Dynamic prop balance

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.

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

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

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?

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.

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?

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

On Wed, 05 May 2004 20:36:12 -0600, MikeM wrote:

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.

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.

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

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.

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.

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.



Thanks,

MikeM


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

On Wed, 05 May 2004 20:36:12 -0600, MikeM wrote:

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.

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.

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.

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

Toecutter and John, thanks for the informative responses. I am
interested in the methods. I have a few more ??

wrote:

snip

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.

Are the accelerometers two axis (X-Y) or single axis?

If single axis, are they mounted so as to be sensitive left-right or
up-down?

Do all balancers use the second accelerometer on the accessory case?

Do the engine mounts allow more up-down, or left-right shake if
the engine-prop are out of balance?


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.

Is the criteria for "best subjective" balance some combination at what
the front and rear accelerometer see?

In other words, if all you had to work with was the front accelerometer,
and you balanced to minimise its amplitude at the fundamental shaft
rotation, could you create a situation that the pilot may interpret as
making things worse?

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.

Is this "good enough" in most cases?

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.

The only "mechanical" explanation I can imagine for 1/2 RPM vibration
modes is that there is a weak cylinder, which in a 4 cycle engine only
fires every other shaft rotation?

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.

Makes perfect sense. I am an engineer and know enough about electronics,
instrumentation, mechanics, & digital signal processing to be totally
dangerous.

snip

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.

Is the 337 necessary to do the ground testing, or only testing during
flight, or to drill a hole and put a bolt into the spinner backing
plate?


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.

I have run into some balancer equipment salesmen at airshows who make
all sort of claims about "you dont have to know anything to use our
equipment" which automatically set off my "bull**** detector".

Appreciate your experience and willingness to share it


MikeM

On Thu, 06 May 2004 09:22:52 -0600, mikem wrote:

Toecutter and John, thanks for the informative responses. I am
interested in the methods. I have a few more ??

Sure thing.

Are the accelerometers two axis (X-Y) or single axis?

The units I am familiar with are all single axis.

If single axis, are they mounted so as to be sensitive left-right or
up-down?

Mounting is the semi-tricky part. The instructions say to mount
securely as close to the front of the engine as possible, with the
"axis" pointing as directly at the center of the crank, perpendicular
to it. I have mounted them off-set somewhat, just makes figuring out
where to add the weights (and the amount of weight) a little trickier.

Do all balancers use the second accelerometer on the accessory case?

The ones that I am familiar with are two channel units. They can be
used one sensor per engine on a twin, or as front/rear, just depends
where you bolt them on.

Do the engine mounts allow more up-down, or left-right shake if
the engine-prop are out of balance?

Here's the deal, with a strobe-type unit, the engine rpm is dialed
into the box (after fine-tuning it is an accurate way of checking the
ship's tach) and the strobe pointed at a piece of reflective tape
placed on the back of the prop blade.

With the engine running at the desired rpm, the stobe will "stop" the
tape somewhere in the circle of rotation, and the box will indicate
the measured amplitude of the vibe.

When the engine is shut down, the prop is rotated to the same position
it was in with the blade "stopped". The sensed "heavy spot" is now
aligned with where the velocimeter is mounted, the weight is added 180
degrees (directly opposing) the velocimeter.

Since the heavy spot is rotating 360 degrees during engine operation I
see no relationship in regard to left/right up/down vibration and the
engine mounts.

A Lycoming "dyna-focal" mounting is supposed to make it easier for the
engine to vibrate "around" it's center of mass, front to rear.
but we're sorta getting in over my head.

Is the criteria for "best subjective" balance some combination at what
the front and rear accelerometer see?

With regard to 1st order vibration, a qualifed yes.

In other words, if all you had to work with was the front accelerometer,
and you balanced to minimise its amplitude at the fundamental shaft
rotation, could you create a situation that the pilot may interpret as
making things worse?

Not typically, I've always assumed that since the front and rear are
for the most part flying in formation with the vibes 180 degrees out
of phase, reducing the front side will tend to reduce the rear side
level also. I know this is an elementary way to look at it, but I'm
kind of a simple guy.

Every once in awhile this is not the case. I am really not sure in the
case of 1/2 order vibration that it gets "worse", but
removing/reducing the 1st order vibe can make others that are present
seem "worse", or at least overall vibe level apparently un-changed.

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.

Is this "good enough" in most cases?

Yes. In most cases, when a single that shows up with anywhere from a
..4-.9 IPS level can be reduced to below .1 (sometimes is can be
reduced to next to nothing) using a front-mounted sensor, there will
be a definite change in the overall vibration level in the cabin. As I
indicated before, if the apparent vibration level stills seems high, a
rear sensor and/or a specturm analyzer can help point a finger in the
right direction, but a dy-bal probably ain't gonna help much.

..1 IPS is the maximum "balanced" level, if on the initial survey run
the level is above 1.0 IPS, adding weights/performing a dy-bal is NOT
recommended. Have only seen this a half dozen or so times. Yanking the
prop and checking the static balance is the first step, then a more
thorough inspection of the prop/engine until a problem is found.

The only "mechanical" explanation I can imagine for 1/2 RPM vibration
modes is that there is a weak cylinder, which in a 4 cycle engine only
fires every other shaft rotation?

That was my thinking also, but personally never came across a flat
cylinder, or any other combustion cycle-related mechanical problem to
back it up this theory. To clarify, I am not saying that you are
incorrect.

Makes perfect sense. I am an engineer and know enough about electronics,
instrumentation, mechanics, & digital signal processing to be totally
dangerous.

Heh, sounds suspiciously like me walking across the ramp pushing a
toolbox carrying a cordless screwdriver with a full charge (totally
dangerous).

Is the 337 necessary to do the ground testing, or only testing during
flight, or to drill a hole and put a bolt into the spinner backing
plate?

Um, er, the flight-testing is gray-area stuff.

The way I understood it at the time, is that if no procedure was in
place, adding (or removing) weights in any manner required a 337.

The thought is that the engine is required to be balanced within
certain limits via the repair/overhaul specs, likewise the propeller.

Since a typical dy-bal will likely put the static balance of the
propeller assy outside of published limits, it is a major alteration.
Likewise as you indicated, if no mechanical provision for securing the
weights is provided, this is also an alteration.

Since I was lucky enuff to have a prop shop next door, I had access to
current technical data (and parts) which allowed me to use the
propeller provisions for static balance weights for a lot of balance
jobs.

I have run into some balancer equipment salesmen at airshows who make
all sort of claims about "you dont have to know anything to use our
equipment" which automatically set off my "bull**** detector".

Heh, tend to have a relatively low tolerance for it myself.

Have only demo'ed the new digital stuff. If you've got a decent local
guy that thinks dy-bals are something he/she wants to get into, ACES
useta keep a couple of demo machines available for demo use. Am not
pimping for ACES, was just one of the digital demos that I remember
doing (was either the 2015 or the 2020). Just did a web search, and
here is a link to dload their operating manuals, if you're interested
in more info on how-to:

http://www.acessystems.com/manuals.htm

Again, it's been awhile since I used them, but they use a photo tach
pickup to spot/stop the reflective tape. After the survey run, the
machine tells you the moment/angular location of the correction weight
needed. The tape is lined up with the optical pickup, and the angular
reference is from this point. A calculator function allowed you to
input the arm (distance from the center of rotation) of the desired
weight mounting point, and would calculate the weight required. It
also allowed you to "split" the location, inputting the angular
locations of the available mounting points if one was not available in
the proper location. The solution would tell you how much weight to
put at each location. A second run would confirm the final solution.

For a basic single velocimeter balance job to below .1, the machine
will do just about all of the heavy-lifting. But beyond that, the tech
has to kinda know what's going on.

Appreciate your experience and willingness to share it

Any time. If I don't hang around here and throw in a little now and
them, I'm going to forget more about what I've learned in GA than what
I already have...

TC

Hello Folks,
Its been awhile since I've checked the group. I'd like to make a few
comments to an old thread from early May.

Toecutter covered the bases pretty well.

a few things I might add:
A fixed pitch prop that is not indexed correctly,not aligned with the
crankthrows as closely as possible, will still leave a 0.3- 0.4 IPS reading
on the aft of the engine. As per constant speed props, I can't say for
sure because its hard for to install one out of position!
However there have been problems with some models initially that were
corrected by the mfg. The Grumman AA-5 to the 5A to the 5B give different
index instructions. Basically same engine and prop on all.
The first 300 serial numbers of the Mooney M20J have a service bulletin to
rearrange the crank flange bushings to bring the prop axis closer to the
crank throws. This changes the engine from an IO-360-A1Bx to an
IO-360-A3Bx, (I forget the suffix's last number). Also somewhere in the
Lycoming Svc Bltn book is one is about the same for some of the Aviat
Husky's.

I always run two channels if possible. It gives me a good idea of the
crank/rod balance quality.
Maybe 1 out of 40 times something else will show up. There been
instances where the prop balance was very good to begin with but didn't the
engine had a rear vibration that needed to be considered.
Anybody remember the rod bearing AD on the Piper Mirage?

The 1/2 order vibration felt in the cockpit is a product of the combustion
pulse. Equipment salesmen will tell you it is camshaft imbalance but that
is a crock. During a flight test, leave the rpm the same but take away
the MP to 15" or less, the 1/2 spike will reduce to minimum. The same with
some of the higher order harmonics.
If the engine is running in good shape, and a strong 1/2 is felt in the
cockpit, then its an isolation problem. It could be lord mounts or
something like an exhaust pipe hitting the cowl. Maybe a rocker box cover
rubbing inside a Bonanza cowl.

Toecutter is right though, some pilots only notice it after a prop balance,
even though it was there all along.
I could write a few pages about customer perceptions of vibrations.

My experience with twins? It does make a difference in the cockpit! I have
a handful of particular stories about twins.

I might had been the culprit that caused the Feds to mandate 337's back in
1990-91. I did some work in another FSDO's turf and they called my FSDO.
I was shut down for two months and the FAA memo calling it a major
alteration was published. Even then though many many operators never
bothered. And many Feds neither.
It use to be stated in the ACES procedure but now is not.
The addition of hardware to a prop bulkhead is an alteration unless it is
permitted in the mfg's maint. manual. At that time, it was only in a few
turboprop manuals. Mooney came out with their own bulletin and checked
every plane off the line. Beech started doing it recently, but you
won't find any note in the logbooks! After I checked a 75 hour SN Baron, I
called the tech rep. I don't think Cessna or Piper bothers.

It is an airframe alteration since the airframe mfg is the final authority
as to what spinner model is installed on the airframe.
Sometime around 1999, a memo out of some office in the Anchorage FSDO
called it a minor alteration and stated it superceded that previous memo.
I have always used boilerplate 337 forms that I fill in the blank. I still
complete them. Heck I use them to renew my IA!

The three major brands of equipment all work on the same principles. It
just the interface to the operator that varies. I've used a Chadwick 192
since 1988. Sure a rookie can use the newer equipment easily, but
experience certainly helps, and helps in the most important of all: customer
service.

Take care
Kent Felkins
Tulsa









wrote in message
...
On Thu, 06 May 2004 09:22:52 -0600, mikem wrote:

Toecutter and John, thanks for the informative responses. I am
interested in the methods. I have a few more ??

Sure thing.

Are the accelerometers two axis (X-Y) or single axis?

The units I am familiar with are all single axis.

If single axis, are they mounted so as to be sensitive left-right or
up-down?

Mounting is the semi-tricky part. The instructions say to mount
securely as close to the front of the engine as possible, with the
"axis" pointing as directly at the center of the crank, perpendicular
to it. I have mounted them off-set somewhat, just makes figuring out
where to add the weights (and the amount of weight) a little trickier.

Do all balancers use the second accelerometer on the accessory case?

The ones that I am familiar with are two channel units. They can be
used one sensor per engine on a twin, or as front/rear, just depends
where you bolt them on.

Do the engine mounts allow more up-down, or left-right shake if
the engine-prop are out of balance?

Here's the deal, with a strobe-type unit, the engine rpm is dialed
into the box (after fine-tuning it is an accurate way of checking the
ship's tach) and the strobe pointed at a piece of reflective tape
placed on the back of the prop blade.

With the engine running at the desired rpm, the stobe will "stop" the
tape somewhere in the circle of rotation, and the box will indicate
the measured amplitude of the vibe.

When the engine is shut down, the prop is rotated to the same position
it was in with the blade "stopped". The sensed "heavy spot" is now
aligned with where the velocimeter is mounted, the weight is added 180
degrees (directly opposing) the velocimeter.

Since the heavy spot is rotating 360 degrees during engine operation I
see no relationship in regard to left/right up/down vibration and the
engine mounts.

A Lycoming "dyna-focal" mounting is supposed to make it easier for the
engine to vibrate "around" it's center of mass, front to rear.
but we're sorta getting in over my head.

Is the criteria for "best subjective" balance some combination at what
the front and rear accelerometer see?

With regard to 1st order vibration, a qualifed yes.

In other words, if all you had to work with was the front accelerometer,
and you balanced to minimise its amplitude at the fundamental shaft
rotation, could you create a situation that the pilot may interpret as
making things worse?

Not typically, I've always assumed that since the front and rear are
for the most part flying in formation with the vibes 180 degrees out
of phase, reducing the front side will tend to reduce the rear side
level also. I know this is an elementary way to look at it, but I'm
kind of a simple guy.

Every once in awhile this is not the case. I am really not sure in the
case of 1/2 order vibration that it gets "worse", but
removing/reducing the 1st order vibe can make others that are present
seem "worse", or at least overall vibe level apparently un-changed.

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.

Is this "good enough" in most cases?

Yes. In most cases, when a single that shows up with anywhere from a
.4-.9 IPS level can be reduced to below .1 (sometimes is can be
reduced to next to nothing) using a front-mounted sensor, there will
be a definite change in the overall vibration level in the cabin. As I
indicated before, if the apparent vibration level stills seems high, a
rear sensor and/or a specturm analyzer can help point a finger in the
right direction, but a dy-bal probably ain't gonna help much.

.1 IPS is the maximum "balanced" level, if on the initial survey run
the level is above 1.0 IPS, adding weights/performing a dy-bal is NOT
recommended. Have only seen this a half dozen or so times. Yanking the
prop and checking the static balance is the first step, then a more
thorough inspection of the prop/engine until a problem is found.

The only "mechanical" explanation I can imagine for 1/2 RPM vibration
modes is that there is a weak cylinder, which in a 4 cycle engine only
fires every other shaft rotation?

That was my thinking also, but personally never came across a flat
cylinder, or any other combustion cycle-related mechanical problem to
back it up this theory. To clarify, I am not saying that you are
incorrect.

Makes perfect sense. I am an engineer and know enough about electronics,
instrumentation, mechanics, & digital signal processing to be totally
dangerous.

Heh, sounds suspiciously like me walking across the ramp pushing a
toolbox carrying a cordless screwdriver with a full charge (totally
dangerous).

Is the 337 necessary to do the ground testing, or only testing during
flight, or to drill a hole and put a bolt into the spinner backing
plate?

Um, er, the flight-testing is gray-area stuff.

The way I understood it at the time, is that if no procedure was in
place, adding (or removing) weights in any manner required a 337.

The thought is that the engine is required to be balanced within
certain limits via the repair/overhaul specs, likewise the propeller.

Since a typical dy-bal will likely put the static balance of the
propeller assy outside of published limits, it is a major alteration.
Likewise as you indicated, if no mechanical provision for securing the
weights is provided, this is also an alteration.

Since I was lucky enuff to have a prop shop next door, I had access to
current technical data (and parts) which allowed me to use the
propeller provisions for static balance weights for a lot of balance
jobs.

I have run into some balancer equipment salesmen at airshows who make
all sort of claims about "you dont have to know anything to use our
equipment" which automatically set off my "bull**** detector".

Heh, tend to have a relatively low tolerance for it myself.

Have only demo'ed the new digital stuff. If you've got a decent local
guy that thinks dy-bals are something he/she wants to get into, ACES
useta keep a couple of demo machines available for demo use. Am not
pimping for ACES, was just one of the digital demos that I remember
doing (was either the 2015 or the 2020). Just did a web search, and
here is a link to dload their operating manuals, if you're interested
in more info on how-to:

http://www.acessystems.com/manuals.htm

Again, it's been awhile since I used them, but they use a photo tach
pickup to spot/stop the reflective tape. After the survey run, the
machine tells you the moment/angular location of the correction weight
needed. The tape is lined up with the optical pickup, and the angular
reference is from this point. A calculator function allowed you to
input the arm (distance from the center of rotation) of the desired
weight mounting point, and would calculate the weight required. It
also allowed you to "split" the location, inputting the angular
locations of the available mounting points if one was not available in
the proper location. The solution would tell you how much weight to
put at each location. A second run would confirm the final solution.

For a basic single velocimeter balance job to below .1, the machine
will do just about all of the heavy-lifting. But beyond that, the tech
has to kinda know what's going on.

Appreciate your experience and willingness to share it

Any time. If I don't hang around here and throw in a little now and
them, I'm going to forget more about what I've learned in GA than what
I already have...

TC