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Ben Jackson
Guest
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 Posted: Mon May 03, 2004 7:32 pm Post subject: Dynamic prop balance |
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My IO-540 with Hartzell compact 2 blade prop is smoother at 2700 RPM
than at cruise (2200-2400 RPM). If I have it dynamically balanced at
cruise will it vibrate more at climb power? Does the fact that it's
smooth at climb power suggest that the engine/prop combo is okay and
there is a problem elsewhere (mounts, etc)?
--
Ben Jackson
<ben (AT) ben (DOT) com>
http://www.ben.com/
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Ron Rosenfeld
Guest
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| Posted: Tue May 04, 2004 12:38 am Post subject: Re: Dynamic prop balance |
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On Mon, 03 May 2004 19:32:21 GMT, [email]ben (AT) ben (DOT) com[/email] (Ben Jackson) wrote:
| Quote: | My IO-540 with Hartzell compact 2 blade prop is smoother at 2700 RPM
than at cruise (2200-2400 RPM). If I have it dynamically balanced at
cruise will it vibrate more at climb power? Does the fact that it's
smooth at climb power suggest that the engine/prop combo is okay and
there is a problem elsewhere (mounts, etc)?
|
How do you know it's the engine? Sometimes there can be vibration in the
airframe at various speeds and/or AOA's.
But it's not unusual for an engine/prop combination to have a "sweet spot".
If your prop has not been balanced, it's a good idea to start there.
Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)
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toecutter1962@yahoo.com
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| Posted: Tue May 04, 2004 4:11 am Post subject: Re: Dynamic prop balance |
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On Mon, 03 May 2004 16:51:07 -0600, mikem <mikem (AT) bogus (DOT) adr> wrote:
snip
| Quote: | 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.
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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
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Roger Long
Guest
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| Posted: Tue May 04, 2004 9:58 am Post subject: Re: Dynamic prop balance |
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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
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toecutter1962@yahoo.com
Guest
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| Posted: Tue May 04, 2004 5:29 pm Post subject: Re: Dynamic prop balance |
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"Roger Long" <rwlong.take.out.obvious.antispam.stuff (AT) maine (DOT) rr.com> wrote
| Quote: | 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.
| Quote: | 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
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G.R. Patterson III
Guest
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| Posted: Wed May 05, 2004 2:30 am Post subject: Re: Dynamic prop balance |
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"toecutter1962 (AT) yahoo (DOT) com" wrote:
| Quote: |
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.
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Roger Long
Guest
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| Posted: Wed May 05, 2004 9:42 am Post subject: Re: Dynamic prop balance |
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| Quote: | 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
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Kyler Laird
Guest
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| Posted: Wed May 05, 2004 2:08 pm Post subject: Re: Dynamic prop balance |
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"Roger Long" <rwlong.take.out.obvious.antispam.stuff (AT) maine (DOT) rr.com> writes:
| Quote: | 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
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Roy Smith
Guest
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| Posted: Wed May 05, 2004 2:18 pm Post subject: Re: Dynamic prop balance |
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In article <hn7nm1-t04.ln1 (AT) snout (DOT) lairds.org>,
Kyler Laird <Kyler (AT) news (DOT) Lairds.org> wrote:
| Quote: | "Roger Long" <rwlong.take.out.obvious.antispam.stuff (AT) maine (DOT) rr.com> 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
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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.
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G.R. Patterson III
Guest
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| Posted: Wed May 05, 2004 2:19 pm Post subject: Re: Dynamic prop balance |
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Roger Long wrote:
| Quote: |
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.
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Roger Long
Guest
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| Posted: Wed May 05, 2004 5:34 pm Post subject: Re: Dynamic prop balance |
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No these were all planes by different owners and we have a self service fuel
station.
--
Roger Long
"Kyler Laird" <Kyler (AT) news (DOT) Lairds.org> wrote
| Quote: | "Roger Long" <rwlong.take.out.obvious.antispam.stuff (AT) maine (DOT) rr.com> 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
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toecutter1962@yahoo.com
Guest
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| Posted: Thu May 06, 2004 1:50 am Post subject: Re: Dynamic prop balance |
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On Wed, 05 May 2004 14:19:08 GMT, "G.R. Patterson III"
<grpphoto (AT) verizon (DOT) net> wrote:
snip
| Quote: | 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"
<grpphoto (AT) comcast (DOT) net> wrote:
| Quote: | 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
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MikeM
Guest
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| Posted: Thu May 06, 2004 2:36 am Post subject: Re: Dynamic prop balance |
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[email]toecutter1962 (AT) yahoo (DOT) com[/email] wrote:
| Quote: | I have checked the vibe level in-flight on numerous occasions, on
aircraft that acted "strange" after a standard dy-bal.
|
How do you get the wire from the accelerometer/position pickup into the
cabin during flight?
| Quote: | 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.
|
Since the only place that you can put a weight is on the prop spinner
backing plate, can Prop balancing do anything except cancel the "first
order" mass imbalance (usually called a "static" balance)?
Wouldn't you have to have access to the other end of the crankshaft to
add weights there in order to be able to do a true multi dimensional
"dynamic balance"?
I'm not trying to be argumentative; just trying to understand the claims
made by the purveyors of prop balancing services.
| Quote: | 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.
|
I have wondered that during a ground run, doesn't the buffeting caused
when the prop tip swings within a couple of inches of the ground induce
wierd vibrations in the moving propeller that might not be there if
the prop where swinging in free air? In other words, wouldn't it be
preferrable to record vibration data during an actual flight, and
then postprocess it into a "where and how much weight" solution later?
| Quote: | 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...
|
What is a typical charge for this service?
Thanks,
MikeM
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John
Guest
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| Posted: Thu May 06, 2004 3:53 am Post subject: Re: Dynamic prop balance |
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On Wed, 05 May 2004 20:36:12 -0600, MikeM <joe (AT) yahoo (DOT) com> wrote:
| Quote: | toecutter1962 (AT) yahoo (DOT) com wrote:
I have checked the vibe level in-flight on numerous occasions, on
aircraft that acted "strange" after a standard dy-bal.
How do you get the wire from the accelerometer/position pickup into the
cabin during flight?
|
ty wraps and 200 MPH tape.
| Quote: | 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.
Since the only place that you can put a weight is on the prop spinner
backing plate, can Prop balancing do anything except cancel the "first
order" mass imbalance (usually called a "static" balance)?
|
You are correct on most engines you do not have access to the rear of
the crankshaft.
| Quote: |
Wouldn't you have to have access to the other end of the crankshaft to
add weights there in order to be able to do a true multi dimensional
"dynamic balance"?
|
Yes
| Quote: |
I'm not trying to be argumentative; just trying to understand the claims
made by the purveyors of prop balancing services.
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.
I have wondered that during a ground run, doesn't the buffeting caused
when the prop tip swings within a couple of inches of the ground induce
wierd vibrations in the moving propeller that might not be there if
the prop where swinging in free air?
|
This would give a 2 X per rev vibration. Mass imbalance gives a 1 X
per rev vibration.
| Quote: | In other words, wouldn't it be
preferrable to record vibration data during an actual flight, and
then postprocess it into a "where and how much weight" solution later?
|
Much more trouble and the chance of removing some paint when removing
the tape holding the wires exists.
| Quote: |
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...
What is a typical charge for this service?
|
Most A&P's that work on helicopters have the balancer needed.
What you are trying to do is balance to 0.1 IPS or less. To do this
requires corrections in the range of a few gram-inches.
This is the weight in grams multiplied by the distance said weight is
from the center of rotation.
All props have some slop in the mounting to the crankshaft. Lets say
you have a prop that weighs 22 Lb or about 10 kilograms. If it has a
0.001 inch slop between the prop and the crankshaft bolts then
0.001*10,000 grams is 10 gram inches. The correction weight would be
2 grams at 5 inches or 10 grams at 1 inch.
To get an acceptable vibration level I used to balance a tail rotor of
a H269 helicopter to within one tenth of a gram. It does not take
many bug splatters to equal this amount!
John
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toecutter1962@yahoo.com
Guest
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| Posted: Thu May 06, 2004 4:45 am Post subject: Re: Dynamic prop balance |
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On Wed, 05 May 2004 20:36:12 -0600, MikeM <joe (AT) yahoo (DOT) com> wrote:
| Quote: | How do you get the wire from the accelerometer/position pickup into the
cabin during flight?
|
Depends on the install. Ty-raps, oil door/cowl flap/cowl, "wing
window"/cabin door and duct tape. Put the reflective tape on the prop
face (instead of the back) and shine the strobe through the window.
| Quote: | 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.
Since the only place that you can put a weight is on the prop spinner
backing plate, can Prop balancing do anything except cancel the "first
order" mass imbalance (usually called a "static" balance)?
Wouldn't you have to have access to the other end of the crankshaft to
add weights there in order to be able to do a true multi dimensional
"dynamic balance"?
I'm not trying to be argumentative; just trying to understand the claims
made by the purveyors of prop balancing services.
|
If you haven't figured it out by now, I am not selling a darn thing,
nor do I tend to avoid controversy. You bring up some valid points,
I'll do my humble best to address them.
I'm not an engineer, just anonymous guy on Usenet that might know how
to balance props-with trial and error, a little elementary physics,
and some phone calls to powerplant and propeller engineers-and has
possibly balanced quite a few over the years. Standard disclaimer-This
info is worth exactly what you paid for it.
As you've indicated, the best way (I'm aware of) to do a vibe survey
is to mount a velocimeter as close to the front of the engine as
practical, and another on the accessory case.
In a perfect world, with a simple 1st order front end "prop" shake,
the rear of the engine will be driven in the opposite direction as the
front (when the front is pulled "up" by the vibe, the rear is being
pushed "down"). Reducing a simple mechanical imbalance by adding
weight to the prop hub/backplate will reduce the amount of vibration
measured at both ends of the engine.
A mechanical imbalance on the rear of the engine will have the same
affect (effect?) on the front. In most cases, reducing the the
measured 1st order vibe at the front will also reduce it at the rear.
In other cases, adding additional weight to take the vibration cycle
"beyond" 0.0 IPS, actually creating vibration 180 degrees out of phase
from the original imbalance (at a reduced level) will reduce the
measured vibration at the rear of the engine by a larger amount. My
thoughts are that this would tend to reduce the 1st order vibes that
are felt from the driver's seat.
The problem (common to most aspects of GA maintenance) is the average
owner wants the job performed yesterday with $$ spent being the
primary consideration.
The quickest way to do that is to reduce the vibe level at the front
(single velocimeter installed) to below .1 IPS. It is also the most
practical/economical method.
A spectrum analyzer comes in handy when diagnosing "strange"
vibrations, or when a massive reduction in 1st order vibes doesn't
seem to make a big difference in the cockpit. The most common cause of
this that I have seen is 1/2 order vibration. The frequency is lower,
it tends to shake the panel (and your backside) a lot harder. We used
an old low-tech box, but it was simple to dial in 1/2 the rpm, and
measure the 1/2 order vibe.
Again, as you've indicated, IMHO this is nothing that can be remedied
by adding weights to the hub/backplate. It just allows you to "see"
that there are higher levels of other vibration frequencies present,
and confirms that the backside is giving the brain good data. I really
think it is impossible to change the levels of these vibrations at non
1/1 frequencies by hanging weights on the front.
Is any of this making sense? Forgive me if it is not, some of this
stuff is hard for me to describe face-to-face, let alone with fingers
and a keyboard.
| Quote: | I have wondered that during a ground run, doesn't the buffeting caused
when the prop tip swings within a couple of inches of the ground induce
wierd vibrations in the moving propeller that might not be there if
the prop where swinging in free air? In other words, wouldn't it be
preferrable to record vibration data during an actual flight, and
then postprocess it into a "where and how much weight" solution later?
|
OK, I have seen what effect (affect?) gusting winds have on dy-bal
jobs, hence the reference to balancing in the hangar. I've often
wondered the same thing. The only in-flight checking I have done has
been on the "weird" ones, and typically did not see a big difference
in the 1st order levels between ground runs and in flight. If I had a
better answer based on personal observation, I would share it.
Again, remember, the average guy just wants it done cheap and in a
hurry, and end up with a turbine-smooth result.
| Quote: | What is a typical charge for this service?
|
No idea. Had a prop shop next door, so had a relatively high volume of
dy-bal jobs. At that time we were more concerned with taking care of
our customers by offering the additional service (balanced with the
capability to do our own 'planes inexpensively) than making a profit.
When I got out of GA several years ago, we charged $150.00 for a basic
single-engine dy-bal which included 1 hour of shop labor. That would
usually cover a survey run, installing "test" weights, a final run,
and basic permanent weight installation. Any additional labor was
charged at the normal shop rate. A twin was $300.00 and included 2
hours of labor.
At that time, due to a relatively well-defined FAA ruling (that was
several years old, just hadn't trickled down to the field) if no
approved maintenance manual procedure for dy-bal/weight attachment was
present, a 337 was required. Not a real big deal to fill out a 337,
but it takes time. And at that time, GA "approved" dy-bal procedures
where pretty much non-existent. Am not sure if that has changed in
recent years.
Our equipment was basic, and experience with how much weight needed to
be added where was a necessity. I had a knack for it, and usually
didn't have to do a lot of weight swapping and additional survey runs.
The primary advantage to the newer digital equipment is that it uses a
optical "tach" pickup, and tells you exactly how much weight to add
and where. A secondary advantage is the ability to do spectrum
analysis. The disadvantage is that with anything other than the
"perfect world" scenario, you're right back to trial-and-error and
experience.
Apologize for the length;
TC
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