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Jabo_68*

Spitfire Magnetic/Gyro Compass

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Posted (edited)

Is the magnetic/gyro compass in the Spitfire functional?

Edited by Jabo_68*
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Yep -- sure does. It's a bit hard to use in VR because it's, well, under your stick more or less -- overall it's just a not as good as say... a heading indicator, but it does* work :).

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Can they be manually set like in Cliffs of Dover?

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18 minutes ago, Jabo_68* said:

Can they be manually set like in Cliffs of Dover?

 

I don't believe so, they're automatically set for you if I'm not mistaken.

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Posted (edited)
7 hours ago, Jabo_68* said:

Is the magnetic/gyro compass in the Spitfire functional?

 

Both Compass and DG (directional gyro) work OK,  with DG automatically synchronized to Compass, working like a "Slave Compass".

RL the synchronization is made manually by pilot after set Compass "red on red" -  set Course Setter "N" coinciding with Compass needle "+" and reading magnetic heading under Compass "Lubber Line" and dial this heading in DG.

 

But in game compass is not much usable - neither is necessary as above (as DG show the same heading all time), is half hidden by control column, and their magnetic needle movement is modeled mirrored,* probable for don't confuse a "casual player" if once he look at compass - as the majority look is in digital HUD information.

 

* RL P-8 compass needle don't point the magnetic heading, but the deviation from the North (since is a naval compass) hence for read magnetic heading is need set the Course Setter (compass movable external scale, not functional in game, see in the linked video bellow.) "red on red" and read Magnetic heading under compass "Lubber Line". See the video under spoiler.

 

AFAIK - Only Spitfire of A2A (for FSX) model P-8 compass correct, DCS and CloD have lacking or wrong modeled details, although they are usable as RL.

 

 

 

 

 
 

 

Edited by Sokol1
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On 8/24/2019 at 9:57 PM, Sokol1 said:

 

Both Compass and DG (directional gyro) work OK,  with DG automatically synchronized to Compass, working like a "Slave Compass".

RL the synchronization is made manually by pilot after set Compass "red on red" -  set Course Setter "N" coinciding with Compass needle "+" and reading magnetic heading under Compass "Lubber Line" and dial this heading in DG.

 

But in game compass is not much usable - neither is necessary as above (as DG show the same heading all time), is half hidden by control column, and their magnetic needle movement is modeled mirrored,* probable for don't confuse a "casual player" if once he look at compass - as the majority look is in digital HUD information.

 

* RL P-8 compass needle don't point the magnetic heading, but the deviation from the North (since is a naval compass) hence for read magnetic heading is need set the Course Setter (compass movable external scale, not functional in game, see in the linked video bellow.) "red on red" and read Magnetic heading under compass "Lubber Line". See the video under spoiler.

 

AFAIK - Only Spitfire of A2A (for FSX) model P-8 compass correct, DCS and CloD have lacking or wrong modeled details, although they are usable as RL.

 

 

  Reveal hidden contents

 

 
 

 

 

That confused me so much when I first flew the Il-2 spit. This gritty and detailed functionality is the one thing I miss from CLOD. :)

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The gyro is also there (in RL aircraft) because over long distances there is a magnetic deviation. An airplane flying a trully straight line would see the heading in the compass slowly drift to one side or another as he flies. So to keep a really straight line over longer distances, the magnetic compass doesn't work. The gyro however is not magnetic, it is .... well, a gyro(scope), so it will stay true on its own. It doesn't know where north is though, not from the start. Thus you use your compass to set your gyro at the start of the flight, possibly using the runway as a "proof". After that the compass becomes superfluous, and over a certain distance, just plain wrong.

 

IIRC in our game maps there is no magnetic deviation, so the compass+gyro interaction becomes unnecessary.

 

I know most people know this already, I'm just posting this for the sake of those who don't.

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Posted (edited)
2 hours ago, danielprates said:

The gyro is also there (in RL aircraft) because over long distances there is a magnetic deviation. An airplane flying a trully straight line would see the heading in the compass slowly drift to one side or another as he flies. So to keep a really straight line over longer distances, the magnetic compass doesn't work. The gyro however is not magnetic, it is .... well, a gyro(scope), so it will stay true on its own. It doesn't know where north is though, not from the start. Thus you use your compass to set your gyro at the start of the flight, possibly using the runway as a "proof". After that the compass becomes superfluous, and over a certain distance, just plain wrong.

 

IIRC in our game maps there is no magnetic deviation, so the compass+gyro interaction becomes unnecessary.

 

I know most people know this already, I'm just posting this for the sake of those who don't.

 

Over long distances, a gyrocompass drifts (or at least, the type used during WW2 does), and is less reliable than a magnetic compass. It does have the advantage however of not being susceptible to the turning errors and effects of acceleration that make a magnetic compass hard to use if the aircraft isn't flying straight and level - though a gyrocompass will become unreliable too if the aircraft is banked beyond the gyro's limits. WW2 pilots set the gyrocompass according to the magnetic compass, at the start of flight, and then periodically checked them against each other (in straight and level flight) adjusting the gyrocompass as needed to match the magnetic bearing. Magnetic deviation needs to be taken into account too, but it can be read from a map - though it changes over time, and maps have to be updated.

 

As for flying long distances in a 'straight line', when navigating on a sphere, the shortest route is a part of a 'great circle', which isn't (unless you are lucky) on a constant magnetic bearing, but a constant bearing from a gyrocompass wouldn't work either, even without the drift. These days, aircraft can use GPS to navigate, and follow 'great circle' courses, and if you check the course against the compass (gyro or magnetic) it varies along the route.

Edited by AndyJWest

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Interesting @AndyJWest. I imagine that ww2 combat planes proceeded as you explained because they were constantly moving about. To my knowledge flights on more conventional, "gentle" routes (as opposed to combat flying) would adopt the procedure I mentioned, isn't it? Gyros drifring in relation to compasses or compasses drifting in relation to gyros is a relative notion, but I always imagined that the one that stays true to a geographical line on the ground has to be the gyro and not the compass. This is a great topic, please elaborate more if I am off the mark here.

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Posted (edited)
4 hours ago, danielprates said:

The gyro is also there (in RL aircraft) because over long distances there is a magnetic deviation. An airplane flying a trully straight line would see the heading in the compass slowly drift to one side or another as he flies. So to keep a really straight line over longer distances, the magnetic compass doesn't work. The gyro however is not magnetic, it is .... well, a gyro(scope), so it will stay true on its own. It doesn't know where north is though, not from the start. Thus you use your compass to set your gyro at the start of the flight, possibly using the runway as a "proof". After that the compass becomes superfluous, and over a certain distance, just plain wrong.

 

IIRC in our game maps there is no magnetic deviation, so the compass+gyro interaction becomes unnecessary.

 

I know most people know this already, I'm just posting this for the sake of those who don't.

 

1 hour ago, AndyJWest said:

Magnetic deviation needs to be taken into account too, but it can be read from a map - though it changes over time, and maps have to be updated.

 

Magnetic declination, not magnetic deviation. 🙂 Magnetic deviation is the effect of nearby metal objects causing the compass to give a false reading (such as the engine).

Edited by LukeFF

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Posted (edited)

The wiki page on heading indicators is relatively comprehensible, and in line with AndyJWest's comments: DGs build up errors from internal friction in the short run, and in long routes due to latitude errors.

 

Of note, strictly speaking the directional gyro is not a true gyrocompass as used in ships at all - it contains a gyro but works on a different principle. Better to call it DG, DI or HI.

 

Opportunity to show an RAF film....

 

 

 

 

Edited by unreasonable

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59 minutes ago, danielprates said:

Interesting @AndyJWest. I imagine that ww2 combat planes proceeded as you explained because they were constantly moving about. To my knowledge flights on more conventional, "gentle" routes (as opposed to combat flying) would adopt the procedure I mentioned, isn't it? Gyros drifring in relation to compasses or compasses drifting in relation to gyros is a relative notion, but I always imagined that the one that stays true to a geographical line on the ground has to be the gyro and not the compass. This is a great topic, please elaborate more if I am off the mark here.

 

Two points: firstly, a gyroscope, when not subject to external forces, keeps its orientation 'in space', rather than in relation to the Earth, which is rotating. This factor alone can lead to a drift of up to 15 degrees an hour with a WW2-style gyrocompass (there are more complex gyrocompasses which don't suffer from this, but they weren't ever used in WW2 fighters, as far as I'm aware). Friction and other effects can cause further drift. A magnetic compass, on the other hand, provided it is held steady, will always align itself with local magnetic north (or will do so with known errors which can be derived in ground tests - most such errors are due to magnetic influence of the aircraft itself). A gyrocompass needs periodic adjustment, and the only thing a fighter pilot had available is the magnetic compass. 

 

Secondly, 'a geographical line on the ground' is (ignoring changes of terrain) an arc of a circle centred on the Earth's centre - part of a 'great circle'. This is the only 'straight line' you can usefully define, and as I said before, neither a magnetic compass bearing nor a gyrocompass bearing will generally follow it.

 

@LukeFF: Yup, 'declination', not 'deviation'.  Should know better.

 

@Unreasonable: Yup: In modern terminology, the WW2 gadget is a 'heading indicator', though nowadays some of those have a flux gate sensor so they can adjust themselves automatically to the Earths magnetic field.

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Posted (edited)
5 hours ago, danielprates said:

IIRC in our game maps there is no magnetic deviation, so the compass+gyro interaction becomes unnecessary.

 

In game both show the same heading all time, hence don't need player interaction, neither is necessary look at compass (besides that is difficult), what's is puzzling is compass operation mirrored. :wacko:

Edited by Sokol1

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with it being synchronised it's not a real british directional gyro, it should go out every couple of turns and have you fiddle with it to get it re-aligned

 

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All gyros drift, to be fair. The engineering that goes into maintaining alignment for gyros which can't be manually adjusted (like, say, on a space probe) gets very interesting indeed.

 

But yes, it is greatly simplified in this sim, particularly the spit's compass, which feels particularly wrong (since it's a compass, but doesn't actually point north, which is pretty freaky :P).

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Wow, so much I had forgotten! 🤔

 

Fighter groups finding their escort position over Berlin or PR pilots being on time / space was truly an unappreciated effort.

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Those are things that most of us (well me, anyway) know with more or less propriety, but time erases that knowledge and you start mixing things up.

 

@AndyJWest one last thing. Imagine I am going for a very long haul. Say, Honolulu to Tokyo on a Constellation (as I am interested in 1940s/50s tech). Flight plans are more complicated than that, I know, but assuming I am going to to a completely straight-line with no radio nav from start to finish, I am forced to pick a heading and just steer on it from end to end. Now: for that I use the gyro or the compass? Or both, and 'how'?

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Posted (edited)
2 hours ago, danielprates said:

Those are things that most of us (well me, anyway) know with more or less propriety, but time erases that knowledge and you start mixing things up.

 

@AndyJWest one last thing. Imagine I am going for a very long haul. Say, Honolulu to Tokyo on a Constellation (as I am interested in 1940s/50s tech). Flight plans are more complicated than that, I know, but assuming I am going to to a completely straight-line with no radio nav from start to finish, I am forced to pick a heading and just steer on it from end to end. Now: for that I use the gyro or the compass? Or both, and 'how'?

 

Again, you can't generally go 'in a straight line' by following either a compass bearing, or a gyrocompass. Not without defining 'a straight line' as something other than the shortest distance between two points on the Earth's surface (an actual straight line would involve digging a tunnel ;)). The shortest distance on the surface is a great circle route, and involves changing your instrument heading as you go. You can follow a Rhumb line on a constant heading, using an instrument (gyrocomass, magnetic compass or whatever) which provides a bearing to magnetic north (or true north for that matter), but that will involve taking a course which is longer than the great circle route.  A Rhumb line is usually a spiral, and when it isn't (i.e. when travelling due north or south ('north' and 'south' as defined either true or magnetic) it is because the spiral coincides with the great circle route.

 

https://en.wikipedia.org/wiki/Rhumb_line

https://en.wikipedia.org/wiki/Great_circle

 

For your Tokyo-Honolulu example,  the distances are:

Rhumb line (using true north, not magnetic, though it won't make much difference) 6279 km. (calculated here: https://planetcalc.com/713/)

Great circle 6191 km (as calculated by the Little NavMap application, which I use for X-Plane https://albar965.github.io/https://albar965.github.io/)

 

There isn't a lot of difference in the Tokyo-Honolulu example, but consider London-Tokyo. The great-circle route looks like this, when plotted on a Mercator-projection map:

LonTok.png

 

If you have trouble believing that is the shortest route, find a globe, and stretch a string between London and Tokyo. The shortest route goes way north. Using a compass, you start by heading NNE, and adjust your heading as you go, finally arriving in Tokyo while heading SSE. 

 

Edit: The same route is a 'straight line' when seen on a globe, from the correct position:

LOn-Tok-Globe.png

 

Edited by AndyJWest

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Posted (edited)
32 minutes ago, AndyJWest said:

 

Again, you can't generally go 'in a straight line' by following either a compass bearing, or a gyrocompass. Not without defining 'a straight line' as something other than the shortest distance between two points on the Earth's surface (an actual straight line would involve digging a tunnel ;)). The shortest distance on the surface is a great circle route, and involves changing your instrument heading as you go. You can follow a Rhumb line on a constant heading, using an instrument (gyrocomass, magnetic compass or whatever) which provides a bearing to magnetic north (or true north for that matter), but that will involve taking a course which is longer than the great circle route.  A Rhumb line is usually a spiral, and when it isn't (i.e. when travelling due north or south ('north' and 'south' as defined either true or magnetic) it is because the spiral coincides with the great circle route.

 

https://en.wikipedia.org/wiki/Rhumb_line

https://en.wikipedia.org/wiki/Great_circle

 

For your Tokyo-Honolulu example,  the distances are:

Rhumb line (using true north, not magnetic, though it won't make much difference) 6279 km. (calculated here: https://planetcalc.com/713/)

Great circle 6191 km (as calculated by the Little NavMap application, which I use for X-Plane https://albar965.github.io/https://albar965.github.io/)

 

There isn't a lot of difference in the Tokyo-Honolulu example, but consider London-Tokyo. The great-circle route looks like this, when plotted on a Mercator-projection map:

LonTok.png

 

If you have trouble believing that is the shortest route, find a globe, and stretch a string between London and Tokyo. The shortest route goes way north. Using a compass, you start by heading NNE, and adjust your heading as you go, finally arriving in Tokyo while heading SSE. 

 

 

I have no trouble at all. Yeah, I get it. I know that a straight line in a mercator projection can be misleading. All you said above is correct of course. What I would like to know is where the compass vs. gyro interaction comes into this.

 

In the example you mentioned a flight would have a flight plan with several legs, with different courses for each leg. Ok. Lets take the geography out of the pilot's job, say, he is just going to follow the courses set in the plan for each leg. What does he use? Gyro? Compass? Both? How? To be more clear: lets imagine that the first leg asks for 300 miles at a 60degrees heading, and leave it there (it's what I meant with the "straight line" expression). How does the pilot proceed with his 1940's gyros and compasses?

Edited by danielprates

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If he is flying straight and level, his gyrocompass and magnetic compass  should show the same thing. If they don't, it is the gyrocompass that he has to assume is wrong. It was aligned on the magnetic compass in the first place, and he doesn't have anything else to set it to (not unless he has a navigator with a sextant, and the right conditions to use it).  You don't use a simple heading-indicator type gyrocompass for long-range navigation. you use it for convenience (it is usually mounted in an easy-to-read position, unlike a magnetic compass which is best placed as far away from ferromagnetic material as is practical), realigning it with the magnetic compass as it drifts.

 

 

 

 

 

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     A magnetic compass will move (change indicated direction) when subject to acceleration or deceleration and may start to spin during sharp or prolonged turns which makes the instrument worthless for instrument flying. After flying combat maneuvers, a magnetic compass can require several minutes to stabilize.

     A gyro stabilized directional instrument (gyro compass, directional indicator, heading indicator, etc.) will provide direction information (which way is the aircraft pointed) during and after violent maneuvers. :salute:

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1 hour ago, Arfsix said:

...   

     A gyro stabilized directional instrument (gyro compass, directional indicator, heading indicator, etc.) will provide direction information (which way is the aircraft pointed) during and after violent maneuvers. :salute:

 

Up to a point. The USAAF P-47 pilot training manual for example says that the gyrocompass "has a limitation of 55° for banks, climbs and glides".

 

 

 

 

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