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 Post subject: Swinging ILS
PostPosted: Tue Jan 02, 2018 6:30 pm 
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Airman

Joined: Thu Sep 16, 2010 9:14 am
Posts: 11
Hello,

I saw this symptom in another post but it did not help.

Using the C182 executing an ILS with less than 10 kt winds and decent frame rates-between 45-60fps-using the auto pilot the C182 swings back and forth across the localizer all the way down final. It doesn't matter whether I use heading mode to capture the final or if the auto pilot does it. It's the same result. I believe I have the latest update as I ran the update utility tool. Before i did i couldn't turn left or right on the ground. After running the update tool I can now taxi, although it seems to be with not much authority. I have not flown a real C182 so I don't know if this is typical behavior. Needing to make a tight turn requires braking action.

I'm using Win 10 Pro 64
P3Dv4.1
Some orbx scenery
Active sky and cloud Art

Thank you very much

Steve


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 Post subject: Re: Swinging ILS
PostPosted: Sun Jan 07, 2018 4:21 pm 
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A2A First Lieutenant
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Joined: Fri Jun 06, 2014 1:06 pm
Posts: 6427
Location: United Kingdom
Hello Steve,

Sorry for the rather late reply to your post. If you're still having problems with this, please can you let us know how far away from the threshold you are when you're attempting to intercept the localizer. Also, what sort of intercept angle are we talking here?

For troubleshooting an issue like this, I'd definitely recommend you don't have ActiveSky connected and use a 'clear skies' weather theme instead.

StevenO wrote:
After running the update tool I can now taxi, although it seems to be with not much authority. I have not flown a real C182 so I don't know if this is typical behavior. Needing to make a tight turn requires braking action.
This is quite normal I gather. The Cessnas have a bungee connecting the nosewheel to the rudder assembly rather than a mechanical linkage, so the need for a bit of differential braking in tight turns is to be expected.

Thanks,
Nick


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 Post subject: Re: Swinging ILS
PostPosted: Wed Jan 10, 2018 1:35 pm 
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Airman

Joined: Thu Sep 16, 2010 9:14 am
Posts: 11
Hello,

Thanks for the reply.

The steering issue seems to be fine .....

As for the ILS, I've never executed an ILS in a real cessna, though I have some amount of time in one. Inside the marker as I get slower and with a 10 knot wind or so there seems to be some oscillation left and right as the A/C stays on the Loc. With no wind, it is not completely gone, but it does seem relative, meaning its there but hardly noticeable. Putting these two together it almost seems "normal." Is this correct behavior for this airplane? If the wind increases will i see a more dramatic "wave" left and right? I'm using less than 30deg to intercept, and just to fly, I'm well outside the marker and established with the autopilot (no left and right behavior) before any of this starts.

Thanks.

Steve


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 Post subject: Re: Swinging ILS
PostPosted: Thu Jan 11, 2018 1:56 pm 
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Chief Master Sergeant
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Joined: Sun May 26, 2013 5:03 am
Posts: 3773
I'd expect some hunting, but it is hard to quantify without a reference. Are you able to share a flight track, using TFTV for instance? In principle, a steady wind with a gradient should mean at most a slight, predictable, non-symmetrical pattern. A perfectly steady wind thorough the coupled approach should not affect the system at all, within some limitations.

-Esa


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 Post subject: Re: Swinging ILS
PostPosted: Sat Jan 13, 2018 5:27 pm 
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Airman

Joined: Thu Sep 16, 2010 9:14 am
Posts: 11
Hello,

Thank you. I would suspect in a steady wind it would be a smooth appch once the auto pilot has "corrected" itself. I have not used this TFTV flight tracker. I will have to download and check it out and then post for your comment.

Thank you very much for the assistance. I will get to it as soon as practicable.

Steve


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 Post subject: Re: Swinging ILS
PostPosted: Tue Jan 16, 2018 1:26 pm 
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Chief Master Sergeant
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Joined: Mon Sep 02, 2013 7:30 pm
Posts: 3249
Location: Typically hanging around N07, 12N, KLDJ, KCDW
So I accompanied a fellow pilot and his CFII as a backseat passenger on an IFR long x-country of 250NM. We shot several approaches including one VOR approach, an LDA and an LNAV.

We were flying in a 172SP with a GFC700 autopilot. The term chasing needles definitely comes to mind to describe how the very sophisticated GFC700 autopilot tracked the non-GPS aids. It was able to keep the needles from going outside a dot, but it just kept oscillating all over the sky unlike a human who would pick a heading and fly it for as stable approach.

Even enroute, 30 miles away from a VOR, it would still wobble around a little to chase the radio signal. We routinely took the AP off to get more comfort. Winds aloft were extremely smooth and generally the approaches had crosswind components of around 8KTS.

The most expensive real world APs on these aircraft do not fly rock steady approaches.

This RW behavior is the same from my experience with a KAP140, STEC-30 and STEC-55X.

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 Post subject: Re: Swinging ILS
PostPosted: Thu Jan 18, 2018 11:49 am 
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Chief Master Sergeant
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Joined: Sun May 26, 2013 5:03 am
Posts: 3773
Oracle427 wrote:
We were flying in a 172SP with a GFC700 autopilot. The term chasing needles definitely comes to mind to describe how the very sophisticated GFC700 autopilot tracked the non-GPS aids. It was able to keep the needles from going outside a dot, but it just kept oscillating all over the sky unlike a human who would pick a heading and fly it for as stable approach.

Even enroute, 30 miles away from a VOR, it would still wobble around a little to chase the radio signal. We routinely took the AP off to get more comfort. Winds aloft were extremely smooth and generally the approaches had crosswind components of around 8KTS.
This is one problem in designing any control system. An issue not present in FSX/P3D is that that the signals themselves are not perfectly steady, but may oscillate, or 'scallop' a little. The response from the autopilot depends on how much the signal twitches, but also on how fast it does. We could use a very smooth response, but then any actual error would not get corrected for before the situation gets worse. A fast response on the other hand results in chasing of even the ever so slightly oscillatory signal.

This can be optimized rather well, as is done in the GPS positioning and in higher-end guidance systems, utilizing a clever set of Kalman filters, but it is not achievable in simple tuned electronics meeting various conditions without much any known good assumptions. Therefore, a simple autopilot is, for a big part, a classical optimization question of a PID response.

-Esa


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 Post subject: Re: Swinging ILS
PostPosted: Thu Jan 25, 2018 10:47 pm 
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Airman First Class

Joined: Sat Feb 21, 2015 10:24 pm
Posts: 61
Location: 90DME YMIA, on 160Deg Radial
I experienced the exact same thing yesterday. Did an ILS approach to RWY 19 at KVPS, in my 182. Swinging from side to side, and getting progressively worse, the nearer the FAF I got. Hadn't got any weather activated either.

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