FGBallonet Class Reference

Detailed Description

Models a ballonet inside a gas cell.

Not intended to be used outside FGGasCell. See FGGasCell for the configuration file format.

Author

Anders Gidenstam

Definition at line 302 of file FGGasCell.h.

#include <FGGasCell.h>

Inheritance diagram for FGBallonet:

Collaboration diagram for FGBallonet:

Public Member Functions
	FGBallonet (FGFDMExec *exec, Element *el, unsigned int num, FGGasCell *parent, const struct FGGasCell::Inputs &input)
void	Calculate (double dt)
	Runs the ballonet model; called by FGGasCell.
double	GetHeatFlow (void) const
	Get the current heat flow into the ballonet.
const FGMatrix33 &	GetInertia (void) const
	Get the moments of inertia of the ballonet.
double	GetMass (void) const
	Get the current mass of the ballonets.
double	GetVolume (void) const
	Get the current volume of the ballonet.
double	GetXYZ (int idx) const
	Get the center of gravity location of the ballonet.
const FGColumnVector3 &	GetXYZ (void) const
	Get the center of gravity location of the ballonet.
Public Member Functions inherited from FGJSBBase
	FGJSBBase ()
	Constructor for FGJSBBase.
virtual	~FGJSBBase ()
	Destructor for FGJSBBase.

Public Attributes
const struct FGGasCell::Inputs &	in

Additional Inherited Members
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 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	CelsiusToRankine (double celsius)
	Converts from degrees Celsius to degrees Rankine.
static constexpr double	Constrain (double min, double value, double max)
	Constrain a value between a minimum and a maximum value.
static bool	EqualToRoundoff (double a, double b)
	Finite precision comparison.
static bool	EqualToRoundoff (double a, float b)
	Finite precision comparison.
static bool	EqualToRoundoff (float a, double b)
	Finite precision comparison.
static bool	EqualToRoundoff (float a, float b)
	Finite precision comparison.
static constexpr double	FahrenheitToCelsius (double fahrenheit)
	Converts from degrees Fahrenheit to degrees Celsius.
static constexpr double	FeetToMeters (double measure)
	Converts from feet to meters.
static const std::string &	GetVersion (void)
	Returns the version number of JSBSim.
static constexpr double	KelvinToCelsius (double kelvin)
	Converts from degrees Kelvin to degrees Celsius.
static constexpr double	KelvinToFahrenheit (double kelvin)
	Converts from degrees Kelvin to degrees Fahrenheit.
static constexpr double	KelvinToRankine (double kelvin)
	Converts from degrees Kelvin to degrees Rankine.
static constexpr double	RankineToCelsius (double rankine)
	Converts from degrees Rankine to degrees Celsius.
static constexpr double	RankineToKelvin (double rankine)
	Converts from degrees Rankine to degrees Kelvin.
static constexpr double	sign (double num)
Static Public Attributes inherited from FGJSBBase
static short	debug_lvl = 1
Static Protected Member Functions inherited from FGJSBBase
static std::string	CreateIndexedPropertyName (const std::string &Property, int index)
Static Protected Attributes inherited from FGJSBBase
static constexpr double	degtorad = M_PI / 180.
static constexpr double	fpstokts = 1.0 / ktstofps
static constexpr double	fttom = 0.3048
static constexpr double	hptoftlbssec = 550.0
static constexpr double	in3tom3 = inchtoft*inchtoft*inchtoft/m3toft3
static constexpr double	inchtoft = 1.0/12.0
static constexpr double	inhgtopa = 3386.38
static const std::string	JSBSim_version = JSBSIM_VERSION " " __DATE__ " " __TIME__
static constexpr double	kgtolb = 2.20462
static constexpr double	kgtoslug = 0.06852168
static constexpr double	ktstofps = 1852./(3600*fttom)
static constexpr double	lbtoslug = 1.0/slugtolb
static constexpr double	m3toft3 = 1.0/(fttom*fttom*fttom)
static const std::string	needed_cfg_version = "2.0"
static constexpr double	psftoinhg = 0.014138
static constexpr double	psftopa = 47.88
static constexpr double	radtodeg = 180. / M_PI
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.

Constructor & Destructor Documentation

FGBallonet()

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

Definition at line 500 of file FGGasCell.cpp.

  : in(input)
{
  string token;
  Element* element;
 
  auto PropertyManager = exec->GetPropertyManager();
  MassBalance = exec->GetMassBalance();
 
  MaxVolume = MaxOverpressure = Temperature = Pressure =
    Contents = Volume = dVolumeIdeal = dU = 0.0;
  Xradius = Yradius = Zradius = Xwidth = Ywidth = Zwidth = 0.0;
  ValveCoefficient = ValveOpen = 0.0;
  BlowerInput = NULL;
  CellNum = num;
  Parent = parent;
 
  // NOTE: In the local system X points north, Y points east and Z points down.
  element = el->FindElement("location");
  if (element) {
    vXYZ = element->FindElementTripletConvertTo("IN");
  } else {
    XMLLogException err(el);
    err << "\nFatal Error: No location found for this ballonet.\n";
    throw err;
  }
  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.
    // FIXME: 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 {
      FGXMLLogging log(el, LogLevel::WARN);
      log << "Unsupported ballonet shape.\n";
      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 {
    XMLLogException err(el);
    err << "\nFatal Error: Ballonet shape must be given.\n";
    throw err;
  }
  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 {
      FGXMLLogging log(el, LogLevel::WARN);
      log << "Invalid initial ballonet fullness value.\n";
    }
  }
  if (el->FindElement("valve_coefficient")) {
    ValveCoefficient =
      el->FindElementValueAsNumberConvertTo("valve_coefficient",
                                            "FT4*SEC/SLUG");
    ValveCoefficient = max(ValveCoefficient, 0.0);
  }
 
  // Initialize state
  if (Temperature == 0.0) {
    Temperature = Parent->GetTemperature();
  }
  if (Pressure == 0.0) {
    Pressure = Parent->GetPressure();
  }
  if (Volume != 0.0) {
    // Calculate initial air 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 air content.
    Contents = Pressure * MaxVolume / (R * Temperature);
  }
 
  Volume = Contents * R * Temperature / Pressure;
 
  // Bind relevant properties
  string property_name, base_property_name;
  base_property_name = CreateIndexedPropertyName("buoyant_forces/gas-cell", Parent->GetIndex());
  base_property_name = CreateIndexedPropertyName(base_property_name + "/ballonet", CellNum);
 
  property_name = base_property_name + "/max_volume-ft3";
  PropertyManager->Tie( property_name, &MaxVolume);
  PropertyManager->GetNode(property_name)->setAttribute( SGPropertyNode::WRITE, false );
 
  property_name = base_property_name + "/temp-R";
  PropertyManager->Tie( property_name, &Temperature);
 
  property_name = base_property_name + "/pressure-psf";
  PropertyManager->Tie( property_name, &Pressure);
 
  property_name = base_property_name + "/volume-ft3";
  PropertyManager->Tie( property_name, &Volume);
 
  property_name = base_property_name + "/contents-mol";
  PropertyManager->Tie( property_name, &Contents);
 
  property_name = base_property_name + "/valve_open";
  PropertyManager->Tie( property_name, &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");
    }
  }
  // Read blower input function
  if (Element* blower = el->FindElement("blower_input")) {
    Element* function_element = blower->FindElement("function");
    BlowerInput = new FGFunction(exec, function_element);
  }
}

~FGBallonet()

~FGBallonet

(

)

Definition at line 669 of file FGGasCell.cpp.

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

Member Function Documentation

Calculate()

void Calculate

(

double

dt

)

Runs the ballonet model; called by FGGasCell.

Definition at line 684 of file FGGasCell.cpp.

{
  const double ParentPressure = Parent->GetPressure(); // [lbs/ft^2]
  const double AirPressure    = in.Pressure;           // [lbs/ft^2]
 
  const double OldTemperature = Temperature;
  const double OldPressure    = Pressure;
  unsigned int i;
 
  //-- Gas temperature --
 
  // The model is based on the ideal gas law.
  // However, it does look a bit fishy. Please verify.
  //   dT/dt = dU / (Cv n R)
  dU = 0.0;
  for (i = 0; i < HeatTransferCoeff.size(); i++) {
    dU += HeatTransferCoeff[i]->GetValue();
  }
  // dt is already accounted for in dVolumeIdeal.
  if (Contents > 0) {
    Temperature +=
      (dU * dt - Pressure * dVolumeIdeal) / (Cv_air * Contents * R);
  } else {
    Temperature = Parent->GetTemperature();
  }
 
  //-- Pressure --
  const double IdealPressure = Contents * R * Temperature / MaxVolume;
  // The pressure is at least that of the parent gas cell.
  Pressure = max(IdealPressure, ParentPressure);
 
  //-- Blower input --
  if (BlowerInput) {
    const double AddedVolume = BlowerInput->GetValue() * dt;
    if (AddedVolume > 0.0) {
      Contents += Pressure * AddedVolume / (R * Temperature);
    }
  }
 
  //-- Pressure relief and manual valving --
  // FIXME: Presently the effect of valving is computed using
  //        an ad hoc formula which might not be a good representation
  //        of reality.
  if ((ValveCoefficient > 0.0) &&
      ((ValveOpen > 0.0) || (Pressure > AirPressure + MaxOverpressure))) {
    const double DeltaPressure = Pressure - AirPressure;
    const double VolumeValved =
      ((Pressure > AirPressure + MaxOverpressure) ? 1.0 : ValveOpen) *
      ValveCoefficient * DeltaPressure * dt;
    // FIXME: Too small values of Contents sometimes leads to NaN.
    //        Currently the minimum is restricted to a safe value.
    Contents =
      max(1.0, Contents - Pressure * VolumeValved / (R * Temperature));
  }
 
  //-- Volume --
  Volume = Contents * R * Temperature / Pressure;
  dVolumeIdeal =
    Contents * R * (Temperature / Pressure - OldTemperature / OldPressure);
 
  // Compute the inertia of the ballonet.
  // Consider the ballonet as a shape of uniform density.
  // FIXME: If the ballonet isn't ellipsoid or cylindrical the inertia will
  //        be wrong.
  ballonetJ.InitMatrix();
  const double mass = Contents * M_air;
  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.
  ballonetJ(1,1) = Ixx;
  ballonetJ(2,2) = Iyy;
  ballonetJ(3,3) = Izz;
  // Transform the moments of inertia to the body frame.
  ballonetJ += MassBalance->GetPointmassInertia(GetMass(), GetXYZ());
}

Here is the call graph for this function:

GetHeatFlow()

double GetHeatFlow ( void  ) const

inline

Get the current heat flow into the ballonet.

Returns

heat flow in lbs ft / sec.

Definition at line 335 of file FGGasCell.h.

{return dU;}       // [lbs ft / sec]

GetInertia()

const FGMatrix33 & GetInertia ( void  ) const

inline

Get the moments of inertia of the ballonet.

Returns

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

Definition at line 328 of file FGGasCell.h.

{return ballonetJ;}

GetMass()

double GetMass ( void  ) const

inline

Get the current mass of the ballonets.

Returns

mass in slug.

Definition at line 323 of file FGGasCell.h.

{return Contents * M_air;}

Here is the caller graph for this function:

GetVolume()

double GetVolume ( void  ) const

inline

Get the current volume of the ballonet.

Returns

volume in ft³.

Definition at line 332 of file FGGasCell.h.

{return Volume;}

GetXYZ() [1/2]

double GetXYZ ( int  idx ) const

inline

Get the center of gravity location of the ballonet.

Returns

CoG location in the structural frame in inches.

Definition at line 319 of file FGGasCell.h.

{return vXYZ(idx);}

GetXYZ() [2/2]

const FGColumnVector3 & GetXYZ ( void  ) const

inline

Get the center of gravity location of the ballonet.

Returns

CoG location in the structural frame in inches.

Definition at line 316 of file FGGasCell.h.

{return vXYZ;}

Here is the caller graph for this function:

Member Data Documentation

in

const struct FGGasCell::Inputs& in

Definition at line 337 of file FGGasCell.h.

---

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

• FGGasCell.h
• FGGasCell.cpp