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JSB Engine spoolup

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

JSB Engine spoolup

Postby it0uchpods » Thu Nov 30, 2017 8:17 pm

Hi
I have a engine, IAE V2500, and I need to modify the spool rate, for this reasons:

It spools much slower at low N1, than faster at high N1 Bypass ratio is a constant value.

After cutoff is false during startup, the engine spools to idle too fast. Being able to dynamically change bypass ratio would solve this -- but is this possible?

Code: Select all
   <milthrust>     26600 </milthrust>
   <bypassratio>     8.1 </bypassratio>
   <tsfc>            0.5 </tsfc>
   <bleed>          0.03 </bleed>
   <idlen1>         22.4 </idlen1>
   <idlen2>         60.9 </idlen2>
   <n1spinup>        0.2 </n1spinup>
   <n2spinup>        1.0 </n2spinup>
   <maxn1>         103.8 </maxn1>
   <maxn2>         101.4 </maxn2>
   <augmented>         0 </augmented>
   <injected>          0 </injected>


Kind Regards,
Josh
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Re: JSB Engine spoolup

Postby wlbragg » Thu Nov 30, 2017 8:23 pm

Is bypassratio being used in a calculation in the JSB FDM functions code section or only initialized and hard-fed into JSB?
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Re: JSB Engine spoolup

Postby it0uchpods » Thu Nov 30, 2017 8:36 pm

I do not know. It is just stated in the engine, that is as far as I know, I believe it is hard fed.

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Josh
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Re: JSB Engine spoolup

Postby Alant » Thu Nov 30, 2017 8:46 pm

http://jsbsim.sourceforge.net/JSBSim/classJSBSim_1_1FGTurbine.html

Says:-

Bypass ratio is used only to estimate engine acceleration time. The
effect of bypass ratio on engine efficiency is already included in
the TSFC value. Feel free to set this parameter (even for turbojets) to
whatever value gives a desired spool-up rate. Default value is 0.

Bypass ration is not currently exposed to the property tree, but iff you put forward a convincing cae on the devel mail list you should be able to get this. I was able to do this for some other FGTurbine properties.

You can find more detail in src\FDM\JSBSim\models\propulsion\FGTurbine.cpp.

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Re: JSB Engine spoolup

Postby it0uchpods » Thu Nov 30, 2017 9:58 pm

Hi Alan
Thank you for your response. Looking over the cpp, it is not user-changeable in-sim. I will bring it up on the mailing list, would be awesome!

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Josh
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Re: JSB Engine spoolup

Postby wkitty42 » Fri Dec 01, 2017 1:54 am

not even using a table like some of the other values are changed from depending on the value of another property?
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: JSB Engine spoolup

Postby it0uchpods » Fri Dec 01, 2017 2:05 am

With all due respect, that is not possible with <bypassratio> (AFAIK)

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Re: JSB Engine spoolup

Postby Soitanen » Fri Dec 01, 2017 7:24 am

It can be easier to set <bypassratio> to momentary reaction to throttles and have your own driver between throttle levers and signal to JSBSim.
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Re: JSB Engine spoolup

Postby it0uchpods » Fri Dec 01, 2017 11:33 am

Hi Soitanen,
That's true, but then it will react odd, when the throttle and engine rpm disagree a lot, aswel, it won't solve the issue of spinning up to quickly on startup.

Thanks however.

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Re: JSB Engine spoolup

Postby Soitanen » Fri Dec 01, 2017 2:59 pm

Yes, you are right. My solution will work only for running engine.
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Re: JSB Engine spoolup

Postby Alant » Fri Dec 01, 2017 7:55 pm

If I understand correctly you wish to modify the spool-up behaviour between cut-off being cleraed and the engine reaching idle speed. You are happy with spool up in the normal engine running condition. Is my understanding correct?

Your post to the devel list did not make this clear.

If you are able to compile FG yourself could you patch/hack FGTurbine.cpp to give the desired response.

You probably will need to change lines 302-304
Code: Select all
      N2 = Seek(&N2, IdleN2, 2.0, N2/2.0);
      N1 = Seek(&N1, IdleN1, 1.4, N1/2.0);
      EGT_degC = Seek(&EGT_degC, in.TAT_c + 363.1, 21.3, 7.3);


to something like the code in lines 221-222.
Code: Select all
  N2 = Seek(&N2, IdleN2 + ThrottlePos * N2_factor, spoolup, spoolup * 3.0);
  N1 = Seek(&N1, IdleN1 + ThrottlePos * N1_factor, spoolup, spoolup * 2.4);


If this works report back to the devel group with a request for a new property that changes just these time constants.

Changing bypassratio affects the engine run phase and will have no effect in the engine start phase.

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Re: JSB Engine spoolup

Postby legoboyvdlp » Thu Oct 11, 2018 9:07 pm

I've attempted this and sent a pull request to JSBSIM on github. If the atmosphere bugs reprlted on the mailing list are fixed this could potentially be in 2019.1, we'll see what happens.
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Re: JSB Engine spoolup

Postby it0uchpods » Fri Oct 12, 2018 4:14 am

Are you sure you posted that in the right place? That makes no sense in this context.

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Re: JSB Engine spoolup

Postby legoboyvdlp » Fri Oct 12, 2018 7:35 am

If the atmosphere bugs take a while to fix then therefore jsbsim will not be merged to flightgear anytime soon, and this could be 2019.2 or even later.
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Re: JSB Engine spoolup

Postby legoboyvdlp » Wed Oct 31, 2018 2:07 pm

Now on next: expected in 2018.4
Example useage:
Code: Select all
   <ignitionn1>     4.8 </ignitionn1>  <!-- added: n1 at which fuel flow starts -->
   <ignitionn2>    23.9 </ignitionn2>  <!-- added: n2 at which fuel flow starts -->
   <idlen1>        23.4 </idlen1>
   <idlen2>        60.9 </idlen2>
   <maxn1>        103.8 </maxn1>
   <maxn2>        101.4 </maxn2>
   <n1spinup>     0.137 </n1spinup>     <!-- added: time from 0% to ignitionn1 -->
   <n2spinup>     0.680 </n2spinup>     <!-- added: time from 0% to ignitionn2 -->
   <n1startrate>  0.365 </n1startrate>  <!-- added: time from ignitionn1 to idlen1 -->
   <n2startrate>  0.724 </n2startrate>  <!-- added: time from ignitionn2 to idlen2 -->
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