Prop Indexing

In rec.aviation.owning Mike Granby wrote:

: Why are fixed-pitch props on PA-28s indexed to the 10-4 position from
: the front?

: a) To help hand propping?
: b) To minimize vibration?
: c) To improve performance?

Interesting question and one that I've had since we got our PA28-180. When we
got ours, it was indexed so TDC was at about 9-3 (from the front). Two other guys'
cherokees were at about 7-1. I liked ours since it was easier to pull through as
though hand-propping. A year ago my mechanic decided it wasn't correct, so we
reindexed ours to be at 7-1. At that time, we consulted the service manual, which
states 8-2... equally incorrect at both locations. We decided to reindex ours to the
lower location anyway. After doing that, a test flight seemed to have less vibration
than we'd had in the past.

So, this brings up more questions:
- Is the 8-2 position listed in the service manual supposed to be from the *cockpit*
view or the front view? If from the cockpit, then your 10-4 assertion is equivalent.
- If my plane (now in the lower, hand-propping-hostile location) is incorrect, why did
the perceived vibration decrease?
- What the f*sck IS the correct orientation?


To propose an answer to your original question, I recall someone (powerflow?)
claiming performance benefits. Maybe it was for a different aircraft, but they
claiming that propwash pulses timed with air intake strokes resulted in a slightly
higher obtainable manifold pressure. Seems possible, as even 1/2" of MP is noticable
in-flight. Still not sure I buy into it though.

-Cory

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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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