Detailed Description

Models a gas cell.

Author: Anders Gidenstam

Configuration File Format:

<buoyant_forces>
  <gas_cell type="{HYDROGEN | HELIUM | AIR}">
    <location unit="{M | IN}">
      <x> {number} </x>
      <y> {number} </y>
      <z> {number} </z>
    </location>
    <x_width unit="{M | IN}"> {number} </x_width>
    <y_radius unit="{M | IN}"> {number} </y_radius>
    <z_radius unit="{M | IN}"> {number} </z_radius>
    <max_overpressure unit="{PA | PSI}"> {number} </max_overpressure>
    <valve_coefficient unit="{M4*SEC/KG | FT4*SEC/SLUG}"> {number} </valve_coefficient>
    <fullness> {number} </fullness>  
    <heat>
      {heat transfer coefficients} [lbs ft / sec]
    </heat>
    <ballonet>
      <location unit="{M | IN}">
        <x> {number} </x>
        <y> {number} </y>
        <z> {number} </z>
      </location>
      <x_width unit="{M | IN}"> {number} </x_width>
      <y_radius unit="{M | IN}"> {number} </y_radius>
      <z_radius unit="{M | IN}"> {number} </z_radius>
      <max_overpressure unit="{PA | PSI}"> {number} </max_overpressure>
      <valve_coefficient unit="{M4*SEC/KG | FT4*SEC/SLUG}"> {number} </valve_coefficient>
      <fullness> {number} </fullness>  
      <heat>
       {heat transfer coefficients} [lb ft / (sec Rankine)]
      </heat>
      <blower_input>
       {input air flow function} [ft^3 / sec]
      </blower_input>
    </ballonet>
  </gas_cell>
</buoyant_forces>

Definition of the gas cell configuration file parameters:

type - One of HYDROGEN, HELIUM or AIR.
location - Location of cell center in the aircraft's structural frame. Currently this is were the forces of the cell is applied.
{x|y|z}_radius - Radius along in the respective direction (both ends).
{x|y|z}_width - Width in the respective direction. NOTE: A 'x', 'y', 'z'-radius/width combination must be specified.
fullness - Initial fullness of the cell, normally [0,1], values >1 initialize the cell at pressure.
max_overpressure - Maximum cell overpressure (excess is automatically valved off).
valve_coefficient - Capacity of the manual valve. The valve is considered to be located at the top of the cell. The valve coefficient determine the flow out of the cell according to: dVolume/dt = ValveCoefficient * DeltaPressure.
heat - Zero or more FGFunction:s describing the heat flow from the atmosphere into the gas cell. Unit: [lb ft / (sec Rankine)]. If there are no heat transfer functions at all the gas cell temperature will equal that of the surrounding atmosphere. A constant function returning 0 results in adiabatic behaviour.
ballonet - Zero or more ballonets, i.e. air bags inside the gas cell. Ballonets are used to maintain the volume of the gas cell and keep its internal pressure higher than that of the surrounding environment.
- location - Location of ballonet center in the aircraft's structural frame.
- {x|y|z}_radius - Radius along in the respective direction (both ends).
- {x|y|z}_width - Width in the respective direction.
- max_overpressure - Maximum ballonet overpressure (excess is automatically valved off).
- valve_coefficient - Capacity of the exit valve between the ballonet and the atmosphere. The valve coefficient determine the flow out of the cell according to: dVolume/dt = ValveCoefficient * DeltaPressure.
- heat - Zero or more FGFunction:s describing the heat flow from the enclosing gas cell into the ballonet. Unit: [lb ft / (sec Rankine)]
- blower_input - One FGFunction describing the air flow into the ballonet. Unit: [ft³ / sec] (at the temperature and pressure of the ballonet.)

Definition at line 166 of file FGGasCell.h.

#include <FGGasCell.h>

Inheritance diagram for FGGasCell:

Collaboration diagram for FGGasCell:

Classes
struct	Inputs

Public Member Functions
	FGGasCell (FGFDMExec exec, Element el, unsigned int num, const struct Inputs &input)
	Constructor.

void	Calculate (double dt)
	Runs the gas cell model; called by BuoyantForces.

int	GetIndex (void) const
	Get the index of this gas cell.

const FGMatrix33 &	GetInertia (void) const
	Get the moments of inertia of the gas cell (including any ballonets)

double	GetMass (void) const
	Get the current mass of the gas cell (including any ballonets)

const FGColumnVector3 &	GetMassMoment (void) const
	Get the moment due to mass of the gas cell (including any ballonets)

double	GetPressure (void) const
	Get the current gas pressure inside the gas cell.

double	GetTemperature (void) const
	Get the current gas temperature inside the gas cell.

double	GetXYZ (int idx) const
	Get the center of gravity location of the gas cell (including any ballonets)

const FGColumnVector3 &	GetXYZ (void) const
	Get the center of gravity location of the gas cell (including any ballonets)

Public Member Functions inherited from FGForce
	FGForce (FGFDMExec *FDMExec)
	Constructor.

virtual	~FGForce ()
	Destructor.

const FGColumnVector3 &	GetActingLocation (void) const

double	GetActingLocationX (void) const

double	GetActingLocationY (void) const

double	GetActingLocationZ (void) const

double	GetAnglesToBody (int axis) const

const FGColumnVector3 &	GetAnglesToBody (void) const

virtual const FGColumnVector3 &	GetBodyForces (void)

double	GetBodyXForce (void) const

double	GetBodyYForce (void) const

double	GetBodyZForce (void) const

const FGColumnVector3 &	GetLocation (void) const

double	GetLocationX (void) const

double	GetLocationY (void) const

double	GetLocationZ (void) const

const FGColumnVector3 &	GetMoments (void) const

double	GetPitch (void) const

TransformType	GetTransformType (void) const

double	GetYaw (void) const

void	SetActingLocation (const FGColumnVector3 &vv)

void	SetActingLocation (double x, double y, double z)
	Acting point of application.

void	SetActingLocationX (double x)

void	SetActingLocationY (double y)

void	SetActingLocationZ (double z)

void	SetAnglesToBody (const FGColumnVector3 &vv)

void	SetAnglesToBody (double broll, double bpitch, double byaw)

void	SetLocation (const FGColumnVector3 &vv)

void	SetLocation (double x, double y, double z)

void	SetLocationX (double x)

void	SetLocationY (double y)

void	SetLocationZ (double z)

void	SetPitch (double pitch)

void	SetTransformType (TransformType ii)

void	SetYaw (double yaw)

const FGMatrix33 &	Transform (void) const

void	UpdateCustomTransformMatrix (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
const struct Inputs &	in

Additional Inherited Members
Public Types inherited from FGForce
enum	TransformType { tNone , tWindBody , tLocalBody , tInertialBody , tCustom }

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Protected Attributes inherited from FGForce
FGFDMExec *	fdmex

std::shared_ptr< FGMassBalance >	MassBalance

FGMatrix33	mT

TransformType	ttype

FGColumnVector3	vActingXYZn

FGColumnVector3	vFn

FGColumnVector3	vMn

FGColumnVector3	vOrient

FGColumnVector3	vXYZn

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.

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__

Constructor & Destructor Documentation

◆ FGGasCell()

FGGasCell	(	FGFDMExec *	exec,
		Element *	el,
		unsigned int	num,
		const struct Inputs &	input
	)

Constructor.

Parameters

exec	Executive a pointer to the parent executive object
el	Pointer to configuration file XML node
num	Gas cell index number.

Definition at line 60 of file FGGasCell.cpp.

  : FGForce(exec), in(input)
{
  string token;
  Element* element;
 
  auto PropertyManager = exec->GetPropertyManager();
  MassBalance = exec->GetMassBalance();
 
  Buoyancy = MaxVolume = MaxOverpressure = Temperature = Pressure =
    Contents = Volume = dVolumeIdeal = 0.0;
  Xradius = Yradius = Zradius = Xwidth = Ywidth = Zwidth = 0.0;
  ValveCoefficient = ValveOpen = 0.0;
  CellNum = num;
 
  // NOTE: In the local system X points north, Y points east and Z points down.
  SetTransformType(FGForce::tLocalBody);
 
  type = el->GetAttributeValue("type");
  if      (type == "HYDROGEN") Type = ttHYDROGEN;
  else if (type == "HELIUM")   Type = ttHELIUM;
  else if (type == "AIR")      Type = ttAIR;
  else                         Type = ttUNKNOWN;
 
  element = el->FindElement("location");
  if (element) {
    vXYZ = element->FindElementTripletConvertTo("IN");
  } else {
    const string s("Fatal Error: No location found for this gas cell.");
    cerr << el->ReadFrom() << endl << s << endl;
    throw BaseException(s);
  }
  if ((el->FindElement("x_radius") || el->FindElement("x_width")) &&
      (el->FindElement("y_radius") || el->FindElement("y_width")) &&
      (el->FindElement("z_radius") || el->FindElement("z_width"))) {
 
    if (el->FindElement("x_radius")) {
      Xradius = el->FindElementValueAsNumberConvertTo("x_radius", "FT");
    }
    if (el->FindElement("y_radius")) {
      Yradius = el->FindElementValueAsNumberConvertTo("y_radius", "FT");
    }
    if (el->FindElement("z_radius")) {
      Zradius = el->FindElementValueAsNumberConvertTo("z_radius", "FT");
    }
 
    if (el->FindElement("x_width")) {
      Xwidth = el->FindElementValueAsNumberConvertTo("x_width", "FT");
    }
    if (el->FindElement("y_width")) {
      Ywidth = el->FindElementValueAsNumberConvertTo("y_width", "FT");
    }
    if (el->FindElement("z_width")) {
      Zwidth = el->FindElementValueAsNumberConvertTo("z_width", "FT");
    }
 
    // The volume is a (potentially) extruded ellipsoid.
    // However, currently only a few combinations of radius and width are
    // fully supported.
    if ((Xradius != 0.0) && (Yradius != 0.0) && (Zradius != 0.0) &&
        (Xwidth  == 0.0) && (Ywidth  == 0.0) && (Zwidth  == 0.0)) {
      // Ellipsoid volume.
      MaxVolume = 4.0  * M_PI * Xradius * Yradius * Zradius / 3.0;
    } else if  ((Xradius == 0.0) && (Yradius != 0.0) && (Zradius != 0.0) &&
                (Xwidth  != 0.0) && (Ywidth  == 0.0) && (Zwidth  == 0.0)) {
      // Cylindrical volume.
      MaxVolume = M_PI * Yradius * Zradius * Xwidth;
    } else {
      cerr << "Warning: Unsupported gas cell shape." << endl;
      MaxVolume =
        (4.0  * M_PI * Xradius * Yradius * Zradius / 3.0 +
         M_PI * Yradius * Zradius * Xwidth +
         M_PI * Xradius * Zradius * Ywidth +
         M_PI * Xradius * Yradius * Zwidth +
         2.0  * Xradius * Ywidth * Zwidth +
         2.0  * Yradius * Xwidth * Zwidth +
         2.0  * Zradius * Xwidth * Ywidth +
         Xwidth * Ywidth * Zwidth);
    }
  } else {
    const string s("Fatal Error: Gas cell shape must be given.");
    cerr << el->ReadFrom() << endl << s << endl;
    throw BaseException(s);
  }
  if (el->FindElement("max_overpressure")) {
    MaxOverpressure = el->FindElementValueAsNumberConvertTo("max_overpressure",
                                                            "LBS/FT2");
  }
  if (el->FindElement("fullness")) {
    const double Fullness = el->FindElementValueAsNumber("fullness");
    if (0 <= Fullness) {
      Volume = Fullness * MaxVolume;
    } else {
      cerr << "Warning: Invalid initial gas cell fullness value." << endl;
    }
  }
  if (el->FindElement("valve_coefficient")) {
    ValveCoefficient =
      el->FindElementValueAsNumberConvertTo("valve_coefficient",
                                            "FT4*SEC/SLUG");
    ValveCoefficient = max(ValveCoefficient, 0.0);
  }
 
  // Initialize state
  SetLocation(vXYZ);
 
  if (Temperature == 0.0) {
    Temperature = in.Temperature;
  }
  if (Pressure == 0.0) {
    Pressure = in.Pressure;
  }
  if (Volume != 0.0) {
    // Calculate initial gas content.
    Contents = Pressure * Volume / (R * Temperature);
 
    // Clip to max allowed value.
    const double IdealPressure = Contents * R * Temperature / MaxVolume;
    if (IdealPressure > Pressure + MaxOverpressure) {
      Contents = (Pressure + MaxOverpressure) * MaxVolume / (R * Temperature);
      Pressure = Pressure + MaxOverpressure;
    } else {
      Pressure = max(IdealPressure, Pressure);
    }
  } else {
    // Calculate initial gas content.
    Contents = Pressure * MaxVolume / (R * Temperature);
  }
 
  Volume = Contents * R * Temperature / Pressure;
  Mass = Contents * M_gas();
 
  // Bind relevant properties
  string property_name, base_property_name;
 
  base_property_name = CreateIndexedPropertyName("buoyant_forces/gas-cell", CellNum);
 
  property_name = base_property_name + "/max_volume-ft3";
  PropertyManager->Tie( property_name.c_str(), &MaxVolume);
  PropertyManager->GetNode()->SetWritable( property_name, false );
  property_name = base_property_name + "/temp-R";
  PropertyManager->Tie( property_name.c_str(), &Temperature);
  property_name = base_property_name + "/pressure-psf";
  PropertyManager->Tie( property_name.c_str(), &Pressure);
  property_name = base_property_name + "/volume-ft3";
  PropertyManager->Tie( property_name.c_str(), &Volume);
  property_name = base_property_name + "/buoyancy-lbs";
  PropertyManager->Tie( property_name.c_str(), &Buoyancy);
  property_name = base_property_name + "/contents-mol";
  PropertyManager->Tie( property_name.c_str(), &Contents);
  property_name = base_property_name + "/valve_open";
  PropertyManager->Tie( property_name.c_str(), &ValveOpen);
 
  Debug(0);
 
  // Read heat transfer coefficients
  if (Element* heat = el->FindElement("heat")) {
    Element* function_element = heat->FindElement("function");
    while (function_element) {
      HeatTransferCoeff.push_back(new FGFunction(exec, function_element));
      function_element = heat->FindNextElement("function");
    }
  }
 
  // Load ballonets if there are any
  if (Element* ballonet_element = el->FindElement("ballonet")) {
    while (ballonet_element) {
      Ballonet.push_back(new FGBallonet(exec,
                                        ballonet_element,
                                        Ballonet.size(),
                                        this, in));
      ballonet_element = el->FindNextElement("ballonet");
    }
  }
 
}

Here is the call graph for this function:

◆ ~FGGasCell()

~FGGasCell ( )

Definition at line 240 of file FGGasCell.cpp.

{
  unsigned int i;
 
  for (i = 0; i < HeatTransferCoeff.size(); i++) delete HeatTransferCoeff[i];
  HeatTransferCoeff.clear();
 
  for (i = 0; i < Ballonet.size(); i++) delete Ballonet[i];
  Ballonet.clear();
 
  Debug(1);
}

Member Function Documentation

◆ Calculate()

void Calculate ( double dt )

Runs the gas cell model; called by BuoyantForces.

Definition at line 255 of file FGGasCell.cpp.

{
  const double AirTemperature = in.Temperature;  // [Rankine]
  const double AirPressure    = in.Pressure;     // [lbs/ft^2]
  const double AirDensity     = in.Density;      // [slug/ft^3]
  const double g = in.gravity;                   // [lbs/slug]
 
  const double OldTemperature = Temperature;
  const double OldPressure    = Pressure;
  unsigned int i;
  const size_t no_ballonets = Ballonet.size();
 
  //-- Read ballonet state --
  // NOTE: This model might need a more proper integration technique.
  double BallonetsVolume = 0.0;
  double BallonetsHeatFlow = 0.0;
  for (i = 0; i < no_ballonets; i++) {
    BallonetsVolume   += Ballonet[i]->GetVolume();
    BallonetsHeatFlow += Ballonet[i]->GetHeatFlow();
  }
 
  //-- Gas temperature --
 
  if (!HeatTransferCoeff.empty()) {
    // The model is based on the ideal gas law.
    // However, it does look a bit fishy. Please verify.
    //   dT/dt = dU / (Cv n R)
    double dU = 0.0;
    for (i = 0; i < HeatTransferCoeff.size(); i++) {
      dU += HeatTransferCoeff[i]->GetValue();
    }
    // Don't include dt when accounting for adiabatic expansion/contraction.
    // The rate of adiabatic cooling looks about right: ~5.4 Rankine/1000ft.
    if (Contents > 0) {
      Temperature +=
        (dU * dt - Pressure * dVolumeIdeal - BallonetsHeatFlow) /
        (Cv_gas() * Contents * R);
    } else {
      Temperature = AirTemperature;
    }
  } else {
    // No simulation of complex temperature changes.
    // Note: Making the gas cell behave adiabatically might be a better
    // option.
    Temperature = AirTemperature;
  }
 
  //-- Pressure --
  const double IdealPressure =
    Contents * R * Temperature / (MaxVolume - BallonetsVolume);
  if (IdealPressure > AirPressure + MaxOverpressure) {
    Pressure = AirPressure + MaxOverpressure;
  } else {
    Pressure = max(IdealPressure, AirPressure);
  }
 
  //-- Manual valving --
 
  // FIXME: Presently the effect of manual valving is computed using
  //        an ad hoc formula which might not be a good representation
  //        of reality.
  if ((ValveCoefficient > 0.0) && (ValveOpen > 0.0)) {
    // First compute the difference in pressure between the gas in the
    // cell and the air above it.
    // FixMe: CellHeight should depend on current volume.
    const double CellHeight = 2 * Zradius + Zwidth;                   // [ft]
    const double GasMass    = Contents * M_gas();                     // [slug]
    const double GasVolume  = Contents * R * Temperature / Pressure;  // [ft^3]
    const double GasDensity = GasMass / GasVolume;
    const double DeltaPressure =
      Pressure + CellHeight * g * (AirDensity - GasDensity) - AirPressure;
    const double VolumeValved =
      ValveOpen * ValveCoefficient * DeltaPressure * dt;
    Contents =
      max(1E-8, Contents - Pressure * VolumeValved / (R * Temperature));
  }
 
  //-- Update ballonets. --
  // Doing that here should give them the opportunity to react to the
  // new pressure.
  BallonetsVolume = 0.0;
  for (i = 0; i < no_ballonets; i++) {
    Ballonet[i]->Calculate(dt);
    BallonetsVolume += Ballonet[i]->GetVolume();
  }
 
  //-- Automatic safety valving. --
  if (Contents * R * Temperature / (MaxVolume - BallonetsVolume) >
      AirPressure + MaxOverpressure) {
    // Gas is automatically valved. Valving capacity is assumed to be infinite.
    // FIXME: This could/should be replaced by damage to the gas cell envelope.
    Contents =
      (AirPressure + MaxOverpressure) *
      (MaxVolume - BallonetsVolume) / (R * Temperature);
  }
 
  //-- Volume --
  Volume = Contents * R * Temperature / Pressure + BallonetsVolume;
  dVolumeIdeal =
    Contents * R * (Temperature / Pressure - OldTemperature / OldPressure);
 
  //-- Current buoyancy --
  // The buoyancy is computed using the atmospheres local density.
  Buoyancy = Volume * AirDensity * g;
 
  // Note: This is gross buoyancy. The weight of the gas itself and
  // any ballonets is not deducted here as the effects of the gas mass
  // is handled by FGMassBalance.
  vFn = {0.0, 0.0, - Buoyancy};
 
  // Compute the inertia of the gas cell.
  // Consider the gas cell as a shape of uniform density.
  // FIXME: If the cell isn't ellipsoid or cylindrical the inertia will
  //        be wrong.
  gasCellJ.InitMatrix();
  const double mass = Contents * M_gas();
  double Ixx, Iyy, Izz;
  if ((Xradius != 0.0) && (Yradius != 0.0) && (Zradius != 0.0) &&
      (Xwidth  == 0.0) && (Ywidth  == 0.0) && (Zwidth  == 0.0)) {
    // Ellipsoid volume.
    Ixx = (1.0 / 5.0) * mass * (Yradius*Yradius + Zradius*Zradius);
    Iyy = (1.0 / 5.0) * mass * (Xradius*Xradius + Zradius*Zradius);
    Izz = (1.0 / 5.0) * mass * (Xradius*Xradius + Yradius*Yradius);
  } else if  ((Xradius == 0.0) && (Yradius != 0.0) && (Zradius != 0.0) &&
              (Xwidth  != 0.0) && (Ywidth  == 0.0) && (Zwidth  == 0.0)) {
    // Cylindrical volume (might not be valid with an elliptical cross-section).
    Ixx = (1.0 / 2.0) * mass * Yradius * Zradius;
    Iyy =
      (1.0 / 4.0) * mass * Yradius * Zradius +
      (1.0 / 12.0) * mass * Xwidth * Xwidth;
    Izz =
      (1.0 / 4.0) * mass * Yradius * Zradius +
      (1.0 / 12.0) * mass * Xwidth * Xwidth;
  } else {
    // Not supported. Revert to pointmass model.
    Ixx = Iyy = Izz = 0.0;
  }
  // The volume is symmetric, so Ixy = Ixz = Iyz = 0.
  gasCellJ(1,1) = Ixx;
  gasCellJ(2,2) = Iyy;
  gasCellJ(3,3) = Izz;
  Mass = mass;
  // Transform the moments of inertia to the body frame.
  gasCellJ += MassBalance->GetPointmassInertia(Mass, GetXYZ());
 
  gasCellM.InitMatrix();
  gasCellM(eX) +=
    GetXYZ(eX) * Mass*slugtolb;
  gasCellM(eY) +=
    GetXYZ(eY) * Mass*slugtolb;
  gasCellM(eZ) +=
    GetXYZ(eZ) * Mass*slugtolb;
 
  if (no_ballonets > 0) {
    // Add the mass, moment and inertia of any ballonets.
    for (i = 0; i < no_ballonets; i++) {
      Mass += Ballonet[i]->GetMass();
 
      // Add ballonet moments due to mass (in the structural frame).
      gasCellM(eX) +=
        Ballonet[i]->GetXYZ(eX) * Ballonet[i]->GetMass()*slugtolb;
      gasCellM(eY) +=
        Ballonet[i]->GetXYZ(eY) * Ballonet[i]->GetMass()*slugtolb;
      gasCellM(eZ) +=
        Ballonet[i]->GetXYZ(eZ) * Ballonet[i]->GetMass()*slugtolb;
 
      gasCellJ += Ballonet[i]->GetInertia();
    }
  }
}

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◆ GetIndex()

int GetIndex ( void ) const

inline

Get the index of this gas cell.

Returns: gas cell index.

Definition at line 190 of file FGGasCell.h.

190{return CellNum;}

◆ GetInertia()

const FGMatrix33 & GetInertia ( void ) const

inline

Get the moments of inertia of the gas cell (including any ballonets)

Returns: moments of inertia matrix in the body frame in slug ft².

Definition at line 209 of file FGGasCell.h.

209{return gasCellJ;}

◆ GetMass()

double GetMass ( void ) const

inline

Get the current mass of the gas cell (including any ballonets)

Returns: gas mass in slug.

Definition at line 204 of file FGGasCell.h.

204{return Mass;}

◆ GetMassMoment()

const FGColumnVector3 & GetMassMoment ( void ) const

inline

Get the moment due to mass of the gas cell (including any ballonets)

Note that the buoyancy of the gas cell is handled separately by the FGForce part and not included here.

Returns: moment vector in the structural frame in lbs in.

Definition at line 216 of file FGGasCell.h.

216{return gasCellM;}

◆ GetPressure()

double GetPressure ( void ) const

inline

Get the current gas pressure inside the gas cell.

Returns: gas pressure in lbs / ft².

Definition at line 224 of file FGGasCell.h.

224{return Pressure;}

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◆ GetTemperature()

double GetTemperature ( void ) const

inline

Get the current gas temperature inside the gas cell.

Returns: gas temperature in Rankine.

Definition at line 220 of file FGGasCell.h.

220{return Temperature;}

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◆ GetXYZ() [1/2]

double GetXYZ ( int idx ) const

inline

Get the center of gravity location of the gas cell (including any ballonets)

Returns: CoG location in the structural frame in inches.

Definition at line 200 of file FGGasCell.h.

200{return vXYZ(idx);}

◆ GetXYZ() [2/2]

const FGColumnVector3 & GetXYZ ( void ) const

inline

Get the center of gravity location of the gas cell (including any ballonets)

Returns: CoG location in the structural frame in inches.

Definition at line 195 of file FGGasCell.h.

195{return vXYZ;}

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Member Data Documentation

◆ in

const struct Inputs& in

Definition at line 226 of file FGGasCell.h.

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

Detailed Description

Configuration File Format:

Classes

Public Member Functions

Public Attributes

Additional Inherited Members

Constructor & Destructor Documentation

◆ FGGasCell()

◆ ~FGGasCell()

Member Function Documentation

◆ Calculate()

◆ GetIndex()

◆ GetInertia()

◆ GetMass()

◆ GetMassMoment()

◆ GetPressure()

◆ GetTemperature()

◆ GetXYZ() [1/2]

◆ GetXYZ() [2/2]

Member Data Documentation

◆ in