flightgear/src/FDM/JSBSim/models/FGLGear.cpp

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

 Module:       FGLGear.cpp
 Author:       Jon S. Berndt
               Norman H. Princen
 Date started: 11/18/99
 Purpose:      Encapsulates the landing gear elements
 Called by:    FGAircraft

 ------------- Copyright (C) 1999  Jon S. Berndt (jsb@hal-pc.org) -------------

 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU Lesser General Public License as published by the Free Software
 Foundation; either version 2 of the License, or (at your option) any later
 version.

 This program is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
 details.

 You should have received a copy of the GNU Lesser General Public License along with
 this program; if not, write to the Free Software Foundation, Inc., 59 Temple
 Place - Suite 330, Boston, MA  02111-1307, USA.

 Further information about the GNU Lesser General Public License can also be found on
 the world wide web at http://www.gnu.org.

FUNCTIONAL DESCRIPTION
--------------------------------------------------------------------------------

HISTORY
--------------------------------------------------------------------------------
11/18/99   JSB   Created
01/30/01   NHP   Extended gear model to properly simulate steering and braking

/%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
INCLUDES
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

#include "FGLGear.h"

namespace JSBSim {

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
DEFINITIONS
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
GLOBAL DATA
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

static const char *IdSrc = "$Id$";
static const char *IdHdr = ID_LGEAR;

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
CLASS IMPLEMENTATION
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

FGLGear::FGLGear(Element* el, FGFDMExec* fdmex, int number) : Exec(fdmex),
                 GearNumber(number)
{
  Element *force_table=0;
  Element *dampCoeff=0;
  Element *dampCoeffRebound=0;
  string force_type="";

  kSpring = bDamp = bDampRebound = dynamicFCoeff = staticFCoeff = rollingFCoeff = maxSteerAngle = 0;
  sSteerType = sBrakeGroup = sSteerType = "";
  isRetractable = 0;
  eDampType = dtLinear;
  eDampTypeRebound = dtLinear;

  name = el->GetAttributeValue("name");
  sContactType = el->GetAttributeValue("type");
  if (sContactType == "BOGEY") {
    eContactType = ctBOGEY;
  } else if (sContactType == "STRUCTURE") {
    eContactType = ctSTRUCTURE;
  } else {
    eContactType = ctUNKNOWN;
  }

  if (el->FindElement("spring_coeff"))
    kSpring = el->FindElementValueAsNumberConvertTo("spring_coeff", "LBS/FT");
  if (el->FindElement("damping_coeff")) {
    dampCoeff = el->FindElement("damping_coeff");
    if (dampCoeff->GetAttributeValue("type") == "SQUARE") {
      eDampType = dtSquare; // default is dtLinear
      bDamp   = el->FindElementValueAsNumberConvertTo("damping_coeff", "LBS/FT/SEC2");
    } else {
      bDamp   = el->FindElementValueAsNumberConvertTo("damping_coeff", "LBS/FT/SEC");
    }
  }

  if (el->FindElement("damping_coeff_rebound")) {
    dampCoeffRebound = el->FindElement("damping_coeff_rebound");
    if (dampCoeffRebound->GetAttributeValue("type") == "SQUARE") {
      eDampTypeRebound = dtSquare; // default is dtLinear
      bDampRebound   = el->FindElementValueAsNumberConvertTo("damping_coeff_rebound", "LBS/FT/SEC2");
    } else {
      bDampRebound   = el->FindElementValueAsNumberConvertTo("damping_coeff_rebound", "LBS/FT/SEC");
    }
  } else {
    bDampRebound   = bDamp;
    eDampTypeRebound = eDampType;
  }

  if (el->FindElement("dynamic_friction"))
    dynamicFCoeff = el->FindElementValueAsNumber("dynamic_friction");
  if (el->FindElement("static_friction"))
    staticFCoeff = el->FindElementValueAsNumber("static_friction");
  if (el->FindElement("rolling_friction"))
    rollingFCoeff = el->FindElementValueAsNumber("rolling_friction");
  if (el->FindElement("max_steer"))
    maxSteerAngle = el->FindElementValueAsNumberConvertTo("max_steer", "DEG");
  if (el->FindElement("retractable"))
    isRetractable = ((unsigned int)el->FindElementValueAsNumber("retractable"))>0.0?true:false;

  ForceY_Table = 0;
  force_table = el->FindElement("table");
  while (force_table) {
    force_type = force_table->GetAttributeValue("type");
    if (force_type == "CORNERING_COEFF") {
      ForceY_Table = new FGTable(Exec->GetPropertyManager(), force_table);
    } else {
      cerr << "Undefined force table for " << name << " contact point" << endl;
    }
    force_table = el->FindNextElement("table");
  }

  sBrakeGroup = el->FindElementValue("brake_group");

  if (maxSteerAngle == 360) sSteerType = "CASTERED";
  else if (maxSteerAngle == 0.0) sSteerType = "FIXED";
  else sSteerType = "STEERABLE";

  Element* element = el->FindElement("location");
  if (element) vXYZ = element->FindElementTripletConvertTo("IN");
  else {cerr << "No location given for contact " << name << endl; exit(-1);}

  if      (sBrakeGroup == "LEFT"  ) eBrakeGrp = bgLeft;
  else if (sBrakeGroup == "RIGHT" ) eBrakeGrp = bgRight;
  else if (sBrakeGroup == "CENTER") eBrakeGrp = bgCenter;
  else if (sBrakeGroup == "NOSE"  ) eBrakeGrp = bgNose;
  else if (sBrakeGroup == "TAIL"  ) eBrakeGrp = bgTail;
  else if (sBrakeGroup == "NONE"  ) eBrakeGrp = bgNone;
  else if (sBrakeGroup.empty()    ) {eBrakeGrp = bgNone;
                                     sBrakeGroup = "NONE (defaulted)";}
  else {
    cerr << "Improper braking group specification in config file: "
         << sBrakeGroup << " is undefined." << endl;
  }

  if      (sSteerType == "STEERABLE") eSteerType = stSteer;
  else if (sSteerType == "FIXED"    ) eSteerType = stFixed;
  else if (sSteerType == "CASTERED" ) eSteerType = stCaster;
  else if (sSteerType.empty()       ) {eSteerType = stFixed;
                                       sSteerType = "FIXED (defaulted)";}
  else {
    cerr << "Improper steering type specification in config file: "
         << sSteerType << " is undefined." << endl;
  }

  RFRV = 0.7;  // Rolling force relaxation velocity, default value
  SFRV = 0.7;  // Side force relaxation velocity, default value

  Element* relax_vel = el->FindElement("relaxation_velocity");
  if (relax_vel) {
    if (relax_vel->FindElement("rolling")) {
      RFRV = relax_vel->FindElementValueAsNumberConvertTo("rolling", "FT/SEC");
    }
    if (relax_vel->FindElement("side")) {
      SFRV = relax_vel->FindElementValueAsNumberConvertTo("side", "FT/SEC");
    }
  }

  State = Exec->GetState();
  LongForceLagFilterCoeff = 1/State->Getdt(); // default longitudinal force filter coefficient
  LatForceLagFilterCoeff  = 1/State->Getdt(); // default lateral force filter coefficient

  Element* force_lag_filter_elem = el->FindElement("force_lag_filter");
  if (force_lag_filter_elem) {
    if (force_lag_filter_elem->FindElement("rolling")) {
      LongForceLagFilterCoeff = force_lag_filter_elem->FindElementValueAsNumber("rolling");
    }
    if (force_lag_filter_elem->FindElement("side")) {
      LatForceLagFilterCoeff = force_lag_filter_elem->FindElementValueAsNumber("side");
    }
  }

  WheelSlipLagFilterCoeff = 1/State->Getdt();

  Element *wheel_slip_angle_lag_elem = el->FindElement("wheel_slip_filter");
  if (wheel_slip_angle_lag_elem) {
    WheelSlipLagFilterCoeff = wheel_slip_angle_lag_elem->GetDataAsNumber();
  }
  
  GearUp = false;
  GearDown = true;
  GearPos  = 1.0;
  useFCSGearPos = false;
  Servicable = true;

// Add some AI here to determine if gear is located properly according to its
// brake group type ??

  State       = Exec->GetState();
  Aircraft    = Exec->GetAircraft();
  Propagate   = Exec->GetPropagate();
  Auxiliary   = Exec->GetAuxiliary();
  FCS         = Exec->GetFCS();
  MassBalance = Exec->GetMassBalance();

  WOW = lastWOW = false;
  ReportEnable = true;
  FirstContact = false;
  StartedGroundRun = false;
  TakeoffReported = LandingReported = false;
  LandingDistanceTraveled = TakeoffDistanceTraveled = TakeoffDistanceTraveled50ft = 0.0;
  MaximumStrutForce = MaximumStrutTravel = 0.0;
  SideForce = RollingForce = 0.0;
  SinkRate = GroundSpeed = 0.0;

  vWhlBodyVec = MassBalance->StructuralToBody(vXYZ);

  vLocalGear = Propagate->GetTb2l() * vWhlBodyVec;

  compressLength  = 0.0;
  compressSpeed   = 0.0;
  brakePct        = 0.0;
  maxCompLen      = 0.0;

  WheelSlip = 0.0;
  TirePressureNorm = 1.0;

  SideWhlVel    = 0.0;
  RollingWhlVel = 0.0;

  SinWheel = 0.0;
  CosWheel = 0.0;

  prevSlipIn  = 0.0;
  prevSlipOut = 0.0;

  Debug(0);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

FGLGear::FGLGear(const FGLGear& lgear)
{
  GearNumber = lgear.GearNumber;
  State    = lgear.State;
  Aircraft = lgear.Aircraft;
  Propagate = lgear.Propagate;
  Auxiliary = lgear.Auxiliary;
  Exec     = lgear.Exec;
  FCS      = lgear.FCS;
  MassBalance = lgear.MassBalance;

  vXYZ = lgear.vXYZ;
  vMoment = lgear.vMoment;
  vWhlBodyVec = lgear.vWhlBodyVec;
  vLocalGear = lgear.vLocalGear;

  WOW                = lgear.WOW;
  lastWOW            = lgear.lastWOW;
  ReportEnable       = lgear.ReportEnable;
  FirstContact       = lgear.FirstContact;
  StartedGroundRun   = lgear.StartedGroundRun;
  LandingDistanceTraveled   = lgear.LandingDistanceTraveled;
  TakeoffDistanceTraveled   = lgear.TakeoffDistanceTraveled;
  TakeoffDistanceTraveled50ft   = lgear.TakeoffDistanceTraveled50ft;
  MaximumStrutForce  = lgear.MaximumStrutForce;
  MaximumStrutTravel = lgear.MaximumStrutTravel;
  SideForce          = lgear.SideForce;
  RollingForce       = lgear.RollingForce;

  kSpring         = lgear.kSpring;
  bDamp           = lgear.bDamp;
  bDampRebound    = lgear.bDampRebound;
  compressLength  = lgear.compressLength;
  compressSpeed   = lgear.compressSpeed;
  staticFCoeff    = lgear.staticFCoeff;
  dynamicFCoeff   = lgear.dynamicFCoeff;
  rollingFCoeff   = lgear.rollingFCoeff;
  brakePct        = lgear.brakePct;
  maxCompLen      = lgear.maxCompLen;
  SinkRate        = lgear.SinkRate;
  GroundSpeed     = lgear.GroundSpeed;
  LandingReported = lgear.LandingReported;
  TakeoffReported = lgear.TakeoffReported;
  name            = lgear.name;
  sSteerType      = lgear.sSteerType;
  sRetractable    = lgear.sRetractable;
  sContactType    = lgear.sContactType;
  eContactType    = lgear.eContactType;
  sBrakeGroup     = lgear.sBrakeGroup;
  eSteerType      = lgear.eSteerType;
  eBrakeGrp       = lgear.eBrakeGrp;
  maxSteerAngle   = lgear.maxSteerAngle;
  isRetractable   = lgear.isRetractable;
  GearUp          = lgear.GearUp;
  GearDown        = lgear.GearDown;
  GearPos         = lgear.GearPos;
  useFCSGearPos   = lgear.useFCSGearPos;
  WheelSlip       = lgear.WheelSlip;
  TirePressureNorm = lgear.TirePressureNorm;
  Servicable      = lgear.Servicable;
  ForceY_Table    = lgear.ForceY_Table;
  CosWheel        = lgear.CosWheel;
  SinWheel        = lgear.SinWheel;
  prevOut         = lgear.prevOut;
  prevIn          = lgear.prevIn;
  prevSlipIn      = lgear.prevSlipIn;
  prevSlipOut     = lgear.prevSlipOut;
  RFRV            = lgear.RFRV;
  SFRV            = lgear.SFRV;
  LongForceLagFilterCoeff = lgear.LongForceLagFilterCoeff;
  LatForceLagFilterCoeff = lgear.LatForceLagFilterCoeff;
  WheelSlipLagFilterCoeff = lgear.WheelSlipLagFilterCoeff;
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

FGLGear::~FGLGear()
{
  Debug(1);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

FGColumnVector3& FGLGear::Force(void)
{
  double t = Exec->GetState()->Getsim_time();
  dT = State->Getdt()*Exec->GetGroundReactions()->GetRate();

  vForce.InitMatrix();
  vMoment.InitMatrix();

  if (isRetractable) ComputeRetractionState();

  if (!GearDown) return vForce; // return the null vForce column vector

  vWhlBodyVec = MassBalance->StructuralToBody(vXYZ); // Get wheel in body frame
  vLocalGear = Propagate->GetTb2l() * vWhlBodyVec; // Get local frame wheel location

  gearLoc = Propagate->GetLocation().LocalToLocation(vLocalGear);
  compressLength = -Exec->GetGroundCallback()->GetAGLevel(t, gearLoc, contact, normal, cvel);

  // The compression length is measured in the Z-axis, only, at this time.

  if (compressLength > 0.00) {

    WOW = true;

    // [The next equation should really use the vector to the contact patch of
    // the tire including the strut compression and not the original vWhlBodyVec.]

    vWhlVelVec      =  Propagate->GetTb2l() * (Propagate->GetPQR() * vWhlBodyVec);
    vWhlVelVec     +=  Propagate->GetVel() - cvel;
    compressSpeed   =  vWhlVelVec(eZ);

    InitializeReporting();
    ComputeBrakeForceCoefficient();
    ComputeSteeringAngle();
    ComputeSlipAngle();
    ComputeSideForceCoefficient();
    ComputeVerticalStrutForce();

    // Compute the forces in the wheel ground plane.

    double sign = RollingWhlVel>0?1.0:(RollingWhlVel<0?-1.0:0.0);
    RollingForce = ((1.0 - TirePressureNorm) * 30 + vLocalForce(eZ) * BrakeFCoeff) * sign;
    SideForce    = vLocalForce(eZ) * FCoeff;

    // Transform these forces back to the local reference frame.

    vLocalForce(eX) = RollingForce*CosWheel - SideForce*SinWheel;
    vLocalForce(eY) = SideForce*CosWheel    + RollingForce*SinWheel;

    // Transform the forces back to the body frame and compute the moment.

    vForce  = Propagate->GetTl2b() * vLocalForce;

// Start experimental section for gear jitter reduction
//
// Lag and attenuate the XY-plane forces dependent on velocity

    double ca, cb, denom;
    FGColumnVector3 Output;

// This code implements a lag filter, C/(s + C) where
// "C" is the filter coefficient. When "C" is chosen at the 
// frame rate (in Hz), the jittering is significantly reduced. This is because
// the jitter is present *at* the execution rate.
// If a coefficient is set to something equal to or less than zero, the filter
// is bypassed.

    if (LongForceLagFilterCoeff > 0) { 
      denom = 2.00 + dT*LongForceLagFilterCoeff;
      ca = dT*LongForceLagFilterCoeff / denom;
      cb = (2.00 - dT*LongForceLagFilterCoeff) / denom;
      Output(eX) = vForce(eX) * ca + prevIn(eX) * ca + prevOut(eX) * cb;
      vForce(eX) = Output(eX);
    }
    if (LatForceLagFilterCoeff > 0) { 
      denom = 2.00 + dT*LatForceLagFilterCoeff;
      ca = dT*LatForceLagFilterCoeff / denom;
      cb = (2.00 - dT*LatForceLagFilterCoeff) / denom;
      Output(eY) = vForce(eY) * ca + prevIn(eY) * ca + prevOut(eY) * cb;
      vForce(eY) = Output(eY);
    }

    prevIn = vForce;
    prevOut = Output;

    if ((fabs(RollingWhlVel) <= RFRV) && RFRV > 0) vForce(eX) *= fabs(RollingWhlVel)/RFRV;
    if ((fabs(SideWhlVel) <= SFRV) && SFRV > 0) vForce(eY) *= fabs(SideWhlVel)/SFRV;

// End section for attentuating gear jitter

    vMoment = vWhlBodyVec * vForce;

  } else { // Gear is NOT compressed

    WOW = false;
    compressLength = 0.0;

    // Return to neutral position between 1.0 and 0.8 gear pos.
    SteerAngle *= max(GetGearUnitPos()-0.8, 0.0)/0.2;

    ResetReporting();
  }

  ReportTakeoffOrLanding();

  // Require both WOW and LastWOW to be true before checking crash conditions
  // to allow the WOW flag to be used in terminating a scripted run.
  if (WOW && lastWOW) CrashDetect();

  lastWOW = WOW;

  return vForce;
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void FGLGear::ComputeRetractionState(void)
{
  double gearPos = GetGearUnitPos();
  if (gearPos < 0.01) {
    GearUp   = true;
    GearDown = false;
  } else if (gearPos > 0.99) {
    GearDown = true;
    GearUp   = false;
  } else {
    GearUp   = false;
    GearDown = false;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void FGLGear::ComputeSlipAngle(void)
{
  // Transform the wheel velocities from the local axis system to the wheel axis system.
  RollingWhlVel = vWhlVelVec(eX)*CosWheel + vWhlVelVec(eY)*SinWheel;
  SideWhlVel    = vWhlVelVec(eY)*CosWheel - vWhlVelVec(eX)*SinWheel;

  // Calculate tire slip angle.
    WheelSlip = atan2(SideWhlVel, fabs(RollingWhlVel))*radtodeg;

  // Filter the wheel slip angle

  double SlipOutput, ca, cb, denom;

  if (WheelSlipLagFilterCoeff > 0) {
    denom = 2.00 + dT*WheelSlipLagFilterCoeff;
    ca = dT*WheelSlipLagFilterCoeff / denom;
    cb = (2.00 - dT*WheelSlipLagFilterCoeff) / denom;

    SlipOutput = ca * (WheelSlip + prevSlipIn) + cb * prevSlipOut;

    prevSlipIn = WheelSlip;
    WheelSlip = prevSlipOut = SlipOutput;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Compute the steering angle in any case.
// This will also make sure that animations will look right.

void FGLGear::ComputeSteeringAngle(void)
{
  double casterLocalFrameAngleRad = 0.0;
  double casterAngle = 0.0;

  switch (eSteerType) {
  case stSteer:
    SteerAngle = degtorad * FCS->GetSteerPosDeg(GearNumber);
    break;
  case stFixed:
    SteerAngle = 0.0;
    break;
  case stCaster:
    // This is not correct for castering gear. Should make steer angle parallel
    // to the actual velocity vector of the wheel, given aircraft velocity vector
    // and omega.
    SteerAngle = 0.0;
    casterLocalFrameAngleRad = acos(vWhlVelVec(eX)/vWhlVelVec.Magnitude());
    casterAngle = casterLocalFrameAngleRad - Propagate->GetEuler(ePsi);
    break;
  default:
    cerr << "Improper steering type membership detected for this gear." << endl;
    break;
  }

  SinWheel      = sin(Propagate->GetEuler(ePsi) + SteerAngle);
  CosWheel      = cos(Propagate->GetEuler(ePsi) + SteerAngle);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Reset reporting functionality after takeoff

void FGLGear::ResetReporting(void)
{
  if (Propagate->GetDistanceAGL() > 200.0) {
    FirstContact = false;
    StartedGroundRun = false;
    LandingReported = false;
    TakeoffReported = true;
    LandingDistanceTraveled = 0.0;
    MaximumStrutForce = MaximumStrutTravel = 0.0;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void FGLGear::InitializeReporting(void)
{
  // If this is the first time the wheel has made contact, remember some values
  // for later printout.

  if (!FirstContact) {
    FirstContact  = true;
    SinkRate      =  compressSpeed;
    GroundSpeed   =  Propagate->GetVel().Magnitude();
    TakeoffReported = false;
  }

  // If the takeoff run is starting, initialize.

  if ((Propagate->GetVel().Magnitude() > 0.1) &&
      (FCS->GetBrake(bgLeft) == 0) &&
      (FCS->GetBrake(bgRight) == 0) &&
      (FCS->GetThrottlePos(0) > 0.90) && !StartedGroundRun)
  {
    TakeoffDistanceTraveled = 0;
    TakeoffDistanceTraveled50ft = 0;
    StartedGroundRun = true;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Takeoff and landing reporting functionality

void FGLGear::ReportTakeoffOrLanding(void)
{
  double deltaT = State->Getdt()*Exec->GetGroundReactions()->GetRate();

  if (FirstContact)
    LandingDistanceTraveled += Auxiliary->GetVground()*deltaT;

  if (StartedGroundRun) {
     TakeoffDistanceTraveled50ft += Auxiliary->GetVground()*deltaT;
    if (WOW) TakeoffDistanceTraveled += Auxiliary->GetVground()*deltaT;
  }

  if ( ReportEnable
       && Auxiliary->GetVground() <= 0.05
       && !LandingReported
       && Exec->GetGroundReactions()->GetWOW())
  {
    if (debug_lvl > 0) Report(erLand);
  }

  if ( ReportEnable
       && !TakeoffReported
       && (Propagate->GetDistanceAGL() - vLocalGear(eZ)) > 50.0
       && !Exec->GetGroundReactions()->GetWOW())
  {
    if (debug_lvl > 0) Report(erTakeoff);
  }

  if (lastWOW != WOW) PutMessage("GEAR_CONTACT: " + name, WOW);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Crash detection logic (really out-of-bounds detection)

void FGLGear::CrashDetect(void)
{
  if ( (compressLength > 500.0 ||
      vForce.Magnitude() > 100000000.0 ||
      vMoment.Magnitude() > 5000000000.0 ||
      SinkRate > 1.4666*30 ) && !State->IntegrationSuspended())
  {
    PutMessage("Crash Detected: Simulation FREEZE.");
    State->SuspendIntegration();
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// The following needs work regarding friction coefficients and braking and
// steering The BrakeFCoeff formula assumes that an anti-skid system is used.
// It also assumes that we won't be turning and braking at the same time.
// Will fix this later.
// [JSB] The braking force coefficients include normal rolling coefficient +
// a percentage of the static friction coefficient based on braking applied.

void FGLGear::ComputeBrakeForceCoefficient(void)
{
  switch (eBrakeGrp) {
  case bgLeft:
    BrakeFCoeff =  ( rollingFCoeff*(1.0 - FCS->GetBrake(bgLeft)) +
                     staticFCoeff*FCS->GetBrake(bgLeft) );
    break;
  case bgRight:
    BrakeFCoeff =  ( rollingFCoeff*(1.0 - FCS->GetBrake(bgRight)) +
                     staticFCoeff*FCS->GetBrake(bgRight) );
    break;
  case bgCenter:
    BrakeFCoeff =  ( rollingFCoeff*(1.0 - FCS->GetBrake(bgCenter)) +
                     staticFCoeff*FCS->GetBrake(bgCenter) );
    break;
  case bgNose:
    BrakeFCoeff =  ( rollingFCoeff*(1.0 - FCS->GetBrake(bgCenter)) +
                     staticFCoeff*FCS->GetBrake(bgCenter) );
    break;
  case bgTail:
    BrakeFCoeff =  ( rollingFCoeff*(1.0 - FCS->GetBrake(bgCenter)) +
                     staticFCoeff*FCS->GetBrake(bgCenter) );
    break;
  case bgNone:
    BrakeFCoeff =  rollingFCoeff;
    break;
  default:
    cerr << "Improper brake group membership detected for this gear." << endl;
    break;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Compute the sideforce coefficients using similar assumptions to LaRCSim for now.
// Allow a maximum of 10 degrees tire slip angle before wheel slides.  At that point,
// transition from static to dynamic friction.  There are more complicated formulations
// of this that avoid the discrete jump (similar to Pacejka).  Will fix this later.

void FGLGear::ComputeSideForceCoefficient(void)
{
  if (ForceY_Table) {

    FCoeff = ForceY_Table->GetValue(WheelSlip);

  } else {

    if (fabs(WheelSlip) <= 10.0) {
      FCoeff = staticFCoeff*WheelSlip/10.0;
    } else if (fabs(WheelSlip) <= 40.0) {
      FCoeff = (dynamicFCoeff*(fabs(WheelSlip) - 10.0)/10.0
                + staticFCoeff*(40.0 - fabs(WheelSlip))/10.0)*(WheelSlip>=0?1.0:-1.0);
    } else {
      FCoeff = dynamicFCoeff*(WheelSlip>=0?1.0:-1.0);
    }
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
// Compute the vertical force on the wheel using square-law damping (per comment
// in paper AIAA-2000-4303 - see header prologue comments). We might consider
// allowing for both square and linear damping force calculation. Also need to
// possibly give a "rebound damping factor" that differs from the compression
// case.

void FGLGear::ComputeVerticalStrutForce(void)
{
  double springForce = 0;
  double dampForce = 0;

  springForce = -compressLength * kSpring;

  if (compressSpeed >= 0.0) {

    if (eDampType == dtLinear)   dampForce = -compressSpeed * bDamp;
    else         dampForce = -compressSpeed * compressSpeed * bDamp;

  } else {

    if (eDampTypeRebound == dtLinear)
      dampForce   = -compressSpeed * bDampRebound;
    else
      dampForce   =  compressSpeed * compressSpeed * bDampRebound;

  }
  vLocalForce(eZ) =  min(springForce + dampForce, (double)0.0);

  // Remember these values for reporting
  MaximumStrutForce = max(MaximumStrutForce, fabs(vLocalForce(eZ)));
  MaximumStrutTravel = max(MaximumStrutTravel, fabs(compressLength));
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

double FGLGear::GetGearUnitPos(void)
{
  // hack to provide backward compatibility to gear/gear-pos-norm property
  if( useFCSGearPos || FCS->GetGearPos() != 1.0 ) {
    useFCSGearPos = true;
    return FCS->GetGearPos();
  }
  return GearPos;
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void FGLGear::bind(void)
{
  char property_name[80];
  if (eContactType == ctBOGEY) {
    snprintf(property_name, 80, "gear/unit[%d]/slip-angle-deg", GearNumber);
    Exec->GetPropertyManager()->Tie( property_name, &WheelSlip );
    snprintf(property_name, 80, "gear/unit[%d]/WOW", GearNumber);
    Exec->GetPropertyManager()->Tie( property_name, &WOW );
    snprintf(property_name, 80, "gear/unit[%d]/wheel-speed-fps", GearNumber);
    Exec->GetPropertyManager()->Tie( property_name, &RollingWhlVel );
    snprintf(property_name, 80, "gear/unit[%d]/z-position", GearNumber);
    Exec->GetPropertyManager()->Tie( property_name, (FGLGear*)this,
                          &FGLGear::GetZPosition, &FGLGear::SetZPosition);
  }

  if( isRetractable ) {
    snprintf(property_name, 80, "gear/unit[%d]/pos-norm", GearNumber);
    Exec->GetPropertyManager()->Tie( property_name, &GearPos );
  }

}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void FGLGear::Report(ReportType repType)
{
  switch(repType) {
  case erLand:
    cout << endl << "Touchdown report for " << name << endl;
    cout << "  Sink rate at contact:  " << SinkRate                << " fps,    "
                                << SinkRate*0.3048          << " mps"     << endl;
    cout << "  Contact ground speed:  " << GroundSpeed*.5925       << " knots,  "
                                << GroundSpeed*0.3048       << " mps"     << endl;
    cout << "  Maximum contact force: " << MaximumStrutForce       << " lbs,    "
                                << MaximumStrutForce*4.448  << " Newtons" << endl;
    cout << "  Maximum strut travel:  " << MaximumStrutTravel*12.0 << " inches, "
                                << MaximumStrutTravel*30.48 << " cm"      << endl;
    cout << "  Distance traveled:     " << LandingDistanceTraveled        << " ft,     "
                                << LandingDistanceTraveled*0.3048  << " meters"  << endl;
    LandingReported = true;
    break;
  case erTakeoff:
    cout << endl << "Takeoff report for " << name << endl;
    cout << "  Distance traveled:                " << TakeoffDistanceTraveled
         << " ft,     " << TakeoffDistanceTraveled*0.3048  << " meters"  << endl;
    cout << "  Distance traveled (over 50'):     " << TakeoffDistanceTraveled50ft
         << " ft,     " << TakeoffDistanceTraveled50ft*0.3048 << " meters" << endl;
    TakeoffReported = true;
    break;
  }
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
//    The bitmasked value choices are as follows:
//    unset: In this case (the default) JSBSim would only print
//       out the normally expected messages, essentially echoing
//       the config files as they are read. If the environment
//       variable is not set, debug_lvl is set to 1 internally
//    0: This requests JSBSim not to output any messages
//       whatsoever.
//    1: This value explicity requests the normal JSBSim
//       startup messages
//    2: This value asks for a message to be printed out when
//       a class is instantiated
//    4: When this value is set, a message is displayed when a
//       FGModel object executes its Run() method
//    8: When this value is set, various runtime state variables
//       are printed out periodically
//    16: When set various parameters are sanity checked and
//       a message is printed out when they go out of bounds

void FGLGear::Debug(int from)
{
  if (debug_lvl <= 0) return;

  if (debug_lvl & 1) { // Standard console startup message output
    if (from == 0) { // Constructor - loading and initialization
      cout << "    " << sContactType << " " << name          << endl;
      cout << "      Location: "         << vXYZ          << endl;
      cout << "      Spring Constant:  " << kSpring       << endl;

      if (eDampType == dtLinear)
        cout << "      Damping Constant: " << bDamp << " (linear)" << endl;
      else
        cout << "      Damping Constant: " << bDamp << " (square law)" << endl;

      if (eDampTypeRebound == dtLinear)
        cout << "      Rebound Damping Constant: " << bDampRebound << " (linear)" << endl;
      else 
        cout << "      Rebound Damping Constant: " << bDampRebound << " (square law)" << endl;

      cout << "      Dynamic Friction: " << dynamicFCoeff << endl;
      cout << "      Static Friction:  " << staticFCoeff  << endl;
      if (eContactType == ctBOGEY) {
        cout << "      Rolling Friction: " << rollingFCoeff << endl;
        cout << "      Steering Type:    " << sSteerType    << endl;
        cout << "      Grouping:         " << sBrakeGroup   << endl;
        cout << "      Max Steer Angle:  " << maxSteerAngle << endl;
        cout << "      Retractable:      " << isRetractable  << endl;
        cout << "      Relaxation Velocities:" << endl;
        cout << "        Rolling:          " << RFRV << endl;
        cout << "        Side:             " << SFRV << endl;
      }
    }
  }
  if (debug_lvl & 2 ) { // Instantiation/Destruction notification
    if (from == 0) cout << "Instantiated: FGLGear" << endl;
    if (from == 1) cout << "Destroyed:    FGLGear" << endl;
  }
  if (debug_lvl & 4 ) { // Run() method entry print for FGModel-derived objects
  }
  if (debug_lvl & 8 ) { // Runtime state variables
  }
  if (debug_lvl & 16) { // Sanity checking
  }
  if (debug_lvl & 64) {
    if (from == 0) { // Constructor
      cout << IdSrc << endl;
      cout << IdHdr << endl;
    }
  }
}

} // namespace JSBSim