Engine Balancing and Resonance Vibration Problem

On Sun, 4 Sep 2005 22:12:12 +0100, AllanFuller
wrote:

Below

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The current engine was put into service in March 2001 with the existing
prop. There were engine vibrations and 13 washers were required to
balance the prop as best as could be. I never felt it was as good and
smooth as it should be considering it was a new engine.

This would be your first clue. Was the propeller inspected at all at
this time? Flush and re-seal? If I needed "13 washers" to balance a
prop on a "new" engine, I would verify the propeller static balance at
a bare minimum. I'm betting that the prop was balanced, it was the
engine with an issue/issues.

This July 2005 (at the time of my annual) we had the prop overhauled at
Memphis Prop at Memphis, Mississippi. They said they balanced the prop.
It was installed and there was engine/prop vibration. There was also a
resonance vibration that has a maximum at 1500 rpm and a second lower
magnitude harmonic vibration at 750 rpm. I brought it to a nearby
airport for dynamic engine/prop balance. The vibration at 2000 rpm was
close to 1 without weights and they thought there was a problem with a
bulkhead crack. We dye tested the bulkhead and there was no crack. We
flipped the prop 180 degrees to see if that reduced the vibration. It
did. This was actually the orientation of the prop when the prop was
removed for the prop overhaul.

What was the IPS before and after the 180 degree "flip"? Again, if I
had overhauled the propeller, I would have removed it, re-verified the
static balance and blade angles. Still sounding like an engine problem
to me.

I brought the plane to another airport for engine balancing. They found
that the vibration was about 0.9 without weights and they were able to
bring the vibration down to about 0.12. Probably could have even
improved on that, but the concern was that they needed about 13
washers. We flipped the prop 180 degrees and found that the vibration
without weights was 1.4. Again the vibration could be lowered with the
use of about 17 washers in the same direction on the bulkhead (same
direction as the balancing of the new engine in 2001 with 13 washers).
There still is this noticeable resonance vibration at 1500 and less
intense resonance vibration at 750 rpm.

Third time, same story. Again, have the static balance and blade
angles on the propeller been verified? Are you aware that
manufacturers of prop dy-bal equipment forbid, repeat FORBID adding
weight/balancing an engine/prop combo that is over 1.0 IPS?

Again, I would guess that the prop is OK, but has it been verified?

The difference in vibration with the two prop orientations is about
0.5. That probably suggests that the major vibration is in the engine
and not static prop balance.

You've pretty much summed it up in one sentence.

People have suggested that the vibration could be caused by a number of
things. The Insight – GEM gauge show even CHT/EGT readings for all
cylinders. The engine mounts were new at the time of the engine
installation in 2001. The governor was overhauled in 2001 and yellow
tagged. One observed problem is that prop cycling at 2000 rpm only
produces a 250 rpm drop and one would expect least 300. Before the new
engine and overhauled governor the engine rpm drop during prop cycling
easily went to 300 and lower if I did not stop it by bring the prop
forward. Currently, At 2300 rpm the prop cycling drop will go to 500
rpm. Is this an oil pump pressure or governor problem? Could this be
the source of the vibration or the resonance vibration at 1500/750?

Find a picture, or do a little digging on the web/in the groups. I
know I've described the engine/governor/prop system at least a couple
of times over the years. On a single, the governor steps up the oil
pressure to "fatten" the pitch to control/reduce RPM. Low/no oil
pressure makes the prop go "flat".

If the engine/governor was not producing sufficient pressure, you
could not reduce engine RPM in flight using the prop control.
(emphasis on the period) Cycling the prop on the ground is a feel-good
crude ops check, not necessarily anything to be used for diagnosis.

My problem is that I am not sure what to do. It is being suggested that
I could replace the motor mounts at about $360 plus labor (They were new
in 2001 and look good/new.). I could have the governor overhauled for
$500 plus labor. I could have the prop sent back to the prop shop to
check their work and re-balance. I will throw money at the problem,
but I want to make smart choices on where to spend the time and money.

It sure sounds to me like you had the exact same problem when the
engine was initially hung/prop installed and the engine mounts
replaced w/new-unless I'm not reading things right. Getting the prop
condition verified is a no-brainer, it should have been done
immediately following the first dy-bal attempt after propeller o-haul.
If the governor is working at TO/cruise, I wouldn't worry about it.

The other fear is that the vibrations are in the engine and related to
the crank shaft recall. I certainly do not want the engine to fail when
I am IMC or at night even if VFR. The resonance vibration coupled with
the recall has me concerned. Having a doctorate in physics probably has
me overly sensitized to the potential problem.

I seriously doubt that the "crank shaft recall" is related to your
particular problem, but I do strongly suspect that your engine is the
source of your vibration.

Using your knowledge in physics (mine is strictly practical, sorry)
and my alleged experience in dy-bals, did any of the by-bal dudes
mount two transducers on your engine? Standard practice when faced
with a shaky weird engine/prop is to mount one 'ducer on the front of
the engine and one on the rear of the accessory case.

If the propeller imbalance is "making" the vibration, the rear 'ducer
should be picking up the vibration 180 degrees out of phase with the
front. The way a dynafocal engine mounting system is designed, in
theory (in a perfect world), the amplitude of the rear should be close
to the amplitude in the front.

Reducing the measured IPS on the front, should also reduce the IPS on
the back-are you following me? It was very, very, very rare, but I did
have a couple of engine/prop combos that I had to play with (trial and
error, coupled with experience) that I used an intentional imbalance
on the propeller/front (by adding too much weight in the "wrong" spot)
to reduce the IPS measured at the rear.

Again-strictly an assumption based on limited data and past
experience-I'm assuming that with the prop/front zero'ed out via
dy-bal you still have measurable vibration present at the engine/rear.

The only other concern that I have is that in another post, you
mentioned that you thought the ring gear support was uni-directional.
It should have no real bearing on what you are experiencing (unless it
has a stack of washers bolted to it), but it can only be installed
properly in one position.

regards;

TC

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