Strange Vibrations

Since the vibration here is most apparent at one particular
RPM, there might be a resonance at work, and resonances are often
weight-related; the weight of the engine, for instance, and then we
would suspect mounts, or maybe an exhaust pipe buzzing against the
cowling.
Could be something else. Pulsating vibration can be caused by the
interaction of two rotating components running at other than
whole-number ratios. In this case, it might be bad generator bearings,
since the gen is gear-driven and (I believe) doesn't run at any even
ratio with the crankshaft. Pulsating rumble in auto engines is
sometimes caused by worn main crank bearings; as the crank turns, it
"walks" around the bearing and causes that sort of noise. Metal in
filters or falling oil pressure are symptoms.

Dan

The O-300D mounts are each tubular with a bonded rubber between the
inside tube and the outside tube. They are oriented so that they will
provide radial stiffness but axial compliance, and arranged so that the
engine torsional vibrations are isolated from the airframe. The
characteristics of the elastomer will change with temperature, and the
mounts on the O-300 engines are all on the warm side of the baffling
system.

Usually the O-300D engines are extremely smooth, unless the prop is out
of balance. Maybe your prop has always been out of balance a little,
and the radial stiffness of each mount may be changing enough to create
a resonant system at certain mount temperatures.

Guess two is maybe a mount is bad after all. Next time you have the
cowl off though, try to twist the engine with a long board in the three
axes (vert lateral, and longitudinal) and look at the deformation of
each mount. Maybe one has become unbonded. Don't put more than about
400 ft lbs into the engine block in any axis though.

Guess three is you might also try to spray silicon on the engine
baffling strips wherever they rub on the cowl. In other words, the
transmission path may not be thru the mounts since you have a rigid
cowl.

Guess four is to check the prop balance. I don't think you have a
serious problem but over a long time will tend to crack the cowl and
baffling etc.

The ~ 1 Hz beating (at ~35 Hz I presume?) you describe is weird. Too
bad you can't get a more precise signature or definition using an
accelerometer into the sound input on a laptop computer.

Guess four is to check the prop balance. I don't think you have a
serious problem but over a long time will tend to crack the cowl and
baffling etc.

And that raises another possibility: a cracked prop. I've heard of
pilots experiencing weird vibration before blade failure. Take a close
look.

Dan

I flew a Cessna 182 (3-blade prop with a Lycoming) that does the same thing
on takeoff. It sounds remarkably like an out of sync twin at low speed just
above the runway. That airplane is only a year old, so the motor mounts
should be OK. Seems fine the rest of the time.

Al,

This may well pose more questions than it answers, but the November 04 issue
of Flying magazine had an article by the editor about a buzzing sound he was
hearing in his Baron when he flew into visible moisture. I think there was
also a reader response by a Mooney owner with the same problem. The noise
was eventually found to have been caused by antenna flutter. In the case of
the Baron, the antenna was replaced with one designed for jet speeds which
solved the problem.

At the very least, this tends to confirm that moisture can be a catalyst for
vibration.

Mal


"Al Gilson" wrote in message
...
We have a 1964 Cessna 172 E with the 145hp Continental 0-300D and a
McCauley fixed pitch prop. Last Saturday, I flew the aircraft for about
an hour. We were at about 3,500 msl with an air temp of about 42F.
There were some low clouds at about 4,200 msl. No problems. Later in
the day, my plane partner took up the aircraft for his biennial flight
review. They also flew around 3,500 msl with the same air temp. A
light drizzle was now falling in places.

They noticed a strange vibration in the aircraft between about 1,700 to
2,200 rpm. They described it as a "pulsing" vibration and likened it to
the vibration/sound made in a twin when the engines are slightly out of
snyc. After the usual checks for carb ice, mixture control, etc., one
speculation was that one of the rubber motor mounts was worn and
transmitting the vibration to the frame.

Needless to say, the flight was shortened.

On Sunday, we took off the cowling and looked for anything loose,
checked the oil for anything shiny, pulled the plugs to see if we had a
fouled plug or cruddy cylinder. Other than some plugs that were showing
some wear and and one with a little lead deposit, all looked OK.

We put the plane back together and ran the engine up. Seemed OK.
Started and ran like it always did. Gagues all read like they always
do. Then, with some severe "pucker factor" we taxied out to the runway
for some very close-in touch and go's. The weather was again about 42F
with a very slight drizzle.

The aircraft powered up normally and climbed as always for two trips
around the pattern. On the third trip, my partner was able to duplicate
the vibration.

At exactly 2,350 rpm, the pulsing vibration occured and I was able to
experience it. It was about 1 cycle per second and did sound/feel like
an out of sync twin. It disappeared at 2,300 or 2,400 rpm. Other than
that, the engine ran as always.

So....any ideas? Has anyone ever experienced this concept? Is this a
mechanical problem or just an alignment of conditions that exacerbates a
principle of physics. Could this be equated to the resonant frequency
of the airplane, airframe, air and moisture conditions? It does make me
wonder since this aircraft has rarely, if ever, been operated by either
of us in any kind of visible moisture.

Thanks in advance for any comments

Al

In my glider, you can hear some of the laminar flow go turbulent even
before you see the
moisture appear. If you do get into light precipitation, like virga, the
noise is very noticeable.
The degradation in performance is immediately obvious. This is not the same
as the
noise of flutter, but it does get your attention. Different gliders,
different effects.


KMH wrote in message ...
Al,

This may well pose more questions than it answers, but the November 04
issue
of Flying magazine had an article by the editor about a buzzing sound he
was
hearing in his Baron when he flew into visible moisture. I think there was
also a reader response by a Mooney owner with the same problem. The noise
was eventually found to have been caused by antenna flutter. In the case of
the Baron, the antenna was replaced with one designed for jet speeds which
solved the problem.

At the very least, this tends to confirm that moisture can be a catalyst
for
vibration.

Mal


"Al Gilson" wrote in message
...
We have a 1964 Cessna 172 E with the 145hp Continental 0-300D and a
McCauley fixed pitch prop. Last Saturday, I flew the aircraft for about
an hour. We were at about 3,500 msl with an air temp of about 42F.
There were some low clouds at about 4,200 msl. No problems. Later in
the day, my plane partner took up the aircraft for his biennial flight
review. They also flew around 3,500 msl with the same air temp. A
light drizzle was now falling in places.

They noticed a strange vibration in the aircraft between about 1,700 to
2,200 rpm. They described it as a "pulsing" vibration and likened it to
the vibration/sound made in a twin when the engines are slightly out of
snyc. After the usual checks for carb ice, mixture control, etc., one
speculation was that one of the rubber motor mounts was worn and
transmitting the vibration to the frame.

Needless to say, the flight was shortened.

On Sunday, we took off the cowling and looked for anything loose,
checked the oil for anything shiny, pulled the plugs to see if we had a
fouled plug or cruddy cylinder. Other than some plugs that were showing
some wear and and one with a little lead deposit, all looked OK.

We put the plane back together and ran the engine up. Seemed OK.
Started and ran like it always did. Gagues all read like they always
do. Then, with some severe "pucker factor" we taxied out to the runway
for some very close-in touch and go's. The weather was again about 42F
with a very slight drizzle.

The aircraft powered up normally and climbed as always for two trips
around the pattern. On the third trip, my partner was able to duplicate
the vibration.

At exactly 2,350 rpm, the pulsing vibration occured and I was able to
experience it. It was about 1 cycle per second and did sound/feel like
an out of sync twin. It disappeared at 2,300 or 2,400 rpm. Other than
that, the engine ran as always.

So....any ideas? Has anyone ever experienced this concept? Is this a
mechanical problem or just an alignment of conditions that exacerbates a
principle of physics. Could this be equated to the resonant frequency
of the airplane, airframe, air and moisture conditions? It does make me
wonder since this aircraft has rarely, if ever, been operated by either
of us in any kind of visible moisture.

Thanks in advance for any comments

Al

It is known that Cessna 182 wing struts will give off a moaning sound when
flying thru rain.
One solution is to check strut attach bolt torque.

Kent Felkins


"KMH" wrote in message
...
Al,

This may well pose more questions than it answers, but the November 04
issue
of Flying magazine had an article by the editor about a buzzing sound he
was
hearing in his Baron when he flew into visible moisture. I think there was
also a reader response by a Mooney owner with the same problem. The noise
was eventually found to have been caused by antenna flutter. In the case
of
the Baron, the antenna was replaced with one designed for jet speeds which
solved the problem.

At the very least, this tends to confirm that moisture can be a catalyst
for
vibration.

Mal


"Al Gilson" wrote in message
...
We have a 1964 Cessna 172 E with the 145hp Continental 0-300D and a
McCauley fixed pitch prop. Last Saturday, I flew the aircraft for about
an hour. We were at about 3,500 msl with an air temp of about 42F.
There were some low clouds at about 4,200 msl. No problems. Later in
the day, my plane partner took up the aircraft for his biennial flight
review. They also flew around 3,500 msl with the same air temp. A
light drizzle was now falling in places.

They noticed a strange vibration in the aircraft between about 1,700 to
2,200 rpm. They described it as a "pulsing" vibration and likened it to
the vibration/sound made in a twin when the engines are slightly out of
snyc. After the usual checks for carb ice, mixture control, etc., one
speculation was that one of the rubber motor mounts was worn and
transmitting the vibration to the frame.

Needless to say, the flight was shortened.

On Sunday, we took off the cowling and looked for anything loose,
checked the oil for anything shiny, pulled the plugs to see if we had a
fouled plug or cruddy cylinder. Other than some plugs that were showing
some wear and and one with a little lead deposit, all looked OK.

We put the plane back together and ran the engine up. Seemed OK.
Started and ran like it always did. Gagues all read like they always
do. Then, with some severe "pucker factor" we taxied out to the runway
for some very close-in touch and go's. The weather was again about 42F
with a very slight drizzle.

The aircraft powered up normally and climbed as always for two trips
around the pattern. On the third trip, my partner was able to duplicate
the vibration.

At exactly 2,350 rpm, the pulsing vibration occured and I was able to
experience it. It was about 1 cycle per second and did sound/feel like
an out of sync twin. It disappeared at 2,300 or 2,400 rpm. Other than
that, the engine ran as always.

So....any ideas? Has anyone ever experienced this concept? Is this a
mechanical problem or just an alignment of conditions that exacerbates a
principle of physics. Could this be equated to the resonant frequency
of the airplane, airframe, air and moisture conditions? It does make me
wonder since this aircraft has rarely, if ever, been operated by either
of us in any kind of visible moisture.

Thanks in advance for any comments

Al