+
+ 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 impact pressure to
+ // avoid "nan" results from sqrt)
+ 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_cal * SG_MPS_TO_KT;
+ double filtered_speed = fgGetLowPass(last_speed_kt,
+ current_speed_kt,
+ dt * RESPONSIVENESS);
+ _speed_node->setDoubleValue(filtered_speed);
+ computeMach();
+
+ if (!_has_overspeed) {
+ return;
+ }
+
+ double lmt = _ias_limit_node->getDoubleValue();
+ if (_pressure_alt->getDoubleValue() > _alt_threshold_node->getDoubleValue()) {
+ double mmo = _mach_limit_node->getDoubleValue();
+ lmt = (filtered_speed/_mach_node->getDoubleValue())* mmo;
+ }
+
+ _airspeed_limit->setDoubleValue(lmt);
+}
+
+void
+AirspeedIndicator::computeMach()
+{
+ if (!_environmentManager) {
+ return;
+ }
+
+ FGEnvironment env(_environmentManager->getEnvironment());
+
+ 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...
+
+ // 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(V_true * SG_MPS_TO_KT );