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Thread: Swing your compass?

  1. #1

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    Swing your compass?

    We are having an interesting discussion about compass cards and compass roses on the J3 forum. I got to wondering - how does Cub Crafters, ACA, or Husky do it? Piper had an indexed turn table, and tied the aircraft to it in level flight attitude.

    My take is that just driving around the typical airport compass rose is so inaccurate you might as well not try. I have tried it. I now know better.

  2. #2

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    I used to have a tail stand, it had 3 six inch casters and strapped to the tail wheel. It was plenty wide as needed to be stable. You could walk the airplane around as needed.

    I have plenty of pictures of it on slide film
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  3. #3
    Gordon Misch's Avatar
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    I used the airport compass rose - did it by hand, not driving around. Engine running or not running didn't seem to make a difference. It's a pain and takes a while, but it worked fine.
    Gordon

    N4328M KTDO
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  4. #4

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    I also found little effect with engine running or not. In one case the crankshaft was still magnetic, it was just that running or not the compass looked at the engine, essentially not airworthy due to disrupting instruments. That was the reason I swung that plane. But with other planes, at least the few I swung both running and not, negligible effect.

  5. #5
    wireweinie's Avatar
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    To do it by the book, the aircraft needs to be set up as in level flight on the rose. Try it with and without the engine running. If it makes little or no difference then just record the one card. If there is a big difference record one card with the engine running and max current draw from the charging system. Then do a second card with the engine running and the charging system and battery shut off.

    Since no one is going to do that, go get a brass screwdriver that fits your compass adjustment screws. On a calm day, fly straight and level as close to MAGNETIC north and south as possible with a GPS. Adjust the appropriate screw to get the compass needle as close as possible to north/south. Do the same with your east/west adjustments. Record these deviations on the card. You can do a second one with the charging system and battery switched off if you find a difference from power on.

    Web
    Life's tough . . . wear a cup.

  6. #6

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    That's kind of how I do it in my J3. I do it with an 18" inch brass screwdriver to make adjustments and the GPS in my lap, I dial in North and East.

    Have Fun!
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  7. #7
    www.SkupTech.com mike mcs repair's Avatar
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    magnetic pole is zinging along to the left at 40 miles a year....

    https://returntonow.net/2019/01/29/n...n-800000-years

  8. #8
    wireweinie's Avatar
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    True. But does it matter much to a compass mounted inside of a steel tube fuselage, that's been welded on with a tig? Maybe more to an aluminum airframe.

    Web
    Life's tough . . . wear a cup.

  9. #9
    BC12D-4-85's Avatar
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    I try to do it on a calm day while on floats so more level. Use the GPS track while going straight as possible. Could be done on wheels or skis if enough room. Painted compass cards can disappear in the winter around here.

    Gary

  10. #10
    Gordon Misch's Avatar
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    Quote Originally Posted by wireweinie View Post
    True. But does it matter much to a compass mounted inside of a steel tube fuselage, that's been welded on with a tig? Maybe more to an aluminum airframe.
    And I'd offer, that for the same reasons, trying to be really, really exact - i.e. level attitude rather than three-point, every electric circuit closed, etc - probably doesn't make much difference. My mag compass is mounted upper left near the front spar root because that's where it seemed happiest. Up there it swung accurately within a couple degrees, and is "pretty accurate" in flight. Anywhere near the panel it always pointed east. Always.
    Gordon

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  11. #11
    coxcub's Avatar
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    Folks,
    This is a topic very close to my heart at present. I am getting close to finishing a Fairchild UC61K and am now clearing up the smaller items. However the mag compass is 'Stuck on 100*' what ever the aircraft heading although when removed it is fine.
    The bracket in which it is mounted is clamped to the main spar carry through tube which was repaired by splicing in a new section. I have come to the conclusion that the welded (probably TIG) sections were not demagnetized.
    My thoughts on a fix are obviously to demagnetize but how? - would passing a magnetic particle NDT yoke along the tube be a way forward?
    Any thoughts on this or any alternates would be very welcome as I am not going to get 'signed off' until the compass at least give an approximate indication of the way I am going.
    Many thanks
    Frank

  12. #12

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    Web's solution is what I suggested. It took us about 30 minutes to adjust and write a card using a GPS and two pilots. It absolutely requires calm winds, but there are days when it is calm.

    Many years ago I tried the compass rose. Even in a J3 it was almost impossible to get the aircraft lined up, and engine running would have been dangerous to say the least. But the poster on the J3 forum needs a compass correction card before his IA will approve a test flight. I tried to suggest the obvious solution to that.

  13. #13
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    Your GPS indicates the direction your airplane is moving. Your compass indicates the direction your airplane is pointing. How do you calculate the difference while in the air?
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  14. #14
    wireweinie's Avatar
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    Fly on a calm day to minimize the difference in magnetic direction and airplane direction.

    Web
    Life's tough . . . wear a cup.
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  15. #15

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    My thoughts on a fix are obviously to demagnetize but how? - would passing a magnetic particle NDT yoke along the tube be a way forward?
    Any thoughts on this or any alternates would be very welcome as I am not going to get 'signed off' until the compass at least give an approximate indication of the way I am going.
    Many thanks
    Frank[/QUOTE]

    I think you need to study up on degaussing airframes. Degaussing loop can be bought or home made.
    You can't get there from here. You have to go over yonder and start from there.

  16. #16
    wireweinie's Avatar
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    If you actually need a magnetic indicator, a remote compass system is about the only one that works reliably in a steel tube fuselage. There are ways of degaussing but after a season or two it seems to show back up. Make sure you have no steel hardware around the compass mount and route power wires away from it first

    Frank. What are your legal requirements for a compass?

    Web
    Life's tough . . . wear a cup.
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  17. #17

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    There are a few really uptight IAs out there. Find one that doesn't have to observe your compass while you demonstrate your parking skills on a compass rose?
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  18. #18
    cruiser's Avatar
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    My GPS works on the ground. No need to fly the airplane.

  19. #19

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    Mine does too but it needs to be moving. One would need a farm field.

  20. #20
    coxcub's Avatar
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    Web,
    It is a requirement over here to have a compass with an in date deviation card so I am stuck with sorting it out.
    Frank

  21. #21
    wireweinie's Avatar
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    Does he have to watch you make up the card? lol

    If you need a mag instrument that works correctly, look into a remote compass system. If you want to stay with the classic whiskey compass you're probably going to have to move it to another location. (I'm assuming that you've already checked for the obvious stuff like the steel hardware) Just remove the compass and hold it in another location to see if it's free from interference. Then find a mount system that works for the location and compass. Other side of the panel? Mount to the center of the windscreen?

    Web
    Life's tough . . . wear a cup.

  22. #22
    cubdriver2's Avatar
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    Quote Originally Posted by coxcub View Post
    Web,
    It is a requirement over here to have a compass with an in date deviation card so I am stuck with sorting it out.
    Frank
    Look under your seat cushion, I bet an old one got lost there sometime in the past

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  23. #23
    BC12D-4-85's Avatar
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    Remove the compass and use it or a hand held surveyor's compass to locate the worse magnetic source's spot. They will swing and point to welds or tubing if held close. If it's localized install the compass away from there if possible. I've used this attached to the compass to help correct local problems. Either bolt or glue it on the compass or mount:

    https://www.aircraftspruce.com/catal...ompensator.php
    https://www.aircraftspruce.com/catal...structions.pdf

    Gary

  24. #24
    Gordon Misch's Avatar
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    I have a set of those compensators, if someone would like to have them.
    Gordon

    N4328M KTDO
    My SPOT: tinyurl.com/N4328M (case sensitive)

  25. #25

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    You guys tickle me. Glad I asked. I feel like I am in very good company with Web and Glenn. I will never feel sorry for a pilot who is grounded for lack of a compass correction card.
    Well, maybe I will feel a little sorry for them . . .

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