JSB N1 N2 spinnup speed

[Octal450]

Hi
While working the new engine system in my A320, I find a flaw:

I have an issue where JSB spins the engines up to n1-spinnup and n2-spinnup values WAY to quickly, and where N2 takes alot longer to reach IDLE than N1. Can I resolve this in JSB? or Do I need to take a Yasim approach and make custom interpolates to startup to cover up this flaw?

Thanks
J

Here is my XML:

Code: Select all

<?xml version="1.0"?>
<!--
  File:     V2522_A5.xml
  Author:   Aero-Matic v 0.81

  Inputs:
    name:           V2522_A5
    type:           turbine
    thrust:         26600 lb
    augmented?      no
    injected?       no
-->

<turbine_engine name="V2527_A5">
  <milthrust>     26600 </milthrust>
  <bypassratio>     2.0 </bypassratio>
  <tsfc>            0.8 </tsfc>
  <bleed>          0.03 </bleed>
  <idlen1>         23.4 </idlen1>
  <idlen2>         63.7 </idlen2>
  <n1spinup>        5.1 </n1spinup>
  <n2spinup>       22.8 </n2spinup>
  <maxn1>         105.8 </maxn1>
  <maxn2>         102.1 </maxn2>
  <augmented>         0 </augmented>
  <injected>          0 </injected>

  <function name="IdleThrust">
   <table>
    <independentVar lookup="row">velocities/mach</independentVar>
    <independentVar lookup="column">atmosphere/density-altitude</independentVar>
    <tableData>
         -10000     0     10000   20000   30000   40000   50000
     0.0  0.0430  0.0488  0.0528  0.0694  0.0899  0.1183  0.1467
     0.2  0.0500  0.0501  0.0335  0.0544  0.0797  0.1049  0.1342
     0.4  0.0040  0.0047  0.0020  0.0272  0.0595  0.0891  0.1203
     0.6  0.0     0.0     0.0     0.0     0.0276  0.0718  0.1073
     0.8  0.0     0.0     0.0     0.0     0.0474  0.0868  0.0900
     1.0  0.0     0.0     0.0     0.0     0.0     0.0552  0.0800
    </tableData>
   </table>
  </function>

  <function name="MilThrust">
   <table>
    <independentVar lookup="row">velocities/mach</independentVar>
    <independentVar lookup="column">atmosphere/density-altitude</independentVar>
    <tableData>
          -10000       0   10000   20000   30000   40000   50000
     0.0   1.2600  1.0000  0.7400  0.5340  0.3720  0.2410  0.1490
     0.2   1.1710  0.9340  0.6970  0.5060  0.3550  0.2310  0.1430
     0.4   1.1500  0.9210  0.6920  0.5060  0.3570  0.2330  0.1450
     0.6   1.1810  0.9510  0.7210  0.5320  0.3780  0.2480  0.1540
     0.8   1.2580  1.0200  0.7820  0.5820  0.4170  0.2750  0.1700
     1.0   1.3690  1.1200  0.8710  0.6510  0.4750  0.3150  0.1950
     1.2   1.4850  1.2300  0.9750  0.7440  0.5450  0.3640  0.2250
     1.4   1.5941  1.3400  1.0860  0.8450  0.6280  0.4240  0.2630
    </tableData>
   </table>
  </function>

</turbine_engine>

[Alant]

Look at the documentation http://jsbsim.sourceforge.net/JSBSim/classJSBSim_1_1FGTurbine.html, in particular the section about bypass ratio.

It is a simplified model of a jet engine, but is surprisinglng acurate.

[Octal450]

OK... Thanks
I tried changing from 2.0 to 5.0 and 0.04... no difference...?

[Alant]

Have you also experimented with n1spinup and n2spinup ?

Alan

[erik]

This is the link I put in every AeromatiC++ generated file for reference:
http://wiki.flightgear.org/JSBSim_Engines#FGTurbine

Erik

[Richard]

This is the link I put in every AeromatiC++ generated file for reference:
http://wiki.flightgear.org/JSBSim_Engines#FGTurbine

I've just amended this to cover details on how the acceleration works at the different phases. Hopefully it makes sense, can you check it please.

[erik]

The code shows this:

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N2 = Seek(&N2, 25.18, N2_spinup, N2/2.0);
  N1 = Seek(&N1, 5.21, N1_spinup, N1/2.0);

The second parameter is the target;
The third the acceleration per time step(*)
The fourth is the deceleration per time step(*).

So deceleration is basically exponential and acceleration is a time-step factor.
Since the FDM runs at 120 Hz most of the time it spools 120 RPM per second faster if spinup is 1.0

Acceleration starts below the target value and deceleration below the target value.

Erik

(*) this makes it model-hz dependent.

[Richard]

(*) this makes it model-hz dependent.

Unless I'm reading the code in Seek wrong I don't think it is. The equation uses TotalDeltaT (which is in seconds) and is set to dt - which is the (fractional) amount of seconds elapsed since the last frame.

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v += in.TotalDeltaT * accel;


and therefore accel is in percent/sec

[erik]

Ah ok, that would make sense.
I was wrong indeed, forget my previous post.

Erik

[Octal450]

Solved, thanks Richard!!!
I thought the N1 N2spinup value was for the spinup max RPM, not for the amount per sec... silly me!

Josh

PS: Still working on the N2 Startup speed