reported FCS applies pitch down with aileron only input with flaps

[Yeti42]

I'm not raising a bug yet, I just want to know if anyone else is experiencing unexpected behavior:

My first flight using the new flight model was an instant action flight on the Syria map, I flew around for 10 minutes or so testing the AOA capabilities and then decided to dump all stores and do some touch-and-go's at Incirlik. To cut a long story short, the landing was fine as was on AOA during finals and touchdown, the problems started when I selected 1/2 flap, military power and rotated after touchdown.  The jet adopted an increasingly nose-high attitude which I was unable to counter with full forward stick and manual nose-down trim. The result was inevitable and the jet departed.

Has anyone else seen behavior similar to this? I've been unable to reproduce the behavior in the 2 short flights since and the track file has been overwritten.
 

[Lord Vader]

Hey @Yeti42

We need to see it happening and the best way is using a track replay.

I just performed a landing with more or less the steps you described and couldn't reproduce it either.

[Yeti42]

17 minutes ago, Lord Vader said:

Hey @Yeti42

We need to see it happening and the best way is using a track replay.

I just performed a landing with more or less the steps you described and couldn't reproduce it either.

Yep, I'll work on reproducing it and remembering to save the track file

[Agg]

Yes, I experienced the exact same thing yesterday. I wanted to see what a take off using zero flaps in the Hornet would look like, and when I got to 130-140 knots the nose pitched up - it might even have pitched up so much that the tail struck the runway. Then it went into a loop, before losing all energy and crashing. I tried a second time to see if I could replicate it and the same thing happened - without any input from me.

[Dragon1-1]

I remember this happening before, basically the FCS would go stupid if you took off with anything other than half flaps. I highly doubt the real plane does that.

[trindade]

2 hours ago, Yeti42 said:

I'm not raising a bug yet, I just want to know if anyone else is experiencing unexpected behavior:

My first flight using the new flight model was an instant action flight on the Syria map, I flew around for 10 minutes or so testing the AOA capabilities and then decided to dump all stores and do some touch-and-go's at Incirlik. To cut a long story short, the landing was fine as was on AOA during finals and touchdown, the problems started when I selected 1/2 flap, military power and rotated after touchdown.  The jet adopted an increasingly nose-high attitude which I was unable to counter with full forward stick and manual nose-down trim. The result was inevitable and the jet departed.

Has anyone else seen behavior similar to this? I've been unable to reproduce the behavior in the 2 short flights since and the track file has been overwritten.
 

Maybe because the trim is set to fly the approach on speed. After the touch&go you have to trim nose down since you are now applying throttle to climb out. 

[Tusky]

hace 8 minutos, trindade dijo:

Maybe because the trim is set to fly the approach on speed. After the touch&go you have to trim nose down since you are now applying throttle to climb out. 

Pressing the "T/O Trim" button once on the ground should be sufficient

[razo+r]

After this patch the FCS applies pitch down if you give aileron only input with the flaps down.

With Flaps half, the additional pitch down happens at about 50% aileron input.

With Flaps full, the additional pitch down happens as soon as you give it any aileron input.

 

In the ground demo track you'll see that with flaps up, both stabilators are moving directly opposed to each other when you give aileron input. No pitch input is given by the FCS.

Once I've put the flaps to half, at around 50% or so aileron only input, you'll see there comes a point where both stabilators will stop their aileron function for a bit and move in pitch only. With full flaps, this moment comes as soon as an input is made, both stabilators will immediately begin to produce a pitch down moment at first, and only with increasing aileron input they'll start to induce a roll.

F-18pitch-ground demo.trkF-18pitch-air demo.trk

[BIGNEWY]

I will ask the team about this.

thanks

threads merged

[Raven (Elysian Angel)]

@BIGNEWY This is a track with the weird take-off behaviour:

I took off at Nellis with AUTO flaps, the FCS went nuts and I was only able to recover by putting the flaps to HALF.

F18 new FCS takeoff 20240223.trk

[Q3ark]

On 2/23/2024 at 12:44 PM, Tusky said:

Pressing the "T/O Trim" button once on the ground should be sufficient

The T/O trim button only works if you have flaps out. 
 

Edit:

 

this isn’t true any more, it works with flaps in any position now.

Edited February 24, 2024 by Q3ark

[Yeti42]

OK, I've managed to reproduce similar behavior as the original issue but not exactly the same.   You'll see in the track file I'm doing lots of approaches with different parameters and some missed approaches,  the last one in the track file is a touch and go which demonstrates the behavior. I don't select gear up and give mill power without touching the stick.  As expected I pitch high but I'm unable to recover even with full stick down and trim, selecting gear up seems to re-instate full pitch authority.   EDIT: attached the tacview file from the original occurrence, this is all I have from the first time it happened but it demonstrates the issue well, note I was applying full nose down at the end when the roller-coaster was occurring 

F18_AOA.miz.trk

Tacview-20240223-120901-DCS-IA-FA18C-Syria-Free Flight.zip.acmi

Edited February 23, 2024 by Yeti42
attached tacview file

[Tusky]

hace 54 minutos, Q3ark dijo:

The T/O trim button only works if you have flaps out. 

Let me rephrase:
1) You land with flaps FULL
2) Once you are on the ground you switch to flaps HALF
3) You press T/O TRIM which should put you back to 12° trim
4) You take off

I haven't performed any of the tests of the OP and mine was just a suggestion.

Edited February 23, 2024 by Tusky

[Yeti42]

56 minutes ago, Tusky said:

Let me rephrase:
1) You land with flaps FULL
2) Once you are on the ground you switch to flaps HALF
3) You press T/O TRIM which should put you back to 12° trim
4) You take off

I haven't performed any of the tests of the OP and mine was just a suggestion.

 

TO trim button was the first thing I thought of, It didn't work, I still had reduced pitch authority when I encountered the first issue, bearing in mind this was after the onset of the issue in flight with the flaps at half.  My suspicion here is that the FCS somehow gets stuck in landing or approach mode and the FCS has an argument with itself when you command pitch down , I've no idea if these FBW  modes are modeled in DCS. 

Edited February 23, 2024 by Yeti42

[Tusky]

hace 29 minutos, Yeti42 dijo:

Trim "reset" (not correct as it doesn't do anything of the kind) by using TO trim button was the first thing I thought of, It didn't work, I still had reduced pitch authority when I encountered the first issue, bearing in mind this was after the onset of the issue in flight with the flaps at half.  My suspicion here is that the FCS somehow gets stuck in landing or approach mode and the FCS has an argument with itself when you command pitch down , I've no idea if these FBW  modes are modeled in DCS. 

It seems this aspect/issue of the flight model has not changed... 

[razo+r]

3 hours ago, BIGNEWY said:

I will ask the team about this.

thanks

threads merged

Are you sure you want to merge these two topics? They may have the same source but are still different issues...

[Yeti42]

16 hours ago, razo+r said:

Are you sure you want to merge these two topics? They may have the same source but are still different issues...

The two threads are different behaviors, they may have the same root cause though.   I have to say looking at the new flight model the improvements outweigh any issues, it still needs fixing, because us a**l people will always find something.    It's not an easy task developing a fly-by-wire model, that's why it takes years and costs millions in the real world

 

[Strider32]

On 2/23/2024 at 11:24 PM, razo+r said:

After this patch the FCS applies pitch down if you give aileron only input with the flaps down.

With Flaps half, the additional pitch down happens at about 50% aileron input.

With Flaps full, the additional pitch down happens as soon as you give it any aileron input.

 

In the ground demo track you'll see that with flaps up, both stabilators are moving directly opposed to each other when you give aileron input. No pitch input is given by the FCS.

Once I've put the flaps to half, at around 50% or so aileron only input, you'll see there comes a point where both stabilators will stop their aileron function for a bit and move in pitch only. With full flaps, this moment comes as soon as an input is made, both stabilators will immediately begin to produce a pitch down moment at first, and only with increasing aileron input they'll start to induce a roll.

F-18pitch-ground demo.trk 451.77 kB · 9 downloads F-18pitch-air demo.trk 996.32 kB · 6 downloads

+1 Noticing this behaviour also.

It does make fine corrections during the landing phase a little more challenging when fighting a pitch down moment

Edited February 25, 2024 by Strider32

[MarkyMark]

this happened to me after a long MP mission returning to the boat. I don’t recall the exact scenario but I think I had to roll dramatically to get some sort of reset and regain control. I could not pitch down no matter what until I rolled out 

[BIGNEWY]

Hi all, 

regarding pitch down tendency with half or full flaps with aileron input, looks like ASI command is making a little overcompensation command. The team will tweak it. 

Thank you for the track replays and reports.