// vertical_speed_indicator.cxx - a regular VSI.
// Written by David Megginson, started 2002.
//
+// Last change by E. van den Berg, 17.02.1013
+//
// This file is in the Public Domain and comes with no warranty.
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#include <simgear/constants.h>
#include <simgear/math/interpolater.hxx>
#include "vertical_speed_indicator.hxx"
#include <Main/fg_props.hxx>
#include <Main/util.hxx>
+//** NOTE: do not change these values. If you change one of them the others need to be changed too */
+//** these values calibrate the VSI at SL. */
+#define Vol_casing 1.25e-4 //m3
+#define A_orifice 7.853982e-9 //m2
+#define Factor_cal 189.145628 //-
-VerticalSpeedIndicator::VerticalSpeedIndicator ( SGPropertyNode *node )
- : _internal_pressure_inhg(29.92),
- name("vertical-speed-indicator"),
- num(0),
- static_port("/systems/static")
-{
- int i;
- for ( i = 0; i < node->nChildren(); ++i ) {
- SGPropertyNode *child = node->getChild(i);
- string cname = child->getName();
- string cval = child->getStringValue();
- if ( cname == "name" ) {
- name = cval;
- } else if ( cname == "number" ) {
- num = child->getIntValue();
- } else if ( cname == "static-port" ) {
- static_port = cval;
- } else {
- SG_LOG( SG_INSTR, SG_WARN, "Error in vertical-speed-indicator config logic" );
- if ( name.length() ) {
- SG_LOG( SG_INSTR, SG_WARN, "Section = " << name );
- }
- }
- }
-}
+using std::string;
-VerticalSpeedIndicator::VerticalSpeedIndicator ()
- : _internal_pressure_inhg(29.92)
+VerticalSpeedIndicator::VerticalSpeedIndicator ( SGPropertyNode *node )
+ : _casing_pressure_Pa(101325),
+ _name(node->getStringValue("name", "vertical-speed-indicator")),
+ _num(node->getIntValue("number", 0)),
+ _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
+ _static_temperature(node->getStringValue("static-temperature", "/environment/temperature-degc"))
{
}
VerticalSpeedIndicator::init ()
{
string branch;
- branch = "/instrumentation/" + name;
- static_port += "/pressure-inhg";
+ branch = "/instrumentation/" + _name;
- SGPropertyNode *node = fgGetNode(branch.c_str(), num, true );
+ SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
_serviceable_node = node->getChild("serviceable", 0, true);
- _pressure_node = fgGetNode(static_port.c_str(), true);
- _speed_node = node->getChild("indicated-speed-fpm", 0, true);
+ _pressure_node = fgGetNode(_static_pressure.c_str(), true);
+ _temperature_node = fgGetNode(_static_temperature.c_str(), true);
+ _speed_fpm_node = node->getChild("indicated-speed-fpm", 0, true);
+ _speed_mps_node = node->getChild("indicated-speed-mps", 0, true);
+ _speed_kts_node = node->getChild("indicated-speed-kts", 0, true);
+ _speed_up_node = fgGetNode("/sim/speed-up", true);
- _serviceable_node->setBoolValue(true);
- // Initialize at ambient pressure
- _internal_pressure_inhg = _pressure_node->getDoubleValue();
+ reinit();
+}
+
+void
+VerticalSpeedIndicator::reinit ()
+{
+ // Initialize at ambient conditions
+ double casing_pressure_inHg = _pressure_node->getDoubleValue();
+ _casing_pressure_Pa = casing_pressure_inHg * SG_INHG_TO_PA;
+ double casing_temperature_C = _temperature_node->getDoubleValue();
+ double casing_temperature_K = casing_temperature_C + 273.15;
+ _casing_density_kgpm3 = _casing_pressure_Pa / (casing_temperature_K * SG_R_m2_p_s2_p_K);
+ _casing_airmass_kg = _casing_density_kgpm3 * Vol_casing;
+ _orifice_massflow_kgps = 0.0;
}
void
VerticalSpeedIndicator::update (double dt)
{
- // model taken from steam.cxx, with change
- // from 10000 to 10500 for manual factor
if (_serviceable_node->getBoolValue()) {
- double pressure = _pressure_node->getDoubleValue();
- _speed_node
- ->setDoubleValue((_internal_pressure_inhg - pressure) * 10500);
- _internal_pressure_inhg =
- fgGetLowPass(_internal_pressure_inhg,
- _pressure_node->getDoubleValue(),
- dt/6.0);
+ double pressure_inHg = _pressure_node->getDoubleValue() ;
+ double pressure_Pa = pressure_inHg * SG_INHG_TO_PA;
+ double speed_up = _speed_up_node->getDoubleValue();
+ double Fsign = 0.;
+ double orifice_mach = 0.0;
+ if( speed_up > 1 )
+ dt *= speed_up;
+
+// This is a thermodynamically correct model of a mechanical vertical speed indicator:
+// It represents an aneroid in a closed (constant volume) casing with the aneroid internal pressure = static pressure
+// The casing has an orifice to static pressure
+// the mass flow through the orifice is calculated using compressible aerodynamics (but adiabatic and of course a perfect gas)
+// using the pressure in the casing and static pressure
+//
+// sadly at very low flows (small VS) in conjunction with the fact discrete timesteps (dt) are used, a numerical instability is formed.
+// this is counteracted by setting the massflow 0 at very small pressure differentials
+// this causes a small funny jump of your VSI when passing through 0...cannot be helped!
+//
+// also note the calibration is only valid for 0ft, so at higher altitudes, the vertical speed is not correct, but would indicate as a real mechanical VSI.
+// Only use for conventional mechanical VSI-s. Dont use in an Air Data Computer.
+//
+// (...and it is supposed to lag!)
+
+ _casing_airmass_kg = _casing_airmass_kg - _orifice_massflow_kgps * dt;
+ double new_density_kgpm3 = _casing_airmass_kg / Vol_casing;
+ _casing_pressure_Pa = _casing_pressure_Pa * pow(new_density_kgpm3 / _casing_density_kgpm3 , SG_gamma);
+ double casing_temperature_K = _casing_pressure_Pa / (new_density_kgpm3 * SG_R_m2_p_s2_p_K);
+
+ if( _casing_pressure_Pa - pressure_Pa > 0.0 ) {
+ Fsign = 1.0; //outflow, pos VS
+ } else {
+ Fsign = -1.0; //inflow, neg VS
+ }
+
+ if( fabs(_casing_pressure_Pa - pressure_Pa) < 0.01 ) {
+ orifice_mach = 0.0;
+ } else {
+ orifice_mach = sqrt(fabs (2.0*SG_cp_m2_p_s2_p_K / (SG_gamma * SG_R_m2_p_s2_p_K) * ( pow(pressure_Pa / _casing_pressure_Pa ,(SG_gamma-1)/SG_gamma ) -1 ) ) );
+ }
+
+ _orifice_massflow_kgps = Fsign * _casing_pressure_Pa / sqrt(casing_temperature_K) * sqrt(SG_gamma/SG_R_m2_p_s2_p_K) * orifice_mach * pow(1+(SG_gamma-1)/2*orifice_mach*orifice_mach,-(SG_gamma+1)/(2*(SG_gamma-1))) * A_orifice;
+
+ double vs_fpm = Fsign * sqrt( fabs( pressure_Pa - _casing_pressure_Pa ) ) * Factor_cal;
+ double vs_kts = vs_fpm / 60 * SG_FPS_TO_KT;
+ double vs_mps = vs_fpm / 60 * SG_FEET_TO_METER;
+
+ _speed_fpm_node
+ ->setDoubleValue(vs_fpm);
+ _speed_kts_node
+ ->setDoubleValue(vs_kts);
+ _speed_mps_node
+ ->setDoubleValue(vs_mps);
+
+ _casing_density_kgpm3 = new_density_kgpm3;
+
}
}
projects / flightgear.git / blobdiff