Trim "sticking"

[Guitarmaster]

Hey Guys.  I want to see if anybody else has experienced this.

On the ground, the trim moves freely in its full travel.  However, with an airload on the stab, the trim seems to get "stuck" and requires either quite a bit of force, or rolling the the opposite direction then go the direction you want.  Sort of like getting a running start.  The problem seems to be toward either travel limit, but mostly nose-down probably because of the high speeds associated with nose-down trim.

The jackscrew and chain have both been lubricated and the torque tube does not seem to be binding anywhere.

Ideas??

[Shadrach]

Mine sometimes does the same at high indicated air speeds. I have been chasing it for a few years. I too have ensured that everything is well lubed and turns freely.  Perhaps there are spots on the jack screw that are worn enough to bind under load.

make sure your MX is actually lubing Jackscrew and not just the indicator that rides on it.

Does anyone know if new parts are available?

 

Edited September 21, 2015 by Shadrach

[takair]

I find mine has tight spots too.  Not as bad as described above, but... I have cleaned, greased, oiled, etc, but doesn't seem to make much difference.  Have not personally had one apart, but it would be interesting to see a cut-away of the mechanism.  That thing has a lot riding on it, would be good to understand what makes it tick.  Anybody have photos or description of the jack screw mechanism break down?  Are there bushings or bearings or anything that can fail?

[Guest]

There is no call for greasing the forward trim gear box, most would have the same grease as when they were built all those years ago.  Might be an item to consider.

Clarence

[takair]

M20Doc

Do you mean where the trim wheel is?  Are there provisions for greasing it?  Also, are there provisions for greasing the box where the jack screw is mounted?  Seems like both are limited to what you can see externally.

[Shadrach]

There is no call for greasing the forward trim gear box, most would have the same grease as when they were built all those years ago.  Might be an item to consider.

Clarence

I knew of a notorious if not infamous vintage Mooney owner (now passed) who claimed that he installed a grease fitting into the box.

[1964-M20E]

I have experienced this as well during descents at high IAS low in the yellow or top of the green.

 

 

[Guitarmaster]

Sounds like a fairly common complaint.  I have thought about removing, cleaning and lubing the jackscrew assembly at the next annual.  This would be a huge PITA.  Right now, I just pack the jackscrew and lube the chain.  That is the only place I can think where a problem could exist.  

I like the idea of a grease fitting on the assembly.

Looking at the exploded view, there are four universal joints.  I am wondering if one (or more) is bad.  Hmmmm.

Edited September 21, 2015 by Guitarmaster

[Guitarmaster]

Mine sometimes does the same at high indicated air speeds. I have been chasing it for a few years. I too have ensured that everything is well lubed and turns freely.  Perhaps there are spots on the jack screw that are worn enough to bind under load.

 

 

I have wondered about it being slightly worn.  It only has 1800 hours on it.  :/

[Guitarmaster]

There is no call for greasing the forward trim gear box, most would have the same grease as when they were built all those years ago.  Might be an item to consider.

Clarence

That will be a point of investigation at the annual this year.

[N201MKTurbo]

Removing the jack screw isn't that hard. After it is out, it isn't that hard to take apart and clean and lube. The only challenge is getting it synced with the front gearbox. That isn't hard with a little forethought. 

Run the trim to the nose up stop. Remove the sheet metal that covers the tail gap and the tail inspection plates. Measure the gap at the bottom of the tail and write it down. Take apart the universal joint just aft of the trim wheel. Slide then torque tube forward until the square coupling on the jack screw comes apart. If you have electric trim you have to remove the drive chain. Remove the trim link in the tail gap. Remove the boot. Remove the two bolts that hold in the jackscrew, they are at the four access holes in the sheet metal. Slide the jack screw forward and remove. Take it all apart and compleatly clean. You can disassemble the bearings by taking the circlips off and removing the balls. Repack the bearings with #6 grease and work as much grease into the screw and nut as possible. Reassemble in the reverse order. Rotate the torque tube until the gap is the same as before you took it apart and put the U-joint back together.

[Conrad]

What about the chain running from the wheel to the gearbox? LASAR just tightened mine up at annual, pointing out that it had become unnecessarily loose. I didn't ask what kind of problem that could cause as I haven't had a problem with it myself, but I imagine it would sound exactly like what you're describing.

[N201MKTurbo]

If the chain is loose, you will just get a bit more backlash.

[N201MKTurbo]

If the behavior changes with air loads, it has to be the screw in the tail. Air loads are huge on the big screw in the tail, that's why it is so big! Air loads have no effect on the front gearbox.

[N201MKTurbo]

I have removed cleaned and re-greased four tail jack screws. I can't imagine why it would ever be necessary to replace it or any part of it. The worst one I saw had lost a few percent of the meat on the threads. Just getting broken in in my opinion. The problem is they are full of fifty year old grease and gunk and are literally glued together. A good clean and lube will do miraculous things for your trim system. 

[Guitarmaster]

Removing the jack screw isn't that hard. After it is out, it isn't that hard to take apart and clean and lube. The only challenge is getting it synced with the front gearbox. That isn't hard with a little forethought. 

Run the trim to the nose up stop. Remove the sheet metal that covers the tail gap and the tail inspection plates. Measure the gap at the bottom of the tail and write it down. Take apart the universal joint just aft of the trim wheel. Slide then torque tube forward until the square coupling on the jack screw comes apart. If you have electric trim you have to remove the drive chain. Remove the trim link in the tail gap. Remove the boot. Remove the two bolts that hold in the jackscrew, they are at the four access holes in the sheet metal. Slide the jack screw forward and remove. Take it all apart and compleatly clean. You can disassemble the bearings by taking the circlips off and removing the balls. Repack the bearings with #6 grease and work as much grease into the screw and nut as possible. Reassemble in the reverse order. Rotate the torque tube until the gap is the same as before you took it apart and put the U-joint back together.

I will be doing this at the annual.  I cleaned out some hard grease in the boot area that was blocking the full trim movement.  I would lay money that the same problem exists inside the jackscrew assembly.

[bonal]

I had to deal with a slight play in the tail assembly last annual so I know the components all to well. Having never seen the equivalent assembly on a modern high power Mooney I am wondering if they are the same and if so such a big difference in the VNe if the tail is one of the limiting factors to that speed where do they get the increase to VNe on the bigger models.

 

[Andy95W]

I had to deal with a slight play in the tail assembly last annual so I know the components all to well. Having never seen the equivalent assembly on a modern high power Mooney I am wondering if they are the same and if so such a big difference in the VNe if the tail is one of the limiting factors to that speed where do they get the increase to VNe on the bigger models.

 

I'll be doing the same next annual.  Did you replace any parts?  Send it to LASAR for refurb?

[bonal]

The first year we flipped the saddle bracket. That gives you a fresh hole for the bolt which tightens things up. The next year we took out the jack screw assembly because there was some play in the tail and you could see the looseness in the screw to block. We replaced the screw part only and that fixed it. The entire assembly is very expensive seems like a good place for a Zirc fitting for easy lubbing but then like with so many things about our 50 plus old airplanes no one ever thought we would keep them flying for so many years when they were designed.

[Shadrach]

I had to deal with a slight play in the tail assembly last annual so I know the components all to well. Having never seen the equivalent assembly on a modern high power Mooney I am wondering if they are the same and if so such a big difference in the VNe if the tail is one of the limiting factors to that speed where do they get the increase to VNe on the bigger models.

 

Vne is a function of many things. In terms of metal Mooneys, I don't believe that the long bodies are any stronger than the short bodies.  What is different are things like wing loading and CG.  Also, there was really little need to certify the B model with a redline of 242KTS.  I doubt that at the time anyone at the factory envisioned the kinds of speeds these airframes are capable of these days.

[1524J]

N201MKTURBO has graciously allowed me to use his instructions and I thought I would add some pics and observations of my own when removing the jackscrew in the tail cone for cleaning.

Run the trim to the nose up stop. Remove the sheet metal that covers the tail gap and the tail inspection plates. Measure the gap at the bottom of the tail and write it down. Take apart the universal joint just aft of the trim wheel.

You'll need to pull the Clevis pins in the pics. Make sure to have new cotter pins when reinstalling. 

There is a inch and a half collar inside the tube with double pins. On mine the ordination of the holes in the collar and tube had to be reinstalled as removed. In other words, if you spin the interior collar 180 degrees, the Clevis pins will not fit. Not hard to remedy, but can a little frustrating until realized.

This is where you measure the distance between the hinge points. The picture is after reinstallation so your measurement will be different.

 

 

Out of an abundance of caution I also marked my verticle stabilizer with a straight egde laying flat on the dorsal fins. You don't want to lose trim orientation.

 Slide then torque tube forward until the square coupling on the jack screw comes apart.

Remove the trim link in the tail gap. This bolt is hidden under the boot. Just pull it back and you'll have access.

 Remove the boot. Remove the two bolts that hold in the jackscrew, they are at the four access holes in the sheet metal.

Once the bolts are removed, you can now remove the jackscrew. It's takes a little work to get the jackscrew out, push down slightly and it'll come out but take it slow. Also, maintain the orientation of the jackscrew. I keep a shop rag handy and marked the top of the main block and both plates on either side with a sharpie. As  N201MKTURBO states, there are very thin shims inside the jackscrew. Don't forget them when you reassemble. 

Double check to make sure your trim is at the same point as when you started by checking your tail cone distance and/or verticle stabilizer. 

As always, get your A&P's blessing before starting and have them check your work.

 

 

 

[carusoam]

1524J,

Nice photos and descriptions!

Best regards,

-a-

[Bartman]

I've been having a problem with trim as well and appreciate the wealth of information on this board. I think mine is the electric trim switch, and it is intermittent both in flight and on the ground. Trim wheel works fine so I have been keeping my hand on the wheel and if it doesn't work electric then I use the manual trim wheel. 

After looking at all of those GREAT step by step instructions and photos above we are going to clean the switch and do the jack screw too. 

Thanks again to N201MKTurbo and 1524J

[1524J]

While you have the sheet metal pieces off the hinge assembly, it's a good time to inspect and comply with service bulletin M20-313A. It's to make sure the filler plate is behind the hinge plate on the aft portion of the assembly. Apparently some were installed incorrectly during assembly. This service bulletin applys to most Mooney models.

[Bartman]

Yeah we did M20-313A a few years ago.

I do appreciate all those great pics to go along with the instructions.