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>
19 // A higher number means more responsive.
20 #define RESPONSIVENESS 50.0
22 AirspeedIndicator::AirspeedIndicator ( SGPropertyNode *node )
24 _name(node->getStringValue("name", "airspeed-indicator")),
25 _num(node->getIntValue("number", 0)),
26 _total_pressure(node->getStringValue("total-pressure", "/systems/pitot/total-pressure-inhg")),
27 _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
28 _has_overspeed(node->getBoolValue("has-overspeed-indicator",false)),
29 _pressure_alt_source(node->getStringValue("pressure-alt-source", "/instrumentation/altimeter/pressure-alt-ft")),
30 _ias_limit(node->getDoubleValue("ias-limit", 248.0)),
31 _mach_limit(node->getDoubleValue("mach-limit", 0.48)),
32 _alt_threshold(node->getDoubleValue("alt-threshold", 13200))
36 AirspeedIndicator::~AirspeedIndicator ()
41 AirspeedIndicator::init ()
44 branch = "/instrumentation/" + _name;
46 SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
47 _serviceable_node = node->getChild("serviceable", 0, true);
48 _total_pressure_node = fgGetNode(_total_pressure.c_str(), true);
49 _static_pressure_node = fgGetNode(_static_pressure.c_str(), true);
50 _density_node = fgGetNode("/environment/density-slugft3", true);
52 _sea_level_pressure_node = fgGetNode("/environment/pressure-sea-level-inhg", true);
53 _oat_celsius_node = fgGetNode("/environment/temperature-degc", true);
55 _speed_node = node->getChild("indicated-speed-kt", 0, true);
56 _tas_node = node->getChild("true-speed-kt", 0, true);
57 _mach_node = node->getChild("indicated-mach", 0, true);
59 // overspeed-indicator properties
61 _ias_limit_node = node->getNode("ias-limit",0, true);
62 _mach_limit_node = node->getNode("mach-limit",0, true);
63 _alt_threshold_node = node->getNode("alt-threshold",0, true);
65 if (!_ias_limit_node->hasValue()) {
66 _ias_limit_node->setDoubleValue(_ias_limit);
69 if (!_mach_limit_node->hasValue()) {
70 _mach_limit_node->setDoubleValue(_mach_limit);
73 if (!_alt_threshold_node->hasValue()) {
74 _alt_threshold_node->setDoubleValue(_alt_threshold);
77 _airspeed_limit = node->getChild("airspeed-limit-kt", 0, true);
78 _pressure_alt = fgGetNode(_pressure_alt_source.c_str(), true);
83 AirspeedIndicator::reinit ()
85 _speed_node->setDoubleValue(0.0);
89 # define FPSTOKTS 0.592484
93 # define INHGTOPSF (2116.217/29.9212)
97 AirspeedIndicator::update (double dt)
99 if (!_serviceable_node->getBoolValue()) {
103 double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
104 double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
105 double r = _density_node->getDoubleValue();
106 double q = ( pt - p ); // dynamic pressure
108 // Now, reverse the equation (normalize dynamic pressure to
109 // avoid "nan" results from sqrt)
110 if ( q < 0 ) { q = 0.0; }
111 double v_fps = sqrt((2 * q) / r);
113 // Publish the indicated airspeed
114 double last_speed_kt = _speed_node->getDoubleValue();
115 double current_speed_kt = v_fps * FPSTOKTS;
116 double filtered_speed = fgGetLowPass(last_speed_kt,
118 dt * RESPONSIVENESS);
119 _speed_node->setDoubleValue(filtered_speed);
120 computeMach(filtered_speed);
122 if (!_has_overspeed) {
126 double lmt = _ias_limit_node->getDoubleValue();
127 if (_pressure_alt->getDoubleValue() > _alt_threshold_node->getDoubleValue()) {
128 double mmo = _mach_limit_node->getDoubleValue();
129 lmt = (filtered_speed/_mach_node->getDoubleValue())* mmo;
132 _airspeed_limit->setDoubleValue(lmt);
136 AirspeedIndicator::computeMach(double ias)
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 = _oat_celsius_node->getDoubleValue() + 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 = _sea_level_pressure_node->getDoubleValue();
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