M20K rigging and AP issues (GFC 500)

[khedrei]

Having some ongoing issues with rigging and my AP.  One is annoying, the other seems to be a bigger problem.  I will try to make it quick.

1980 M20K 231, Merlyn wastegate, LB engine, no rudder trim.  Full Garmin setup, G500TXi, 750xi, 650, GFC500, G5, etc

GFC500 AP servos were replaced late last year under the Garmin extended warrenty.  I had already paid for one to be changed before Garmin fessed up to their mistake and then had all of them (including the one already replaced in 2022) replaced.  After this I noticed some annoying pitch twitching.  It was small, and generally wouldn't show more than 100FPM up or down but often no indication on the VSI, just felt the annoying twitch.  It was very much a twitch rather than an oscellation, and quickly over.  I never remember this AP doing this before the servos were changed.  

When they changed out the servos, I had them check the rigging of the ailerons, as the plane seemed to roll to the right with the AP off and the controls were also permenantly tilted to the right.  It has been like this since I got it in 2017 but I figured this was a reputable shop, and while it was in, I wanted to fix as much as possible.  I conceeded that the control yoke being straight was less important than the rigging being straight.  They made what seemed to be fairly large adjustements on the ailerons on both sides.  Yoke still not straight though.  When I got the plane back, I noticed the twitching, and that the plane still pulled to the right with the AP off.

Back to the shop, flew with their pilot who used to own the place before he retired and left it to his family and he acknowledged that there was a problem but he believed it to be in the rigging of the tail because if left rudder was used in level flight, the plane would fly straight.  I agreed, and they proceeded to make some adjustments as well as clean and lube the controls, and adjust gain setting for the AP twitching.

I have the plane back and it took a while for me to get a good test flight in smooth air to test it, but the pull to the right is very much there.  In fact it is probably worse than before.  At cruise flight, trimmed, power set, A/S steady, if I disconnect the AP the plane will yaw to the right pretty aggressivly and be over to 20 degrees of bank within a few seconds.  It seems that I can hold it straight with some left rudder.

I think my space might be full but I will try to attach some pics and videos but a quick summary:

-The right aileron is up slightly (but noticably) higher in straight cruise flight regardless whether I have the ball centred with rudder

-The airplane DOES fly about 4-7 knots faster in cruise flight with the same power settings.  I do the same trip often so this is easy to see.  I always found this plane to be slow for this model compared to others, so maybe we are on the right track with the rigging.

-Last trip (which is where I noticed the rigging isse the most) the plane seemed to be constantly banked to the left by a degree or so, I assume because the ball is to the left so the AP is fighting a slip and the YD can't put enough pressure.  I know the YD is not rudder trim but I would have thought that with a slight deviation it would keep the ball centred.

-Will flying around misrigged destroy these servos and if so, now quickly?  I'm trying to get this sorted before a huge trip I am planning in June.

-We originally thought the twitching was caused by moisture in the static line because it didn't happen in PIT mode and still doesn't, but they told me they blew out the system.  It's slightly better now than it was at first, but not as good as I remember.

Two questions

1. Does anyone with this model airplane without rudder trim have the issue in cruise flight in smooth air where they MUST use left rudder to prevent an agressive turn to the right?  Or should this plane fly "Pretty damn straight" hands and feet off?

2. Has anyone else noticed any of this "twitching" in pitch with the AP on in ALT mode.  It is even present in a VS climb or decsent.  Should this thing be on rails in smooth air or are my expectations too high?

They are trying to figure out what to do next.  They are a pretty good shop but if they are going to keep charging me and not fix this I have to figure out another route.  Any input that I can share with them would be greatly appreciated.

 

Kevin

Edited April 30 by khedrei

[LANCECASPER]

29 minutes ago, khedrei said:

Having some ongoing issues with rigging and my AP.  One is annoying, the other seems to be a bigger problem.  I will try to make it quick.

1980 M20K 231, Merlyn wastegate, LB engine, no rudder trim.  Full Garmin setup, G500TXi, 750xi, 650, GFC500, G5, etc

GFC500 AP servos were replaced late last year under the Garmin extended warrenty.  I had already paid for one to be changed before Garmin fessed up to their mistake and then had all of them (including the one already replaced in 2022) replaced.  After this I noticed some annoying pitch twitching.  It was small, and generally wouldn't show more than 100FPM up or down but often no indication on the VSI, just felt the annoying twitch.  It was very much a twitch rather than an oscellation, and quickly over.  I never remember this AP doing this before the servos were changed.  

When they changed out the servos, I had them check the rigging of the ailerons, as the plane seemed to roll to the right with the AP off and the controls were also permenantly tilted to the right.  It has been like this since I got it in 2017 but I figured this was a reputable shop, and while it was in, I wanted to fix as much as possible.  I conceeded that the control yoke being straight was less important than the rigging being straight.  They made what seemed to be fairly large adjustements on the ailerons on both sides.  Yoke still not straight though.  When I got the plane back, I noticed the twitching, and that the plane still pulled to the right with the AP off.

Back to the shop, flew with their pilot who used to own the place before he retired and left it to his family and he acknowledged that there was a problem but he believed it to be in the rigging of the tail because if left rudder was used in level flight, the plane would fly straight.  I agreed, and they proceeded to make some adjustments as well as clean and lube the controls, and adjust gain setting for the AP twitching.

I have the plane back and it took a while for me to get a good test flight in smooth air to test it, but the pull to the right is very much there.  In fact it is probably worse than before.  At cruise flight, trimmed, power set, A/S steady, if I disconnect the AP the plane will yaw to the right pretty aggressivly and be over to 20 degrees of bank within a few seconds.  It seems that I can hold it straight with some left rudder.

I think my space might be full but I will try to attach some pics and videos but a quick summary:

-The right aileron is up slightly (but noticably) higher in straight cruise flight regardless whether I have the ball centred with rudder

-The airplane DOES fly about 4-7 knots faster in cruise flight with the same power settings.  I do the same trip often so this is easy to see.  I always found this plane to be slow for this model compared to others, so maybe we are on the right track with the rigging.

-Last trip (which is where I noticed the rigging isse the most) the plane seemed to be constantly banked to the left by a degree or so, I assume because the ball is to the left so the AP is fighting a slip and the YD can't put enough pressure.  I know the YD is not rudder trim but I would have thought that with a slight deviation it would keep the ball centred.

-Will flying around misrigged destroy these servos and if so, now quickly?  I'm trying to get this sorted before a huge trip I am planning in June.

-We originally thought the twitching was caused by moisture in the static line because it didn't happen in PIT mode and still doesn't, but they told me they blew out the system.  It's slightly better now than it was at first, but not as good as I remember.

Two questions

1. Does anyone with this model airplane without rudder trim have the issue in cruise flight in smooth air where they MUST use left rudder to prevent an agressive turn to the right?  Or should this plane fly "Pretty damn straight" hands and feet off?

2. Has anyone else noticed any of this "twitching" in pitch with the AP on in ALT mode.  It is even present in a VS climb or decsent.  Should this thing be on rails in smooth air or are my expectations too high?

They are trying to figure out what to do next.  They are a pretty good shop but if they are going to keep charging me and not fix this I have to figure out another route.  Any input that I can share with them would be greatly appreciated.

 

Kevin

 

First I don't know of any Mooney 231s that had rudder trim. (Some Mooney 252s and Encores that had a KFC150 autopilot were installed with STec Yaw dampers and rudder trim. )

 

@khedrei Have you had the latest revision done?

 

[Pinecone]

For the pitch twitching, Garmin just released an SB allowing some change in gain settings to deal with it.

Is the shop that did the rigging one that really knowns Mooneys?  You might want to try a top notch Mooney rigging shop.

[hubcap]

The M20K 231 does not have rudder trim. My M20K 231 is equipped similarly to yours and it does require a small amount of left rudder pressure to keep the ball centered and it does pickup a knot or two of airspeed. However, I have no issues with the pitch and the GFC 500. Mine is as steady as a rock.

[EricJ]

A small adjustment on one of the wing flaps may fix the wing-down issue.   Get the airplane to fly straight and then adjust the ailerons so that the yoke is level.

[khedrei]

Thanks for the replies so far.

 

@LANCECASPER I didn't think any 231's had rudder trim, just wanted to make sure people knew my plane did not.  If they were to offer me advice, I wanted to include as much info as possible beforehand.

@EricJ I will look into the flaps being a potential problem.  

I will inquire about the Rev 5 on the GFC500.  That could be the fix for it.

Does anyone know of a good Mooney shop in the area of say... Michigan, Indiana, Ohio, Pensylvania, upstate New York?  Basically somewhere I could get to within an hour or 2 of Toronto.  We have less options here in Canada and if these guys can't fix it then I might be able to get faster and more reliable service in the U.S.  I gotta get this fixed by the end of this month as my big trip in June will surely destroy the servos if things are that far out of rig.

 

 

[Andy95W]

There’s a Mooney Service Center in Kitchener that’s run by a highly knowledgeable guy.  Have you tried there?

[Mooney in Oz]

On 5/1/2024 at 5:01 AM, khedrei said:

2. Has anyone else noticed any of this "twitching" in pitch with the AP on in ALT mode.  It is even present in a VS climb or decsent.  Should this thing be on rails in smooth air or are my expectations too high?

I had the same twitching issues as you describe. Since I had Rev 5 Gain Addendum installed, my J flies rock solid with no more twitch. IAS mode in climb and descent is also vastly improved. Very happy.

[Z W]

Been through the rigging issue, and I have the same autopilot.  1982 K model.

Plane rigging was way off when we got it.  Wing wanted to drop sharply to the right when released, and the ball was always about 1/4 to the right in straight and level flight.  Hand flying approaches was always a challenge, and with the old KAP-150 autopilot on, the plane flew around in a slight slip all the time to track straight.

What I learned is that many mechanics go changing aileron and rudder settings when the problem starts somewhere else.  The proper way to fix it is to start with the gear doors, make sure they are flush when closed and adjusted properly.  Then the flaps, make sure they are in the proper position and equal.  After that, set the ailerons, rudder and elevator (not sure which order).  There are specifications for how to do it in the service manual, and they call for the use of rigging travel boards. 

If you don't start in the right place, or just start changing things to chase the turn and bank indicator, each adjustment just puts it further out of whack.  You need an experienced mechanic with set of proper rigging boards, most likely. Once everything is properly centered, test fly and make very minor adjustments if necessary. None were required for me.

Side note - do all this without anything like a large iPad or VFR GPS bolted to the yoke throwing off the balance.

Dave Green at Sarasota Avionics in Florida did all this and now the plane flies beautifully.  Like on rails, hands off.  Autopilot is also very solid and has always been.  It was installed after the rigging was fixed.

Good luck, hope you can get it figured out.

[PT20J]

Do this in the proper order

1. Get it rigged correctly

2. Remove any excessive control friction 

3. Make sure there are no static leaks

4. Then if there are still issues try the new gains. If you do this first, you may be masking other issues.

[khedrei]

Thanks @Z W

Spoke to the shop. They seem willing to take the plane in again and have a look at things and they want to make it right.

He mentioned possibly installing a rudder trim. I saw the one he is talking about and I really don't like it. I hope they can straighten the tail properly without having to resort to cutting a hole in my rudder to install a goofy trim tab.  Florida is really a hike but ill do it if I have to.  It's going to cost 3k just for the trip. Ideally I find somewhere closer if these guys can't figure it out. 

[Z W]

3 hours ago, khedrei said:

Spoke to the shop. They seem willing to take the plane in again and have a look at things and they want to make it right.

He mentioned possibly installing a rudder trim. I saw the one he is talking about and I really don't like it. I hope they can straighten the tail properly without having to resort to cutting a hole in my rudder to install a goofy trim tab.  

Is there a STC to put rudder trim on a K?  Never heard of that.

I wouldn't let anyone install rudder trim unless all other efforts had failed.  Just another system to manage and maintain that you shouldn't need.  I'm sure there are shops closer than Florida that have done it before.

Hope you get it figured out.

[LANCECASPER]

14 hours ago, Z W said:

Is there a STC to put rudder trim on a K?  Never heard of that.

For sure Stec had an STC to put rudder trim/yaw damper on the M20K 252 models that had the Bendix King autopilot. I don't know whether the STC covered 231 K models. 

I had this on an 1997 K model but never felt it was necessary. Putting rudder trim on to mask over something else isn't a good idea.  No shop that knows how to rig a Mooney would suggest this.

[khedrei]

I'm pretty sure he's talking about this product for trim.  I definitely don't want it.

 

http://www.aerotriminc.com/id2.html

 

It shouldn't be needed on a properly rigged airplane. 

[Bigdaddie]

I could be wrong (and I'm wrong a lot), but isn't there a provision to bend the aft lower skin on the rudder to address trim issues?  I don't know if I read this in the service manual or it was word of mouth or forum.  I'd let a Mooney mechanic do this though. 

[EricJ]

2 minutes ago, Bigdaddie said:

I could be wrong (and I'm wrong a lot), but isn't there a provision to bend the aft lower skin on the rudder to address trim issues?  I don't know if I read this in the service manual or it was word of mouth or forum.  I'd let a Mooney mechanic do this though. 

Yes. that's in the SMM, at least for the J model.   It is done sparingly, though, since aluminum work hardens and fatigues every time it is bent, so my understanding is it is really intended to only be done once or twice in the life of the rudder, from what I can gather, although the SMM does not indicate any limitations in this regard.   

[PT20J]

You do all the rigging by the SMM first and bend metal afterwards to make small  tweaks if necessary. 
 

Shoptalk - rigging.pdf

[khedrei]

Thank you very much for this documemt @PT20J im going to print it off for them to use.

 

[Marc_B]

Regarding the S-Tec yaw damper/rudder trim: I had this in my 97 M20K Encore. It mounts in the same location as the Garmin GFC 500 yaw damper. So you have to forego the Garmin YD if you want to keep rudder trim. This is different attachment from the Mooney rudder trim seen on all long bodies that had an L bracket attached to the bell cranks and the servo motor mounted in a different place that didn’t interfere with the Garmin YD. This appears to be really simple to add to other Mooneys without rudder trim (heck, most of the IPC graphics and parts are the same on the M20K as well as thru later long bodies) however there is no STC or FAA process unless you go with field approval. Sounds way to expensive for little gain so I opted to have no rudder trim in lieu of Garmin YD that was installed.

But from someone who went from rudder trim to no rudder trim, it was a non issue with the Encore 220hp. The only time I’d use rudder trim was on takeoff. Now my foot accommodates and all is good. Otherwise my Mooney is trimmed pretty well and usually slips or skids are cause by me, not the Mooney. 
I do think the Garmin YD integrates well and “tightens” things up. Not horribly noticeable but noticeable if you’re looking for it. 

[khedrei]

Figured I would update this thread.

I was able to solve the rigging problems.  I had to be my own advocate a bit and push the shop to dig deeper in the manual.  They admitted they were not experts at rigging a mooney, and also admitted they did not know that the trailing edges could be bent on the rudder or ailerons.  They thanked me for pushing them towards the solution, admitted they learned something and covered that visit under warrenty so I ended up paying for some fuel as well as AP software updates but the rigging was basically paid for in the previous visit and covered under warrenty for this visit.  I was very happy with their professionalism and their willing to aknowledge their errors.  I wish more people could be like that.  Going on a long trip very soon so I will find out if the AP update smoothed out the pitch twitch or not.  

In the end, they told me that the ailerons needed another slight adjustment, the right flap got dropped 1 degree (a bit annoying cause looking out the window in cruise I can tell it's a bit lower) and the trailing edge was bent (I believe twice) to find the sweet spot.  The plane now flies dead straight at 28" 2500RPM and about 3/4 fuel with hands and feet off.

Thanks for everyones input and for posting the paragraph regarding the rigging.