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In a message dated 7/30/03 5:59:57 PM Pacific Daylight Time, "Scott percival" <percivalsd@hotmail.com> writes:
.....I believe that Hartzell don't recommend the 68" prop on the 200hp none (sic) counter weighted engine only the 72"? .....
Believe it or not, if Hartzell recommends against the use of a particular prop on a particular engine, there is at least one (and possibly more) sound engineering reason for that position. They do a great deal of highly competent research on various engine/prop combinations. The short blades have a higher resonant frequency and the angle-valve 360 without the 6th and 8th order absorbing counterweights have a grotesque torsional excitation profile.
Perhaps you may have noticed that on certificated aircraft with the non-counterweighted 360's, (which use a prop which has been vibration-surveyed and stamped "OK"), there's an RPM band right in the middle of the operating range which is marked in yellow on the tach, with POH notations recommending against operating in that yellow zone. Do you suppose they did that in an effort to use up an oversupply of yellow paint?
(If memory serves, Hartzell doesnt like the HC-F2YR-1F/F7068-2 used on the counterweighted 360 either.)
......are there people using the 68" on 200hp none counter weighted engines and do we have a safe history with this usage? ......
What possible difference could the answer to that question make? It's like: Are there people successfully jumping off bridges and not getting killed?? Any combination you decide to bolt together is "safe" until the point at which it sheds a blade segment.
If you want to ignore learned and knowledgeable recommendations on the basis of wishful thinking and perpetual motion, that's what experimental aviation enables you to do. But experience has shown that MANY of these recommendations are written in blood.
Jack Kane
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