1 // vertical_speed_indicator.cxx - a regular VSI.
2 // Written by David Megginson, started 2002.
4 // Last change by E. van den Berg, 17.02.1013
6 // This file is in the Public Domain and comes with no warranty.
12 #include <simgear/constants.h>
13 #include <simgear/math/interpolater.hxx>
15 #include "vertical_speed_indicator.hxx"
16 #include <Main/fg_props.hxx>
17 #include <Main/util.hxx>
19 //** NOTE: do not change these values. If you change one of them the others need to be changed too */
20 //** these values calibrate the VSI at SL. */
21 #define Vol_casing 1.25e-4 //m3
22 #define A_orifice 7.853982e-9 //m2
23 #define Factor_cal 189.145628 //-
27 VerticalSpeedIndicator::VerticalSpeedIndicator ( SGPropertyNode *node )
28 : _casing_pressure_Pa(101325),
29 _name(node->getStringValue("name", "vertical-speed-indicator")),
30 _num(node->getIntValue("number", 0)),
31 _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
32 _static_temperature(node->getStringValue("static-temperature", "/environment/temperature-degc"))
36 VerticalSpeedIndicator::~VerticalSpeedIndicator ()
41 VerticalSpeedIndicator::init ()
44 branch = "/instrumentation/" + _name;
46 SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
47 _serviceable_node = node->getChild("serviceable", 0, true);
48 _pressure_node = fgGetNode(_static_pressure.c_str(), true);
49 _temperature_node = fgGetNode(_static_temperature.c_str(), true);
50 _speed_fpm_node = node->getChild("indicated-speed-fpm", 0, true);
51 _speed_mps_node = node->getChild("indicated-speed-mps", 0, true);
52 _speed_kts_node = node->getChild("indicated-speed-kts", 0, true);
53 _speed_up_node = fgGetNode("/sim/speed-up", true);
59 VerticalSpeedIndicator::reinit ()
61 // Initialize at ambient conditions
62 double casing_pressure_inHg = _pressure_node->getDoubleValue();
63 _casing_pressure_Pa = casing_pressure_inHg * SG_INHG_TO_PA;
64 double casing_temperature_C = _temperature_node->getDoubleValue();
65 double casing_temperature_K = casing_temperature_C + 273.15;
66 _casing_density_kgpm3 = _casing_pressure_Pa / (casing_temperature_K * SG_R_m2_p_s2_p_K);
67 _casing_airmass_kg = _casing_density_kgpm3 * Vol_casing;
68 _orifice_massflow_kgps = 0.0;
72 VerticalSpeedIndicator::update (double dt)
74 if (_serviceable_node->getBoolValue()) {
75 double pressure_inHg = _pressure_node->getDoubleValue() ;
76 double pressure_Pa = pressure_inHg * SG_INHG_TO_PA;
77 double speed_up = _speed_up_node->getDoubleValue();
79 double orifice_mach = 0.0;
83 // This is a thermodynamically correct model of a mechanical vertical speed indicator:
84 // It represents an aneroid in a closed (constant volume) casing with the aneroid internal pressure = static pressure
85 // The casing has an orifice to static pressure
86 // the mass flow through the orifice is calculated using compressible aerodynamics (but adiabatic and of course a perfect gas)
87 // using the pressure in the casing and static pressure
89 // sadly at very low flows (small VS) in conjunction with the fact discrete timesteps (dt) are used, a numerical instability is formed.
90 // this is counteracted by setting the massflow 0 at very small pressure differentials
91 // this causes a small funny jump of your VSI when passing through 0...cannot be helped!
93 // 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.
94 // Only use for conventional mechanical VSI-s. Dont use in an Air Data Computer.
96 // (...and it is supposed to lag!)
98 _casing_airmass_kg = _casing_airmass_kg - _orifice_massflow_kgps * dt;
99 double new_density_kgpm3 = _casing_airmass_kg / Vol_casing;
100 _casing_pressure_Pa = _casing_pressure_Pa * pow(new_density_kgpm3 / _casing_density_kgpm3 , SG_gamma);
101 double casing_temperature_K = _casing_pressure_Pa / (new_density_kgpm3 * SG_R_m2_p_s2_p_K);
103 if( _casing_pressure_Pa - pressure_Pa > 0.0 ) {
104 Fsign = 1.0; //outflow, pos VS
106 Fsign = -1.0; //inflow, neg VS
109 if( fabs(_casing_pressure_Pa - pressure_Pa) < 0.01 ) {
112 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 ) ) );
115 _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;
117 double vs_fpm = Fsign * sqrt( fabs( pressure_Pa - _casing_pressure_Pa ) ) * Factor_cal;
118 double vs_kts = vs_fpm / 60 * SG_FPS_TO_KT;
119 double vs_mps = vs_fpm / 60 * SG_FEET_TO_METER;
122 ->setDoubleValue(vs_fpm);
124 ->setDoubleValue(vs_kts);
126 ->setDoubleValue(vs_mps);
128 _casing_density_kgpm3 = new_density_kgpm3;
133 // end of vertical_speed_indicator.cxx