Dynamic prop balance

On Mon, 03 May 2004 16:51:07 -0600, mikem wrote:

snip

Prop balancing is done on the ground, with the engine running WOT at
whatever RPM is attainable during a static run. For most aircraft,
you cannot get the engine up to climb RPM during balancing.

snip

I have checked the vibe level in-flight on numerous occasions, on
aircraft that acted "strange" after a standard dy-bal.

The newer balancing equipment can log the vibe signature through a
wide frequency range and is useful in these cases also. The older CH
spectrum analyzer could also do a survey of a range of frequencies.

It would seem to me (and forgive me, it's been a few years) that on a
constant-speed propeller that achieving cruise rpm is quite do-able.
I'm thinking that we useta set the rpm approx. 100 rpm over the
desired "cruise" setting using the throttle, and retarded it using the
prop control.

Our thoughts were that balancing at cruise rpm would tend to reduce
the vibration level at the point that the airplane spent most of its
time in the air.

After several years of balancing inside of a hangar (aircraft securely
chained down), a couple of us could guess the IPS to within about .2
on the initial engine run.

On most singles, a reduction of less than about .4 IPS (at 1/1) is not
noticeable from the pilot's seat. Half-order vibrations are much more
noticeable/visible as instrument panel shake, and seem to be a
"harder" vibration. Unfortunately, a dy-bal doesn't do much for a
half-order vibration.

Never had a light/medium twin that "felt" any different, even after a
..7-.8 IPS reduction.

A proper dy-bal job performed by a knowledgeable technician is a
decent value. We did all the company aircraft mainly because we owned
the box...

TC

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.

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?

--
Roger Long

"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

" wrote:

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".

With a four-banger, the pistons tend to stop halfway along the bores. That would put
the front throw either up or down. That would mean that the prop stops in the
vertical position when it's indexed this way. Correct?

George Patterson
If you don't tell lies, you never have to remember what you said.

With a four-banger, the pistons tend to stop halfway along the bores. That
would put
the front throw either up or down. That would mean that the prop stops in
the
vertical position when it's indexed this way. Correct?

It's actually about 45 degrees from vertical. A whole tie down row
happened to fill up with Skyhawks the other day. While walking to my plane,
I noticed that all the props were at the same angle. It looked like someone
had gone out and arranged them. Rather a pretty picture actually.

--
Roger Long

"Roger Long" om writes:

While walking to my plane,
I noticed that all the props were at the same angle. It looked like someone
had gone out and arranged them.

This *is* done sometimes, isn't it? It's something I've suspected but never
investigated.

The reason I suspect it is that I think that I've noticed my props having
been turned after an FBO has moved it. I assumed that they want the props
near horizontal for towing. That would be especially true for a single
(tractor) engine plane.

--kyler

In article ,
Kyler Laird wrote:

"Roger Long" om writes:

While walking to my plane,
I noticed that all the props were at the same angle. It looked like someone
had gone out and arranged them.

This *is* done sometimes, isn't it? It's something I've suspected but never
investigated.

The reason I suspect it is that I think that I've noticed my props having
been turned after an FBO has moved it. I assumed that they want the props
near horizontal for towing. That would be especially true for a single
(tractor) engine plane.

--kyler

Many line service departments have a trick of turning the props vertical
(or horizontal, take your pick) after they fuel the plane. It makes it
easy to look down a line of tied-down planes and see if any need to get
topped off. This works well in a situation where there are standing
orders to refuel automatically, which would often be the case at a
flight school or club.

No these were all planes by different owners and we have a self service fuel
station.

--
Roger Long

"Kyler Laird" wrote in message
...
"Roger Long" om writes:

While walking to my plane,
I noticed that all the props were at the same angle. It looked like
someone
had gone out and arranged them.

This *is* done sometimes, isn't it? It's something I've suspected but
never
investigated.

The reason I suspect it is that I think that I've noticed my props having
been turned after an FBO has moved it. I assumed that they want the props
near horizontal for towing. That would be especially true for a single
(tractor) engine plane.

--kyler

Roger Long wrote:

With a four-banger, the pistons tend to stop halfway along the bores. That
would put
the front throw either up or down. That would mean that the prop stops in
the
vertical position when it's indexed this way. Correct?

It's actually about 45 degrees from vertical.

That indicates that the prop was indexed to allow hand-propping, not indexed for the
least vibration.

George Patterson
If you don't tell lies, you never have to remember what you said.

On Wed, 05 May 2004 14:19:08 GMT, "G.R. Patterson III"
wrote:

snip

That indicates that the prop was indexed to allow hand-propping, not indexed for the
least vibration.

sig snip

On Wed, 05 May 2004 02:30:47 GMT, "G.R. Patterson III"
wrote:

With a four-banger, the pistons tend to stop halfway along the bores. That would put
the front throw either up or down. That would mean that the prop stops in the
vertical position when it's indexed this way. Correct?

Unsure. Honestly have never really thought about it from this
perspective.

As you posted above, I seem to remember when indexed according to "the
book", the descending blade would typically stop approximately 45
degrees from the "top" (vertical)..

It would seem reasonable to me that this would be one bolt hole away
(in the direction of rotation) from being aligned with the #1 throw.

I'm sorry I cannot be more specific, it's been several years since I
left GA. If you don't "use it", you eventually start to "lose it", I
guess.

TC