Discussion:
heading: magnetic or not?
(too old to reply)
Major A
2002-12-01 20:49:00 UTC
Permalink
Just another question: I noticed that the different places in which
heading plays a role (the different indicators as well as the
autopilot) use magnetic and geographic heading at will. Are there any
plans to unify them and/or come up with a convention of what heading
is magnetic and what isn't?

Andras

===========================================================================
Major Andras
e-mail: ***@users.sourceforge.net
www: http://andras.webhop.org/
===========================================================================
David Megginson
2002-12-01 21:57:29 UTC
Permalink
Post by Major A
Just another question: I noticed that the different places in which
heading plays a role (the different indicators as well as the
autopilot) use magnetic and geographic heading at will. Are there any
plans to unify them and/or come up with a convention of what heading
is magnetic and what isn't?
Are you sure? Here's what you should find in the 172:

- The magnetic compass displays magnetic heading, but it is strongly
affected by acceleration and turning errors: it will be accurate
only in straight-and-level steady flight (or sitting flat on the
ground).

- The directional gyro is purely relative and doesn't care what it
displays: you can set it to true, magnetic, or anything in-between.
It will drift during flight, however, so you have to reset it from
time to time. Its starting point is fairly arbitrary -- make sure
you adjust it to what you want before you take off.

- The autopilot in the 172 tries to keep the orange heading bug at the
top of the directional gyro, period. If you set the DG to the true
heading (as you would in the Arctic), then the autopilot uses true
heading; if the DG has drifted, the AP will be off by the amount of
the drift. Bigger planes have more sophisticated flight-management
systems, but we're not modelling those properly yet.

- The ADF needle simply points to the ADF station (in real life, +/-10
degrees -- it's not all that accurate).

- The VOR indicators show (roughly) what radial the plane is on.
That's not necessarily the direction you're flying, or even exactly
the direction to the VOR.

The only place you can get the true heading directly is the HUD, which
isn't really meant to simulate anything you'll find in a small plane
-- it's just a (useful) developer's tool or a user's toy. Usually it
should be turned off.


All the best,


David
--
David Megginson, ***@megginson.com, http://www.megginson.com/
Major A
2002-12-01 23:02:15 UTC
Permalink
Post by David Megginson
- The autopilot in the 172 tries to keep the orange heading bug at the
top of the directional gyro, period. If you set the DG to the true
heading (as you would in the Arctic), then the autopilot uses true
Ever flown a (real) 172 across the North Pole? :-)
Post by David Megginson
The only place you can get the true heading directly is the HUD, which
isn't really meant to simulate anything you'll find in a small plane
-- it's just a (useful) developer's tool or a user's toy. Usually it
should be turned off.
Oh. I used the HUD quite a bit in the past because it gives me
information I really want given the limits of the input devices I have
(mouse and keyboard...).

How about the HSI on the A4 panel? It appears to be fixed at true
heading, and it doesn't seem to be adjustable (it's supposed to be a
gyro, so it should be).

Andras

===========================================================================
Major Andras
e-mail: ***@users.sourceforge.net
www: http://andras.webhop.org/
===========================================================================
David Megginson
2002-12-01 23:57:34 UTC
Permalink
Post by Major A
Post by David Megginson
- The autopilot in the 172 tries to keep the orange heading bug at the
top of the directional gyro, period. If you set the DG to the true
heading (as you would in the Arctic), then the autopilot uses true
Ever flown a (real) 172 across the North Pole? :-)
No, but the Arctic starts a long way from the North Pole. Many people
in Canada fly 172's in Northern Domestic Airspace (roughly north of
60, but it zigzags a lot), where you use true rather than magentic
headings. The same may apply to parts of Alaska.
Post by Major A
How about the HSI on the A4 panel? It appears to be fixed at true
heading, and it doesn't seem to be adjustable (it's supposed to be a
gyro, so it should be).
I don't use the A4, so I haven't noticed. It should probably be
showing magnetic heading. Usually, an HSI is slaved to a separate
magnetic sensor that keeps it aligned, so that you don't have to
adjust it manually (it's also very expensive, at least for a small
plane).


All the best,


David
--
David Megginson, ***@megginson.com, http://www.megginson.com/
Andy Ross
2002-12-02 00:11:57 UTC
Permalink
Post by David Megginson
Post by Major A
Post by David Megginson
- The autopilot in the 172 tries to keep the orange heading bug at the
top of the directional gyro, period. If you set the DG to the true
heading (as you would in the Arctic), then the autopilot uses true
Ever flown a (real) 172 across the North Pole? :-)
No, but the Arctic starts a long way from the North Pole. Many people
in Canada fly 172's in Northern Domestic Airspace (roughly north of
60, but it zigzags a lot), where you use true rather than magentic
headings. The same may apply to parts of Alaska.
It's worth pointing out that a DG will work fine in the polar regions.
Other than precession (which has a 24 hour period -- hardly a huge
source of error), there's no way for it to "know" that it's over the
pole. It will even work fine on the pole itself, in the sense that it
will always point toward the same meridian.

It won't tumble or do anything silly. Only floating point euler angle
computations have trouble at the poles; real hardware is a little more
robust. :)

Andy
--
Andrew J. Ross NextBus Information Systems
Senior Software Engineer Emeryville, CA
***@nextbus.com http://www.nextbus.com
"Men go crazy in conflagrations. They only get better one by one."
- Sting (misquoted)
Norman Vine
2002-12-02 00:31:38 UTC
Permalink
Post by Andy Ross
It's worth pointing out that a DG will work fine in the polar regions.
Other than precession (which has a 24 hour period -- hardly a huge
source of error), there's no way for it to "know" that it's over the
pole. It will even work fine on the pole itself, in the sense that it
will always point toward the same meridian.
It won't tumble or do anything silly. Only floating point euler angle
computations have trouble at the poles; real hardware is a little more
robust. :)
FWIW
I would be interested in test cases where FGFS has a 'pole' problem

Best

Norman
Norman Vine
2002-12-02 00:26:49 UTC
Permalink
Post by David Megginson
Post by Major A
Post by David Megginson
- The autopilot in the 172 tries to keep the orange heading bug at the
top of the directional gyro, period. If you set the DG to the true
heading (as you would in the Arctic), then the autopilot uses true
Ever flown a (real) 172 across the North Pole? :-)
No, but the Arctic starts a long way from the North Pole. Many people
in Canada fly 172's in Northern Domestic Airspace (roughly north of
60, but it zigzags a lot), where you use true rather than magentic
headings.
FYI
This page demonstrates why a magnetic compass is not the necessarily
very useful near the pole
http://www.geolab.nrcan.gc.ca/geomag/e_nmpole.html

Cheers

Norman
Jim Wilson
2002-12-04 03:31:53 UTC
Permalink
Post by Major A
How about the HSI on the A4 panel? It appears to be fixed at true
heading, and it doesn't seem to be adjustable (it's supposed to be a
gyro, so it should be).
Do you mean the AI or the HSI? The AI isn't adjustable because when I set it
up there wasn't a way to recieve clicks in a 3D cockpit. It's on my list.
All the other instruments are just "standards" (for the c172 mostly) from the
Aircraft/Instruments directory and not unique to the A4 at all (well maybe the
AoA and ASI are "unique").

Best,

Jim

Norman Vine
2002-12-01 23:34:55 UTC
Permalink
Post by David Megginson
The only place you can get the true heading directly is the HUD, which
isn't really meant to simulate anything you'll find in a small plane
-- it's just a (useful) developer's tool or a user's toy.
Note David is talking from a 'small plane' < C172> perspective.

There are much more sophisticated platforms modeled in FGFS
where the HUD is quite appplicable

Best

Norman
David Megginson
2002-12-01 23:59:38 UTC
Permalink
Post by Norman Vine
Post by David Megginson
The only place you can get the true heading directly is the HUD, which
isn't really meant to simulate anything you'll find in a small plane
-- it's just a (useful) developer's tool or a user's toy.
Note David is talking from a 'small plane' < C172> perspective.
Yes, as I noted. This would also include all but the most
sophisticated transport planes. For fighter jets, a HUD is sometimes
appropriate, but I don't think we're modelling any specific real-word
HUD's right now. It's a *great* development and debugging tool,
though.


All the best,


David
--
David Megginson, ***@megginson.com, http://www.megginson.com/
Continue reading on narkive:
Loading...