[BUG] Spoolup time of J79 engines too quick

[frumpy]

Hi there,

I noticed when in idle on the ground, snapping the thrustlever forward takes less than a second to go to 100% RPM.

In the manual "emergency procedures" for the J79 (found here: https://u.pcloud.link/publink/show?code=kZnVU2VZ3jUQji5iaE8obXrXLyCPfRvVkHV0 ) there is a table on page 16 that says from idle-90% it takes a second, from 90-100% another 2 seconds.

 

Also: there a a few different indications on the engines in the sim, than described in the manual:

Idle EGT in the manual is at 400°-500°, while in the sim it's 350°.

Idle nozzle position should be slightly closed, while in the sim it's full open. With afterburner, it should be more closed too (in the sim it's almost open, should be 5/8th to 7/8th).

With afterburner, the EGT should stabilize after ~2s at 625+-10° too. I tested at 15°OAT and got 670°.

This is taken from the J79 manual for the F-4C. Thanks for checking!

Edited May 24, 2024 by frumpy

[Schwokol]

Nice catch. Seems the engine simulation needs some work, but i’m sure they’ll iron it out. The F-14 has remarkably realistic engine modeling. Its spool-up time is fast, but not instantaneous; you can jockey your throttles to full mil and it will still take a couple of seconds for the RPM to react and stabilize

Edited May 29, 2024 by Schwokol

[Longiron]

Could be wrong but it seems like AB is all or nothing also.

I can hear the distinct pops of the different stages but the amount of thrust you get seems the same.

I was also pushing M1.5 and the inlet ramps didn't move.

[CF104]

On 5/24/2024 at 1:06 AM, frumpy said:

Hi there,

I noticed when in idle on the ground, snapping the thrustlever forward takes less than a second to go to 100% RPM.

In the manual "emergency procedures" for the J79 (found here: https://u.pcloud.link/publink/show?code=kZnVU2VZ3jUQji5iaE8obXrXLyCPfRvVkHV0 ) there is a table on page 16 that says from idle-90% it takes a second, from 90-100% another 2 seconds.

 

Also: there a a few different indications on the engines in the sim, than described in the manual:

Idle EGT in the manual is at 400°-500°, while in the sim it's 350°.

Idle nozzle position should be slightly closed, while in the sim it's full open. With afterburner, it should be more closed too (in the sim it's almost open, should be 5/8th to 7/8th).

With afterburner, the EGT should stabilize after ~2s at 625+-10° too. I tested at 15°OAT and got 670°.

This is taken from the J79 manual for the F-4C. Thanks for checking!

 

 

 

 

 

Hi,

One thing to consider is that the F-4C has the J79-GE-15 which has a different nozzle and lower operating temperatures than the J79-GE-17 in the F-4E. The F-4E J79-GE-17(without low smoke) has the following parameters:

Idle EGT after start is 220°-420°C

Idle nozzle position is approximately 7/8. Idle to 80% is mechanically scheduled linear to just below 1/2. 80%to Mil is either mechanically or temperature scheduled to below 1/4. In afterburner the nozzle is temperature scheduled to control the max EGT below 678°C @ 15°C OAT. This is accomplished by varying the nozzle area (anywhere between 5/8 at min and full open at max A/B) as required to maintain this temperature. See the following youtube video. It's a -19 on the test stand with the same nozzle, control system and temps as the -17.

 

Cheers,

John

Edited May 28, 2024 by CF104

[MysteriousHonza]

16 hours ago, CF104 said:

Hi,

One thing to consider is that the F-4C has the J79-GE-15 which has a different nozzle and lower operating temperatures than the J79-GE-17 in the F-4E. The F-4E J79-GE-17(without low smoke) has the following parameters:

Idle EGT after start is 220°-420°C

Idle nozzle position is approximately 7/8. Idle to 80% is mechanically scheduled linear to just below 1/2. 80%to Mil is either mechanically or temperature scheduled to below 1/4. In afterburner the nozzle is temperature scheduled to control the max EGT below 678°C @ 15°C OAT. This is accomplished by varying the nozzle area (anywhere between 5/8 at min and full open at max A/B) as required to maintain this temperature. See the following youtube video. It's a -19 on the test stand with the same nozzle, control system and temps as the -17.

 

Cheers,

John

 

Do you know if there are mechanical changes allowing for almost instant spool time compared to F4C engine installation?

[CF104]

5 hours ago, MysteriousHonza said:

Do you know if there are mechanical changes allowing for almost instant spool time compared to F4C engine installation?

Hi,

The J79 acceleration is completely controlled by the Main Fuel Control on the engine. It has an internal scheduling and governing system that can't be influenced by aircraft mounted controls. As it stands the HB engine acceleration modelling is too fast. I've attached the engine burst acceleration chart for the -15 (C/D) which shows a 4.5 second accel time from idle to Mil. I can't find the -17/-19 chart but I'll keep looking.

In the meantime, here is another YouTube video of a TF-104G (J79-GE-11 which is essentially the same as a -15) and you can see the burst check does take a good 4.5 to 5 seconds from idle to Mil. As soon as I can find some -17/-19 information, I'll post it here.

 

Cheers, 

John

[CF104]

Hi All,

Here is the J79-GE-17 test cell idle to mil guaranteed acceleration time chart. It shows that the fastest accel time is 5 seconds at 50°F(10°C) which is at the bottom of the bell curve. cooler or warmer temps will cause longer acceleration times.  Cheers, John

 

[Raven (Elysian Angel)]

5 hours ago, CF104 said:

idle to mil guaranteed acceleration time chart

Isn't that something that's more done for maintenance purposes, so the people who operate the engines know what the tolerances are?
Perhaps it's me, but the wording "guaranteed acceleration time" leads me to believe it's manufacturer speak for: "it won't spool up any slower than this". Meaning, in practice it's likely faster 
I wonder what the SMEs have to say about it...

[CF104]

1 hour ago, Raven (Elysian Angel) said:

Isn't that something that's more done for maintenance purposes, so the people who operate the engines know what the tolerances are?
Perhaps it's me, but the wording "guaranteed acceleration time" leads me to believe it's manufacturer speak for: "it won't spool up any slower than this". Meaning, in practice it's likely faster 
I wonder what the SMEs have to say about it...

Maintenance are the ones who ensure that these numbers are met. The guaranteed part is a goal to be achieved with minimal gains on improvement once it's met. Rarely will they be exceeded by any significant margin. I've been in aviation maintenance for the last 42 years and working on turbine powered aircraft for the last 34. In my experience one may occasionally see a up to a 1 second improvement on the acceleration numbers but usually they're bang on the numbers for a good condition engine. Even if they come close to the -11 engine as shown in the YouTube TF-104G video or the -15 burst chart it'll be way more realistic than the instant response it currently has.

Cheers,

John

Edited May 29, 2024 by CF104

[Raven (Elysian Angel)]

Ok thanks for that explanation. Let's see if this gets looked at then  

[Marduk879]

Nice job guys!

I think we got everything that is needed to fix this bug.

I even think data provided here could be a basis for a improvement of this module - Implementing a temperature based acceleration meaning the mission temperature itself would actualy have effect on your spool time (from 5seconds to 8seconds according to the provided graph).

[Victory205]

6 hours ago, Marduk879 said:

Nice job guys!

I think we got everything that is needed to fix this bug.

I even think data provided here could be a basis for a improvement of this module - Implementing a temperature based acceleration meaning the mission temperature itself would actualy have effect on your spool time (from 5seconds to 8seconds according to the provided graph).

Be careful of test cell data label “estimated thrust transient time”. Installed performance incorporates intake design limitations on what is achievable.

[CF104]

16 minutes ago, Victory205 said:

Be careful of test cell data label “estimated thrust transient time”. Installed performance incorporates intake design limitations on what is achievable.

And taking this into account, this will mean performance on aircraft will be equal to or less than the test cell and never better. It's usually less once installed into the airframe. So far this is the only concrete data I can find as it's not in the -1 for the F-4E. If the J79-GE-17 testing information from TO2J-J79-96-9 won't do, then what document is out there to support the instantaneous throttle response as modeled? What needs to be found is a full maintenance test runup card for the -17 as installed. But as it stands the modeling is not correct. 

Cheers, John

[Johnny Dioxin]

You guys have far too much time on your hands.

[CF104]

7 hours ago, Marduk879 said:

Nice job guys!

I think we got everything that is needed to fix this bug.

I even think data provided here could be a basis for a improvement of this module - Implementing a temperature based acceleration meaning the mission temperature itself would actualy have effect on your spool time (from 5seconds to 8seconds according to the provided graph).

It's actually based on compressor inlet temperature and not just the mission temp. So at an altitude of 36000' @ Mach 0.85, the ram rise is about 30°C. The OAT at that altitude is at a relatively constant -54°C. Subtract the ram rise and the airframe is actually feeling -24°C. There's lot's of variables to make that chart work over Mach and OAT to give proper compressor inlet temps.

Cheers, John

7 minutes ago, Johnny Dioxin said:

You guys have far too much time on your hands.

I'm an Aircraft Maintenance Engineer so this just comes naturally.

Edited May 30, 2024 by CF104

[Marduk879]

On 5/30/2024 at 10:44 PM, CF104 said:

It's actually based on compressor inlet temperature and not just the mission temp. So at an altitude of 36000' @ Mach 0.85, the ram rise is about 30°C. The OAT at that altitude is at a relatively constant -54°C. Subtract the ram rise and the airframe is actually feeling -24°C. There's lot's of variables to make that chart work over Mach and OAT to give proper compressor inlet temps.

Cheers, John

I understand.

I think mission creators cannot set any other value than air temperature in the mission. So what else can you do about it in ME right?

The system that converts the mission temp into inlet temp is something that would need to be done on the module side.

 

Also i dont see anything about this in the newest changelog from HB. Hopefuly its gonna get fixed soon.

Edited June 5, 2024 by Marduk879

[IronMike]

Will be hopefully fixed in the next patch, and is already in testing.

Thank you all!

[CF104]

6 hours ago, Marduk879 said:

I understand.

I think mission creators cannot set any other value than air temperature in the mission. So what else can you do about it in ME right?

The system that converts the mission temp into inlet temp is something that would need to be done on the module side.

 

Also i dont see anything about this in the newest changelog from HB. Hopefuly its gonna get fixed soon.

 

This is something that will definitely have to be programmed in the module. 

Cheers, John

[SgtPappy]

On 5/30/2024 at 4:40 PM, Johnny Dioxin said:

You guys have far too much time on your hands.

I always find it funny when someone judges someone else for being passionate about something - particularly when one nerd judges someone for being too nerdy.

If they were being rude about it, that's one thing but you're in one of the nerdiest corners of the internet.

Let people be themselves or go somewhere else.

[Victory205]

2 hours ago, SgtPappy said:

I always find it funny when someone judges someone else for being passionate about something - particularly when one nerd judges someone for being too nerdy.

If they were being rude about it, that's one thing but you're in one of the nerdiest corners of the internet.

Let people be themselves or go somewhere else.

I had the reputation of having a “sunny disposition” until I got involved with flight simmers.

It’s difficult to understand the contemptuous mindset.

[SgtPappy]

4 hours ago, Victory205 said:

I had the reputation of having a “sunny disposition” until I got involved with flight simmers.

It’s difficult to understand the contemptuous mindset.

I think just being on the internet does that to everyone.

At any rate, it's good to know that this particular engine quirk will be corrected on an already spectacular module.

[Johnny Dioxin]

On 6/6/2024 at 3:26 PM, SgtPappy said:

I always find it funny when someone judges someone else for being passionate about something - particularly when one nerd judges someone for being too nerdy.

If they were being rude about it, that's one thing but you're in one of the nerdiest corners of the internet.

Let people be themselves or go somewhere else.

I'm ever so sorry for having a sense of humour,. Guys like you are the reason I stay away from these forums more than ever when it used to be like my second home. You judge me about judging others? Which I wasn't doing in any case.

Topic over for me. Have a nice day.

[SgtPappy]

1 hour ago, Johnny Dioxin said:

I'm ever so sorry for having a sense of humour,. Guys like you are the reason I stay away from these forums more than ever when it used to be like my second home. You judge me about judging others? Which I wasn't doing in any case.

Topic over for me. Have a nice day.

Hey I was totally just kidding, come back 

 

Edited June 10, 2024 by SgtPappy