fixed Consistently wrong engine behavior depending on previous throttle position

[bkthunder]

Long standing bug, (already reported with track, but missing link due to new forum).

I thought it was going to get fixed with the wind bug, but it's still here. 

 

Very easy to reproduce and check. 

 

Going from IDLE to e.g. 90% RPM, the values for fuel flow, EGT and nozzle position are completely different than if you go MIL and then back to 90%. 

In other words, if you throttle BACK from MIL to xxRPM, you get higher thrust (and higher EGT, Fuel flow etc), than if you were IDLE and throttle UP to  xxRPM

 

This is consistent at any altitude/speed/temperature.

 

The practical result is I can be flying at 90% RPM and my wingman will fly 200kts fatser or much slower at the same 90% RPM. Pretty bizzare bug.

 

[Shadow KT]

Just tested this, if you go from idle to 90%, EGT stabilized around 400 C. Went from 90 to 100% and back to 90%, now EGT is stabilized at 550C and I am holding around 20 knots more speed.F-5 Thrust-EGT.trk

Edited January 28, 2021 by Shadow KT

[bkthunder]

Thanks for adding the track!

[NineLine]

I'll check it out, thanks for the track.

[NineLine]

Reported, thanks guys!

[bkthunder]

@NineLine do you think this will be fixed in a timely fashion or is it going to take years like for the wind bug? I can't wait to go back to the F-5...

[NineLine]

I havent had an update yet, but I will continue to bug them, I cant give a timeline, but my personal preference would not to leave it for 5 years.

[blaster182]

It would be great to see this fixed soon. Thanks!

[bkthunder]

Are we going to see a fix with 1.2.7?

 

 

EDIT: just to add more debug info, I am pretty sure this is a bug with the nozzles behavior. 

It's basically as if at the MIL detent there's a switch that only closes the nozzles, and at the IDLE detent there's a switch that opens the nozzles.

 

In fact, going from IDLE through basically 99.5% of the throttle range (excluding after burner), the nozzles only go from 55% at IDLE to about 38% at 100%RPM (again, just a hair before full MIL power). So they stay open all the time.

Conversely, once you hit the MIL detent, the nozzles close up to 10%, and even if you throttle back all the way to 80%RPM, they still stay closed at 10%. Hence the increased thrust, EGT and fuel consumption despite the same RPM.

 

 

 

 

Edited April 13, 2021 by bkthunder

[bkthunder]

Jus checked, no fix for this in 1.2.7

[nessuno0505]

My last purchase has been the a-10c II (10 dollars). My intention was to buy something new with this discount, but I think I will not. There's some old stuff that is mostly abandoned. An older module's support like this is unacceptable. Something has to radically change, and this time for real. I suggest an heavy bug fix for an older module, one for each month, or something similar. Otherwise it's just words.

[Sideburns]

On 4/22/2021 at 5:58 PM, nessuno0505 said:

My last purchase has been the a-10c II (10 dollars). My intention was to buy something new with this discount, but I think I will not. There's some old stuff that is mostly abandoned. An older module's support like this is unacceptable. Something has to radically change, and this time for real. I suggest an heavy bug fix for an older module, one for each month, or something similar. Otherwise it's just words.

 

Ditto, spending freeze due to lack of good progress on old bugs like this, and also newer modules.

[bkthunder]

Latest update: Version 2.7.1.64310 

The bug has not been solved. 

 

 

F-5 engine oddities 2.trk

[bkthunder]

Still an issue as of the lastest update. 

In the time this has been an issue and reported (it was reported long ago in the "old" forum), ED has released a multitude of products. 

F-18, F-16, Hind, P-47, Fw-190, Channel map, super carrier, yak-52 etc. Surely you have different devs for different tasks, however, and whatever your internal work organization is, it really stinks that you leave your released products in a state of decay while you triumphantly announce EA release after EA release (that all need to be supported, too!).

 

Like the wind bug that you fixed after years and years (thank you for that!), this is a SEVERE bug. The nozzle scheduling is broken and it results in unpredictable engine thrust.

Will you finally give a feedback to your F-5 customers with a plan to fix it, and stick to it??

 

Thanks

 

 

 

[Maxthrust]

 

Can anyone overlay the in game nozzle position below and show how it is wrong?

 

Edited June 21, 2021 by Maxthrust

[bkthunder]

Here it is. Keep in mind the problem is that nozzle shceduling changes based on if you were previously at MIL or at IDLE.  

 

Starting from IDLE (yellow line), the values are near the chart even if there are discrepancies. 

Once you reach MIL, the nozzle closes up (purple line) and remains closed all the way down to 80%, then resumes normal scheduling. 

 

This is the issue. 

[Machalot]

5 hours ago, bkthunder said:

Here it is. Keep in mind the problem is that nozzle shceduling changes based on if you were previously at MIL or at IDLE.  

 

Starting from IDLE (yellow line), the values are near the chart even if there are discrepancies. 

Once you reach MIL, the nozzle closes up (purple line) and remains closed all the way down to 80%, then resumes normal scheduling. 

 

This is the issue. 

How did you define the values for your x axis? 

[bkthunder]

6 hours ago, Machalot said:

How did you define the values for your x axis? 

By looking at the RPM value

[Machalot]

7 hours ago, bkthunder said:

By looking at the RPM value

Do you know if the results are dependent on altitude or Mach?

[Maxthrust]

7 hours ago, bkthunder said:

By looking at the RPM value

 X axis is the linear arc distance between the labelled detents. In order to test this one must have a linear throttle axis between the idle and mil detents via the user curve setting option.

 

nevertheless the nozzle position seem to be a function of rpm, not throttle, and therefore the OP made his point with the discrepancy.

 

Could ED comment on these findings please? 

[Machalot]

7 minutes ago, Maxthrust said:

 X axis is the linear arc distance between the labelled detents. In order to test this one must have a linear throttle axis between the idle and mil detents via the user curve setting option.

 

nevertheless the nozzle position seem to be a function of rpm, not throttle, and therefore the OP made his point with the discrepancy.

According to the original graph, from idle to MIL the nozzle position can be expressed as a unique function of throttle or RPM (all three values are uniquely related).  But it appears based on bkthunder's test data that nozzle position is not a unique function of RPM nor of throttle, and there is either another variable in play, or it has memory (i.e. is path-dependent).

[Svend_Dellepude]

The nozzle schedule certainly is very inconsistent. It's very noticeable when landing, where you can have too much power or no power at all with the same throttle position depending where the throttle came from.

[bkthunder]

Guys can I suggest that we keep the conversation to the point, which has been clearly demonstrated and explained in a multitude of ways, and is clear to ED as well. 

 

You can discuss what are the parameters, the speed, the mach, the pressure etc. but all of this has nothing to do with the bug being reported here, that is: the J85 in DCS has not one nozzle schedule (as per ED's own manual!), but TWO DIFFERENT SCHEDULES that are dependent on whether you previously hit the IDLE stop or the MIL stop.

It's easy to test, so please go ahead and see for yourself.

We can open a separate thread if other parameters are wrong, but this is the obvious one that's probably throwing everything else out of place (it affects EGT and fuel flow as well).

 

There is not a single J85 in  existence (and most probably no other engine wither) where the nozzle behaves in two different ways based on previous throttle position. Not a single one.

It serves no function.

It makes for unpredictable engine thrust > flight parameters.

It is not how the J85 works.

It's a bug, and big one at that.

 

ED has to fix their existing products, especially when the FM/engines have big issues like these, that's all.

[DoorMouse]

This only effects the thrust as you retreat the throttle? But resets once you hit mil? Or does this also effect your AB thrust capabilities? 

Edited June 22, 2021 by DoorMouse

[Maxthrust]

4 hours ago, Machalot said:

According to the original graph, from idle to MIL the nozzle position can be expressed as a unique function of throttle or RPM (all three values are uniquely related).  But it appears based on bkthunder's test data that nozzle position is not a unique function of RPM nor of throttle, and there is either another variable in play, or it has memory (i.e. is path-dependent).

 

Nozzle operation is quite complex:

 

VARIABLE EXHAUST

1

NOZZLE OPERATION

Variable exhaust nozzle operation is controlled by throttle position and EGT. When the throttle is advanced slowly to MIL, nozzle opening decreases toward 0% until approximately 85% rpm. At this pofnt, the nozzle remains constant at a fixed cruise flat position ( 16% to 22%) until the throttle is advanced to where the nozzle starts to 

close toward 0%. The engine delivers best cruise power performance with minimum fuel consumption when on the cruise flat. When the throttle is advanced beyond the cruise flat toward MIL rpm, the nozzle continues to close until an EGT above 670° ± 5° is momentarily reached. The nozzle then opens via the T5 amplifier control to maintain EGT within limits. This is called Ts modulation. Just prior to T5 modulation, the nozzle is still mechanically controlled by the throttle. A throttle setting just prior to Ts modulation improves fuel consumption rates. When Ts modulation occurs, the nozzle opens slightly. During a rapid throttle burst from IDLE to MIL or MAX, the nozle closes to 43% to 53%, and stays at that opening momentarily. Nozzle hesitation at this point during acceleration minimizes exhaust back pressure to provide rapid acceleration and to preclude compressor stall. The nozzle then closes to 0% to 3% until T5 modulation occurs. At high altitude, low airspeed, when a throttle burst from IDLE or cruise to MIL or MAX is made, the nozzle opens toward the 43% to 53% area, then closes to approximately 7% to 12% to minimize rpm rollback and compressor stall prior to Ts modulation. During a throttle burst to AB range at low altitude, the main afterburner fuel flow is delayed by a sequence valve, momentarily causing the nozzle to pause (approximately 6% to 14% above MIL steady-state nozzle position) to allow afterburner pilot fuel to light off first; permitting a softer afterburner lightoff, 

28% to 38% to maintain safe EGT operation. 

This nozzle position is known as the T5 lockout area. At high altitudes and low airspeeds, MIL nozzle opening may be larger and EGT lower than observed at low altitudes

and high airspeeds. During ground operation at MIL power, nozzle opening should be approximately 0%. As the throttle advances into the AB range, openin~ should approximate 25% to 50% in mmimum afterburner, increasing to approximately 80% at maximum afterburner. Nozzle indication of 75% or hi~her indicates a fullopen nozzle (nozzle-limited) condition. Under this condition, fuel flow to the affected engine is reduced to maintain EGT within limits. If the

Ts amplifier fails during MIL or AB power, retard the throttle to maintain EGT within limits if flight conditions permit.

 

Ts AMPLIFIER SYSTEM

The T5 amplifier system maintains a preset

turbine discharge EGT within allowable limits

during MIL and AB power operation by varying

the exhaust nozzle opening. Operation is

automatic with ac power supplied by the engine

tachometer generator. If EGT is higher

than the reference temperature, the amplifier

causes the nozzle to open; if lower, the nozzle

closes. The system operates primarily in MIL

and AB power ranges.