"Roger Long" om wrote in message ...
It sounds like you could answer this question.

We had the prop on our 172N dynamically balanced a couple years ago and the
plane was smooth as silk. The prop was put back on rotated about 40 degrees
from the original position because that is where it was smoothest. This
resulted in it always stopping vertical which was a pain using the towbar.

That shop went out of business and we just had the engine rebuilt after
premature lifter failure. The new shop insisted on putting the prop back
according to the service manual instead of the index marks. It's hard to
tell with three months of no flying in between but my impression is that it
is not as smooth.

Another shop I talked to about rotating the prop back agreed that
positioning according to the shop manual is important (especially on a 4 jug
engine) because of internal torsional vibration and stress issues that do
not create noticeable airframe vibration.

Have discussed this issue with Lycoming and TCM, also Hartzell and
McCauley. The prop people say that from a vibration standpoint, they
prefer to have a blade aligned with the front crankshaft "throw". The
engine people say that it really doesn't make any difference as far as
they are concerned, and that it is an "airframe" issue.

Have noticed in a lot of cases with a fixed pitch install (as you've
indicated, changing the indexing is easy) that the factory's
instructions tend to yield a blade in close to the ideal hand-propping
position.

On a constant-speed installation, often indexing choices are limited.
On some (all?) TCM's, there are indexing pins that limit you to a 180
degree swap. On Lycoming's there is a short prop bolt bushing in the
crank that mates up with a matching impression on a Hartzell hub, am
thinking on the McCauley's you have more than one option. On the
PA31-350 McCauley "conversion" following the instructions causes one
blade to point straight down, we always installed them flipped 180 to
stagger the 2 downward facing blades.

Have indexed a few when balancing, never had an instance where it made
a big difference that I recall.

We don't have anything I would call vibration. It's more of a buzz that you
hear rather than feel.

Do you think we should have the prop rebalanced?

I guess I always figured if you were going to go the dy-bal route,
engine overhaul or prop overhaul/rework (or both) is a good time to
get "re"-balanced. Our policy was to charge approx. 1/2 the cost if no
adjustment was needed. I have talked to other shops that had the same
policy.

Another general recommendation for a dy-bal job is to grease the hub
(if applicable to a constant-speed prop install) and dress the leading
edges and touch-up the paint prior to dy-bal. Just never made a lot of
sense to me to start filing and painting soon after a dy-bal.

During a engine overhaul on a fixed-pitch application, at the minimum
I recommend getting the prop static balance checked while it's off.

Hope some of this helps;

TC