Mass Balance Calculation

[rickechler]

Hello, are the inertia products required for the entire aircraft, empty weight, including wings, tail, gear etc? Then pilot, fuel, and other point masses are calculated separately?

Thanks!

From FGMassBalance.cpp

static FGMatrix33 ReadInertiaMatrix(Element* document)
{
double bixx, biyy, bizz, bixy, bixz, biyz;

bixx = biyy = bizz = bixy = bixz = biyz = 0.0;
if (document->FindElement("ixx"))
bixx = document->FindElementValueAsNumberConvertTo("ixx", "SLUGFT2");
if (document->FindElement("iyy"))
biyy = document->FindElementValueAsNumberConvertTo("iyy", "SLUGFT2");
if (document->FindElement("izz"))
bizz = document->FindElementValueAsNumberConvertTo("izz", "SLUGFT2");
if (document->FindElement("ixy"))
bixy = document->FindElementValueAsNumberConvertTo("ixy", "SLUGFT2");
if (document->FindElement("ixz"))
bixz = document->FindElementValueAsNumberConvertTo("ixz", "SLUGFT2");
if (document->FindElement("iyz"))
biyz = document->FindElementValueAsNumberConvertTo("iyz", "SLUGFT2");

// Transform the inertia products from the structural frame to the body frame
// and create the inertia matrix.
if (document->GetAttributeValue("negated_crossproduct_inertia") == string("false"))
return FGMatrix33( bixx, bixy, -bixz,
bixy, biyy, biyz,
-bixz, biyz, bizz );
else
return FGMatrix33( bixx, -bixy, bixz,
-bixy, biyy, -biyz,
bixz, -biyz, bizz );

[seanmcleod]

Hello, are the inertia products required for the entire aircraft, empty weight, including wings, tail, gear etc? Then pilot, fuel, and other point masses are calculated separately?

Yes, take a look just a bit further down in FGMassBalance::Run()

jsbsim/src/models/FGMassBalance.cpp

Lines 220 to 236 in e4aa2a9

	// Calculate new total moments of inertia
	// At first it is the base configuration inertia matrix ...
	mJ = baseJ;
	// ... with the additional term originating from the parallel axis theorem.
	mJ += GetPointmassInertia( lbtoslug * EmptyWeight, vbaseXYZcg );
	// Then add the contributions from the additional pointmasses.
	mJ += CalculatePMInertias();
	mJ += in.TankInertia;
	mJ += in.GasInertia;
	Ixx = mJ(1,1);
	Iyy = mJ(2,2);
	Izz = mJ(3,3);
	Ixy = -mJ(1,2);
	Ixz = -mJ(1,3);
	Iyz = -mJ(2,3);

[rickechler]

Thank you! What elements are typically included in the base configuration?

[seanmcleod]

Pretty much as you mentioned, i.e. the empty weight configuration of the aircraft.