Magnetic field

[Guy Buchanan]

OK it took me some digging, but I found a place that will rent a tool you can use for demagnitizing large structures. This link takes you to a place that rents contour probes. These are used for NDT, but work as demagnetizers. When I put my IV together the fuselage around the instrument panel was magnetized enough to render my compass useless. I rented one of these and remove enough of the magnetism to make the compass accurate without compensation. They're dead simple to use, but you will demagnetize anything and everything else in the area, so you want to do it before installing any avionics, and better yet without installing the engine charging and ignition system, or any other digital stuff.


I used one of these dirt simple gauss meters to check out the actual fuselage magnetism.

[HighWing]

A question, I guess. When I was first building - 1993 - I installed hard points for a VOR antenna, but in the five years before first flight early GPS units, rather primitive by today's standard, became available and the VOR antenna was never installed. Then, I am trying to remember using the E6B calculator in the Kitfox. I know I did in the 70s when first flying, but it is never in the cockpit today. Charts, yes, but as back-ups only. The thought, how often do I check the compass - probably never. But then my flying has been mostly in the five western states with mountains to give general orientation. Am I doing something wrong?

[cap01]

ten years ago when i made the instrument panel , i mounted the compass in the panel and also a ram mount for a gps. the gps blocks the view of the compass so i have never even seen the compass , besides i have no idea how accurate it is since I've never swung it. i usually have two sources of heading info in the cockpit if i need it, the i pad with flyq and the garmin. i really like the chart updates on flyq. the paper chars are relegated to sunshade in the overhead.

[nyrikki]

In a steel frame airplane or really any airplane you will want to consider paying the extra for a compass like Sirs that has corrections for coefficients A, B C, and D.

Coefficient:

A: will be a consistent error, and you can just rotate the indicator

B: will fix the front to back magnetism, when you swing your plane on a compass rose it will be an error that is the sine of your local flux lines.

C: Is like B but is caused by athwartship magnetism and will produce an error that is the cosine of the heading.

D: Is typically caused by electronics and will typically be twice the sine of the heading.

E: Is caused by soft iron and will be twice the cosine and shouldn't be an large issue on tube and frame aircraft and I don't think any common aircraft compasses offer this correction anyway but is usually fixed with large iron spheres on ships.

Take your plane to an airport with a good compass calibration pad; turn on all of your gear and have someone help you swing your plane and chart the error you can get way more accurate than most cards will be set even in commercial planes.

You will need to find someone who is good at math to help you out, but the error will approximately model the following. One of these days someone should produce a phone app or web page that will let you play with Fourier transforms to guess the corrections, but by hand rotating the plane with a plumb bobs taped to the nose and tail to line up on the lines you can work them out through brute force.

Deviation ≈ A + B sin θ + C cos θ + D sin 2*θ + E cos 2*θ

If you use a compass with just N-S and E-W adjusters (like most airpath models) you will need to depend on the card. If you spend the extra money on a model with 4 magnet correction abilities or better it will take a bit more effort to set but will be far nicer in the long run (sirs as an example).

I think the cost difference is around $100 so weigh that with the convenience of not needing a card.

[n85ae]

I have a Sirs and spent a lot of time tweaking it, and getting it just right, and
never ever even look at it ... I guess with the number of things that provide
heading information these days I simply don't use it.

I almost always align DG on runway heading, takeoff and then start using
GPS track. When I notice a bit of DG drift I more often than not correct it with
a combination of track, and land line information (roads, etc)

It would be a big effort to degauss the airframe for something that I bet you
won't use very much ...

Jeff