Mystery Vibration - 74 Cherokee 140

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 *
************************************************** ***********************

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

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

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 *
************************************************** ***********************

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

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"