1966 M20E Compass Problem

[N9335M]

I recently purchased a 1966 M20E. It seems the compass is somehow slaved to the post in the middle of the windshield. It will not deviate from 220deg; however, when I moved it away, it swings freely. Does anyone know how to demagnetize the center post? If not, I guess I need to have it mounted elsewhere. I'm new to Mooneys. Thank you in advance!

[madjano]

Degausser, shop may or may not have one. Or bust open that old tube TV in the attic.

Sent from my SM-G981V using Tapatalk

[Andy95W]

It’s a common problem that’s been around a long time.

Here are a Service Bulletin and a Service Instruction that address the issue.  Both documents have been around since the 1960s.

https://www.mooney.com/wp-content/uploads/2020/12/SBM20-150A.pdf

https://www.mooney.com/wp-content/uploads/2020/12/SIM20-23A.pdf

Edited August 22, 2023 by Andy95W

[Jim Peace]

Question:  Where is Earths magnetic north pole?

[EricJ]

5 minutes ago, Jim Peace said:

Question:  Where is Earths magnetic north pole?

It's migrating to Russia.

[Jim Peace]

5 minutes ago, EricJ said:

It's migrating to Russia.

simpler....if the north pole of a bar magnet, a compass, points towards what we call the top of the earth....and opposites attract with magnetism....then.......come on now I know you can figure this out....

[ArtVandelay]

Question:  Where is Earths magnetic north pole?

Trick question, Antarctica.

[EricJ]

Pedantry.

Another example:   Which direction does electrical current flow?

[MikeOH]

1 hour ago, EricJ said:

Pedantry.

Another example:   Which direction does electrical current flow?

Electron or hole current?

[EricJ]

3 minutes ago, MikeOH said:

Electron or hole current?

Pick a point of view, and pick the two with that point of view that agree with each other.   i.e., two with that point of view will agree with each other, the other two will agree with the other point of view:

1.  The FAA.
2.  The US Navy
3.  The mathematics of using traditional charge convention and energy flow.
4.  The physics and forces produced by electromagnetic fields and resulting power flow.

[N201MKTurbo]

I wonder if there is a correlation between the belief in electron flow and the prohibition on touch and gos?

[Hank]

40 minutes ago, EricJ said:

Pick a point of view, and pick the two with that point of view that agree with each other.   i.e., two with that point of view will agree with each other, the other two will agree with the other point of view:

1.  The FAA.
2.  The US Navy
3.  The mathematics of using traditional charge convention and energy flow.
4.  The physics and forces produced by electromagnetic fields and resulting power flow.

And this is why I'm a MECHANICAL engineer--crazy Electrical types can't make up their minds about how their own stuff works!!

[Yetti]

3 hours ago, EricJ said:

Pedantry.

Another example:   Which direction does electrical current flow?

The way the phases pull it down the line.   Ha ha you messed up the question and did not state AC or DC.

[Yetti]

Just now, Yetti said:

The way the phases pull it down the line.   Ha ha you messed up the question and did not state AC or DC.

Which reminds me of the time I turned in my program on microfiche.   The TA got all pissy and I just said  you did not specify the output format.

[EricJ]

7 minutes ago, Yetti said:

The way the phases pull it down the line.   Ha ha you messed up the question and did not state AC or DC.

No, instantaneous current is the same. 

[N201MKTurbo]

1 hour ago, EricJ said:

No, instantaneous current is the same. 

Current is current no matter which way it is flowing, unless it does….

[cliffy]

I think I have a sheet of MU metal that is supposed to block the magnetism if wrapped around the center post. 

It would take a full compass swing to verify if it actually worked enough to bring the compass into swing spec.

[N9335M]

Thank you all for your help, and excellent discussion on magnetism and electricity. I bought a degausser. I will give it a try after it arrives tomorrow and let you know how it worked.

I'm embarrassed to point out that I flew the plane a few times before realizing the compass wasn't working. Foreflight gives me sufficient information that I seldom use the compass.

[PT20J]

The windshield tube in my J is non-magnetic (stainless steel). Is this true for all models/years?

[Yetti]

On 8/22/2023 at 9:35 PM, EricJ said:

No, instantaneous current is the same. 

I am going to go with where it is measured.

Safety

National Program

Understanding Lightning: Making Connection With the Ground

Weather.gov > Safety > Understanding Lightning: Making Connection With the Ground

During the development of the stepped leader, the positive charge on the ground increases. As the branches of the stepped leader approach the ground, the electrical forces between the leader tips and the tops of tall objects on the ground also continues to increase.

Eventually, these forces cause the air above the taller objects to become more conductive, to ionize. As it does, negative charge starts moving from the air to the taller objects and into the ground. The result of the negative charge moving toward the ground, is that the channel of air directly above the taller objects becomes positively charged. This is usually referred to as an upward streamer. Eventually, the negatively-charged, downward-moving stepped leader makes contact with one of the upward developing positive streamers.

When this contact is made, the lightning channel is complete and charges can flow rapidly from the cloud toward the ground. It takes only a fraction of a second to go from the stepped leader initiation to the final connection.

[Hank]

10 hours ago, PT20J said:

The windshield tube in my J is non-magnetic (stainless steel). Is this true for all models/years?

No. Many vintage models are the same material as the rest of the cockpit cage, and the whole thing can be magnetized. 

[bcg]

Thank you all for your help, and excellent discussion on magnetism and electricity. I bought a degausser. I will give it a try after it arrives tomorrow and let you know how it worked.
I'm embarrassed to point out that I flew the plane a few times before realizing the compass wasn't working. Foreflight gives me sufficient information that I seldom use the compass.

Be careful, if you overdo it with the degausser it's possible to make the problem worse. I would suggest trying the alloy sheet wrapped around the center post first, it's an inexpensive experiment in aviation money.

https://www.aircraftspruce.com/catalog/inpages/magneticcompassshield.php

Sent from my Pixel 6a using Tapatalk

[Fly Boomer]

On 8/23/2023 at 11:12 PM, N9335M said:

Thank you all for your help, and excellent discussion on magnetism and electricity. I bought a degausser. I will give it a try after it arrives tomorrow and let you know how it worked.

I'm embarrassed to point out that I flew the plane a few times before realizing the compass wasn't working. Foreflight gives me sufficient information that I seldom use the compass.

Please heed the advice from @bcg above.  Any kind of electrical device used inside your airplane has the potential to create as many problems as it cures if not used properly.

[N201MKTurbo]

It is important with a degausser to operate it properly. The degausser works by inducing an AC magnetic field into the structure to try to randomize the local magnetic fields. The degausser must be moved away from the structure while it is energized. If you turn off the degausser while in close proximity to the structure, it will assume the polarity of the degausser when it was switched off. If you pull it away before deenergizing it, the effects diminish rapidly with distance and the fields stay randomized.

You also have to keep moving it while close to the structure. It only works by leaving the magnetic poles in a random state as the degausser moves away from something, such as along a tube.

[N201MKTurbo]

On 8/24/2023 at 7:42 AM, PT20J said:

The windshield tube in my J is non-magnetic (stainless steel). Is this true for all models/years?

I'm pretty sure it is aluminum, not stainless.