Control wheel/yoke elevator pressure

[Hogface]

I am chasing an autopilot issue. The autopilot is a Stec55x. It flies, in VS mode, and approaches with no issues.

Altitude hold however, and you get a heck of a porpoise. I am wondering if my elevator yoke pressure is to high. Mine tested today (fish scale) to 28 lbs. to get deflection.

Has or could anyone else measure there's?

We have exhausted about everything else, from presure transducer to start voltage in the servo. It does with the trim motor on or off. Unit mounted very stable, and seats firmly. Tried different sources of static air. Validate 12vdc stability out of the unit.

Down to yoke pressure...

[carusoam]

Your mechanic has cleaned and lubricated all the control and trim parts???

I would expect old grease sticks up the system pretty well. At least enough to confuse an A/P...

Best regards,

-a-

[gacoon]

I had this issue once on my STEC 50 with Alt Hold.  Avionics shop adjusted/tightened up something and it cured it.  Go to a shop that knows, STEC - hopefully its just some alignment adjustements.

[mike_elliott]

I am chasing an autopilot issue. The autopilot is a Stec55x. It flies, in VS mode, and approaches with no issues.

Altitude hold however, and you get a heck of a porpoise. I am wondering if my elevator yoke pressure is to high. Mine tested today (fish scale) to 28 lbs. to get deflection.

Has or could anyone else measure there's?

We have exhausted about everything else, from presure transducer to start voltage in the servo. It does with the trim motor on or off. Unit mounted very stable, and seats firmly. Tried different sources of static air. Validate 12vdc stability out of the unit.

Down to yoke pressure...

There is a good chance your pitch servo armature is dirty. Have your avionics shop make up a 9 pin connector pigtale, plug it in the pitch servo and run the armature in both directions for a minute ea. 

That should fix you up.

[OR75]

just to restate the obvious:

 

 

I am almost sure you have an auto trim with an S-Tec 55x.

 

If you disconnect the A/P using the yoke button, is the plane in an out of trim condition ? 

[Marauder]

How far is your nose down trim when it autotrims? Mine will be close to full nose down.

Sent using Tapatalk

[Jeff_S]

I have an S-TEC 55 as well, and was experiencing some porpoising in altitude hold. Upon inspection, the vinyl tubing that was connecting the air pressure parts was old and cracked and screwing up the built-in altimeter, and this air leakage was causing the porpoise.  So have that all checked out as well. (Disclaimer: I'm sure I messed up some of the terminology there but hopefully you get the gist.)

[carusoam]

I'm putting money on Jeff's potential solution!

-a-

[Hogface]

Ok...wanted to give everyone an update.

Since I had already tested the start voltage, and lines in cockpit checked, and unit seated, alt transducer checked, I did a deep dive in the tail.

The plane is apart for annual, so timing was good. The bungees in the tail were terrible...years of grease sprayed on them by unsuspecting mechanics. We temporarily sprayed cleaner into the bungees and the elevator became much easier to move after just a few sprays and up and down movement.

I had them removed and now soaking in chemical cleaner.

While I don't know if this was the problem, I can assure you it wasn't helping. Going to also test the rest of the yoke while the elevator is disconnected too.

I also cleaned the yokes and they slide much easier.

After annual, the IFR certification get done....

Fingers crossed that root has been found..

I will update the group, and thanks for the help!

[Piloto]

I had the same problem with the Century 31 AP. In my case the cause of the problem was yoke shaft binding. Just lubricated both yoke shafts with silicon lubricant and problem solved. You can check for binding by moving the elevator surface up and down by hand. Check for smooth deflection with no binding.

 

José .

[Hogface]

just an update....

Annual complete..

Turbo inlet probe, gear horn, and cleaning of the bungee's

 

The yoke pressure is half of what it was (around14lbs) as near as can be measured.

 

Waiting on a faring cover for the step and I will be able to test fly it....

 

The elevator at the tail moves much easier and the bungees required overnight soaking... more to follow...

 

here is a shot during annual.

 

 

[Awqward]

I got porpoising towards the end of last flight...after 3hrs....I have the KAP150 but hopefully some of the suggestion above are still applicable...

[Hogface]

Update:

Mooney flew way differently, the significant pitch is now gone,and the yokes move much easier. Check yours bungees, grease is bad, Teflon is good.

However it still oscillates +_60 feet and never settles in.(at 200fpm, but not the 1000fpm oscillations)

During pitot static test found signicant leak on back of airspeed indicator, and seems to be a small hole in the diaphragm on airspeed indicator, so it will get sent in..It appears to be allowing small puffs of pitot air to hit static system and we were seeing the +_60 feet in pitot system test equipment similar to the autopilot.

Also the Stec transducer was vertical instead of mounted horizontal which I guess is a big big no no..

These will be corrected and I will advise after that, but so far big progress.

Thanks

[Jeff_S]

I figured there had to be an air leak in there somewhere...as I said, that's what my major cause was. I still do get some oscillation in very still air. This was quite noticeable on both my legs to-from Sun n Fun last week, cruising in rock still air I would start to get those oscillations again. When there is any degree of turbulence, even just a very light chop, it evens right out. I'm in for my IFR cert next month so I'll have them check again for leaks.

[PMcClure]

Have you looked into your AI? I have had this type of problem on 2 different A/C and each time it was the AI. 

[Jeff_S]

Have you looked into your AI? I have had this type of problem on 2 different A/C and each time it was the AI. 

 

S-TEC  are rate-driven autopilots so they are completely independent of the AI. They perform all altitude functions as a result of pressure transducers in the pitot-static system and turn functions result from an independent gyro in the turn coordinator.  In fact, the vacuum pump on my plane only drives a stupid little independent AI that I seldom look at unless I'm practicing my commercial maneuvers.  Sure is fun to see that thing slew over to 60 degrees on steep turns though!