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flightgear/src/Instrumentation/airspeed_indicator.cxx

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Added pitot system and new airspeed indicator.
2002-09-28 20:48:53 +00:00
// airspeed_indicator.cxx - a regular VSI.
// Written by David Megginson, started 2002.
//
// This file is in the Public Domain and comes with no warranty.
#include <math.h>
#include <simgear/math/interpolater.hxx>
#include "airspeed_indicator.hxx"
#include <Main/fg_props.hxx>
#include <Main/util.hxx>
AirspeedIndicator::AirspeedIndicator ()
{
}
AirspeedIndicator::~AirspeedIndicator ()
{
}
void
AirspeedIndicator::init ()
{
_serviceable_node =
fgGetNode("/instrumentation/airspeed-indicator/serviceable",
true);
_total_pressure_node =
fgGetNode("/systems/pitot/total-pressure-inhg", true);
_static_pressure_node =
fgGetNode("/systems/static/pressure-inhg", true);
_speed_node =
fgGetNode("/instrumentation/airspeed-indicator/indicated-speed-kt",
true);
}
#ifndef SEA_LEVEL_DENSITY_SLUGFG3
# define SEA_LEVEL_DENSITY_SLUGFT3 0.002378
#endif
#ifndef FPSTOKTS
# define FPSTOKTS 0.592484
#endif
Patch from Alex Perry: Ok, I found the problem. You're computing the dynamic pressure in "psf" and adding it to the static pressure in "inHg" to form the total pressure. The attached patch is the simple fix to the source. With that fix, failing the pitot while in cruise at 3k' will cause the airspeed to indicate beyond redline during climb ... well before 4k'. Thus, a pitot problem can be detected on any IFR altitude change. Similarly, failing the static (with working pitot) while cruising 4k' causes the airspeed to indicate beyond redline during a descent well before reaching 3k' (during which, of course, the ALT looks fine). Thus, a static failure can be detected before the aircraft breaks out of the pilot tolerance range and is blatantly conspicuous soon after.
2002-10-10 18:15:22 +00:00
#ifndef INHGTOPSF
# define INHGTOPSF (2116.217/29.9212)
#endif
Added pitot system and new airspeed indicator.
2002-09-28 20:48:53 +00:00
void
AirspeedIndicator::update (double dt)
{
if (_serviceable_node->getBoolValue()) {
double pt = _total_pressure_node->getDoubleValue();
double p = _static_pressure_node->getDoubleValue();
Patch from Alex Perry: Ok, I found the problem. You're computing the dynamic pressure in "psf" and adding it to the static pressure in "inHg" to form the total pressure. The attached patch is the simple fix to the source. With that fix, failing the pitot while in cruise at 3k' will cause the airspeed to indicate beyond redline during climb ... well before 4k'. Thus, a pitot problem can be detected on any IFR altitude change. Similarly, failing the static (with working pitot) while cruising 4k' causes the airspeed to indicate beyond redline during a descent well before reaching 3k' (during which, of course, the ALT looks fine). Thus, a static failure can be detected before the aircraft breaks out of the pilot tolerance range and is blatantly conspicuous soon after.
2002-10-10 18:15:22 +00:00
double q = ( pt - p ) * INHGTOPSF; // dynamic pressure
Added pitot system and new airspeed indicator.
2002-09-28 20:48:53 +00:00
// Now, reverse the equation
double v_fps = sqrt((2 * q) / SEA_LEVEL_DENSITY_SLUGFT3);
_speed_node->setDoubleValue(v_fps * FPSTOKTS);
}
}
// end of airspeed_indicator.cxx
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