Suport for Three-spool/Multi-spool turbofan/turboprop

[www2000]

The majority of Turbofan/Turbojet/Turboprop in today's aircraft having 2-spool and this reflect the current state of jsbsim.
In this idea is to upgrade jsbsim to support three-spool[1] and n-spool configs for Turbofan/Turbojet/Turboprop.
The default is 2-spool setup for backward compatibility the older config files when the spools={n} Attributes in the turbine_engine tag is missing in the XML config.
beside this the <spools n={number}> ... </spools> make the XML code more readable.

[1] https://en.wikipedia.org/wiki/Category:Three-spool_turbofan_engines

xml template with the change:

<?xml version="1.0"?> 
 <turbine_engine name="{string}" spools="{ number > 2}">
  <milthrust unit="{LBS | N}"> {number} </milthrust>
  <maxthrust unit="{LBS | N}"> {number} </maxthrust>
  <bypassratio> {number} </bypassratio>
  <bleed> {number} </bleed>
  <tsfc> {number} </tsfc>
  <atsfc> {number} </atsfc>
  <augmented> {0 | 1} </augmented>
  <augmethod> {0 | 1 | 2} </augmethod>
  <injected> {0 | 1} </injected>
  <injection-time> {number} </injection-time>
  <function name="IdleThrust"> </function>
  <function name="MilThrust">  </function>
  <function name="AugThrust">  </function>
  <function name="Injection">  </function>
  
  <spools n={number}>
    <ignitionn> {number} </ignitionn>
    <idlen> {number} </idlen>
    <spinup> {number} </spinup>
    <startrate> {number} </startrate>
    <max> {number} </max>
  </spools>
  
  <!-- alias for spools for backward compatibility (optional and will be ignore by detection of the <spool> tag) -->
  <!-- alias for n1 spool <spools n=1> ... </spools> -->
  <ignitionn1> {number} </ignitionn1>
  <idlen1> {number} </idlen1>
  <n1spinup> {number} </n1spinup>
  <n1startrate> {number} </n1startrate>
  <maxn1> {number} </maxn1>
  
  <!-- alias for n2 spool <spools n=2> ... </spools> -->
  <ignitionn2> {number} </ignitionn2>
  <idlen2> {number} </idlen2>
  <n2spinup> {number} </n2spinup>
  <n2startrate> {number} </n2startrate>
  <maxn2> {number} </maxn2>
  
 </turbine_engine>

[bcoconni]

Hi @www2000,

Just to avoid confusion: the turbine model in JSBSim is not an engineering model. No physics or thermodynamics laws are modeled: the spool rotation speeds n1 and n2 are simply faked based on a simple linear relationship between idling and the maximum value:

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 124 to 129 in dafcaad

	if (Running && !Starved) {
	phase = tpRun;
	N1_factor = MaxN1 - IdleN1;
	N2_factor = MaxN2 - IdleN2;
	N2 = IdleN2 + ThrottlePos * N2_factor;
	N1 = IdleN1 + ThrottlePos * N1_factor;

The same goes for all the other engine parameters (EGT, oil temp, etc.). So handling a third spool could just be the matter of adding a new property n3 to your XML model. This property would output whatever value you deem plausible and the realism would be just as good as if this third value was computed in the C++ code.

That being said, if you'd like to redesign the XML code for better legibility that's fine and you are welcome to submit a pull request. As long as they were properly written and did not break anything we have accepted all the pull requests that were adding features to the turbine model.