M20C Trim Issue

[tim417]

I'm having a strange issue with my trim and I am having my A&P look at it, however I wanted to the hive mind input to see what specific recommendations for my mechanic to follow, as he isn't sure what is going on either. 

The issue is with the trim/elevator behavior during landing. As you know, as you deploy flaps in the Mooney, it pitches the nose downward. So with every notch of flaps I deploy (or pumps, as it were,) I pull the yoke back to compensate for the pitch down, and trim off the pressure. Around the second to third pump, after I pull the yoke back, I try to trim off the pressure but the pressure doesn't release. I give it a couple partial turns of the wheel  as I am used to, but nothing happens. Then a few seconds later, the yoke/elevator abruptly releases the pressure and it can be startling. 

Everything during preflight as far as controls feel like they are working properly. They are free and clear and move smoothly. The trim wheel moves normally. This behavior just started a few months ago and hasn't seemed to have gotten better or worse. It doesn't matter if I manually adjust trim using the wheel or use the trim servo of the GFC500 system. 

The empennage behaves normally with the trim adjustment on the ground as does the elevator movement. However, I am suspecting that maybe the elevator bungees have something to do with this issue? 
 

[PT20J]

47 minutes ago, tim417 said:

The empennage behaves normally with the trim adjustment on the ground as does the elevator movement. However, I am suspecting that maybe the elevator bungees have something to do with this issue? 

You might be on to something. The bungees are the only part of the trim system that is not rigidly connected to the trim wheel. They should be lubed every annual. I might set the trim to the landing position and then check the elevator for anything that impedes its movement. 

[N201MKTurbo]

The elevator bungees could have been lubed with all kinds of things over the years. They attract dirt which can get caked in there. It might be good to remove them, take them apart and give them a good cleaning and painting. Just measure  the nut positions and duplicate on reassembly.

[Echo]

Interesting. I don’t recover with yoke pull after flap engagement. I roll trim and nose comes up and yoke pressure is relieved. 

[Kelpro999]

 You might try rolling trim back to approximately where you notice issues then lift and push down the stinger with some force and observe.

[cliffy]

The nuts at the back of the bungee units have nothing to do with any adjustment of the elevator angles They only snug up and impart a slight preload on the opposing springs inside the barrel. Any adjustment of the elevator angles to set the correct UP elevator angle at the correct STABILIZER setting ( see the TCDS) is done by the forward Heim joint connection by screwing in or out on that part of the bungee

Now you MIGHT have a bungee spring stuck or even broken and you will have to disassemble the rear end of the bungees to inspect that and clean and relube. If you remove the bungees you will need to rerig the elevators and the proper UP angle at the proper STABILIZER setting per the TCDS.

You might also check behind your instrument panel to see if any wires have fallen and are hooking onto the elevator controls 

Another area still is to check the 4  hole link at the back of the jack screw to see if it is worn. One side moves and can bet worn enough to be sloppy

and loose when you pull up and down on the tail end (extending and retracting the jack screw in its threads). 

[N201MKTurbo]

5 minutes ago, cliffy said:

The nuts at the back of the bungee units have nothing to do with any adjustment of the elevator angles They only snug up and impart a slight preload on the opposing springs inside the barrel. Any adjustment of the elevator angles to set the correct UP elevator angle at the correct STABILIZER setting ( see the TCDS) is done by the forward Heim joint connection by screwing in or out on that part of the bungee

Now you MIGHT have a bungee spring stuck or even broken and you will have to disassemble the rear end of the bungees to inspect that and clean and relube. If you remove the bungees you will need to rerig the elevators and the proper UP angle at the proper STABILIZER setting per the TCDS.

You might also check behind your instrument panel to see if any wires have fallen and are hooking onto the elevator controls 

Another area still is to check the 4  hole link at the back of the jack screw to see if it is worn. One side moves and can bet worn enough to be sloppy

and loose when you pull up and down on the tail end (extending and retracting the jack screw in its threads). 

The nuts on the bungees have very little to do with the control forces, but a little bit. They can change the spring rate, a little bit.

if you totally jack them up, it won’t kill you, and you may never know the difference. Your plane would probably not fly as nice as it could.

[cliffy]

The nuts do snug down to a shoulder on the rod capturing the two springs 

[N201MKTurbo]

1 minute ago, cliffy said:

The nuts do snug down to a shoulder on the rod capturing the two springs 

It has been a long time since I took one apart. I recall they had a threaded rod with a Lot of threads left. They may not all be the same.

[cliffy]

There are actually 2 different Mooney drawings (as the parts manual says for different models) and I have both Mooney drawings somewhere

Just don't know where I put them 15 years ago when Mooney Engineering gave them to me. 

[Fritz1]

Check the ball that swivels in the instrument with the steering joke shaft, make sure it moves freely and does not wedge the shaft in any position, clean as needed, trim to landing position, move the elevator by hand about the position where it is during landing, it should drop under its own weight and not bind in any position, trace the elevator linkage from the elevator to the joke, see if anything is binding or broken, lube everything according to service manual, do not lube the ball or the joke shaft, just catches dirt

[Fritz1]

ball swivels in instrument panel

[Fly Boomer]

I understand that this topic was a question about the M20C, but some of the posts in this topic demonstrate a deep understanding of how these things work.  Both @PT20J and @N201MKTurbo were discussing bungees, which I don't think I have, but I'm interested in understanding how the rigging on my M20K works.

The first image below is from the M20J SMM I have and, under "Elevator Removal and Installation", item D seems straightforward enough.  The second image is from the 2014 M20K SMM for my airplane.  Here, item D adds "Set elevators (L & R) to be even with horizontal stabilizer.  Adjust at 915007 HEIM bearings at elevator horns."

Now, I understand that the K has the bob weights while the J doesn't.  Because of that, my elevators hang down against the stops unless someone is pulling on the yoke, or 200 MPH air is helping them align with the horizontal stabilizer.

So, what does the SMM mean by "Set elevators ... even with stabilizer"?  Do they want some kind of clamp to keep them in that position?

 

 

[PT20J]

All of the airplanes before the M20K had trim assist bungees. They function to bias the elevator neutral point in the direction of trim reducing the need for incidence change of the horizontal stabilizer to eliminate stick force at the trimmed airspeed. The K and after have a bob weight (to increase stick force per g) and a downspring that changes tension with stabilizer trim movement to perform the same function.

[Fly Boomer]

22 minutes ago, PT20J said:

All of the airplanes before the M20K had trim assist bungees. They function to bias the elevator neutral point in the direction of trim reducing the need for incidence change of the horizontal stabilizer to eliminate stick force at the trimmed airspeed. The K and after have a bob weight (to increase stick force per g) and a downspring that changes tension with stabilizer trim movement to perform the same function.

I understand that I have the bob weight, but it seems that that bob weight makes it impossible to follow the instruction

"Set elevators (L & R) to be even with horizontal stabilizer.
Adjust at 915007 HEIM bearings at elevator horns."

My elevators don't want to be even with the horizontal stabilizer.

[N201MKTurbo]

1 minute ago, Fly Boomer said:

I understand that I have the bob weight, but it seems that that bob weight makes it impossible to follow the instruction

"Set elevators (L & R) to be even with horizontal stabilizer.
Adjust at 915007 HEIM bearings at elevator horns."

My elevators don't want to be even with the horizontal stabilizer.

I think they just want you to even them up. If they are not even, they will give you a rolling tendency.

[Fly Boomer]

2 minutes ago, N201MKTurbo said:

I think they just want you to even them up. If they are not even, they will give you a rolling tendency.

So something like clamp one side, and then set the other side to match?

[PT20J]

18 minutes ago, Fly Boomer said:

I understand that I have the bob weight, but it seems that that bob weight makes it impossible to follow the instruction

"Set elevators (L & R) to be even with horizontal stabilizer.
Adjust at 915007 HEIM bearings at elevator horns."

My elevators don't want to be even with the horizontal stabilizer.

If you have a bob weight, then you don't have a M20J and so the M20J SMM does not apply.

[Fly Boomer]

Just now, PT20J said:

If you have a bob weight, then you don't have a M20J and so the M20J SMM does not apply.

That quote is from the M20K SMM.  I was comparing the J and K because one has the bob weight (elevators hang down), and the other does not have the bob weight (elevators line up with horizontal stabilizer).

[N201MKTurbo]

20 minutes ago, Fly Boomer said:

So something like clamp one side, and then set the other side to match?

 I think you are over thinking it. Raise one side with your hand till it is aligned with the stab. Then  look at the other one and if it is high or low, adjust it till they come up even.

[Fly Boomer]

Just now, N201MKTurbo said:

 I think you are over thinking it. Raise one side with your hand till it is aligned with the stab. Then  look at the other one and if it is high or low, then adjust it till they come up even.

Makes sense.  Thanks.