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.
12 #include <simgear/math/interpolater.hxx>
14 #include "airspeed_indicator.hxx"
15 #include <Main/fg_props.hxx>
16 #include <Main/util.hxx>
17 #include <Environment/environment_mgr.hxx>
18 #include <Environment/environment.hxx>
21 // A higher number means more responsive.
22 #define RESPONSIVENESS 50.0
24 AirspeedIndicator::AirspeedIndicator ( SGPropertyNode *node )
26 _name(node->getStringValue("name", "airspeed-indicator")),
27 _num(node->getIntValue("number", 0)),
28 _total_pressure(node->getStringValue("total-pressure", "/systems/pitot/total-pressure-inhg")),
29 _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
30 _has_overspeed(node->getBoolValue("has-overspeed-indicator",false)),
31 _pressure_alt_source(node->getStringValue("pressure-alt-source", "/instrumentation/altimeter/pressure-alt-ft")),
32 _ias_limit(node->getDoubleValue("ias-limit", 248.0)),
33 _mach_limit(node->getDoubleValue("mach-limit", 0.48)),
34 _alt_threshold(node->getDoubleValue("alt-threshold", 13200))
36 _environmentManager = NULL;
39 AirspeedIndicator::~AirspeedIndicator ()
44 AirspeedIndicator::init ()
47 branch = "/instrumentation/" + _name;
49 SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
50 _serviceable_node = node->getChild("serviceable", 0, true);
51 _total_pressure_node = fgGetNode(_total_pressure.c_str(), true);
52 _static_pressure_node = fgGetNode(_static_pressure.c_str(), true);
53 _density_node = fgGetNode("/environment/density-slugft3", true);
54 _speed_node = node->getChild("indicated-speed-kt", 0, true);
55 _tas_node = node->getChild("true-speed-kt", 0, true);
56 _mach_node = node->getChild("indicated-mach", 0, true);
58 // overspeed-indicator properties
60 _ias_limit_node = node->getNode("ias-limit",0, true);
61 _mach_limit_node = node->getNode("mach-limit",0, true);
62 _alt_threshold_node = node->getNode("alt-threshold",0, true);
64 if (!_ias_limit_node->hasValue()) {
65 _ias_limit_node->setDoubleValue(_ias_limit);
68 if (!_mach_limit_node->hasValue()) {
69 _mach_limit_node->setDoubleValue(_mach_limit);
72 if (!_alt_threshold_node->hasValue()) {
73 _alt_threshold_node->setDoubleValue(_alt_threshold);
76 _airspeed_limit = node->getChild("airspeed-limit-kt", 0, true);
77 _pressure_alt = fgGetNode(_pressure_alt_source.c_str(), true);
80 _environmentManager = (FGEnvironmentMgr*) globals->get_subsystem("environment");
84 # define FPSTOKTS 0.592484
88 # define INHGTOPSF (2116.217/29.9212)
92 AirspeedIndicator::update (double dt)
94 if (!_serviceable_node->getBoolValue()) {
98 double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
99 double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
100 double r = _density_node->getDoubleValue();
101 double q = ( pt - p ); // dynamic pressure
103 // Now, reverse the equation (normalize dynamic pressure to
104 // avoid "nan" results from sqrt)
105 if ( q < 0 ) { q = 0.0; }
106 double v_fps = sqrt((2 * q) / r);
108 // Publish the indicated airspeed
109 double last_speed_kt = _speed_node->getDoubleValue();
110 double current_speed_kt = v_fps * FPSTOKTS;
111 double filtered_speed = fgGetLowPass(last_speed_kt,
113 dt * RESPONSIVENESS);
114 _speed_node->setDoubleValue(filtered_speed);
115 computeMach(filtered_speed);
117 if (!_has_overspeed) {
121 double lmt = _ias_limit_node->getDoubleValue();
122 if (_pressure_alt->getDoubleValue() > _alt_threshold_node->getDoubleValue()) {
123 double mmo = _mach_limit_node->getDoubleValue();
124 lmt = (filtered_speed/_mach_node->getDoubleValue())* mmo;
127 _airspeed_limit->setDoubleValue(lmt);
131 AirspeedIndicator::computeMach(double ias)
133 if (!_environmentManager) {
137 FGEnvironment env(_environmentManager->getEnvironment());
139 // derived from http://williams.best.vwh.net/avform.htm#Mach
140 // names here are picked to be consistent with those formulae!
142 double oatK = env.get_temperature_degc() + 273.15; // OAT in Kelvin
143 double CS = 38.967854 * sqrt(oatK); // speed-of-sound in knots at altitude
144 double CS_0 = 661.4786; // speed-of-sound in knots at sea-level
145 double P_0 = env.get_pressure_sea_level_inhg();
146 double P = _static_pressure_node->getDoubleValue();
148 double DP = P_0 * (pow(1 + 0.2*pow(ias/CS_0, 2), 3.5) - 1);
149 double mach = pow(5 * ( pow(DP/P + 1, 2.0/7.0) -1) , 0.5);
151 // publish Mach and TAS
152 _mach_node->setDoubleValue(mach);
153 _tas_node->setDoubleValue(CS * mach);
156 // end of airspeed_indicator.cxx