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GrimPhoenix9349 avatar image
GrimPhoenix9349 asked GrimPhoenix9349 edited

Is it possible to adjust spool rate?

Having reviewed this topic:

https://devsupport.flightsimulator.com/questions/966/jet-engine-model-equation-fuel-flow-and-fuel-consu.html


I would like to add that the current spool rate for jet engines appears not to be adjustable (at least, I have not found a way!) . In FSX, this was achieved by manipulating the N2 v FF table, but as noted in the above topic if you set that to adjustable ( '=1') in MSFS and make any adjustments, the engine fails to work entirely.


If we use '=0' for that table, the spool rate is reversed in comparison to a normal jet engine behaviour - currently it is highly responsive in the low RPM range and slow to respond in the high RPM range. A real jet has the higher responsiveness in the higher band's of RPM, hence military aircraft often use airbrakes and higher power settings in formation or in the circuit. Also, given that different jets can have vastly different ranges from idle to max, and thus different bands of responsiveness, it would be really useful not only to have this corrected in the default jet engine model but also to have an adjustable table that we can use to refine an individual engine's behaviour.

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screenshot-20220529-004609-opera.jpg

You mean more than 6 months?

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FlyingRaccoon avatar image
FlyingRaccoon answered

Hello @GrimPhoenix9349

Sorry, we missed that one.
We are aware of the of the issue with the spool rate of our turbine and that's something we plan to address.

But using the corrected_N2_from_ff_table should work. When you say your "engine fails to work entirely", what do you mean?
If the turbine is shutting down, that's something we experienced as well and it was caused by the N2 being below the min_N2_for_fuel_flow threshold so make sure you're always above this threshold when N1 goes above low_idle_N1 (moment when the starter stops).

Hope this helps.
Regards,
Sylvain

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MiGMan_Flight_Sim_Museum avatar image
MiGMan_Flight_Sim_Museum answered

Spool rates are really important in fast jets, especially around the circuit.

Enabling this would make Asobo's Top Gun Maverick F/A-18E much more realistic ! ! !

And hence more fun.


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GrimPhoenix9349 avatar image
GrimPhoenix9349 answered GrimPhoenix9349 edited

@FlyingRaccoon Hi Sylvain,

Initial settings:

min_N2_for_fuel_flow = 15

min_N2_for_combustion = 19

Lowest point on the N2 axis is 20

Use_N2_to_N1 has been set to both 1 and 0, when set to 1 the two defined line graphs are close together across the range.


The turbine is not 'shutting down' as such - it is stagnating, and the rpm will neither increase nor decrease. I have tried a variety of settings, each one keeping n2 above the minimum settings. The only difference in behaviour is that the rpm sits solidy without moving at different rpm values depending on the lowest point on the N2 axis. I have also tried adjusting the angle of the graph line, but this also made no effect.

I have managed , just by altering the fuel flow v N2 curve, to get a running engine with no throttle response at 30%, 45%, 60% and 70% rpm, the last two being significantly above the idle point, the fact that the first two (slightly below and significantly below idle) maintain a running engine suggest that fuel flow is not the issue but that the use of the table is?

(apologies for my own late response, I have an ongoing family situation which is eating up virtually all of my spare time).

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Hello @GrimPhoenix9349

Can you provide me with the engines.cfg file that produces the problem so I can investigate please?

Regards,
Sylvain

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GrimPhoenix9349 avatar image GrimPhoenix9349 FlyingRaccoon ♦♦ ·
Happy to, but how would I go about sending you that file?
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Check section 3 here, where we explain how to provide private content.

How to report a bug or crash - MSFS DevSupport (flightsimulator.com)

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FlyingRaccoon avatar image
FlyingRaccoon answered

Hello @GrimPhoenix9349

Regarding the use of the corrected_n2_from_ff_table, I think the issue comes from your low_idle_n2 and low_idle_n1 values set respectively to 55 and 50, being too high for the actual N1 and N2 you get from your configuration.

So when your engine is in idle, the fuel flow (idle_fuel_flow) is limited to 634 pounds per hour.
With a static_thrust of 5200, the interpolation of your corrected_n2_from_ff_table gives an N2 around 20 and an N1 around 9. Both of those being below your low idle values, the turbine is considered in a "starting" status and the fuel flow is still limited to 634 pph.
That explains why the RPM are stagnating.
You want to lower those idle values or raise the N2 you get from the idle fuel flow.

Does that make sense?

Regards,
Sylvain

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GrimPhoenix9349 avatar image
GrimPhoenix9349 answered

Hi Sylvain,


Thanks for your answer - it makes sense in a way but I do not think that it is actually the problem as (referring back to my earlier comment) you will see that it is possible to achieve exactly the same behaviour at 70%rpm, simply by following your suggestion of raising the N2 from the idle fuel flow on the graph. It will probably go higher but that's as far as I went when testing it. I raised the fuel flow curve significantly at the same RPM points, and have been unable to get response to the throttle.


The N1 and N2 idle values are correct to the book, as is the idle fuel flow. Static thrust of 5,200 lb is at max rpm so shouldn't affect the idle fuel flow. 634 PPH is the Hawk's idle fuel flow at sea level / standard ISA / 52%rpm. I am using real-world figures throughout.

The tables are exact to those which worked in FSX / P3D and gave correct behaviour and figures. I know MSFS is not either of those sims, but would expect the option to use tables from them to maintain the same behaviours as there are no documented changes in the SDK which would necessitate alteration.



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FlyingRaccoon avatar image
FlyingRaccoon answered FlyingRaccoon edited

Hello @GrimPhoenix9349

"The tables are exact to those which worked in FSX / P3D and gave correct behaviour and figures"

Do you know any external resource where this table and how to use it is described?
I can see it comes from FSX in the code, but can't find any explanation on how it was supposed to be used previously. The code is exactly the same though.

BTW, our doc seems to be faulty and pair should be inverted compared to what's stated in the doc I think.

Here's how it works in MSFS: this table is sampled by interpolating using a corrected_FF / static_thrust ratio. In the pairs provided, the first value is the ff/thrust ratio and the second is the corresponding N2.
In your case, the FF seems to range from 634 pph (idle) to around 3700 pph and static thrust is 5200.
So if I configure the corrected_n2_from_ff_table to have a section of it ranging from 0.122:55 to 0.712:max n2 (104? 110?), I end up with what seems to be the behavior you are looking for.
Engine ends up idling at the N1 & N2 values you indicated and RPM are covering the whole range when using throttle.

I am interested to have your feedback on this.

Regards,
Sylvain

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GrimPhoenix9349 avatar image
GrimPhoenix9349 answered GrimPhoenix9349 edited

Hi Sylvain,


Okay, I have it working but it is very rough - no smoothness in the rpm as per previous sims, see the following videos:

https://drive.google.com/file/d/10UOBiH-R0a4AvnIHNTU2TnxML8QU-plM/view?usp=sharingff-v-n2.jpg


and

https://drive.google.com/file/d/1ZuK-7b99nQ89zs8v6lplTwBYHyl9u2nq/view?usp=sharing

ff-v-n2-2.jpgVery slightly different curves, but both are much smoother than were necessary in the previous sims (see reference material later) yet you can see the jerky nature in the videos.


Whilst you say the code is exactly the same as FSX, I think there must be something affecting this table (conflict with the new code?) because the behaviour whilst broadly similar is not as we had before and if it were identical it should have worked with the original table, as below, which works perfectly in FSX and P3D. In the table below, there are several points before the idle (which is actually roughly in the same relative ff/n2 position, just further up the curve) and these are used to control the spool up during engine start with the highlighted point being a fraction under 20% rpm where combustion is first achieved. There is then a fairly straight section which leads up to the idle % at which point the curve flattens for slow rpm response before rising towards the higher rpm bands to gain faster response times. The information you have given me above and the 'range' requested for it to work has removed the startup section of the spool rate, so there is no longer control over that.

ff-v-n2-fsx.jpg


Regarding descriptive resources, there is a decent thread on FS Developer:

https://www.fsdeveloper.com/forum/threads/table-1505-what-it-was-intended-to-do-and-how-it-should-be-set-up.389153/

The first post is probably the most useful, though there is comment further on that "The spool rate is interpolated to achieve a smooth spin up " which may also be what is not happening here in MSFS. Worth noting that the curves shown in that thread are not at all smooth and yet in-sim they achieve smooth rpm movement.


I will keep exploring options with the table now that rpm is moving (thank you), but as it currently works it is still not acceptable for anything on sale to the public.


Thanks again for your time on this.


ff-v-n2.jpg (23.5 KiB)
ff-v-n2-2.jpg (24.2 KiB)
ff-v-n2-fsx.jpg (22.7 KiB)
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