stresses were
acceptable.
 
However, there is an STC which allows the installation of the Lycoming 10:1
pistons (LW-11487-S from the HIO-360-D1A) to increase the engine power
output. Hartzell discovered that the installation of those STC'd pistons
significantly altered the torsional characteristics of the IO-360-A engine,
to the point that it drove the blade root stresses beyond the endurance limit
of the material. That engine, with its pendulous torsional absorber
counterweights, might be expected to be "torsionally tolerant" (for a
4-cylinder engine). The fact that a relatively small change in engine
configuration caused such a significant change in vibratory stress gives some
insight into just how critical this engine/propeller interaction can be, and
the kind of risks people take making changes to this stuff without
understanding what's involved.

Jack Kane
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