src/Instrumentation/airspeed_indicator.cxx

   1 // airspeed_indicator.cxx - a regular pitot-static airspeed indicator.
   2 // Written by David Megginson, started 2002.
   3 //
   4 // This file is in the Public Domain and comes with no warranty.
   5
   6 #include <math.h>
   7
   8 #include <simgear/math/interpolater.hxx>
   9
  10 #include "airspeed_indicator.hxx"
  11 #include <Main/fg_props.hxx>
  12 #include <Main/util.hxx>
  13
  14
  15 // A higher number means more responsive.
  16 #define RESPONSIVENESS 50.0
  17
  18 AirspeedIndicator::AirspeedIndicator ( SGPropertyNode *node )
  19     :
  20     _name(node->getStringValue("name", "airspeed-indicator")),
  21     _num(node->getIntValue("number", 0)),
  22     _total_pressure(node->getStringValue("total-pressure", "/systems/pitot/total-pressure-inhg")),
  23     _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg"))
  24 {
  25 }
  26
  27 AirspeedIndicator::~AirspeedIndicator ()
  28 {
  29 }
  30
  31 void
  32 AirspeedIndicator::init ()
  33 {
  34     string branch;
  35     branch = "/instrumentation/" + _name;
  36
  37     SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
  38     _serviceable_node = node->getChild("serviceable", 0, true);
  39     _total_pressure_node = fgGetNode(_total_pressure.c_str(), true);
  40     _static_pressure_node = fgGetNode(_static_pressure.c_str(), true);
  41     _density_node = fgGetNode("/environment/density-slugft3", true);
  42     _speed_node = node->getChild("indicated-speed-kt", 0, true);
  43 }
  44
  45 #ifndef FPSTOKTS
  46 # define FPSTOKTS 0.592484
  47 #endif
  48
  49 #ifndef INHGTOPSF
  50 # define INHGTOPSF (2116.217/29.9212)
  51 #endif
  52
  53 void
  54 AirspeedIndicator::update (double dt)
  55 {
  56     if (_serviceable_node->getBoolValue()) {
  57         double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
  58         double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
  59         double r = _density_node->getDoubleValue();
  60         double q = ( pt - p );  // dynamic pressure
  61
  62         // Now, reverse the equation (normalize dynamic pressure to
  63         // avoid "nan" results from sqrt)
  64         if ( q < 0 ) { q = 0.0; }
  65         double v_fps = sqrt((2 * q) / r);
  66
  67                                 // Publish the indicated airspeed
  68         double last_speed_kt = _speed_node->getDoubleValue();
  69         double current_speed_kt = v_fps * FPSTOKTS;
  70         _speed_node->setDoubleValue(fgGetLowPass(last_speed_kt,
  71                                                  current_speed_kt,
  72                                                  dt * RESPONSIVENESS));
  73     }
  74 }
  75
  76 // end of airspeed_indicator.cxx