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