Modifying inertia and cg

[fernandafenelon]

Hello everyone,

I would like to run my model for several CGs and mass' configuration.

Is possible to create a variable in XML that defines the inertia and manipulate it via Python?

[seanmcleod]

Currently JSBSim only supports numbers and not properties for the entries you're interested in.

jsbsim/src/models/FGMassBalance.h

Lines 83 to 112 in 8852bf5

	<h3>Configuration File Format for \<mass_balance> Section:</h3>
	@code{.xml}
	<mass_balance negated_crossproduct_inertia="true|false">
	<ixx unit="{SLUG*FT2 | KG*M2}"> {number} </ixx>
	<iyy unit="{SLUG*FT2 | KG*M2}"> {number} </iyy>
	<izz unit="{SLUG*FT2 | KG*M2}"> {number} </izz>
	<ixy unit="{SLUG*FT2 | KG*M2}"> {number} </ixy>
	<ixz unit="{SLUG*FT2 | KG*M2}"> {number} </ixz>
	<iyz unit="{SLUG*FT2 | KG*M2}"> {number} </iyz>
	<emptywt unit="{LBS | KG"> {number} </emptywt>
	<location name="CG" unit="{IN | FT | M}">
	<x> {number} </x>
	<y> {number} </y>
	<z> {number} </z>
	</location>
	[<pointmass name="{string}">
	<form shape="{tube | cylinder | sphere | ball}">
	<radius unit="{IN | FT | M}"> {number} </radius>
	<length unit="{IN | FT | M}"> {number} </length>
	</form>
	<weight unit="{LBS | KG}"> {number} </weight>
	<location name="{string}" unit="{IN | FT | M}">
	<x> {number} </x>
	<y> {number} </y>
	<z> {number} </z>
	</location>
	</pointmass>
	... other point masses ...]
	</mass_balance>
	@endcode

You could edit your aircraft's xml file and replace the number with a variable name, e.g. variable_ixx and variable_cg_x and then in your python you could perform a search a replace for these variables by reading your template aircraft xml file and writing out a new aircraft file which you then use each time to start up your simulation run in python.

[aregis74]

Hy @seanmcleod, I am seeing your answer about mass property entries and just to make sure if I understood.
So, there is no way to use something like fdm['inertia/cg-x-in'] = {number} as variable in python?
About the same subject, how jsbsim compute the inertia moments and product of inertia for fuel?
Regards

[seanmcleod]

Yep, take a look at the source code and you'll see that all the properties are read-only.

jsbsim/src/models/FGMassBalance.cpp

Lines 397 to 427 in 8852bf5

	void FGMassBalance::bind(void)
	{
	typedef double (FGMassBalance::*PMF)(int) const;
	PropertyManager->Tie("inertia/mass-slugs", this,
	&FGMassBalance::GetMass);
	PropertyManager->Tie("inertia/weight-lbs", this,
	&FGMassBalance::GetWeight);
	PropertyManager->Tie("inertia/empty-weight-lbs", this,
	&FGMassBalance::GetEmptyWeight);
	PropertyManager->Tie("inertia/cg-x-in", this,1,
	(PMF)&FGMassBalance::GetXYZcg);
	PropertyManager->Tie("inertia/cg-y-in", this,2,
	(PMF)&FGMassBalance::GetXYZcg);
	PropertyManager->Tie("inertia/cg-z-in", this,3,
	(PMF)&FGMassBalance::GetXYZcg);
	PropertyManager->Tie("inertia/ixx-slugs_ft2", this,
	&FGMassBalance::GetIxx);
	PropertyManager->Tie("inertia/iyy-slugs_ft2", this,
	&FGMassBalance::GetIyy);
	PropertyManager->Tie("inertia/izz-slugs_ft2", this,
	&FGMassBalance::GetIzz);
	PropertyManager->Tie("inertia/ixy-slugs_ft2", this,
	&FGMassBalance::GetIxy);
	PropertyManager->Tie("inertia/ixz-slugs_ft2", this,
	&FGMassBalance::GetIxz);
	PropertyManager->Tie("inertia/iyz-slugs_ft2", this,
	&FGMassBalance::GetIyz);
	typedef int (FGMassBalance::*iOPV)() const;
	PropertyManager->Tie("inertia/print-mass-properties", this, (iOPV)0,
	&FGMassBalance::GetMassPropertiesReport);
	}

Hence my suggestion that you write some code to do a template match search and replace in your aircraft config file.

One option that does allow you to write/update mass properties is the support for point masses.

jsbsim/src/models/FGMassBalance.cpp

Lines 431 to 448 in 8852bf5

	// This function binds properties for each pointmass object created.
	//
	void FGMassBalance::PointMass::bind(FGPropertyManager* PropertyManager,
	unsigned int num) {
	string tmp = CreateIndexedPropertyName("inertia/pointmass-weight-lbs", num);
	PropertyManager->Tie( tmp.c_str(), this, &PointMass::GetPointMassWeight,
	&PointMass::SetPointMassWeight);
	tmp = CreateIndexedPropertyName("inertia/pointmass-location-X-inches", num);
	PropertyManager->Tie( tmp.c_str(), this, eX, &PointMass::GetPointMassLocation,
	&PointMass::SetPointMassLocation);
	tmp = CreateIndexedPropertyName("inertia/pointmass-location-Y-inches", num);
	PropertyManager->Tie( tmp.c_str(), this, eY, &PointMass::GetPointMassLocation,
	&PointMass::SetPointMassLocation);
	tmp = CreateIndexedPropertyName("inertia/pointmass-location-Z-inches", num);
	PropertyManager->Tie( tmp.c_str(), this, eZ, &PointMass::GetPointMassLocation,
	&PointMass::SetPointMassLocation);
	}

[aregis74]

Ok, thanks a lot.
How does jsbsim compute the inertia moments and product of inertia for fuel?

[seanmcleod]

How does jsbsim compute the inertia moments and product of inertia for fuel?

@aregis74 have you taken a look at the source code?