Ramifications of Cherokee Trim Jackcrew Failure

If the stabilator on the Aztec is the same as that on the Cherokee, the
antiservo tab isn't the sole thing driving the stabilator, although it looks
like it. If you take the top half of the tail cone off, you can see a large
tubular rod extending forward into the tail from the main pivot for the
stabilator. I'm told that this has a counterweight on the end inside the
tail, and that the main control (don't know if it's a rod or cables) from
the yokes attach to this. The only cables going to the trim jackscrew are
the ones from the trim control between the seats. If the trim jackscrew
stripped out, you would not lose the ability to move the stabilator with the
yoke, but you might lose all antiservo action of the tab, depending on the
failure mode. This may or may not be a complete disaster. The reason there
is antiservo action is so that the forces on the stabilator don't result in
positive feedback as you move the stabilator away from the neutral position.
Since the center of pressure moves with AOA, without the antiservo action
the normal aerodynamic action would be to push the stabilator farther from
neutral the farther you move it, the opposite of what you want. At least
that's the way I understand it. Can you fly with it like that? Don't know.
Don't want to find out.

We're rapidly reaching the conclusion that we should get it sooner rather
than later.

--
Bob (Chief Pilot, White Knuckle Airways)

I don't have to like Bush and Cheney (Or Kerry, for that matter) to love
America

"pacplyer" wrote in message
om...
Bob, I'm kinda worried about this. On my snub-nose Aztec the
stabilator has a similar "antiservo" surface that "drives" the entire
stabilator. By pulling on the yoke you are first moving that tab
which in result "flys" the entire tail to a new pitch command
position. Loss of control of that tab suface (i.e. trim jackscrew
unexpectedly strips) at least on my bird, will not just loose trim,
but could, if it shears past the limit, loose all longitudinal pitch
control.

Bob Chilcoat wrote...
If the stabilator on the Aztec is the same as that on the Cherokee, the
antiservo tab isn't the sole thing driving the stabilator...

What Packy described is a servo tab, not an anti-servo
tab. A servo tab coming loose will definitely cause loss
of control. An anti-servo tab coming loose isn't *necessarily*
as catastrophic but I wouldn't push it just the same. Free-floating
surfaces that *can* backdrive the control surface aren't a good thing.

Dave 'aeroservoelastic' Hyde

It should be fixed properly, Its the "maybe it will hold out" Or
"Perhaps the failure mode is not severe" thinking that gets folks in
trouble. Instead try thinking "perhaps it might fail on short final"
or "Perhaps it might fail while flying with someones beloved
father/husband" Just spend the time and or money and get it fixed....



As I mentioned in a couple of other threads in RAP recently, the trim
jackscrew is very worn in our '74 Archer. The wear seems to be nearly half
of the thread depth. In other words, you can move the screw laterally
inside the drum about half of the height of a thread. It can be rocked
angularly inside the drum something like 5-7 degrees. Our A&P can't get to
it to replace it until mid July, so I'm trying to decide if we should ground
the plane, rush it done somewhere else, or wait until the next annual (nine
months). Our A&P tells us that he's never seen one fail, although he has
seen the cable break, which is pretty much of a non-event.

Of course, if the cable breaks, the trim is probably left stuck in some mid
position, not full up or down trim. I can see why this might not be
catastophic.

OTOH, a failure of the jackscrew itself might result in the trim slipping
all the way to full up or down, and this might be more difficult to deal
with. I've never tried to fly with trim at the extremes, so I don't really
know how much effort is needed to stay in control. I know we can just try
it, but that will put more stress on the jackscrew, so I'm not sure I want
to run the experiment on this airplane. Anyone out there with a Cherokee
want to run the experiment for me and let me know what you find?

Another possible failure mode is for the screw to strip so badly that some
or all antiservo function of the tab is lost. This could result in extreme
difficulty in controlling pitch. Anyone have thoughts on how bad this might
be? I have trouble imagining the screw stripping rapidly to the point that
the screw could slide freely up and down inside the drum, but it might be
possible. How can we find out how bad this would be? Is the antiservo
function essential for flight control, or is it just to improve the "feel"
of the controls?

An final concern is that with the extreme looseness we have now, the flutter
margins might be significantly compromised. Stabilator flutter could
certainly result in catastrophic failure modes! This is another potential
ramification of flying with it in the current state. Any thoughts on how to
research this possibility?

Finally, are there other failure modes I haven't though of? Anyone think
that this can wait until the annual? I'd love some thoughts. I have a lot
of respect for you folks, although I realize that free advice is worth just
what I paid for it. Thanks.

"nauga" wrote in message nk.net...
Bob Chilcoat wrote...
If the stabilator on the Aztec is the same as that on the Cherokee, the
antiservo tab isn't the sole thing driving the stabilator...

What Packy described is a servo tab, not an anti-servo
tab. A servo tab coming loose will definitely cause loss
of control. An anti-servo tab coming loose isn't *necessarily*
as catastrophic but I wouldn't push it just the same. Free-floating
surfaces that *can* backdrive the control surface aren't a good thing.

Dave 'aeroservoelastic' Hyde


Dave 'aeroservoelastic' Hyde is correct. It's a servo tab on my plane
not an anti-servo like Bobs. Thanks for the correction. For some
reason it didn't occur to me that the Archer has the same system that
is on the Warrior. Is that right? So then Bob's system and my system
are markedly different. I last instructed in the Warrior in 83'.
You'd think I could remember these things. If the trim jackscrew
cleans off all the threads kinda like on alaska airlines, you may
still have some elevator authority on the archer, that's true. But
how stable is it going to be as you slow down for approach and put out
flaps? Is it going to slam back and forth against the trim limit
positions? Will the trim limit stops hold? The Alaska guys kept
dicking with it trying to simulate the trim needed for an approach
when it let go. After that accident most of us resolved to not try
and free up a jammed stab as procedure dictates. Just head for a
lakebed and land flaps up. On the point about the second control
tube that comes off the counter balance side, on my airplane this
turns into a cable run. But for sure, if my servo tab bolt fell out,
my airplane would just turn into a pointed tool box. So you mothers
stay away from it, understand?

Interesting conversation Bob.

pac

these are all just my hazy opinons only

On Thu, 3 Jun 2004 16:47:43 -0400, "Bob Chilcoat"
wrote:

If the stabilator on the Aztec is the same as that on the Cherokee, the
antiservo tab isn't the sole thing driving the stabilator, although it looks
like it. If you take the top half of the tail cone off, you can see a large
tubular rod extending forward into the tail from the main pivot for the
stabilator. I'm told that this has a counterweight on the end inside the
tail, and that the main control (don't know if it's a rod or cables) from
the yokes attach to this. The only cables going to the trim jackscrew are
the ones from the trim control between the seats. If the trim jackscrew
stripped out, you would not lose the ability to move the stabilator with the
yoke, but you might lose all antiservo action of the tab, depending on the
failure mode. This may or may not be a complete disaster. The reason there
is antiservo action is so that the forces on the stabilator don't result in
positive feedback as you move the stabilator away from the neutral position.
Since the center of pressure moves with AOA, without the antiservo action
the normal aerodynamic action would be to push the stabilator farther from
neutral the farther you move it, the opposite of what you want. At least
that's the way I understand it. Can you fly with it like that? Don't know.
Don't want to find out.

We're rapidly reaching the conclusion that we should get it sooner rather
than later.

I have pulled one of these apart to replace the bobbin.
your description is exactly correct.
btw when you replace the bobbin, if the trim indicator doesnt seem to
be in exactly the old positions you have come to know when you
reassemble it all, then old son you have done what we did and put it
in upside down. it is almost totally symetrical and the only
indication that it is in upside down is that the trim indicator sits
out of position.

the counterweight is a large lump of cast iron.

amazing to think that the entire tailplane is held to the fuselage by
two AN3 hinge bolts isnt it :-)
Stealth Pilot

Stealth Pilot wrote:
: amazing to think that the entire tailplane is held to the fuselage by
: two AN3 hinge bolts isnt it :-)
: Stealth Pilot

I thought the same thing when I replaced both of mine (and the associated
bearings) last fall. Especially since the ones in there were a bit corroded/worn.
Metal fatigue bad!

Of course, just for grins I did the math. IIRC, it was a 1/4" bolt in double
shear. Using the shear strength for AN bolts and four shear joints, I got something
like 15000 lbs. So, you could lift the plane fully loaded by the tailfeathers, shake
vigorously at 6G's, and still be within the yield limit.

(Don't try this at home)...

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

"pacplyer" wrote in message The Alaska guys kept
dicking with it trying to simulate the trim needed for an approach
when it let go. So you mothers stay away from it, understand?

On my first Azwreck, a C model, I had a runaway trim. I ran the electric for
some nose up trim while advancing the power during the take-off roll. I
didn't notice that the trim kept running. It ran to the stop and jammed. She
rotated much earlier than normal. Turning the overhead crank didn't do
anything. It took a lot of muscle but I flew it 20 miles to the next airport
(the departure airport being amongst the top ten busiest GA airports). Some
pattern nazi started in on my for not using the radio while landing
cross-wise to everyone else. After getting the plane on the ground, I went
off on him in way that would have made Uncle Bob proud.

D.

"Capt.Doug" wrote:

Some pattern nazi started in on my for not using the radio while landing
cross-wise to everyone else. After getting the plane on the ground, I went
off on him in way that would have made Uncle Bob proud.

D.
++++++++++++++++++++++++++++++++++

Geez,
Capt Doug, kudos to you.

You make these tired old eyes moist
and put a big lump in my throat.

Thanks,

Unla' BOb

"Capt.Doug" wrote in message ...
"pacplyer" wrote in message The Alaska guys kept
dicking with it trying to simulate the trim needed for an approach
when it let go. So you mothers stay away from it, understand?

On my first Azwreck, a C model, I had a runaway trim. I ran the electric for
some nose up trim while advancing the power during the take-off roll. I
didn't notice that the trim kept running. It ran to the stop and jammed. She
rotated much earlier than normal. Turning the overhead crank didn't do
anything. It took a lot of muscle but I flew it 20 miles to the next airport
(the departure airport being amongst the top ten busiest GA airports). Some
pattern nazi started in on my for not using the radio while landing
cross-wise to everyone else. After getting the plane on the ground, I went
off on him in way that would have made Uncle Bob proud.

D.

Wow, you must not of had any fat chicks in the back. Interesting to
know big tricepts can override full up trim. Different cause, but
that's what killed those guys in the diesel-8 as I recall. They gave
the S/O a t/o and when he rotated he held on kinda tight and ran the
stab to full nose up. (not a runaway trim, just a runaway plumber
thumb.) The last thing on the voice recorder was the captain
repeatedly yelling for him to "push forward!" Apparently the captain
didn't notice the trim running. (it was orig quiet, I think) The tail
was so huge on that thing and its a different type (THS) that full
down elevator couldn't overcome the stab position.

What's with your MD's crooked elevators anyway? Everytime the winds
blowing it scares us guys with normal tail feathers. ;-)

pac

"pacplyer" wrote in message What's with your MD's crooked elevators
anyway? Everytime the winds blowing it scares us guys with normal tail
feathers. ;-)

Ye of little faith....
The elevators float up and down freely and independently of each other. We
have no control over that. Direct your attention to the inboard tabs on the
elevators for the truth. The tabs are cable operated (the 'dc' in DC-9
stands for 'direct cable'). We can lose all hydraulics and electrical
without affecting the primary flight controls. No Sioux City for us!

D.