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Why is artificial horizon so badly wrong at times?


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On 4/16/2017 at 12:17 PM, Ala13_ManOWar said:

 It's supposed to auto-erect after a while of stable flying and a real vídeo where that was clearly seen was posted.

I just emptied a tank without the horizon auto correcting. Spitfire has behaved like this the 2-3 years I had it. It's getting old. If it acted like this in real life it would be completely useless.

 

I waited for it to be fixed a long while, without saying anything. I thought about making a long flight to test the "auto correct". Didn't set out to do this but flew a long while straight 100 nm at least, and noted still all wrong.

Not a perfect test, stalled a bit, but didn't notice any correction.

 

A way to "correct" it is to make wild acrobatics opposite direction, but that's ridiculous and can only get it approximately anyway.

 

 

 


Edited by -0303-

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2 hours ago, -0303- said:

I just emptied a tank without the horizon auto correcting. Spitfire has behaved like this the 2-3 years I had it. It's getting old. If it acted like this in real life it would be completely useless.

All the warbirds we have are declared to be non all weather. Do you wonder why? 🙃

 

S!

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I can't believe a real life instrument would be completely useless.

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23 minutes ago, -0303- said:

I can't believe a real life instrument would be completely useless.

Believe it. There is a reason we practiced partial panel back in the day. Gyroscopic air- driven instruments fail early and often. Makes for an exciting and sometimes fatal excursion. I have experienced multiple in flight gyro failures and a double vacuum pump failure in a Seneca. A pilot I worked with briefly spun to his death after an attitude gyro failure at night over West Texas many years back.

475th Fighter Group Discord https://discord.gg/xkKsApD

 

 

 

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But they don't fail 100% of the time do they? The P-51 horizon is usable because it can be reset, you can get 15 minutes (or something) out of it, getting through a cloud maybe. Once the Spitfire is out of sync, which it is sometimes from takeoff (says some people, cant confirm), it's just gone.

 

Quote

It's messed up even right after I start the engine.

 

(personally I spun up the battery driven ball & turn in gliders just to listen to it ... never mind)

 

As mentioned, I've had the Spitfire for 2-3 years and I've kept quiet wondering if it's something I just don't get on how to use it, or for it to be "fixed". It's getting old.


Edited by -0303-

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Real thing video. Note, it's effed up immediately after engine start but after  time cut (warm up?) it's straight (auto correct or reset?).

 

On landing it's almost straight and on taxiing one can see it auto corrects while taxiing straight, gets slightly off when turning and corrects again when taxiing straight again, on engine off it's perfectly straight.

 

Fastforwarded the actual flying, I just imagine it would straighten if he didn't turn all the time.

 

Unrelated, we're missing the loud pneumatic hiss when taking up the flap. Just spawned rwy Idle to verify, no nothing, just the lever click.

 

 


Edited by -0303-
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1 hour ago, -0303- said:

Real thing video. Note, it's effed up immediately after engine start but after  time cut (warm up?) it's straight (auto correct or reset?).

 

On landing it's almost straight and on taxiing one can see it auto corrects while taxiing straight, gets slightly off when turning and corrects again when taxiing straight again, on engine off it's perfectly straight.

 

Fastforwarded the actual flying, I just imagine it would straighten if he didn't turn all the time.

 

Unrelated, we're missing the loud pneumatic hiss when taking up the flap. Just spawned rwy Idle to verify, no nothing, just the lever click.

 

 

 

According to this video (startup part), it aligns at around 30º per 20 seconds. But in a 30 second fragment before 7:09 it doesn't seem to align at all, despite level-ish flight. 

In any case, I fail to understand the reasoning behind giving a pilot such an instrument without manual align function. The one with "they'll mess it up" fails at remembering the pilots were the most educated servicemen in the field, learning an immence array of information from meteorology to exact construction and functioning principles of every mechanism on board -- so quirks of "align" function of one of the many cockpit instruments is a drop in a sea. 

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They are not vulching... they are STRAFING!!! :smartass::thumbup:

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I think it corrects, if just slightly, between 6:34 - 7:15.

Rewatched the startup more closely, one can visibly see it moving.

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Scouring the net ... found 360' view 48 min video Dover to Dunkirk where horizon is visible. I might, at some point, gather the energy to watch all of it ... at 2x speed.

It stays good for a good long while early on. Fastforward to 20:00, it's effed up, but at 25:00 it's straight again. Stays fine during turns until 27:15 when he rolls and it's effed up again. Generally seems it stays fine until a roll or looping, after which it takes ~5 min to get straight again.

 

Generally, not all horizon does the dancing as in Hatfield's video above. The astronaut who besides space, also writes hit songs and fly Spitfire, I hate him so much... A thought, probably wrong: Are restored "modern" Spitfires required to have a calibrated top shape horizon instrument?  Not much need for it VFR.

 

 


Edited by -0303-

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Hmm... I wrote "less than perfect test" above. Now trying to mess up the horizon for a proper test. I didn't manage to mess it up.

My "less than perfect test" included a screaming dive from 30K as finish. May have influenced final result.

 

Maybe the horizon works after all. :unsure:

I need to do a better proper test before I state anything. Ignore my complaint above for now. :Flush:

Out of time for more tests right now...

 

What I want do to is to screw up it up properly and then fill the tank, fly straight and see if it corrects.


Edited by -0303-

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Now I did a proper test

 

It was painful. It raised more questions. Next post might be an error report. I recorded a long track with three tests. I flew 4x speed. Best way to present is probably a video in 3x 30 minute chunks (sped up 10x maybe), which is also a lot of work.

 

1) Took off, flew straight and smooth Batumi - Sochi. Horizon (nearly) perfect all the way. Ever so slightly off after full stop.

 

2) Refueled and did donuts on Sochi rwy until I managed to get horizon crooked. Locked brakes and let it sit 30+ minutes. Horizon did not correct at all (Hadley's real Mk IX does visibly correct itself on the ground according to video).

 

3) Took off and flew back to Batumi (4x), starting with a crooked horizon. Here's weirdness and new questions. Horizon swings opposite directions depending if turning left or turning right (*1). Having turned either direction, it does not correct itself (as it should) if keep going straight. But turning other direction one can make it straight again, if turning just the right amount, if turning further, horizon gets crooked in opposite direction.

 

*1) Same thing happens on the ground, doing donuts it gets crooked after 90-180 degrees, but straight again after 360 degrees. Eventually I managed to get it crooked while also pointing along the rwy so I could take off with horizon crooked and do the flight back to Batumi test.

 

~

 

Test 2) is conclusive enough I think, it doesn't work as it should. Not quite sure what happens in test 3) but it doesn't straighten out while flying straight, so that's seems wrong.

 

Another point not explored is pitch. I did another long track first (can't use because stupidly landing on carrier and replay hits the sea) where I noted pitch wise the horizon was all wrong after a long smooth flight (1x time) Batumi - Guadata. So pitch correction is also not right. If anything it seemed pitch got worse and worse (horizon receding downwards).

 

Trying to summarize. If it is correct, it stays correct (see test 1). If provoked "wrong" it stays wrong, no matter what.

 

Track is very long and quite boring. Uploaded here, object oriented, so I don't lose track of the track.

spitfire_horizon.trk


Edited by -0303-

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  • 2 weeks later...

The bug thread is "reported" and locked. Add a few comments here I can't add in the locked bug thread (also inappropriate longish gyro discussion for strictly bug description thread).

1) It's not Chris Hadley (I got in my head it was Hadley) flying the Mk IX in 1st video. It's some guy named Rick Voler.

2) I speculated in the bug thread that low RPM gave insufficient suction and that's why horizon drifts. Watching the "Hadley" video, one can see the horizon correcting on startup at just 800 rpm (first 40 seconds and after landing).

Watching carefully RPM is the upper right "clock" instrument (marked 0, 2, 4, 6, 8 ) and below right is the "boost" instrument. Squinting hard at opportune moments (sun from behind), one can read "RPM" and "Boost" and also their readings make sense while flying. He takes off with boost 7 and max out 3000 rpm. He smoothly increases boost and only reach 7 and 3000 after takeoff it seems. In DCS I just jam it to ~8 instantly ... Then rpm is reduced to 2650.

Point being, horizon seems to correct already with idling 800 rpm so ... it should NOT drift at idle?

In DCS it drifts on idle (very little) and when engine shuts down (much more so). Should it drift when engine shuts down? I don't understand enough to know. On one hand it stops spinning (after a while) so there's no spinning-rigidity but is there a simple gravity thing that makes it straight when shut down?

Hunting youtube gyro horizon videos ...

Below is a good series for people so inclined.

Video 1, guy shows an electric (helicopter) gyro self corrects way faster than it needs to, even if mounted in an SR71 flying over the curvature of the earth. Hmmm ... 22 degrees in 10 min (slower than the Spitfire's 90 degrees in 9 minutes) and 29 degrees in 6 min in opposite direction.

Having watched all 8 videos (weirdly ordered in reverse 8, 7, 6 ... 1), this is a very good explanatory series (playlist) of artificial horizon gyros from a professional jet pilot. He'll explain in 2nd video (7) why it corrects faster in one direction. I'm getting the impression that self correction is an inherent basic feature of how every (mechanical) artificial horizon works. It self corrects level with gravity (video 3 (6)).

Video 5(4) explains how self correction works. First, it is slightly gravity balanced to become straight, but that is not the primary correction mechanism. Spinning generates air pressure (even in an electrical gyro). Small opposed air pressure ports are opened or closed by gravity controlled "pendulous vanes". When the horizon is aligned all air ports are half open applying zero corrective pressure. Obviously the gravity controlled "pendulous vanes" are subject to inertia, they don't "know" if a force comes from "plain" gravity or if the plane is being thrown around.

Summarizing. An important concept I struggled slightly with before getting. The internal mechanism will always strive toward "true level". So even, for example, if you put the aircraft (the instrument casing) in an extended climb, it will NOT align with the climb, it will always align with "true level" and hence always show a correct indication throughout the climb.

He goes into details. The instrument balance (gravity) calibration is simple and elegant. It's just two big screws, one for roll, one for pitch, fixed with loctite.

This electrical gyro is different from the Spitfire gyro in at least one respect, it can be rolled without tumbling.

Getting tempted to test all DCS Warbirds horizon gyros to see if they will self correct...


Edited by -0303-

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On 10/2/2021 at 11:12 PM, -0303- said:

Getting tempted to test all DCS Warbirds horizon gyros to see if they will self correct...

I'd be interested in your thoughts on the artificial horizon, I find myself ignoring it most of the time due to inaccuracy.

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On 10/3/2021 at 6:12 AM, -0303- said:

Summarizing. An important concept I struggled slightly with before getting. The internal mechanism will always strive toward "true level". So even, for example, if you put the aircraft (the instrument casing) in an extended climb, it will NOT align with the climb, it will always align with "true level" and hence always show a correct indication throughout the climb.

 

 

To be precise, the 'level' is the g force vector. So the gyro will get out of alignment in a prolonged turn, but correct itself when flying straight and level. Not to confuse with precession errors. 

 

Anyway, the DCS implementation of the attitude gyros is poor and carries over the same long standing bugs from airplane to airplane. I tried to explain it here, and this problem was reported multiple times since F5 first release 

 

 


Edited by some1

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I haven't tested it in all warbirds, but definitely in the Mossie the horizon does not self erect (same goes to the Spit).
I found and gathered a few videos and pictures here (with a picture of the horizon used in the Spit):

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  • 2 weeks later...

What happens in a sustained (coordinated) turn? The plane and passengers (including gyro horizon) perceives the lift force (green) perpendicular to the wings. The horizon gyro is initially aligned with the true gravity vector (blue). The centrifugal force (yellow) pressures the gyro to realign parallel with the lift vector (green). This doesn't happen, much...

BWkyG.png

Maximum error is induced by a 180 degree turn while a full 360 cancels out the error. I've been told to think of it like this: In a 360 degree turn, the whole plane, the instrument case and the gimbals rotates around the rigid-in-space gyro, pulling it, literally, in all directions. Authoritative sources: pilot school quiz-let and instrument retailer. On leveling wings after 180 degrees, the small error in roll and pitch is quickly erased by the gyro's self erection (aligning with the gravity vector).

Two excellent videos (in pedagogically explaining... stuff):

1) [21:06] Artificial Horizon Of Aircraft | Working Principles Of Artificial Horizon | Lecture 29
First half pneumatic gyro horizons, aka "classic horizons": pneumatic, spin ccw viewed from above and 110 roll limited (every Warbird?). Effect in coordinated, slip or skid turns and acceleration / deceleration. Then electric gyros, the same with differences.

 

2) [28:03] Gyrocompass showing effect of Earth rotation
Don't be put off by the initial man cave experiment vibe. At 8:20 he starts an excellent illustrated explanation of what precession actually is and how to understand and predict it's effects. Also, as title says, a (great) explanation of the gyro-compass.


Edited by -0303-

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What annoys me is the FW190, and I assume the ME109, instrument works perfectly. I was quite happily using it to fly through clouds in the Anton... Good luck trying that in the Spit or Mozzie 


Edited by Krupi

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On 10/22/2021 at 12:42 PM, Krupi said:

What annoys me is the FW190, and I assume the ME109, instrument works perfectly. I was quite happily using it to fly through clouds in the Anton... Good luck trying that in the Spit or Mozzie 

 

No they don't. Not Bf109, not FW190. Not a single Warbird as far as I know, not F-5, not F-86 artificial horizon works as they should. Seems a general problem. Probably a lot more if tested.

An easy test: Cage horizon, roll to some degree (45, 60...), uncage horizon and roll level. Now flight straight and level. The artificial horizon should reorient itself towards the gravity vector within a reasonable time (*1). Typical "self erection" rates are ~2 - 8 degrees per minute.

The two videos I posted above are the best I come across in explaining how they should work.

~

*1) For the real Spitfire Mk 9 video above (Dover to Dunkirk), the time was 9 minutes (watch 18:05 - 27:02). That was a worst case situation, longest time I think. It started near vertical and recovery from right bias take longer time then left bias due to the earths rotation.


Edited by -0303-

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Well from my experience I was able to use it successfully when flying through a cloud which is a far cry from my experience with the Spit and Mozzie. 

I will test it some more. 


Edited by Krupi

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3 hours ago, Krupi said:

Well from my experience I was able to use it successfully when flying through a cloud ...

 

Yes, if the horizon is aligned it'll stay that way for a while in DCS. Not what I'm addressing. Look at my first link: "On some modules, once the artificial horizon developps an error it is unable to correct it."

In a real plane, If you fly "watercombed", no rolls, no loops or anything that "tumbles" the artificial horizon you'll never ever have to cage it because it continuously auto erects itself towards the gravity vector (~2-8 degrees per minute). DCS artificial horizons (Warbirds at least) does not do this. They should.

In the Spitfire the error is obvious because of the inability to cage. In P-51, Bf 109 you cage and don't think about it. But caging offers the opportunity to easily get the horizon un-aligned for testing purposes:

Uncage the horizon while rolled (30, 45, 60 degrees something). Roll back to level and the horizon will be un-aligned (30, 45, 60 ...). Now just fly straight and level and the artificial horizon should correct itself back to true alignment with the gravity vector (perpendicular to ground). The real Spitfire (Dover to Dunkirk  timestamp 18:05-27:02) in the video above took 9 minutes.

Note, you do not in any way need to fly perfectly straight and level to allow the horizon to correct itself. That would be an insane demand.


Edited by -0303-
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Sorry, I see your point now. 

Yes something is definitely amiss. 

Might be worthwhile in getting this placed in the bug area so that it is officially addressed. 

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