Compass problem

[Clark1961]

I seem to have a compass that varies heading according to engine rpm. Not good. I noticed the behavior during runup a few flights ago and just sort of filed the info. Now I'm thinkin' that it isn't normal and should be fixed. It's not like I need the compass to get lost since I manage that on my own but it might be nice if it was accurate...

Obviously I need to see if it does it with the alternator on and off. Any other things I should be looking at?

(now back to shoveling snow - look out Wichita it's headed your way)

 

[kmead]

Any chance it could be out of fluid, they get pretty screwy when they're dry. Try tapping on the side, it should not bounce around.

HUT is inline with ICT for the snow, this winter wont end.

Do I get any award for making the 100th post? Maybe I will at 10,000!

Kevin

 

[Skip Miller]

Similar thread on the red board. The suggestion was made that the tach cable is magnetized and needs to be degaussed.

-Skip

*edit* Well, I can't find the thread over on Red. Anyhow, wherever it was.....

 

[Greg Bockelman]

Similar thread on the red board. The suggestion was made that the tach cable is magnetized and needs to be degaussed.

-Skip

I would like to know how that could possibly happen and why that would affect the compass with varying RPM.

 

[Skip Miller]

I would like to know how that could possibly happen and why that would affect the compass with varying RPM.

Greg, so would I. I was just trying to pass on a link to a similar problem being discussed but now I can't find it.

-Skip

 

[Skip Miller]

OK, found it. FWIW. -Skip

http://forums.aopa.org/showthread.php?t=53035&highlight=degauss

 

[ScottM]

Do I get any award for making the 100th post? Maybe I will at 10,000!

You got a tittle change. Look under your name.

 

[Clark1961]

The compass is in ok shape. I had the compass swung about a month ago. According to the old compass card the last swing was 1988 or so and it showed very small errors. The new compass card has errors as large as ten degrees.

I've been thinking about degaussing a la the Sac-Skyranch big ole electromagnent. Dunno if the big degauss is better or worse than degaussing individual parts. Probably less work to pull the compass and do the big degauss.

 

[gismo]

The compass is in ok shape. I had the compass swung about a month ago. According to the old compass card the last swing was 1988 or so and it showed very small errors. The new compass card has errors as large as ten degrees.

I've been thinking about degaussing a la the Sac-Skyranch big ole electromagnent. Dunno if the big degauss is better or worse than degaussing individual parts. Probably less work to pull the compass and do the big degauss.

First thing I'd try is turning off the alternator and if that doesn't work turning off the battery master. I also cannot see how a magetized tach cable could cause this problem but current flowing through a wire definitely could.

If you determine that it's not electrical, remove the compass from it's mounting and set it on some soft foam. That will isolate it from any vibration which might affect it. If this temporarily solves the problem start looking for something that's changed the balance of the engine and prop or a problem with the engine's vibration isolators.

 

[SCCutler]

What Lance said. I bet good moolah that it's the ammeter wire (one side) too close to the compass.

 

[Clark1961]

First thing I'd try is turning off the alternator and if that doesn't work turning off the battery master. I also cannot see how a magetized tach cable could cause this problem but current flowing through a wire definitely could.

If you determine that it's not electrical, remove the compass from it's mounting and set it on some soft foam. That will isolate it from any vibration which might affect it. If this temporarily solves the problem start looking for something that's changed the balance of the engine and prop or a problem with the engine's vibration isolators.

I'll give the electrical system stuff a try. The battery cable from the alternator looks like it was replaced at some point but all that wiring is low in the panel as near as I can tell. It looks like everything is run at bus level with vertical wires to various components. The alternator cable is strapped to that bundle and near as I can tell the wires for the ammeter are run directly up to the meter itself. Of course I can't be 100% certain about that description. Of course the radio stack is directly under the compass so those wires are close. Can't say I've notice the compass swing when keying the mic but I haven't been thinking about looking for that either.

I had the prop dynamically balanced after overhaul last fall and the engine is pretty-darn-smoooooooooth now (0.05 ips). The before/after difference on the prop balance was rather startling. I was glad the work was done. Of course now, 80 hours later, the prop is leaking so I get to do it all again in the near future. Hopefully the prop shop will warantee the seal work at least.

 

[gismo]

I'll give the electrical system stuff a try. The battery cable from the alternator looks like it was replaced at some point but all that wiring is low in the panel as near as I can tell. It looks like everything is run at bus level with vertical wires to various components. The alternator cable is strapped to that bundle and near as I can tell the wires for the ammeter are run directly up to the meter itself. Of course I can't be 100% certain about that description. Of course the radio stack is directly under the compass so those wires are close. Can't say I've notice the compass swing when keying the mic but I haven't been thinking about looking for that either.

I had the prop dynamically balanced after overhaul last fall and the engine is pretty-darn-smoooooooooth now (0.05 ips). The before/after difference on the prop balance was rather startling. I was glad the work was done. Of course now, 80 hours later, the prop is leaking so I get to do it all again in the near future. Hopefully the prop shop will warantee the seal work at least.

Is this a MaCaulley prop leaking red fluid or is it just grease? If the former, you're probably already not airworthy as this usually means a cracked hub (had that happen to one of my props), if the latter it could just be that they put too much grease into the hub.

What you should be looking for electrically is a medium to large current flowing through a wire that's not balanced by a return (ground wire) somewhere near the compass. The wire could also be close to some ferrous material that feeds the magnetic field to the compass area. Keying a radio will virtually never deflect a compass from the RF but the wire feeding the power to the radio could due to the significant increase in current when transmitting (like 5X) but that normally wouldn't be very dependent on RPM. Usually the only current that's significantly affected by RPM is the field of the alternator although the alternator output could also change with RPM if there is a big load on it. You might also want to check the condition of your battery, if it had a shorted cell the alternator would be dumping a lot of current into it and that current could affect the compass. The alternator itself can generate a magnetic field but normally this is fairly well contained. Turning everything off should eliminate all electrically genrated magnetic fields so if the compass still changes with RPM it's very unlikely due to an electrical current. That pretty much leaves vibration. You can have a perfectly balanced prop and still get high frequency vibration coupling into the airframe, especically if the engine mount isolators are failing and/or some part of the engine or exhaust is touching the airframe.

 

[Clark1961]

Is this a MaCaulley prop leaking red fluid or is it just grease? If the former, you're probably already not airworthy as this usually means a cracked hub (had that happen to one of my props), if the latter it could just be that they put too much grease into the hub.

What you should be looking for electrically is a medium to large current flowing through a wire that's not balanced by a return (ground wire) somewhere near the compass. The wire could also be close to some ferrous material that feeds the magnetic field to the compass area. Keying a radio will virtually never deflect a compass from the RF but the wire feeding the power to the radio could due to the significant increase in current when transmitting (like 5X) but that normally wouldn't be very dependent on RPM. Usually the only current that's significantly affected by RPM is the field of the alternator although the alternator output could also change with RPM if there is a big load on it. You might also want to check the condition of your battery, if it had a shorted cell the alternator would be dumping a lot of current into it and that current could affect the compass. The alternator itself can generate a magnetic field but normally this is fairly well contained. Turning everything off should eliminate all electrically genrated magnetic fields so if the compass still changes with RPM it's very unlikely due to an electrical current. That pretty much leaves vibration. You can have a perfectly balanced prop and still get high frequency vibration coupling into the airframe, especically if the engine mount isolators are failing and/or some part of the engine or exhaust is touching the airframe.

The prop is the original equipment Hartzell and the leakage appears to be engine oil. Thanks for the additional info on the electrical system. The battery is fairly new and appears to hold a charge well with voltage in the 12.3 to 12.5 range with the master on. Ok, that's a cheap voltage meter in the cigar lighter but it's typically close to the multi-meter when I'm futzing around with the system.

 

[Clark1961]

Checked the compass response during run-up on Saturday. The response is tied to the charging circuit.

The circuit is pretty simple with just a field wire (alternator master, OVR, and VR), alternator, battery wire, and ammeter. The battery wire runs from the alt to the left side of the fireware, through the firewall at about the top of the pedals and then back to the main wire bundle that runs across the panel at bus height. The ammeter wires tie to the battery wire in the wiring bundle near as I can tell.

You folks s'pose there's any hope for eliminating the field. Maybe I could wrap the wiring bundle in some flexible magnetic field shield material but that looks like a major pain.

Maybe I just have to live with a wonky whiskey compass and start looking for a remote reading compass? It looks like something maybe available from Century...

 

[Dan Thomas]

Checked the compass response during run-up on Saturday. The response is tied to the charging circuit.

The circuit is pretty simple with just a field wire (alternator master, OVR, and VR), alternator, battery wire, and ammeter. The battery wire runs from the alt to the left side of the fireware, through the firewall at about the top of the pedals and then back to the main wire bundle that runs across the panel at bus height. The ammeter wires tie to the battery wire in the wiring bundle near as I can tell.

You folks s'pose there's any hope for eliminating the field. Maybe I could wrap the wiring bundle in some flexible magnetic field shield material but that looks like a major pain.

Maybe I just have to live with a wonky whiskey compass and start looking for a remote reading compass? It looks like something maybe available from Century...

Every light airplane with a full electrical system will have a charging circuit and a compass and most have no problems. Your airplane didn't have problems when it left the factory. Either someone has messed with the wiring, or there's a bad ground sending current through the airframe under or near the compass. I'd look at the maintenance logs to see what changes might have been made to the system. If nothing there, then use a set of battery booster cables to connect the instrument panel to the firewall or engine mount and try it again. Riveted lap joints will oxidize or corrode and cause all sorts of ground-loop issues and send current through stuff that shouldn't be getting it. I'd also check to see if the ground straps on the shock-mounted gyro panel are busted or loose.

Varying RPM really shouldn't make much electrical difference. The current flow from the alternator won't change much between about 1000 RPM and redline; it's the field current that will vary and that comes from the voltage regulator directly to the alternator. And field current is only two or three amps. The regulator will likely be on the firewall and no field wiring will come anywhere near the compass. More likely I'd be looking for vibration issues; get the RPM up to where the compass starts telling lies, then put your finger against the front, then the side, then the top to see if it settles down when it's damped. No jewellry while doing this, of course.

Dan

 

[Skip Miller]

it's the field current that will vary and that comes from the voltage regulator directly to the alternator.

And field current will vary inversely with rpm: more rpm, less field current and therefore less magnetic field. It doesn't sound like the problem is field current to me.

-Skip

 

[gismo]

Checked the compass response during run-up on Saturday. The response is tied to the charging circuit.

The circuit is pretty simple with just a field wire (alternator master, OVR, and VR), alternator, battery wire, and ammeter. The battery wire runs from the alt to the left side of the fireware, through the firewall at about the top of the pedals and then back to the main wire bundle that runs across the panel at bus height. The ammeter wires tie to the battery wire in the wiring bundle near as I can tell.

You folks s'pose there's any hope for eliminating the field. Maybe I could wrap the wiring bundle in some flexible magnetic field shield material but that looks like a major pain.

Maybe I just have to live with a wonky whiskey compass and start looking for a remote reading compass? It looks like something maybe available from Century...

Did you confirm this by turning the alternator off?

Either there's a conductor carrying much of the alternator output running near the compass or there's a magnetic path through some ferrous material bringing the magnetic field closer to the compass than the wiring itself. Magnetic field strength falls off very quickly with distance unless the poles are widely spaced and that's not the case with a single wire.

Did you confirm this by turning the alternator off?

 

[Clark1961]

Did you confirm this by turning the alternator off?

Either there's a conductor carrying much of the alternator output running near the compass or there's a magnetic path through some ferrous material bringing the magnetic field closer to the compass than the wiring itself. Magnetic field strength falls off very quickly with distance unless the poles are widely spaced and that's not the case with a single wire.

Did you confirm this by turning the alternator off?

Hmmm, there seems to be an echo in your post.

Yes, I confirmed it by turning the alternator off. To whit: run-up rpm with alternator on results in compass swing, identical run-up with alternator off results in no compass swing. Ammeter needle movement conincides with compass swing. I'm fairly confident (reasonably certain in fact) that the charging circuit is at the heart of the matter.

Well, at least vibration can be eliminated as a cause.

The conductor which carries the alternator output is the battery wire I descrbed in the previous post. As mentioned, that wire runs in the wiring bundle which is at buss level behind the panel. It appears that I could re-route that wire so that it doesn't run under the panel. It'd be a major pain but it could be done. The wire does run close to the engine support truss on the engine side of the firewall.

I'm thinkin' that a ferrous material hunt under the panel is the next order of business. Most of the avionics are ancient and I suspect non-magnetic. Some equipment has come and gone through the years so maybe something ferrous got added. There have been a couple suprises under the panel that I've had to correct so another goof isn't out of the question.

Thanks for the feedback folks.

 

[gismo]

Hmmm, there seems to be an echo in your post.

Electromagnetic interference.