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
  19 AirspeedIndicator::AirspeedIndicator ()
  20 {
  21 }
  22
  23 AirspeedIndicator::~AirspeedIndicator ()
  24 {
  25 }
  26
  27 void
  28 AirspeedIndicator::init ()
  29 {
  30     _serviceable_node =
  31         fgGetNode("/instrumentation/airspeed-indicator/serviceable",
  32                   true);
  33     _total_pressure_node =
  34         fgGetNode("/systems/pitot/total-pressure-inhg", true);
  35     _static_pressure_node =
  36         fgGetNode("/systems/static/pressure-inhg", true);
  37     _density_node = fgGetNode("/environment/density-slugft3", true);
  38     _speed_node =
  39         fgGetNode("/instrumentation/airspeed-indicator/indicated-speed-kt",
  40                   true);
  41 }
  42
  43 #ifndef FPSTOKTS
  44 # define FPSTOKTS 0.592484
  45 #endif
  46
  47 #ifndef INHGTOPSF
  48 # define INHGTOPSF (2116.217/29.9212)
  49 #endif
  50
  51 void
  52 AirspeedIndicator::update (double dt)
  53 {
  54     if (_serviceable_node->getBoolValue()) {
  55         double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
  56         double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
  57         double r = _density_node->getDoubleValue();
  58         double q = ( pt - p );  // dynamic pressure
  59
  60         // Now, reverse the equation (normalize dynamic pressure to
  61         // avoid "nan" results from sqrt)
  62         if ( q < 0 ) { q = 0.0; }
  63         double v_fps = sqrt((2 * q) / r);
  64
  65                                 // Publish the indicated airspeed
  66         double last_speed_kt = _speed_node->getDoubleValue();
  67         double current_speed_kt = v_fps * FPSTOKTS;
  68         _speed_node->setDoubleValue(fgGetLowPass(last_speed_kt,
  69                                                  current_speed_kt,
  70                                                  dt * RESPONSIVENESS));
  71     }
  72 }
  73
  74 // end of airspeed_indicator.cxx