Mystery Vibration - 74 Cherokee 140

[Mike Spera]

Well, I finally have one. The weird vibration out of nowhere. About 6-7
flights ago, i noticed a very slight rhythmic vibration during a smooth
straight and level flight. About 4-5 seconds of smooth flight and then
2-3 seconds of vibration. I chalked it up to the air conditions. Over
the years, I have felt a lot of different vibes depending on the day.
Each successive flight since then it has become more noticeable. The
pilot wife even noticed it, and she has no mechanical inclinations.

Tried dropping mags off with no effect. Uncowled the beast and the
mounts and accessory brackets all look good. Cold or hot engine has no
effect. It is there from take off to landing in 1 hour plus flights.
Mixture appears to have no effect. I tried running through several tanks
of fuel to see if I got a bad load, no change. Checked the airframe for
any loose parts, none found. We have not done anything recently that
could cause it (replacement, maintenance, adjustment, plug change,
etc.). Plugs all clean, harnesses in good shape.

In bouncy conditions, it comes and goes randomly. Feels a bit like a
very light engine stumble or prop out of balance. It is better or worse
depending on RPM, but it is there pretty much between 2250 and the 2700
red line. Cannot tell at lower rpms, the thing usually was not very
smooth throttled back.

A couple of flights ago, I climbed to smooth air and it was not there,
unless I applied a bit of forward pressure on the yoke. Once back down
in the mild bumps, it was there again. Today, I climbed up to about
5700ft to find smooth air. Running 70% power or a bit less at 2475rpm
and again, it was very smooth without a hint of vibration. I tried to
induce it by pushing forward on the yoke, but this time it had no effect.

I'm going to do a compression check with the wrench. Pulling the prop
through after a flight yields 2 successive cyls with more compression
that the other two, but not a dramatic difference. I know how a 60lb.
and under cylinder feels (all too well unfortunately). Never noticed any
vibration or power difference when we had soft cylinders before. The
thing is making what feels to be full power.

Thoughts?
Thanks,

Mike

[nrp]

Maybe an intake manifold system leak?

Signs would be - does leaning the mixture aggravate it? and does carb
heat alleviate it?

It is a fundamental vibration (once/revolution)?

[Capt. Geoffrey Thorpe]

"Mike Spera" > wrote in message
ink.net...
> Well, I finally have one. The weird vibration out of nowhere. About 6-7
> flights ago, i noticed a very slight rhythmic vibration during a smooth
> straight and level flight. About 4-5 seconds of smooth flight and then 2-3
> seconds of vibration. I chalked it up to the air conditions. Over the
> years, I have felt a lot of different vibes depending on the day. Each
> successive flight since then it has become more noticeable. The pilot wife
> even noticed it, and she has no mechanical inclinations.


Ok, not that I know anything, but it really sounds like something is
resonating. And, it's getting worse.

Are you sure you are looking at the right end of the airplane? If it's a
resonance, it could be something somewhere else that is coming loose.

Start by giving the wings or tail a shake and listen for rattles???

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

[Fly]

Ahem....
A little bit of flutter is developing
pressing on the control yoke is the clue you give.
How would that affect the engine operation?

Randon vibration outof an engine is commonly valve train related.

Kent Felkins
Tulsa


"Mike Spera" > wrote in message
ink.net...
> Well, I finally have one. The weird vibration out of nowhere. About 6-7
> flights ago, i noticed a very slight rhythmic vibration during a smooth
> straight and level flight. About 4-5 seconds of smooth flight and then 2-3
> seconds of vibration. I chalked it up to the air conditions. Over the
> years, I have felt a lot of different vibes depending on the day. Each
> successive flight since then it has become more noticeable. The pilot wife
> even noticed it, and she has no mechanical inclinations.
>
> Tried dropping mags off with no effect. Uncowled the beast and the mounts
> and accessory brackets all look good. Cold or hot engine has no effect. It
> is there from take off to landing in 1 hour plus flights. Mixture appears
> to have no effect. I tried running through several tanks of fuel to see if
> I got a bad load, no change. Checked the airframe for any loose parts,
> none found. We have not done anything recently that could cause it
> (replacement, maintenance, adjustment, plug change, etc.). Plugs all
> clean, harnesses in good shape.
>
> In bouncy conditions, it comes and goes randomly. Feels a bit like a very
> light engine stumble or prop out of balance. It is better or worse
> depending on RPM, but it is there pretty much between 2250 and the 2700
> red line. Cannot tell at lower rpms, the thing usually was not very smooth
> throttled back.
>
> A couple of flights ago, I climbed to smooth air and it was not there,
> unless I applied a bit of forward pressure on the yoke. Once back down in
> the mild bumps, it was there again. Today, I climbed up to about 5700ft to
> find smooth air. Running 70% power or a bit less at 2475rpm and again, it
> was very smooth without a hint of vibration. I tried to induce it by
> pushing forward on the yoke, but this time it had no effect.
>
> I'm going to do a compression check with the wrench. Pulling the prop
> through after a flight yields 2 successive cyls with more compression that
> the other two, but not a dramatic difference. I know how a 60lb. and under
> cylinder feels (all too well unfortunately). Never noticed any vibration
> or power difference when we had soft cylinders before. The thing is making
> what feels to be full power.
>
> Thoughts?
> Thanks,
>
> Mike


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[J. Severyn]

"Mike Spera" > wrote in message
ink.net...
> Well, I finally have one. The weird vibration out of nowhere. About 6-7
> flights ago, i noticed a very slight rhythmic vibration during a smooth
> straight and level flight. About 4-5 seconds of smooth flight and then 2-3
> seconds of vibration. I chalked it up to the air conditions. Over
snip

Mike,
Have you checked the torque on the prop mounting bolts?
John Severyn
KLVK

[Kyle Boatright]

Is it possible that something has sagged under your cowling and is
intermittantly rattling against something else? An exhaust system or a fuel
or oil line?

[Doug]

Check the engine mounts. They can cause vibration.

[The Visitor]

Just a wild guess but could a control cable, like the elevator cable be
popping out of place around a pulley somewhere. Or the rudder? I am
thinking that perhaps with that happening a surface might be free to
vibrate because the tension is not what it should be.

John

Mike Spera wrote:
> Well, I finally have one. The weird vibration out of nowhere. About 6-7
> flights ago, i noticed a very slight rhythmic vibration during a smooth
> straight and level flight. About 4-5 seconds of smooth flight and then
> 2-3 seconds of vibration. I chalked it up to the air conditions. Over
> the years, I have felt a lot of different vibes depending on the day.
> Each successive flight since then it has become more noticeable. The
> pilot wife even noticed it, and she has no mechanical inclinations.
>
> Tried dropping mags off with no effect. Uncowled the beast and the
> mounts and accessory brackets all look good. Cold or hot engine has no
> effect. It is there from take off to landing in 1 hour plus flights.
> Mixture appears to have no effect. I tried running through several tanks
> of fuel to see if I got a bad load, no change. Checked the airframe for
> any loose parts, none found. We have not done anything recently that
> could cause it (replacement, maintenance, adjustment, plug change,
> etc.). Plugs all clean, harnesses in good shape.
>
> In bouncy conditions, it comes and goes randomly. Feels a bit like a
> very light engine stumble or prop out of balance. It is better or worse
> depending on RPM, but it is there pretty much between 2250 and the 2700
> red line. Cannot tell at lower rpms, the thing usually was not very
> smooth throttled back.
>
> A couple of flights ago, I climbed to smooth air and it was not there,
> unless I applied a bit of forward pressure on the yoke. Once back down
> in the mild bumps, it was there again. Today, I climbed up to about
> 5700ft to find smooth air. Running 70% power or a bit less at 2475rpm
> and again, it was very smooth without a hint of vibration. I tried to
> induce it by pushing forward on the yoke, but this time it had no effect.
>
> I'm going to do a compression check with the wrench. Pulling the prop
> through after a flight yields 2 successive cyls with more compression
> that the other two, but not a dramatic difference. I know how a 60lb.
> and under cylinder feels (all too well unfortunately). Never noticed any
> vibration or power difference when we had soft cylinders before. The
> thing is making what feels to be full power.
>
> Thoughts?
> Thanks,
>
> Mike

[[email protected]]

The Visitor > wrote:
: Just a wild guess but could a control cable, like the elevator cable be
: popping out of place around a pulley somewhere. Or the rudder? I am
: thinking that perhaps with that happening a surface might be free to
: vibrate because the tension is not what it should be.

Since you had a relationship with forward yoke pressure and the vibration,
check the stabilator. The bearings/bolts that hold it on are subject to wear and
corrosion... I replaced the two on mine. Grab the end of the stabilator and try to
pull it back/forth and up/down (without rotatating). It should not move at all... no
"tick-tick" back and forth.

Mine weren't too bad, but the bolts had a bit of rust and a slight wear on
them. Just to provoke respect for this oft-ignored point of wear on PA28's....
exactly two 1/4" bolts are what pivot the stabilator and hold the whole thing on.

Other things to check back there are the trim surface. We had to replace ours
due to cracks where it connects to the arm that actuates it. There's also a bushing
in the bottom of the trim jackscrew.

Just some random thoughts... doesn't really sound like engine stuff.
-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[The Visitor]

wrote:

>
> Since you had a relationship with forward yoke pressure and the vibration,
> check the stabilator. The bearings/bolts that hold it on are subject to wear and
> corrosion... I replaced the two on mine. Grab the end of the stabilator and try to
> pull it back/forth and up/down (without rotatating). It should not move at all... no
> "tick-tick" back and forth.


I remember finding some play in the stab on my Seneca. So the shop
replaced the bushings. I am going to look in the tech logs to see if the
bolts were also done. If not, I'll have it done. It's a 83 model.
125,000 lp tensile strength on those bolts though I believe.

John

[[email protected]]

Plugs might be clean but the resistor in them can fail or
conduct intermittently, weakening the spark. It won't usually show up
during a mag check, but in cruise, if the mags aren't timed exactly the
same, the plug that fires first causes a pressure rise in the cylinder
that the defective plug now has trouble sparking against. Plugs need to
be tested in a pressure-type spark tester, and even then they can act
up when hot.

Dan

[[email protected]]

The Visitor > wrote:
: I remember finding some play in the stab on my Seneca. So the shop
: replaced the bushings. I am going to look in the tech logs to see if the
: bolts were also done. If not, I'll have it done. It's a 83 model.
: 125,000 lp tensile strength on those bolts though I believe.

Yeah... when I was replacing the bolts/bearings on mine, I did a quick
calculation just to make sure. I don't remember the exact numbers, but each one is in
double-shear (both sides of the bearing). That's 4 cross-sections of 1/4" to split
the load over. Not super-special bolts, either... just regular AN stuff. Anyway,
from what I remember, my computation on the shear strength of the bolts concluded that
I could pick up the airplane loaded at gross weight by just the tail, and then shake
it with 4G's before the bolts would shear.... :)

Now, that doesn't say anything about excess material for corrosion resistance,
stress risers due to corrosion, or other strength-reduction due to wear. As long as
it's in good shape however, nothing to worry about... just seemed too small. :)

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[The Visitor]

Thank you for the comments. They are most reassuring.

John

wrote:
> The Visitor > wrote:
> : I remember finding some play in the stab on my Seneca. So the shop
> : replaced the bushings. I am going to look in the tech logs to see if the
> : bolts were also done. If not, I'll have it done. It's a 83 model.
> : 125,000 lp tensile strength on those bolts though I believe.
>
> Yeah... when I was replacing the bolts/bearings on mine, I did a quick
> calculation just to make sure. I don't remember the exact numbers, but each one is in
> double-shear (both sides of the bearing). That's 4 cross-sections of 1/4" to split
> the load over. Not super-special bolts, either... just regular AN stuff. Anyway,
> from what I remember, my computation on the shear strength of the bolts concluded that
> I could pick up the airplane loaded at gross weight by just the tail, and then shake
> it with 4G's before the bolts would shear.... :)
>
> Now, that doesn't say anything about excess material for corrosion resistance,
> stress risers due to corrosion, or other strength-reduction due to wear. As long as
> it's in good shape however, nothing to worry about... just seemed too small. :)
>
> -Cory
>

[Jay Honeck]

> Thank you for the comments. They are most reassuring.

I'll "second" that -- thanks for that data, Cory!

I've always looked at those stabilator attach points and quickly averted my
eyes, cringing. They look soooo small!

Of course, take a look at how a 172s wing attaches, and you'll never fly one
of those again. Yet they've never had a wing failure.

All I can say is: Thank goodness for engineers who can figure this safety
stuff out.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"