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Engine surge oscillation at taxi


Machalot
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I noticed the engine sound and pressure ratio have a noticeable oscillation when the throttle is in the flight idle detent or slightly above it. I have only noticed it during taxi. My friend even heard it while watching me in an external view on a multiplayer server.  Anybody else? 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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What was the OAT?

 

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Posted (edited)

Outside air temperature? I didn't record it, but it was on the Hoggit training Caucasus server so probably close to standard. 


Edited by Machalot

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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On 8/15/2021 at 10:47 AM, Machalot said:

I noticed the engine sound and pressure ratio have a noticeable oscillation when the throttle is in the flight idle detent or slightly above it. I have only noticed it during taxi. My friend even heard it while watching me in an external view on a multiplayer server.  Anybody else? 

Yeah i have this sound.

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  • 1 month later...
I noticed the engine sound and pressure ratio have a noticeable oscillation when the throttle is in the flight idle detent or slightly above it. I have only noticed it during taxi. My friend even heard it while watching me in an external view on a multiplayer server.  Anybody else? 

I’ve had this too, there seem to be a sweet spot where this occur, and if you’re lucky enough you’ve set your flight idle at that spot ;)
I adjusted the throttle curve slightly to get out of the spot where the oscillation occur.

There shouldn’t be a ground idle vs flight idle spot, the throttle should increase thrust linearly.
- The Mil thrust range is about 50% of the throttle range, last 50% is the AB zones. But I understand why it’s done the way it’s done, not many throttles have adjustable detents and there would be serious crying if the AB kicked in at 50% range of peoples throttles :)
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There's no flight idle detent in the real jet? Why did they add one? I find it really annoying because ground idle is too low to taxi, but flight idle is too fast, so I'm constantly jumping across that detent 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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2 hours ago, Machalot said:

There's no flight idle detent in the real jet? Why did they add one? I find it really annoying because ground idle is too low to taxi, but flight idle is too fast, so I'm constantly jumping across that detent 

There is a push-through detent between ground and flight idle, and the thrust increase is linear from ground idle to max dry.

The upside down "T" are "physical" detents where you have to lift or push the throttle.
The triangles are push through detents

AJS37 Throttle positions.png


Edited by MYSE1234
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2 hours ago, Machalot said:

Did the real jets have the same issue with taxi thrust? 

I remember reading that it has a relatively high thrust output at flight idle, but if what we have in DCS is accurate or not I do not know.
Flight idle should be about 14,3kN according to "SFI FPL AJS37, Del 1".

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17 hours ago, Machalot said:

Did the real jets have the same issue with taxi thrust? 

Just did some tests at low speed and calculated the thrust to be about 13,9kN, so pretty close to the 14,3kN from the manual. And there are probably some losses that I'm not compensating for, so I'd say it's accurate as is.


Edited by MYSE1234

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There's no flight idle detent in the real jet? Why did they add one? I find it really annoying because ground idle is too low to taxi, but flight idle is too fast, so I'm constantly jumping across that detent 

The detent is there, what I meant was the detent is not a on/off switch between ground and flight idle. In DCS if you move the throttle slowly from ground to flight idle the rpm increase is very sudden, it should increase like when you move it in the flight idle to max Mil regime.
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4 hours ago, outbaxx said:


The detent is there, what I meant was the detent is not a on/off switch between ground and flight idle. In DCS if you move the throttle slowly from ground to flight idle the rpm increase is very sudden, it should increase like when you move it in the flight idle to max Mil regime.

That makes sense. Maybe this should be a separate bug report? 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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12 minutes ago, outbaxx said:


No I don’t think so, if they can fix the oscillation spot I don’t think anyone will have any issues with it really.

I find it annoying because there is no comfortable taxi thrust, it's either too much or too little. 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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I find it annoying because there is no comfortable taxi thrust, it's either too much or too little. 

That is a good argument!
I would think that they made it the way it is to harmonize between all the different throttles being used by us?
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39 minutes ago, outbaxx said:


That is a good argument!
I would think that they made it the way it is to harmonize between all the different throttles being used by us?

Many other aircraft have keybindable throttle detents. I think a good implementation would use a button, which when held would allow pushing through the detents.  If the button is not pressed, the throttle would be able to cross a detent.

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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Reading your last findings, it appears to me that the "sweetspot" of your throttle basically makes the engine thrust jump between the two, hence the oscillation. A detent for that, by button press for example, indeed could be a solution.

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1 hour ago, IronMike said:

Reading your last findings, it appears to me that the "sweetspot" of your throttle basically makes the engine thrust jump between the two, hence the oscillation. A detent for that, by button press for example, indeed could be a solution.

We might be combining two issues. First is the intermittent oscillation in thrust.  My throttle has no physical detents.  It was set slightly above the Viggen's idle detent, my input did not change when this was happening, and it did not appear to be jumping back and forth across the idle detent.  This seems like an instability or limit cycle in the engine control software model.  I'll keep a close eye on this next time it occurs to make sure I'm not wrong about my throttle position. 

 

The second is the fat deadzone between ground idle and flight idle that creates a big jump in thrust with no linear region.  This makes it hard to maintain a good taxi speed, forcing the pilot to frequently move the throttle into flight idle to gain speed, and then back to ground idle to keep from over speeding, ad infinitum.

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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42 minutes ago, Machalot said:

We might be combining two issues. First is the intermittent oscillation in thrust.  My throttle has no physical detents.  It was set slightly above the Viggen's idle detent, my input did not change when this was happening, and it did not appear to be jumping back and forth across the idle detent.  This seems like an instability or limit cycle in the engine control software model.  I'll keep a close eye on this next time it occurs to make sure I'm not wrong about my throttle position. 

 

The second is the fat deadzone between ground idle and flight idle that creates a big jump in thrust with no linear region.  This makes it hard to maintain a good taxi speed, forcing the pilot to frequently move the throttle into flight idle to gain speed, and then back to ground idle to keep from over speeding, ad infinitum.

 

What I meant was more that this fat deadzone will create an oscillation as the virtual representation of your physical stick (being still and in place at the "brink" of this deadzone) is jumping back and forth, but it could be two separate issues. Kinda, you rest your stick right in the spot that makes the thrust jump between idle and gaining speed, if that makes sense.


Edited by IronMike

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5 minutes ago, IronMike said:

 

What I meant was more that this fat deadzone will create an oscillation as the virtual representation of your physical stick (being still and in place at the "brink" of this deadzone) is jumping back and forth, but it could be two separate issues. Kinda, you rest your stick right in the spot that makes the thrust jump between idle and gaining speed, if that makes sense.

 

Ok, I see what you mean. That's definitely possible. I'll look closely at the control overlay next time it happens. 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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1 minute ago, Machalot said:

Ok, I see what you mean. That's definitely possible. I'll look closely at the control overlay next time it happens. 

 

To test that, see if you get oscillations below or above said spot range as well. If yes, likely I am wrong.


Edited by IronMike

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On 9/24/2021 at 1:22 PM, IronMike said:

 

To test that, see if you get oscillations below or above said spot range as well. If yes, likely I am wrong.

 

@IronMike

I could not reproduce the oscillations by putting the throttle near the detent.

 

However, tonight I had a recurrence of the oscillation with the throttle all the way at minimum in ground idle.  The oscillation had a very regular period of about 1 second, and It was accompanied by a flashing XTANK BRA warning indicator on the left panel during each cycle.  The RPM needle bounced up and down a few hundred RPMs.  If I pushed the throttle forward past the first detent into flight idle, the oscillation and flashing XTANK BRA went away.  It was at the beginning of a ~30 minute multiplayer session, so I have a track file but it is large.  I spent a few minutes fiddling with waypoints and mission planning, but once I started taxiing I noticed the oscillation and stopped to make some observations.  If you would like me to send it to you in a PM, let me know.

 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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8 hours ago, Machalot said:

@IronMike

I could not reproduce the oscillations by putting the throttle near the detent.

 

However, tonight I had a recurrence of the oscillation with the throttle all the way at minimum in ground idle.  The oscillation had a very regular period of about 1 second, and It was accompanied by a flashing XTANK BRA warning indicator on the left panel during each cycle.  The RPM needle bounced up and down a few hundred RPMs.  If I pushed the throttle forward past the first detent into flight idle, the oscillation and flashing XTANK BRA went away.  It was at the beginning of a ~30 minute multiplayer session, so I have a track file but it is large.  I spent a few minutes fiddling with waypoints and mission planning, but once I started taxiing I noticed the oscillation and stopped to make some observations.  If you would like me to send it to you in a PM, let me know.

 

 

Thank you, if the track is large, likely it is borked unfortunately, but you can send it nonetheless. the xtank bra might be a pointer though, so adding your observation to the tracker. Thanks for testing it!

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