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C172p horizon meter thing keeps getting "de-synced"

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C172p horizon meter thing keeps getting "de-synced"

Postby nulln » Wed May 03, 2017 3:09 pm

Image

I don't know how to explain it, but this thing keeps happening. See the horizon meter that shows how level you are (what's the name of that even)? It keeps breaking for me and doing this. It keeps getting "de-synced" with the horizon. Why is it doing that?
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby Bomber » Wed May 03, 2017 3:52 pm

Are you flying a plane who's engine cuts out with negative g's ?

Could it be that there's a direct link to vacuum generated by a working engine.... and there's no vacuum reservoir in the circuit.

I'm at work here so could be miles off base..

Simon.
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby wlbragg » Wed May 03, 2017 4:56 pm

Are you flying a plane who's engine cuts out with negative g's ?

Yes it does, but I think it is fuel flow.

This is the 2nd time I have heard this issue relating to the c172p and I can "easily" reproduce it.

So I did a search on the property driving the AI (Attitude Indicator) and it is coming from instrumentation/attitude-indicator/indicated-roll-deg. I did a "find in file" search on "indicated-roll-deg" and it is not touched anywhere else in the c172p code. So this appears to be coming from JSBSim internals. I looked at the same property in another JSBSim aircraft that doesn't have any AI system modeled and I cannot reproduce this behavior, its instrumentation/attitude-indicator/indicated-roll-deg property stays "synced" to the horizon.

I will bring this up to the developers on the dev repo but would appreciate anyone that can shed some light on this behavior!
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby Parnikkapore » Wed May 03, 2017 5:34 pm

BTW, the engine is either running or just have cut here. Look at the tachometer!
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby dany93 » Wed May 03, 2017 5:50 pm

Is it after a spin and stall or something like this?
This instrument is gyroscopic. If it is subjected to hard jolts it de-synchronizes for a while. It cannot follow (keep its absolute orientation) with very brutal attitude changes.
Normally it recovers after a few minutes (4 - 5 mn).
Last edited by dany93 on Wed May 03, 2017 6:30 pm, edited 3 times in total.
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby wlbragg » Wed May 03, 2017 6:04 pm

Is this behavior hard coded in JSBSim?

If so, why this aircraft and not, say the J3Cub (looking solely at the property tree)?

The behavior of instrumentation/attitude-indicator/indicated-roll-deg act differently between the two aircraft, from what I can tell.
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby dany93 » Wed May 03, 2017 6:17 pm

Yes, I strongly think that this is hard coded (in FG itself rather than JSBSim only).
I think that this behaviour is realistic (at least qualitatively).
It is much more likely to happen on an aircraft which has the stall and spin behavior implemented (only a few ones in FG). Normal movements (in the flight envelope with the controls) are slow enough to be followed by a gyroscopic instrument.
I've seen this on several FG aircraft with stall and spin. Recovering of the indication after about 4 - 5 mn.
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby stuart » Thu May 04, 2017 12:45 am

Hi,

Yes, I think this behaviour is for a "gyro tumble" due to excessive roll and pitch movements. Aerobatic aircraft often have an option to "cage" the gyro before performing aerobatics to stop the gyro from moving.

Had you performed particularly violent maneuvers - high-G, or negative-G perhaps?

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Re: C172p horizon meter thing keeps getting "de-synced"

Postby wlbragg » Thu May 04, 2017 1:12 am

Had you performed particularly violent maneuvers - high-G, or negative-G perhaps?

Yes, I did to verify that is the cause and it is. My question is why does the c172p behave this way, but the j3cub doesn't (I'm going by the property and not the animation)?
it is much more likely to happen on an aircraft which has the stall and spin behavior implemented

As @dany93 points out I think this is the case with the j3, that it doesn't have the "stall and spin behavior implemented". I would like to understand how and why that affects the internals of either FG or JSBSim, that it would affect the way the gyro behaves?
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby nulln » Thu May 04, 2017 7:05 am

Thanks for your replies everyone. I guess it's because I do really sharp turns by banking the plane really steep, almost vertically (And now you should understand my skill level :P)! I feel stupid now lol.

(BTW: When I see it happening, its like the gyro system is glitching on my panel. That's what made me think it was a glitch.)
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Re: C172p horizon meter thing keeps getting "de-synced"

Postby Johan G » Sun May 07, 2017 11:42 am

Glitching like in really fast changes or glitching like as in that it freezes momentarily?
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