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[REPORTED]Low altitude, Low fuel, High speed wobbling


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Hey,

I have found a bug in F-16.

 

While being low on fuel <2000lbs, and relatively low alt <3000 ft, and flying above Mach 1 speed plane starts to wobble. It's impossible to counter it.

 

To be honest, moving any surface during this event or when it is about to start wobbling will cause much stronger wobble later.

 

I've tried it only with no pylons loadout.

 

Problem doesn't happen when plane has more than 2000lbs fuel.

 

There are 2 short tracks, in 2nd one I have turned autopilot on (alt and course hold).

 

F-16C_low_alt_FBW_bug .trk

 

F-16C_low_alt_autopiloton_FBW_bug .trk

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Just an update

 

Maximum operating airspeed is 800 knots from sea level to 30,000 feet MSL. Above 30,000 feet MSL, the aircraft is limited to 2.025 mach.

 

The team will investigate behaviour outside of the allowed envelope. When I know more I will pass it on.

 

Thanks


Edited by BIGNEWY

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For info, it is an engine limitation not an air-frame limitation.

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  • 2 weeks later...

F16C low level high speed

 

Is this a bug or something like that happen in the real F16C at low level - high speed ??

 

https://gofile.io/?c=HwYdKr

 

The aircraft starts to shake like hell :D

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Thank you

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I had this happen to me as well, shortly after I bought the Viper. I created a mission where it was clean wing, and full fuel. Quite a lot of fun when you start the takeoff roll and hear the burner kick in!

 

Anyway, I went low level, about 150 feet over water on full burner with the altitude hold turned on. Don't recall my fuel state, but it was not bingo. As I went faster I started to oscillate side to side as well, which got progressively worse. I pulled back the throttle, but was still oscillating badly. Got to the point that it blacked out my pilot, and when he came to, I was inverted but stable and still flying.

 

I didn't save that track, but I'll see if I can duplicate it.


Edited by Diesel_Thunder

Aircraft: A-10A, A-10C, A-10C II, P-51D, F-16C, F/A-18C, F-15C

Modules: Super Carrier, NTTR, Persian Gulf, WWII Assets

 

PC: MSI 990FXA Gaming, FX-8120 3.1 GHz, 32GB Patriot Viper RAM, GTX980 4GB, 27" 1080p curved Samsung monitor, Saitek X-45 HOTAS, Opentrack

 

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Just tried it again, and I was able to induce the wild oscillation repeatedly at various fuel states. Above 800 KIAS, kicking the rudder will set in motion the oscillations.

 

I have been unable to correct it with normal control inputs. My only reliable way out of it is to throttle back and put the speed brakes out. The oscillation starts to dampen out, and the aircraft recovers around 500 KIAS.

 

EDIT: I did try one where I went low level and full burner, setting the altitude hold and no control inputs. Got just above 900 KIAS and it stayed straight as an arrow the whole time.

f-16 oscillation.trk


Edited by Diesel_Thunder

Aircraft: A-10A, A-10C, A-10C II, P-51D, F-16C, F/A-18C, F-15C

Modules: Super Carrier, NTTR, Persian Gulf, WWII Assets

 

PC: MSI 990FXA Gaming, FX-8120 3.1 GHz, 32GB Patriot Viper RAM, GTX980 4GB, 27" 1080p curved Samsung monitor, Saitek X-45 HOTAS, Opentrack

 

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Just tried it again, and I was able to induce the wild oscillation repeatedly at various fuel states. Above 800 KIAS, kicking the rudder will set in motion the oscillations.

 

I have been unable to correct it with normal control inputs. My only reliable way out of it is to throttle back and put the speed brakes out. The oscillation starts to dampen out, and the aircraft recovers around 500 KIAS.

 

EDIT: I did try one where I went low level and full burner, setting the altitude hold and no control inputs. Got just above 900 KIAS and it stayed straight as an arrow the whole time.

 

The limit of the jet is 800KIAS or 2.025 whichever is lower. So do not fly it out of this region or you can expect strange things..your flying yourself out of the flight envelope of the Viper.

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Hi!

The limit of the jet is 800KIAS or 2.025 whichever is lower. So do not fly it out of this region or you can expect strange things..your flying yourself out of the flight envelope of the Viper.

 

It is an airspeed limitation, and this limitation is dictated by engine diagrams (c.f. Dash-1 - OPERATING LIMITATIONS - AIRSPEED LIMITATIONS - Refer to Figure 5-3 ; 5-5 ; 5-6)

So it is not out of the flight envelope, but outside of the engine envelope.

 

Regards


Edited by Dee-Jay

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Hi!

 

 

It is an airspeed limitation, and this limitation is dictated by engine diagrams (c.f. Dash-1 - OPERATING LIMITATIONS - AIRSPEED LIMITATIONS - Refer to Figure 5-3 ; 5-5 ; 5-6)

So it is not out of the flight envelope, but outside of the engine envelope.

 

Regards

 

Although I do not really agree with you...does it really matter?..you should not go past those limits...for all I care you could end up in the twilight zone...

 

If you read the lines above those drawings you listed, it reads: Maximum operating airspeed is 800 knots from sea level to 30,000 feet MSL. Above 30,000 feet MSL, the AIRCRAFT is limited to 2.05 mach.

 

The aircraft as in structure including engine.

 

So do we really want ED to come up with the correct flight model/behavior when limits are exceeded? My answer: no


Edited by Bouli306
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That are aerodynamic and autopilot limitations. Look for why they built the Tornado - even that fighter has its limitations. You can also watch on youtube (F-14 Tomcat Flyby Explosion and Crash as Fighter Jet in Supersonic Boom Flyover USS John Paul Jones)

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Hi!

 

look for Dutch Roll and watch "岐阜基地航空祭2019 F-2 F-15 F-4 ダッチロール比較 Dutch roll JASDF Gifu Air show" on Youtube

 

No dutch roll here. Look at the rudder, yaw is commanded. If we had dutch roll like this with that amplitude at that speed, obviously, a/c would be in trouble and pilot will have landed ASAP.

 

 

Although I do not really agree with you...does it really matter?..

I gave my sources, if you have some other clues/reference, I'll take them, and will be happy to consider them.

 

So do we really want ED to come up with the correct flight model/behavior when limits are exceeded?
Does DCS model a/c damage in case of runway excursion? Yes. Is the pilot supposed to roll in the grass with an F-16? Nope.

... as long as you can reach that airspeed, IMHO, we should not witness such a resonance which is typical from numerical instability with FLCS computation.

 

Regards


Edited by Dee-Jay

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Hi!

 

I gave my sources, if you have some other clues/reference, I'll take them, and will be happy to consider them.

 

Well your source is a fairly known book to me, only the MLU one. Said limits are hard limits LM does not want you to exceed. You say it is a engine limit i doubt that. Simply because the PW has the same limits. If it is engine related at all i think it would primarly be because of the F-16 intake.

 

I understand that the Viper will not explode when you are flying 809kts or 2.06M. But what i do know is that FCF flights the pilot checks the FLCS dampening by giving small inputs on rudder and roll to see if the FLCS willl dampen the input. It might have something to do with that dampening effect when flying too fast. or it might have something to do with the LEF's that can not go up more than 2 degrees...

 

I am just saying that myself and you do not know what should happen to a real Viper when flying too fast because it never regularly been testflown..un- and intentionally.

 

Does DCS model a/c damage in case of runway excursion? Yes. Is the pilot supposed to roll in the grass with an F-16? Nope.

... as long as you can reach that airspeed, IMHO, we should not witness such a resonance which is typical from numerical instability with FLCS computation.

Regards

 

Thing is that your opinion is just an opinion like mine. I am just saying you want ED to model something other than the jet is reacting at the moment to an overspeed while you have no clue other than your opnion. That is why i say to leave it till somebody tells and proofs how the Viper should react.

 

Runway excursions: we know a few instances of that to model this in DCS.

 

The Viper, engine or the combination of the two are not designed to go over the speedlimits so also not the FLCS...why should they.

 

This is it for me..i'm out.


Edited by Bouli306
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If it is engine related at all i think it would primarly be because of the F-16 intake.

Same thoughts here. I would even bet on "only" rather than "primarly".

 

I am just saying that myself and you do not know what should happen to a real Viper when flying too fast because it never regularly been testflown..un- and intentionally.

By reading the FLCS diagrams, we can have a fair idea that "yaw damper" would not enter in resonance. At least not that way, it would be a major and unacceptable flow in FLCS code/design.

However is right ... if you know a (certified) a/c departing that way in yaw beyond actual structural limits so close to its VNE, give me a call please, so I would never attempt to fly it IRL.

 

The Viper, engine or the combination of the two are not designed to go over the speedlimits so also not the FLCS...why should they.

Because a pilot may, especially in combat, go accidentally beyond those limits. Don't you think/know? :) ... shall he loose the control for a little over-speed? ... I don't think so. Otherwise, LM didn't applied the right safely factor to it a/c's limitations. (... because indeed, a real structural limitation is always (way) above the published value. Most of the time, it is close to 150% ... not for all limitations agree, such as FTIT for example.

 

Two different POV. That is fine also. And to be honest, I don't really care. I do not choose what ED likes to consider what is relevant and what is not, I run "on my own business". ;)

 

 

Regards.


Edited by Dee-Jay

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You can experience a high frequency long period yaw oscillation well below 800kts just by kicking the rudder. Neither the frequency nor the period can match a normal dutch roll mode. I'm not sure this is a wanted behaviour and may result in a bad control quality rating according to MIL-STD-1797A, which provides guidance in evaluating aircraft controllability and handling.

 

Some users in the forum and reddit, like Curly and mav-jp, suggest that the FLCS used by DCS F-16C may be based on NASA TP1538:

 

The issues has more to do with the viper under preforming. The DCS viper is using an early NASA FLCS. Which is problematic due to the pitch rate integrator not allowing maximum pitch rate through the envelope. Remember that FLCs was designed to prevent the so called deep stall not maximum performance. It was seen as the starting off point for designing the modern FLCS.

(https://forums.eagle.ru/showpost.php?p=4096586&postcount=82)

 

This is not unreasonable since yaw oscillation can happen if the Yaw Axis Gain is too high for the current speed/altitude. And in NASA TP1538, the Yaw Axis Gain is a fixed 0.5 and is unsuitable for high speed flying since it seems that the gains are simplified for subsonic flight. In Analog and Digital FLCS, the Yaw Axis Gain is scheduled with Qc/Ps which takes speed/altitude into account.

 

1640742815_yawaxisgainschedule.jpg.3eb7fa75d9784acc8d0b53a74480e922.jpg

 

Maybe the pitch axis has the same problem of gain scheduling, but I could also be wrong and the problem may come from elsewhere. :joystick:


Edited by LJQCN101
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EFM / FCS developer, Deka Ironwork Simulations.

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Not saying it is the same problem/cause in DCS ... but some years ago, we had exactly the same issue. Exactly the same behavior at the same speed range.

It was because of an FM computation indexed of "frame rate". This was solved by using adaptive algorithms.

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Not saying it is the same problem/cause in DCS ... but some years ago, we had exactly the same issue. Exactly the same behavior at the same speed range.

It was because of an FM computation indexed of "frame rate". This was solved by using adaptive algorithms.

 

Yeah timestep/dt can be a cause too. I remembered the adaptive dt back then.

EFM / FCS developer, Deka Ironwork Simulations.

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  • 5 weeks later...

Rudder wobble since last update

 

Hi all,

 

 

I've noticed at high speed at low to medium altitudes but most noticeably at very low altitudes regardless if loadout my Viper oscillates left and right. Could be like buffeting.

 

It feels like it's wobbling.

Does it in and out of AP.

Have recalibrated my hotas.

Have moved the rudder axis.

 

Anyone else getting a slight left and right wobble??

 

Matt

 

Sent from my IN2013 using Tapatalk


Edited by HR-Crumble
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Yes but I think this has already been reported?

 

-S

Okay that's good to know as for a while there I was thinking my new hotas was on the blink.

 

I've never come across the problem until after last week's update.

 

Sent from my IN2013 using Tapatalk

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