Compass Swing with GPS

[GAHorn]

It's true that even with no wind, the track over the ground is not necessarily the same as the heading. But the track is what you're really interested in. For example, if an airplane is way out of rig and flies through the air in a skid, you'd fly the compass in the direction you want to go, the track, and not the heading.

Rudy

But Rudy, the compass so calibrated is only good for the few minutes that the winds aloft are exactly the same as during the moments the compass was compensated. Next flight....compass errors again.
If your compass is within 5 degrees it's probably good enough for navigation over land. (Think about it, ...so your compass is calibrated EXACTLY ...using whatever method you finally convinced yourself was valid. Now...three days....three weeks....three years later.....with different winds.....so WHAT! Your heading, based upon the perfectly calibrated compass will no longer be the same exact heading as the day you calibrated the compass. What will you do? YOu'll fly a selected heading, and you'll observe the landmarks using pilotage, and you'll correct your heading to derive a track upon your desired course.)
But I understand your wish for a compass as accurately calibrated as possible. That objective is achieved by using an official compass rose. (Which by the way, was calibrated on a certain date, the magnetic variation being known on that date, and the variation changes will have been documented since that date, and published/illustrated on a current sectional chart. But for most purposes, as long as the paint on the ramp is legible, it's close enough. Still another reason to not sweat exact-reading magnetic compasses.)

[rudymantel]

George, what I'd REALLY like is a nice RMI, slaved off a remote mounted flux gate compass (which guess would still have to be calibrated/swung once in a while)...

[GAHorn]

Lindbergh had a good compass... an Earth Inductor compass. It used a spinning aeronometer (a small windmill) to spin an armature that used the earth's magnetic lines of flux to induce (hence "Inductor") a current in an armature (like a generator which has no field coils....it used the earth's magnetism to be the field) and the gauge was a meter which converted the electrical current to a readout depicting the current as direction.
It automatically derived magnetic direction because of the lines of force. (The earth's core is largely molten iron, which being magnetic imparts lines of force to the surface of the globe. Because of the earth's rotation about it's axis...i.e., spin....the lines of force change ever so slightly, but predictably....hence the published isogonic lines on the sectional chart which change in variation each year or so. About once every 25 million years the earth's poles completely swap polarity.)
Disclaimer: Don't quote me on the 25 million. I don't really recall the exact date ...but it's a long time and is proven by geological evidence.

[JTS]

I agree with George on this one, as far as not sweating a slightly inaccurate compass. If your trying to achieve accuracy, it's probably to fly good old fashioned basic navigation, in which case any deviation the compass may have will be dealt with the same as a little extra, or not quite as much winds aloft, using whatever method of track correction you choose.

Best Regards,
Jody

[Walker]

[zero.one.victor]

I have heard of people in pre-GPS days swinging their compass in flight by flying section lines out in the farm country. Same caveat applies re: no wind conditions, pretty easy to determine by a lack of wind-drift flying said section lines.

Eric

[jrenwick]

I have heard of people in pre-GPS days swinging their compass in flight by flying section lines out in the farm country. Same caveat applies re: no wind conditions, pretty easy to determine by a lack of wind-drift flying said section lines.

I've done this with my J3. It works OK for a slow aircraft like that, but you only get four deviation points, not the twelve you need to fill out your card. Wind drift is not much of a factor, because even if you're drifting, you can still point the nose parallel to a road. Turbulence is a factor: it works best in smooth air. Variation is a factor: it's easiest to do in a place where variation is negligible, because roads are aligned with true directions, not magnetic.

Whether or not GPS is available has nothing to do with it. GPS is not a compass!! Your track over the ground is not the same as your heading, and heading is what you have to calibrate your compass to!

Best Regards,
John