X-Git-Url: https://git.mxchange.org/?a=blobdiff_plain;f=src%2FInstrumentation%2Fairspeed_indicator.cxx;h=2879f73b74bb6269b305517d16d0b452d72e053a;hb=6acf1c12fc02d6dfd29de894b033ce163640d7ae;hp=0622f2103adfc2ac9d4c49880ccd597f80da1c28;hpb=28ec7db69df926370c36334f293d69459ee01962;p=flightgear.git

diff --git a/src/Instrumentation/airspeed_indicator.cxx b/src/Instrumentation/airspeed_indicator.cxx
index 0622f2103..2879f73b7 100644
--- a/src/Instrumentation/airspeed_indicator.cxx
+++ b/src/Instrumentation/airspeed_indicator.cxx
@@ -1,10 +1,16 @@
 // airspeed_indicator.cxx - a regular pitot-static airspeed indicator.
 // Written by David Megginson, started 2002.
+// Last modified by Eric van den Berg, 09 Dec 2012
 //
 // This file is in the Public Domain and comes with no warranty.
 
+#ifdef HAVE_CONFIG_H
+#  include "config.h"
+#endif
+
 #include <math.h>
 
+#include <simgear/constants.h>
 #include <simgear/math/interpolater.hxx>
 
 #include "airspeed_indicator.hxx"
@@ -39,7 +45,7 @@ AirspeedIndicator::~AirspeedIndicator ()
 void
 AirspeedIndicator::init ()
 {
-    string branch;
+    std::string branch;
     branch = "/instrumentation/" + _name;
 
     SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
@@ -76,13 +82,11 @@ AirspeedIndicator::init ()
     _environmentManager = (FGEnvironmentMgr*) globals->get_subsystem("environment");
 }
 
-#ifndef FPSTOKTS
-# define FPSTOKTS 0.592484
-#endif
-
-#ifndef INHGTOPSF
-# define INHGTOPSF (2116.217/29.9212)
-#endif
+void
+AirspeedIndicator::reinit ()
+{
+    _speed_node->setDoubleValue(0.0);
+}
 
 void
 AirspeedIndicator::update (double dt)
@@ -91,24 +95,25 @@ AirspeedIndicator::update (double dt)
         return;
     }
     
-    double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
-    double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
-    double r = _density_node->getDoubleValue();
-    double q = ( pt - p );  // dynamic pressure
+    double pt = _total_pressure_node->getDoubleValue() ;
+    double p = _static_pressure_node->getDoubleValue() ;
+    double qc = ( pt - p ) * SG_INHG_TO_PA ;  // Impact pressure in Pa, _not_ to be confused with dynamic pressure!!!
 
-    // Now, reverse the equation (normalize dynamic pressure to
+    // Now, reverse the equation (normalize impact pressure to
     // avoid "nan" results from sqrt)
-    if ( q < 0 ) { q = 0.0; }
-    double v_fps = sqrt((2 * q) / r);
+    qc = std::max( qc , 0.0 );
+    // Calibrated airspeed (using compressible aerodynamics) based on impact pressure qc in m/s
+    // Using calibrated airspeed as indicated airspeed, neglecting any airspeed indicator errors.
+    double v_cal = sqrt( 7 * SG_p0_Pa/SG_rho0_kg_p_m3 * ( pow( 1 + qc/SG_p0_Pa  , 1/3.5 )  -1 ) );
 
                             // Publish the indicated airspeed
     double last_speed_kt = _speed_node->getDoubleValue();
-    double current_speed_kt = v_fps * FPSTOKTS;
+    double current_speed_kt = v_cal * SG_MPS_TO_KT;
     double filtered_speed = fgGetLowPass(last_speed_kt,
                                              current_speed_kt,
                                              dt * RESPONSIVENESS);
     _speed_node->setDoubleValue(filtered_speed);
-    computeMach(filtered_speed);
+    computeMach();
 
     if (!_has_overspeed) {
         return;
@@ -124,7 +129,7 @@ AirspeedIndicator::update (double dt)
 }
 
 void
-AirspeedIndicator::computeMach(double ias)
+AirspeedIndicator::computeMach()
 {
   if (!_environmentManager) {
     return;
@@ -132,21 +137,24 @@ AirspeedIndicator::computeMach(double ias)
   
   FGEnvironment env(_environmentManager->getEnvironment());
   
-  // derived from http://williams.best.vwh.net/avform.htm#Mach
-  // names here are picked to be consistent with those formulae!
-
-  double oatK = env.get_temperature_degc() + 273.15; // OAT in Kelvin
-  double CS = 38.967854 * sqrt(oatK); // speed-of-sound in knots at altitude
-  double CS_0 = 661.4786; // speed-of-sound in knots at sea-level
-  double P_0 = env.get_pressure_sea_level_inhg();
-  double P = _static_pressure_node->getDoubleValue();
+  double oatK = env.get_temperature_degc() + SG_T0_K - 15.0 ;         // OAT in Kelvin
+  oatK = std::max( oatK , 0.001 );                                // should never happen, but just in case someone flies into space...
+  double c = sqrt(SG_gamma * SG_R_m2_p_s2_p_K * oatK);                 // speed-of-sound in m/s at aircraft position
+  double pt = _total_pressure_node->getDoubleValue() * SG_INHG_TO_PA;  // total pressure in Pa
+  double p = _static_pressure_node->getDoubleValue() * SG_INHG_TO_PA;  // static pressure in Pa
+  p = std::max( p , 0.001 );                                           // should never happen, but just in case someone flies into space...
+  double rho = _density_node->getDoubleValue() * SG_SLUGFT3_TO_KGPM3;  // air density in kg/m3
+  rho = std::max( rho , 0.001 );                                      // should never happen, but just in case someone flies into space...
   
-  double DP = P_0 * (pow(1 + 0.2*pow(ias/CS_0, 2), 3.5) - 1);
-  double mach = pow(5 * ( pow(DP/P + 1, 2.0/7.0) -1) , 0.5);
+  // true airspeed in m/s
+  pt = std::max( pt , p );
+  double V_true = sqrt( 7 * p/rho * (pow( 1 + (pt-p)/p , 1/3.5 ) -1 ) );
+  // Mach number; _see notes in systems/pitot.cxx_
+  double mach = V_true / c;
   
   // publish Mach and TAS
   _mach_node->setDoubleValue(mach);
-  _tas_node->setDoubleValue(CS * mach);
+  _tas_node->setDoubleValue(V_true * SG_MPS_TO_KT );
 }
 
 // end of airspeed_indicator.cxx