133 lines
5.3 KiB
C++
133 lines
5.3 KiB
C++
// 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 //-
|
|
|
|
using std::string;
|
|
|
|
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::~VerticalSpeedIndicator ()
|
|
{
|
|
}
|
|
|
|
void
|
|
VerticalSpeedIndicator::init ()
|
|
{
|
|
string branch;
|
|
branch = "/instrumentation/" + _name;
|
|
|
|
SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
|
|
_serviceable_node = node->getChild("serviceable", 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);
|
|
|
|
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)
|
|
{
|
|
if (_serviceable_node->getBoolValue()) {
|
|
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;
|
|
|
|
}
|
|
}
|
|
|
|
// end of vertical_speed_indicator.cxx
|