Strange Vibrations

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

I've noticed what you describe with our Pawnee IO540 fixed pitch prop at
full power with a glider in tow. It appears just after lift off (rotation
with a tailwheel) as I accelerate from 55 to about 60 and get the pitch
established to maintain 60 in the climb. When all the accelerations are
established and into a steady state climb.. it goes away. I attributed it to
the engine just "coming to life" as the speed gets established between 55-65
on initial climb.

I cannot duplicate it at any other time.. although it seems present with or
without a glider in tow. Lasts less then 15-20 seconds.

BT

"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

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.

How many hours on the engine? How old are the engine mounts?

As the rubber mounts age, the engine sags a bit, and they become stiffer,
transferring more vibration to the airframe. You may have just developed a
resonant at that frequency range? And perhaps the unusual atmospheric
conditions made it more apparent?

Or, it could be something more serious in the offing.

Ain't airplane ownership fun?

;-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Hi Al,

My Boss has a '63 Cessna with the 160 Continental that started
having the same problem you describe with about 300 hrs left on it. He flew
it another 100 hrs and then had the engine rebuilt since it needed it
anyway. After the reinstall the vibrations are gone. The IA said the
vibration was probably the engine mounts getting old and not so much the
engine.

Paul Davis


"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

Al Gilson wrote:

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.

Have the same plane, no symptoms yet... However our previous C152 had
something like that, only during takeoff turn to downwind and everything
was spooling and speeding up. After replacement with Lord engine mounts,
problem went away...

Look at the cowling to spinner angle, if the engine is sagging in the
mounts, you should be able to see it there...

On 2005-04-05, Jay Honeck wrote:
As the rubber mounts age, the engine sags a bit, and they become stiffer,
transferring more vibration to the airframe.

Is it really worth replacing the rubber mounts on a mid time engine with
a lot (10-15 years) of calendar time on it?

--
Ben Jackson

http://www.ben.com/

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

As Jay asked:

There are about 500 SMOH on this engine and it was overhauled about 5 or
6 years ago. The engine mounts all look good with the exception of one
that has some very slight surface cracking.

Of course, we had plenty of ownership fun poking around in the engine
compartment and then raising our heart rates during our test flight.

What is interesting is that this all came on at once.

Still curious.

Al



Jay Honeck wrote:
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.


How many hours on the engine? How old are the engine mounts?

As the rubber mounts age, the engine sags a bit, and they become stiffer,
transferring more vibration to the airframe. You may have just developed a
resonant at that frequency range? And perhaps the unusual atmospheric
conditions made it more apparent?

Or, it could be something more serious in the offing.

Ain't airplane ownership fun?

;-)

Ben Jackson wrote:
On 2005-04-05, Jay Honeck wrote:

As the rubber mounts age, the engine sags a bit, and they become stiffer,
transferring more vibration to the airframe.


Is it really worth replacing the rubber mounts on a mid time engine with
a lot (10-15 years) of calendar time on it?

Why not? If it turns out not to be the problem it's one less thing to
buy at the overhaul.

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.