How to use n1spindown/n2spindown

[Octal450]

Hi,
I am trying to adjust the shutdown speed of the FGTurbine, and are using FlightGear v Next (2020.4).

However, these tags seem to have no effect on the turbine. No matter how low I set them, the turbine still shuts down at a wrong speed. I'm not sure if I am doing something wrong?

A weird thing also, with these tags in place and smaller values set, the "set-running" property no longer works. The engine blips in RPM and then shuts back down.

The tags I am using are <n1spindown> and <n2spindown>. The spinup versions work perfectly.

	<milthrust>60690</milthrust>
	<tsfc>0.544</tsfc>
	<bleed>0</bleed> <!-- Set by pneumatic system -->
	<ignitionn1>2.8</ignitionn1>
	<ignitionn2>29.4</ignitionn2>
	<n1spinup>0.091</n1spinup>
	<n2spinup>0.958</n2spinup>
	<n1spindown>0.01</n1spindown>
	<n2spindown>0.02</n2spindown>
	<n1startrate>0.578</n1startrate>
	<n2startrate>1.193</n2startrate>
	<idlen1>23.7</idlen1>
	<idlen2>62.9</idlen2>
	<maxn1>117.5</maxn1>
	<maxn2>112.5</maxn2>
	<augmented>0</augmented>
	<injected>0</injected>

Kind Regards,
Josh

[seanmcleod]

How did you come up with your very small spin down values?

So taking a look, these are the default values if you don't specify any.

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 74 to 75 in 056bb71

	N1_spinup = 1.0; N2_spinup = 3.0; IgnitionN1 = 5.21; IgnitionN2 = 25.18; N1_start_rate = 1.4; N2_start_rate = 2.0;
	N1_spindown = 2.0; N2_spindown = 2.0;

And they're used in the call to Seek().

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 172 to 184 in 056bb71

	double FGTurbine::Off(void)
	{
	Running = false;
	FuelFlow_pph = Seek(&FuelFlow_pph, 0, 1000.0, 10000.0);
	// some engines have inlets that close when they are off. So, if a flag is true disable windmilling
	if (disableWindmill == false) {
	// Need a small non-zero increment for acceleration otherwise acceleration will be 0 if N1 = 0
	N1 = Seek(&N1, in.qbar/10.0, N1/2.0 + 0.1, N1/N1_spindown);
	N2 = Seek(&N2, in.qbar/15.0, N2/2.0 + 0.1, N2/N2_spindown);
	} else {
	N1 = Seek(&N1, 0, N1/2.0, N1/N1_spindown);
	N2 = Seek(&N2, 0, N2/2.0, N2/N2_spindown);
	}

And this is the implementation of Seek().

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 400 to 410 in 056bb71

	double FGTurbine::Seek(double *var, double target, double accel, double decel) {
	double v = *var;
	if (v > target) {
	v -= in.TotalDeltaT * decel;
	if (v < target) v = target;
	} else if (v < target) {
	v += in.TotalDeltaT * accel;
	if (v > target) v = target;
	}
	return v;
	}

Note how the deceleration factor is calculated when calling Seek() - N1/N1_spindown, so given your very small spin down values I would expect the engine to spin down faster than normal, i.e. when you don't specify a spin down value.

Now I noticed in the SpinUp() function the spin up values aren't used as a denominator.

jsbsim/src/models/propulsion/FGTurbine.cpp

Lines 273 to 278 in 056bb71

	double FGTurbine::SpinUp(void)
	{
	Running = false;
	FuelFlow_pph = 0.0;
	N2 = Seek(&N2, IgnitionN2, N2_spinup, N2/2.0);
	N1 = Seek(&N1, IgnitionN1, N1_spinup, N1/2.0);

[Octal450]

Hi Sean,
I completely was unaware that this was denominator... I'll admit I was ignorant and didn't check the code. I assumed it worked like the spinup values.

There doesn't seem to be any documentation on the thing: https://jsbsim.sourceforge.net/JSBSim/classJSBSim_1_1FGTurbine.html hence how I missed it.

I will recalculate everything and try again tomorrow.

Thank you!
Josh

[seanmcleod]

I guess we should also double-check why the spinup and spindown values are used differently and whether they shouldn't be consistent.

[Octal450]

Since it's been so many years since implemented (2018) I think we probably should just leave it how it is and just document that it works that way.

Kind Regards,
Josh

[Octal450]

You know what - it's actually better like this. That way the shutdown curve is somewhat realistic instead of linear.

Kind Regards,
Josh

[seanmcleod]

Forgetting about backwards compatibility for now, but doesn't this also apply to the spin up case, i.e. it should also be used as a denominator?

[Octal450]

No, spin up is driven by the starter either air or electric etc. it’s a fairly linear rise. Back compat is simple, use either or add a new prop. Not that anyways has used it that I’m aware of anyways since it’s undocumented.

Kind Regards,
Josh

[Octal450]

One more thing - would it be possible to have this value on a table lookup? So that my panel state loading system can override the value with a very fast value for quickly shutting down the engine - as the real takes long. For startup, set-running works fine but I guess also having spinup and startrate on a table might be useful.

Kind Regards,
Josh