Detailed Description

Models weight, balance and moment of inertia information.

Maintains a vector of point masses. Sums the contribution of all, and provides this to FGPropagate. Loads the <mass_balance> section of the aircraft configuration file. There can be any number of <pointmasses>. Each can also have a shape which - if present - causes an associated moment of inertia to be calculated based on the shape. Note that a cylinder is solid, a tube is hollow, a ball is solid and a sphere is hollow.

The inertia tensor must be specified in the structural frame (x axis positive aft, y axis positive out of the right wing and z axis upward). The sign of the inertia cross products are optional by JSBSim. if negated_crossproduct_inertia == "true", then define: ixy = -integral( x * y * dm ), ixz = -integral( x * z * dm ), iyz = -integral( y * z * dm ). else if negated_crossproduct_inertia == "false", then define: ixy = integral( x * y * dm ), ixz = integral( x * z * dm ), iyz = integral( y * z * dm ). default is negated_crossproduct_inertia = "true". We strongly recommend defining negated_crossproduct_inertia = "false", which is consistent with the specifications in the field of flight dynamics.

Configuration File Format for <mass_balance> Section:

<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>

@see Stevens and Lewis, "Flight Control & Simulation"
@see Bernard Etkin, " Dynamics Of Atmosferic Flight"
@see https://en.wikipedia.org/wiki/Moment_of_inertia#Inertia_tensor
@see https://www.mathworks.com/help/physmod/sm/ug/specify-custom-inertia.html

Definition at line 124 of file FGMassBalance.h.

#include <FGMassBalance.h>

Inheritance diagram for FGMassBalance:

Collaboration diagram for FGMassBalance:

Classes
struct	Inputs

Public Member Functions
	FGMassBalance (FGFDMExec *)

void	AddPointMass (Element *el)

double	GetDeltaXYZcg (int axis) const

const FGColumnVector3 &	GetDeltaXYZcg (void) const

double	GetEmptyWeight (void) const

const FGMatrix33 &	GetJ (void) const
	Returns the inertia matrix expressed in the body frame.

const FGMatrix33 &	GetJinv (void) const
	Returns the inverse of the inertia matrix expressed in the body frame.

double	GetMass (void) const

void	GetMassPropertiesReport (int i)

FGMatrix33	GetPointmassInertia (double mass_sl, const FGColumnVector3 &r) const
	Computes the inertia contribution of a pointmass. More...

const FGColumnVector3 &	GetPointMassMoment (void)

double	GetTotalPointMassWeight (void) const

double	GetWeight (void) const

double	GetXYZcg (int axis) const

const FGColumnVector3 &	GetXYZcg (void) const
	Returns the coordinates of the center of gravity expressed in the structural frame.

bool	InitModel (void) override

bool	Load (Element *el) override

bool	Run (bool Holding) override
	Runs the Mass Balance model; called by the Executive Can pass in a value indicating if the executive is directing the simulation to Hold. More...

void	SetAircraftBaseInertias (const FGMatrix33 &BaseJ)

void	SetBaseCG (const FGColumnVector3 &CG)

void	SetEmptyWeight (double EW)

FGColumnVector3	StructuralToBody (const FGColumnVector3 &r) const
	Conversion from the structural frame to the body frame. More...

Public Member Functions inherited from FGModel
	FGModel (FGFDMExec *)
	Constructor.

	~FGModel () override
	Destructor.

virtual SGPath	FindFullPathName (const SGPath &path) const

FGFDMExec *	GetExec (void)

const std::string &	GetName (void)

unsigned int	GetRate (void)
	Get the output rate for the model in frames.

bool	InitModel (void) override

void	SetPropertyManager (std::shared_ptr< FGPropertyManager > fgpm)

void	SetRate (unsigned int tt)
	Set the ouput rate for the model in frames.

Public Member Functions inherited from FGModelFunctions
std::string	GetFunctionStrings (const std::string &delimeter) const
	Gets the strings for the current set of functions. More...

std::string	GetFunctionValues (const std::string &delimeter) const
	Gets the function values. More...

std::shared_ptr< FGFunction >	GetPreFunction (const std::string &name)
	Get one of the "pre" function. More...

bool	Load (Element el, FGFDMExec fdmex, std::string prefix="")

void	PostLoad (Element el, FGFDMExec fdmex, std::string prefix="")

void	PreLoad (Element el, FGFDMExec fdmex, std::string prefix="")

void	RunPostFunctions (void)

void	RunPreFunctions (void)

Public Member Functions inherited from FGJSBBase
	FGJSBBase ()
	Constructor for FGJSBBase.

virtual	~FGJSBBase ()
	Destructor for FGJSBBase.

void	disableHighLighting (void)
	Disables highlighting in the console output.

Public Attributes
struct JSBSim::FGMassBalance::Inputs	in

Additional Inherited Members
Public Types inherited from FGJSBBase
enum	{ eL = 1 , eM , eN }
	Moments L, M, N.

enum	{ eP = 1 , eQ , eR }
	Rates P, Q, R.

enum	{ eU = 1 , eV , eW }
	Velocities U, V, W.

enum	{ eX = 1 , eY , eZ }
	Positions X, Y, Z.

enum	{ ePhi = 1 , eTht , ePsi }
	Euler angles Phi, Theta, Psi.

enum	{ eDrag = 1 , eSide , eLift }
	Stability axis forces, Drag, Side force, Lift.

enum	{ eRoll = 1 , ePitch , eYaw }
	Local frame orientation Roll, Pitch, Yaw.

enum	{ eNorth = 1 , eEast , eDown }
	Local frame position North, East, Down.

enum	{ eLat = 1 , eLong , eRad }
	Locations Radius, Latitude, Longitude.

enum	{ inNone = 0 , inDegrees , inRadians , inMeters , inFeet }
	Conversion specifiers.

Static Public Member Functions inherited from FGJSBBase
static const std::string &	GetVersion (void)
	Returns the version number of JSBSim. More...

static constexpr double	KelvinToFahrenheit (double kelvin)
	Converts from degrees Kelvin to degrees Fahrenheit. More...

static constexpr double	CelsiusToRankine (double celsius)
	Converts from degrees Celsius to degrees Rankine. More...

static constexpr double	RankineToCelsius (double rankine)
	Converts from degrees Rankine to degrees Celsius. More...

static constexpr double	KelvinToRankine (double kelvin)
	Converts from degrees Kelvin to degrees Rankine. More...

static constexpr double	RankineToKelvin (double rankine)
	Converts from degrees Rankine to degrees Kelvin. More...

static constexpr double	FahrenheitToCelsius (double fahrenheit)
	Converts from degrees Fahrenheit to degrees Celsius. More...

static constexpr double	CelsiusToFahrenheit (double celsius)
	Converts from degrees Celsius to degrees Fahrenheit. More...

static constexpr double	CelsiusToKelvin (double celsius)
	Converts from degrees Celsius to degrees Kelvin. More...

static constexpr double	KelvinToCelsius (double kelvin)
	Converts from degrees Kelvin to degrees Celsius. More...

static constexpr double	FeetToMeters (double measure)
	Converts from feet to meters. More...

static bool	EqualToRoundoff (double a, double b)
	Finite precision comparison. More...

static bool	EqualToRoundoff (float a, float b)
	Finite precision comparison. More...

static bool	EqualToRoundoff (float a, double b)
	Finite precision comparison. More...

static bool	EqualToRoundoff (double a, float b)
	Finite precision comparison. More...

static constexpr double	Constrain (double min, double value, double max)
	Constrain a value between a minimum and a maximum value.

static constexpr double	sign (double num)

Static Public Attributes inherited from FGJSBBase
static char	highint [5] = {27, '[', '1', 'm', '\0' }
	highlights text

static char	halfint [5] = {27, '[', '2', 'm', '\0' }
	low intensity text

static char	normint [6] = {27, '[', '2', '2', 'm', '\0' }
	normal intensity text

static char	reset [5] = {27, '[', '0', 'm', '\0' }
	resets text properties

static char	underon [5] = {27, '[', '4', 'm', '\0' }
	underlines text

static char	underoff [6] = {27, '[', '2', '4', 'm', '\0' }
	underline off

static char	fgblue [6] = {27, '[', '3', '4', 'm', '\0' }
	blue text

static char	fgcyan [6] = {27, '[', '3', '6', 'm', '\0' }
	cyan text

static char	fgred [6] = {27, '[', '3', '1', 'm', '\0' }
	red text

static char	fggreen [6] = {27, '[', '3', '2', 'm', '\0' }
	green text

static char	fgdef [6] = {27, '[', '3', '9', 'm', '\0' }
	default text

static short	debug_lvl = 1

Protected Member Functions inherited from FGModel
bool	Upload (Element *el, bool preLoad)
	Uploads this model in memory. More...

Static Protected Member Functions inherited from FGJSBBase
static std::string	CreateIndexedPropertyName (const std::string &Property, int index)

Protected Attributes inherited from FGModel
unsigned int	exe_ctr

FGFDMExec *	FDMExec

std::string	Name

std::shared_ptr< FGPropertyManager >	PropertyManager

unsigned int	rate

Protected Attributes inherited from FGModelFunctions
FGPropertyReader	LocalProperties

std::vector< std::shared_ptr< FGFunction > >	PostFunctions

std::vector< std::shared_ptr< FGFunction > >	PreFunctions

Static Protected Attributes inherited from FGJSBBase
static constexpr double	radtodeg = 180. / M_PI

static constexpr double	degtorad = M_PI / 180.

static constexpr double	hptoftlbssec = 550.0

static constexpr double	psftoinhg = 0.014138

static constexpr double	psftopa = 47.88

static constexpr double	fttom = 0.3048

static constexpr double	ktstofps = 1852./(3600*fttom)

static constexpr double	fpstokts = 1.0 / ktstofps

static constexpr double	inchtoft = 1.0/12.0

static constexpr double	m3toft3 = 1.0/(fttomfttomfttom)

static constexpr double	in3tom3 = inchtoftinchtoftinchtoft/m3toft3

static constexpr double	inhgtopa = 3386.38

static constexpr double	slugtolb = 32.174049
	Note that definition of lbtoslug by the inverse of slugtolb and not to a different constant you can also get from some tables will make lbtoslug*slugtolb == 1 up to the magnitude of roundoff. More...

static constexpr double	lbtoslug = 1.0/slugtolb

static constexpr double	kgtolb = 2.20462

static constexpr double	kgtoslug = 0.06852168

static const std::string	needed_cfg_version = "2.0"

static const std::string	JSBSim_version = JSBSIM_VERSION " " __DATE__ " " __TIME__

Member Function Documentation

◆ GetPointmassInertia()

FGMatrix33 GetPointmassInertia	(	double	mass_sl,
		const FGColumnVector3 &	r
	)		const

inline

Computes the inertia contribution of a pointmass.

Computes and returns the inertia matrix of a pointmass of mass slugs at the given vector r in the structural frame. The units should be for the mass in slug and the vector in the structural frame as usual in inches.

Parameters

mass_sl	the mass of this single pointmass given in slugs
r	the location of this single pointmass in the structural frame

Definition at line 161 of file FGMassBalance.h.

   {
     FGColumnVector3 v = StructuralToBody( r );
     FGColumnVector3 sv = mass_sl*v;
     double xx = sv(1)*v(1);
     double yy = sv(2)*v(2);
     double zz = sv(3)*v(3);
     double xy = -sv(1)*v(2);
     double xz = -sv(1)*v(3);
     double yz = -sv(2)*v(3);
     return FGMatrix33( yy+zz, xy, xz,
                        xy, xx+zz, yz,
                        xz, yz, xx+yy );
   }

◆ Run()

bool Run ( bool Holding )

overridevirtual

Runs the Mass Balance model; called by the Executive Can pass in a value indicating if the executive is directing the simulation to Hold.

Parameters

Holding if true, the executive has been directed to hold the sim from advancing time. Some models may ignore this flag, such as the Input model, which may need to be active to listen on a socket for the "Resume" command to be given.

Returns: false if no error

Reimplemented from FGModel.

Definition at line 181 of file FGMassBalance.cpp.

 {
   double denom, k1, k2, k3, k4, k5, k6;
   double Ixx, Iyy, Izz, Ixy, Ixz, Iyz;
  
   if (FGModel::Run(Holding)) return true;
   if (Holding) return false;
  
   RunPreFunctions();
  
   double ChildFDMWeight = 0.0;
   for (size_t fdm=0; fdm<FDMExec->GetFDMCount(); fdm++) {
     if (FDMExec->GetChildFDM(fdm)->mated) ChildFDMWeight += FDMExec->GetChildFDM(fdm)->exec->GetMassBalance()->GetWeight();
   }
  
   Weight = EmptyWeight + in.TanksWeight + GetTotalPointMassWeight()
     + in.GasMass*slugtolb + ChildFDMWeight;
  
   Mass = lbtoslug*Weight;
  
 // Calculate new CG
  
   vXYZcg = (EmptyWeight*vbaseXYZcg
             + GetPointMassMoment()
             + in.TanksMoment
             + in.GasMoment) / Weight;
  
   // Track frame-by-frame delta CG, and move the EOM-tracked location
   // by this amount.
   if (vLastXYZcg.Magnitude() == 0.0) vLastXYZcg = vXYZcg;
   vDeltaXYZcg = vXYZcg - vLastXYZcg;
   vDeltaXYZcgBody = StructuralToBody(vLastXYZcg) - StructuralToBody(vXYZcg);
   vLastXYZcg = vXYZcg;
  
   // Compensate displacements of the structural frame when the mass distribution
   // is modified while the aircraft is in contact with the ground.
   if (FDMExec->GetHoldDown() || in.WOW)
     Propagate->NudgeBodyLocation(vDeltaXYZcgBody);
  
 // 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);
  
 // Calculate inertia matrix inverse (ref. Stevens and Lewis, "Flight Control &
 // Simulation")
  
   k1 = (Iyy*Izz - Iyz*Iyz);
   k2 = (Iyz*Ixz + Ixy*Izz);
   k3 = (Ixy*Iyz + Iyy*Ixz);
  
   denom = 1.0/(Ixx*k1 - Ixy*k2 - Ixz*k3 );
   k1 = k1*denom;
   k2 = k2*denom;
   k3 = k3*denom;
   k4 = (Izz*Ixx - Ixz*Ixz)*denom;
   k5 = (Ixy*Ixz + Iyz*Ixx)*denom;
   k6 = (Ixx*Iyy - Ixy*Ixy)*denom;
  
   mJinv = { k1, k2, k3,
             k2, k4, k5,
             k3, k5, k6 };
  
   RunPostFunctions();
  
   Debug(0);
  
   return false;
 }

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◆ StructuralToBody()

FGColumnVector3 StructuralToBody ( const FGColumnVector3 & r ) const

Conversion from the structural frame to the body frame.

Converts the location given in the structural frame coordinate system to the body frame. The units of the structural frame are assumed to be in inches. The unit of the result is in ft.

Parameters

r	vector coordinate in the structural reference frame (X positive aft, measurements in inches).

Returns: vector coordinate in the body frame, in feet.

Definition at line 364 of file FGMassBalance.cpp.

 {
   // Under the assumption that in the structural frame the:
   //
   // - X-axis is directed afterwards,
   // - Y-axis is directed towards the right,
   // - Z-axis is directed upwards,
   //
   // (as documented in http://jsbsim.sourceforge.net/JSBSimCoordinates.pdf)
   // we have to subtract first the center of gravity of the plane which
   // is also defined in the structural frame:
   //
   //   FGColumnVector3 cgOff = r - vXYZcg;
   //
   // Next, we do a change of units:
   //
   //   cgOff *= inchtoft;
   //
   // And then a 180 degree rotation is done about the Y axis so that the:
   //
   // - X-axis is directed forward,
   // - Y-axis is directed towards the right,
   // - Z-axis is directed downward.
   //
   // This is needed because the structural and body frames are 180 degrees apart.
  
   return FGColumnVector3(inchtoft*(vXYZcg(1)-r(1)),
                          inchtoft*(r(2)-vXYZcg(2)),
                          inchtoft*(vXYZcg(3)-r(3)));
 }

The documentation for this class was generated from the following files:

Detailed Description

Configuration File Format for <mass_balance> Section:

Classes

Public Member Functions

Public Attributes

Additional Inherited Members

Member Function Documentation

◆ GetPointmassInertia()

◆ Run()

◆ StructuralToBody()