1 // airspeed_indicator.cxx - a regular pitot-static airspeed indicator.
2 // Written by David Megginson, started 2002.
4 // This file is in the Public Domain and comes with no warranty.
8 #include <simgear/math/interpolater.hxx>
10 #include "airspeed_indicator.hxx"
11 #include <Main/fg_props.hxx>
12 #include <Main/util.hxx>
13 #include <Environment/environment_mgr.hxx>
14 #include <Environment/environment.hxx>
17 // A higher number means more responsive.
18 #define RESPONSIVENESS 50.0
20 AirspeedIndicator::AirspeedIndicator ( SGPropertyNode *node )
22 _name(node->getStringValue("name", "airspeed-indicator")),
23 _num(node->getIntValue("number", 0)),
24 _total_pressure(node->getStringValue("total-pressure", "/systems/pitot/total-pressure-inhg")),
25 _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
26 _has_overspeed(node->getBoolValue("has-overspeed-indicator",false)),
27 _pressure_alt_source(node->getStringValue("pressure-alt-source", "/instrumentation/altimeter/pressure-alt-ft")),
28 _ias_limit(node->getDoubleValue("ias-limit", 248.0)),
29 _mach_limit(node->getDoubleValue("mach-limit", 0.48)),
30 _alt_threshold(node->getDoubleValue("alt-threshold", 13200))
32 _environmentManager = NULL;
35 AirspeedIndicator::~AirspeedIndicator ()
40 AirspeedIndicator::init ()
43 branch = "/instrumentation/" + _name;
45 SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
46 _serviceable_node = node->getChild("serviceable", 0, true);
47 _total_pressure_node = fgGetNode(_total_pressure.c_str(), true);
48 _static_pressure_node = fgGetNode(_static_pressure.c_str(), true);
49 _density_node = fgGetNode("/environment/density-slugft3", true);
50 _speed_node = node->getChild("indicated-speed-kt", 0, true);
51 _tas_node = node->getChild("true-speed-kt", 0, true);
52 _mach_node = node->getChild("indicated-mach", 0, true);
54 // overspeed-indicator properties
56 _ias_limit_node = node->getNode("ias-limit",0, true);
57 _mach_limit_node = node->getNode("mach-limit",0, true);
58 _alt_threshold_node = node->getNode("alt-threshold",0, true);
60 if (!_ias_limit_node->hasValue()) {
61 _ias_limit_node->setDoubleValue(_ias_limit);
64 if (!_mach_limit_node->hasValue()) {
65 _mach_limit_node->setDoubleValue(_mach_limit);
68 if (!_alt_threshold_node->hasValue()) {
69 _alt_threshold_node->setDoubleValue(_alt_threshold);
72 _airspeed_limit = node->getChild("airspeed-limit-kt", 0, true);
73 _pressure_alt = fgGetNode(_pressure_alt_source.c_str(), true);
76 _environmentManager = (FGEnvironmentMgr*) globals->get_subsystem("environment");
80 # define FPSTOKTS 0.592484
84 # define INHGTOPSF (2116.217/29.9212)
88 AirspeedIndicator::update (double dt)
90 if (!_serviceable_node->getBoolValue()) {
94 double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
95 double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
96 double r = _density_node->getDoubleValue();
97 double q = ( pt - p ); // dynamic pressure
99 // Now, reverse the equation (normalize dynamic pressure to
100 // avoid "nan" results from sqrt)
101 if ( q < 0 ) { q = 0.0; }
102 double v_fps = sqrt((2 * q) / r);
104 // Publish the indicated airspeed
105 double last_speed_kt = _speed_node->getDoubleValue();
106 double current_speed_kt = v_fps * FPSTOKTS;
107 double filtered_speed = fgGetLowPass(last_speed_kt,
109 dt * RESPONSIVENESS);
110 _speed_node->setDoubleValue(filtered_speed);
111 computeMach(filtered_speed);
113 if (!_has_overspeed) {
117 double lmt = _ias_limit_node->getDoubleValue();
118 if (_pressure_alt->getDoubleValue() > _alt_threshold_node->getDoubleValue()) {
119 double mmo = _mach_limit_node->getDoubleValue();
120 lmt = (filtered_speed/_mach_node->getDoubleValue())* mmo;
123 _airspeed_limit->setDoubleValue(lmt);
127 AirspeedIndicator::computeMach(double ias)
129 if (!_environmentManager) {
133 FGEnvironment env(_environmentManager->getEnvironment());
135 // derived from http://williams.best.vwh.net/avform.htm#Mach
136 // names here are picked to be consistent with those formulae!
138 double oatK = env.get_temperature_degc() + 273.15; // OAT in Kelvin
139 double CS = 38.967854 * sqrt(oatK); // speed-of-sound in knots at altitude
140 double CS_0 = 661.4786; // speed-of-sound in knots at sea-level
141 double P_0 = env.get_pressure_sea_level_inhg();
142 double P = _static_pressure_node->getDoubleValue();
144 double DP = P_0 * (pow(1 + 0.2*pow(ias/CS_0, 2), 3.5) - 1);
145 double mach = pow(5 * ( pow(DP/P + 1, 2.0/7.0) -1) , 0.5);
147 // publish Mach and TAS
148 _mach_node->setDoubleValue(mach);
149 _tas_node->setDoubleValue(CS * mach);
152 // end of airspeed_indicator.cxx