Wierd vibration -- Prop or engine, balance or not???

Hi Chuck

I suggest that the first thing you do is check alignment/tracking.
Why? Because it takes one minute and costs nothing, and if nothing else
it eliminates a probable cause.

So. Get yourself a cardboard box, and stand it on something that allows
the prop to touch the box.

Place the box so it is exactly touching the prop.
Rotate to the other blade.
Does that also touch the same?
If it does then it ain't tracking.
If it doesn't - get that checked first.

HTH

Tony

In article ,
Chuck wrote:

Hi guys & gals,

I purchased an older Cherokee 180 with a Lycoming O-360A4A and
Sensitech propeller. Unfortunately, I'm not sure on of the prop model
as I haven't pulled the cone yet.

Since I purchased my Cherokee, there has been a slight vibration in it
while running and flying. The first mechanic I had went for a ride
with me and immediately said the prop needed a "balance" job.

But since then, another very experienced A&P has told me that
fixed-pitch props are never "balanced". This guy told me that balance
jobs are done on constant-speed props and not fixed-pitch.

So, does anyone have experience with a balance job on a fixed-pitch
prop? Or has anyone had a vibration problem that turned out to be a
fixed-pitch prop balance problem???

I ask because it would take a week to get the equipment in and of
course, cost several hundred for the prop balance. I'd hate to spend
that money and still have the vibration problem.

Thanks for any suggestions or information you guys can offer.



Chuck
N7398W




--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE

Toecutter gave you good advice.

If your engine is running in tip top shape. the vibration cause
probabilities are 1, dynamic balance, 2, rubber isolation mounts (lord or
barry), 3, interference problems, exhaust hiting cowling.
Interference problems can be checked easily and cheaply!

Without other instructions, install the prop with the blades aligned with
the crank throws as close as possible. Most 4 cyl Lycomings and 4 cyl
Continentals have two blade prop installed on the bushings at 1:00 & 7:00
facing from the front looking aft. Or from the pilots seats 11:00t & 5:00.


Many fixed pitch props are never checked after new installations. And a
metal fixed pitch prop will take a different set after 3 or 4 thousand
hours. Theblade angle may not match closely and the static balance be
different.

My area, props shops charge a hundred bucks for a static balance check and
blade angle check. An overhaul is about $500.

Toecutter is right in that many Sensenich props models are not to be
shortened because the vibration characteristics change.


Since you asked, you are obligated to inform us of what solves you problem.

good luck
Kent Felkins

Kent - Do you by chance know why the prop blades should be in the same
plane as the crank throws? Your description (1 & 7 oclock) is at odds
with that).

In 4 cylinder engines, I think I can see a basis for a 45 degree
offset to prevent second harmonic engine yaw from coupling into the
airframe as first harmonic due to the different swinging inertia of a
two blade prop.

Does anyone else know why or have a different reason for prop
orientation to the crank throws? I know the orientation is critical
on a 4 cylinder 172. A local flying club almost tore their newly
majored engine apart chasing engine first harmonics before finding out
the mechanic had put the prop on incorrectly.

I'd like to believe the balance people would have caught that sort of
thing rather than hang compensating weights on the spinner.

Another cheap check is to trace one of the blades on a piece of
cardboard and put it against the other blade.
If they match, Great!
After years of nick dressing your blades may not even be close to the
same dimensions/weights.

Dave

wrote:

Kent - Do you by chance know why the prop blades should be in the same
plane as the crank throws? Your description (1 & 7 oclock) is at odds
with that).

In 4 cylinder engines, I think I can see a basis for a 45 degree
offset to prevent second harmonic engine yaw from coupling into the
airframe as first harmonic due to the different swinging inertia of a
two blade prop.

Does anyone else know why or have a different reason for prop
orientation to the crank throws? I know the orientation is critical
on a 4 cylinder 172. A local flying club almost tore their newly
majored engine apart chasing engine first harmonics before finding out
the mechanic had put the prop on incorrectly.

I'd like to believe the balance people would have caught that sort of
thing rather than hang compensating weights on the spinner.

in reply to:

- Kent, Do you by chance know why the prop blades should be in the same
plane as the crank throws? Your description (1 & 7 oclock) is at odds
with that).

In 4 cylinder engines, I think I can see a basis for a 45 degree
offset to prevent second harmonic engine yaw from coupling into the
airframe as first harmonic due to the different swinging inertia of a
two blade prop.

I'm not an engineer, I have not done a modal analysis and do not know how
the 2nd harmonic will act.
45 degrees? The position is 30 degrees after TDC or BDC which ever crank
throw you are watching.
I have no idea why Lycoming does not put a bushing exactly at TDC. Though I
do recall seeing one there in the lightweight flange models.
Perhaps a Commanche. Or maybe the crank shop installed the bushing in the
wrong hole after regrind!

A prop installed on the 90 degree bushing can display a very acceptable
dynamic balance reading on the front end of the engine. However the engine
and prop assembly as a whole can be roughly moving in a dynamic couple.
This couple will be apparent if a second vibration sensor is used on the
aft of the engine. A differential of 0.30 - 0.40 Inches per second
is not unusual.

The pros do not always get it right the first time either. Some examples:
The first 300 of the Mooney 201's have a service bulletin that moves the
index bushings in the crank flange. Changes the IO-360 A1xx to a
IO-360-A3xx. A Lycoming service bltn say about the same for some Aviat
Huskys. Grumman AA-5A service manual is different from the AA-5B. 5a is
before TDC, 5b is after.

Perhaps the timing of the combustion impulse in consideration with the prop
vibe survey is a factor. Faint rumour is that it will be difficult to
certify a metal prop on a 4 cyl Lyc with electronic ignition. Did Van's
aircraft get Hartzell to bless their's?

And finally in reply to


I'd like to believe the balance people would have caught that sort of
thing rather than hang compensating weights on the spinner.

Been around aviation long?

Besides, heck, if a flying club was smart enough to know they were chasing
1st order vibres, they should have been savy to suspect prop index.
And another besides...... a lot of balance technicians are barely
acquainted with theory and don't need more than minimum experience.
Their box tells them how to do it!

Kent Felkins

Since you asked, you are obligated to inform us of what solves you problem.

good luck
Kent Felkins


OK Kent, I'll let the group know how it comes out.

Since this prop only has ~700 hours since new, I don't think that its
been modified or severely filed down. But you never know...

Looks like my first checks are prop dimensions, blade shape
comparison, and indexing. Those should be fairly quick & easy to do.
And if one of those doesn't expose the problem -- its time to call the
experts for a dynamic balance.

Thanks for the suggestions Kent.


Chuck

Chuck wrote:
: OK Kent, I'll let the group know how it comes out.

: Since this prop only has ~700 hours since new, I don't think that its
: been modified or severely filed down. But you never know...

: Looks like my first checks are prop dimensions, blade shape
: comparison, and indexing. Those should be fairly quick & easy to do.
: And if one of those doesn't expose the problem -- its time to call the
: experts for a dynamic balance.

Chuck, as an owner of an older Cherokee 180 as well, I'd also look at the
motor mount Lord isolators. There is very little clearance between the
green tubular bracket at the back of the carburettor (which holds the
throttle & mixture cable sheathes) an dthe motor mount tubes. If the lord
mounts sag even a little this green tubular bracket touches the motor mount
and makes a very impressive vibration.
--
Aaron Coolidge (N9376J)

"Aaron Coolidge" wrote in message
...
Chuck, as an owner of an older Cherokee 180 as well, I'd also look at the
motor mount Lord isolators. There is very little clearance between the
green tubular bracket at the back of the carburettor (which holds the
throttle & mixture cable sheathes) an dthe motor mount tubes. If the lord
mounts sag even a little this green tubular bracket touches the motor
mount
and makes a very impressive vibration.
--
Aaron Coolidge (N9376J)


Thanks for the tip Aaron. I'll remember to look at the speciifc point.

Many people do not realize the amount an engine moves while running. It is
even pulled forward some.

Try to have at least 1/2 inch clearance or more.
Kent Felkins