PC Rudder servo cable tension

[TheTurtle]

Anybody know how to set or check how much slack should be in the cables that connect the rudder servos to the the rudder?  Mine both seem like they have a bunch of slack.  Just replaced all the red and green hose to all the servos, taped the servos, had the pilot valve and thumb disconnect rebuilt and its still not working correctly.

I checked the install manual and dont see that info anywhere.

Edited March 1, 2017 by TheTurtle

[Hank]

Those aren't cables, they are vacuum tubes. Our Mooneys don't have any cables, except that big one from the battery to the ground.    The red and green tubes should not be in tension,they are just thin wall plastic tubing.

The servo is the big black cylinder that you retaped, and it should be connected to the ~1-1/2" dia steel tube that runs though the rear of the fuselage. Mine are right inside the large avionics panel. I can stop by the plane tomorrow evening and check things if you'd like, send pictures, etc. I'm more familiar with the aileron servos, having had a problem with one of them. Talk about bad access to the mounting screw!

[Hank]

Question:  what do you mean by "it's still not working correctly"?

The PC system is designed to hold wings level attitude when you let go of the yoke to get a drink, look at a sectional, flip through approach plates, etc. Pressing the thumb button vents the vacuum in the line, relieving pressure from the servos so that it's easier for you to maneuver; releasing the button re-activates the system.

Edited March 1, 2017 by Hank

[TheTurtle]

2 minutes ago, Hank said:

 

The servo is the big black cylinder that you retaped, and it should be connected to the ~1-1/2" dia steel tube that runs though the rear of the fuselage.

SO....  what pulls on the rudder shaft when the servo activates???  2 cables...

As I said I just ran new red and green tubing.  Im intimately familiar with them after 2 days upside down on my head running them and obviously realize they arent pulling on anything.

in the rear access where the battery is theres 2 servos.  Each servo has a cable that connects it to the rudder shaft.  Both those cables are really loose.  The cables, wires whatever you want to call them are what pulls on the shaft that connects to the rudder.

As far as whats not working... Not really anything to do with the rudder servos but during a quick ground test today the yoke only goes left when turning right.  not the other direction when turning left.  I just noticed the cables on the rudder servos are really loose and while im beating on this stupid system I figure I should work out that issue as well.

Im going to have to vacuum test the system which Ive been dreading doing.  Maybe one of the aileron servos is bad.  Or maybe my TC is bad.

No kidding on the aileron servos.  That took me a solid 2 hours to get the hose on the copilots side connected.  The other side went on fine.  maybe the green hose is stiffer or something but man what a pain.

[TheTurtle]

theres also a cable on the vacuum operated step just to pick nits

[Hank]

My aileron servos have a short piece of flat chain connected to the bellcrank, it's pretty loose and floppy. I'll look tomorrow and see what's in the tailcone. For the record, my step is fixed. My cowl flaps are fixed. My landing gear is electric, and so are the flaps. Trim is manual . . . Should work the same as yours.

[TheTurtle]

1 minute ago, Hank said:

My aileron servos have a short piece of flat chain connected to the bellcrank, it's pretty loose and floppy. I'll look tomorrow and see what's in the tailcone. For the record, my step is fixed. My cowl flaps are fixed. My landing gear is electric, and so are the flaps. Trim is manual . . . Should work the same as yours.

yea the aileron servos have chains.  the rudder servos has a cable that connects to a spot on the rudder shaft.  Of course I dont have any pics of them..  Ill try to grab some tonight.

 

[takair]

I think you are talking about the short length of cables between the servo and the push-pull tube.  These are essentially self tensioning.  When the servos have vacuum on them, the piston essentially disappears (like a scared turtle) into its cylinder.  When you shut down and move the plane, they can pop out and the cables look loose.  I suspect you have a leak somewhere.  Leak check is the next step, ideally with a portable vacuum system.

[RLCarter]

If memory serves, one servo pulls forward and the other pulls aft, I would think that there needs to be enough slack to where when the rudder is fully deflected the servo that is being pulled apart still has some movement before being stretched past its limits...... Purely a guess on my part

[TheTurtle]

Just now, takair said:

I think you are talking about the short length of cables between the servo and the push-pull tube.  These are essentially self tensioning.  When the servos have vacuum on them, the piston essentially disappears (like a scared turtle) into its cylinder.  When you shut down and move the plane, they can pop out and the cables look loose.  I suspect you have a leak somewhere.  Leak check is the next step, ideally with a portable vacuum system.

i have a vacuum pump for AC systems.  My issue is it pulls 30 inches.  I need to figure out some kind of needle valve so I can bleed off and only pull 5 inches.

both cables feel to loose but I havent tested it with somebody actually pushing on the rudder pedals which I assume would make one tight and one loose and the opposite with the other pedal. 

[TheTurtle]

Just now, RLCarter said:

If memory serves, one servo pulls forward and the other pulls aft, I would think that there needs to be enough slack to where when the rudder is fully deflected the servo that is being pulled apart still has some movement before being stretched past its limits...... Purely a guess on my part

thats correct. and takairs post just made me go duh..  I need somebody to fully deflect the rudders while I look at those cables.  I assume one will be loose and the other tight and visa versa when the other rudder pedal is stomped.

[takair]

Don't have anyone do that unless you are jacked up.  The force to move the nose wheel while on the ground, without movement will eventually break a weld.  Moving the rudder alone won't make the piston go back in, you are pushing against a flexible cable.  You can simply push the piston into the housing.  With the rudder straight, the face will roughly be flush with the lip.  You can also start the engine and have someone look inside, or shut down without turning the rudder and look inside to insure they are tensioned.  As for the pump, you can try to get a salvage regulator and put it in line.  I don't think I would want to use the one in the aircraft to drop that much differential.  

[RLCarter]

8 minutes ago, TheTurtle said:

i have a vacuum pump for AC systems.  My issue is it pulls 30 inches.  I need to figure out some kind of needle valve so I can bleed off and only pull 5 inches.

both cables feel to loose but I havent tested it with somebody actually pushing on the rudder pedals which I assume would make one tight and one loose and the opposite with the other pedal. 

The A/C pump should work, air-cons are sealed so it just keeps pulling, the vacuum system is open and pulls air through the gyro's

[Yetti]

As I recall the rear connecting cables are pretty loose and floppy when sitting on the ground.   You could also use a mitey mite pump to pull the vacuum.  Sounds like one of the wings has a leak

[Browncbr1]

You deflect the surface all the way in one direction and make the servo on the other side all the way out.   With this, there should be about an inch of slop.   Repeat other side.    When sitting on the ground, it looks sloppy.  When vacuum is applied to both servos, the slop goes away.   

I'm not an A&P

[TheTurtle]

i figured out my issue.  the red rudder servo behind the battery box I didnt retape because it was hard to get to.  Well of course that one had a huge leak in it...  rubber was pulled up off the servo on the bottom.  had a huge gap.  Really should have just sucked on each hose before connecting it to the Tee as a test...  Retaped it and ground test shows my yoke moving both directions!!  going to test fly it today.

[Hank]

Yay! You'll really like having a functional PC system.