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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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Please send your photos and drawings to marvkaye@olsusa.com.