From 4d146352d702ebfdb53a390eb4b406a30fc17cc8 Mon Sep 17 00:00:00 2001
From: david <david>
Date: Wed, 12 Mar 2003 20:28:54 +0000
Subject: [PATCH] Simplify the pitot system and ASI -- the indicated airspeed
 should be more reasonable now.

---
 src/Instrumentation/airspeed_indicator.cxx | 14 ++++++--------
 src/Instrumentation/airspeed_indicator.hxx |  2 ++
 src/Systems/pitot.cxx                      | 10 +++++++---
 3 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/src/Instrumentation/airspeed_indicator.cxx b/src/Instrumentation/airspeed_indicator.cxx
index 5ae754332..2b80fbd96 100644
--- a/src/Instrumentation/airspeed_indicator.cxx
+++ b/src/Instrumentation/airspeed_indicator.cxx
@@ -34,15 +34,12 @@ AirspeedIndicator::init ()
         fgGetNode("/systems/pitot/total-pressure-inhg", true);
     _static_pressure_node =
         fgGetNode("/systems/static/pressure-inhg", true);
+    _density_node = fgGetNode("/environment/density-slugft3", 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
@@ -55,14 +52,15 @@ void
 AirspeedIndicator::update (double dt)
 {
     if (_serviceable_node->getBoolValue()) {
-        double pt = _total_pressure_node->getDoubleValue();
-        double p = _static_pressure_node->getDoubleValue();
-        double q = ( pt - p ) * INHGTOPSF;      // dynamic pressure
+        double pt = _total_pressure_node->getDoubleValue() * INHGTOPSF;
+        double p = _static_pressure_node->getDoubleValue() * INHGTOPSF;
+        double r = _density_node->getDoubleValue();
+        double q = ( pt - p );  // dynamic pressure
 
         // Now, reverse the equation (normalize dynamic pressure to
         // avoid "nan" results from sqrt)
         if ( q < 0 ) { q = 0.0; }
-        double v_fps = sqrt((2 * q) / SEA_LEVEL_DENSITY_SLUGFT3);
+        double v_fps = sqrt((2 * q) / r);
 
                                 // Publish the indicated airspeed
         double last_speed_kt = _speed_node->getDoubleValue();
diff --git a/src/Instrumentation/airspeed_indicator.hxx b/src/Instrumentation/airspeed_indicator.hxx
index 1047608cb..e416e7395 100644
--- a/src/Instrumentation/airspeed_indicator.hxx
+++ b/src/Instrumentation/airspeed_indicator.hxx
@@ -23,6 +23,7 @@
  * /instrumentation/airspeed-indicator/serviceable
  * /systems/pitot[0]/total-pressure-inhg
  * /systems/static[0]/pressure-inhg
+ * /environment/density-slugft3
  *
  * Output properties:
  *
@@ -44,6 +45,7 @@ private:
     SGPropertyNode_ptr _serviceable_node;
     SGPropertyNode_ptr _total_pressure_node;
     SGPropertyNode_ptr _static_pressure_node;
+    SGPropertyNode_ptr _density_node;
     SGPropertyNode_ptr _speed_node;
     
 };
diff --git a/src/Systems/pitot.cxx b/src/Systems/pitot.cxx
index daa20edec..5cfc6188b 100644
--- a/src/Systems/pitot.cxx
+++ b/src/Systems/pitot.cxx
@@ -41,6 +41,10 @@ PitotSystem::unbind ()
 # define INHGTOPSF (2116.217/29.9212)
 #endif
 
+#ifndef PSFTOINHG
+# define PSFTOINHG (1/INHGTOPSF)
+#endif
+
 #ifndef KTTOFPS
 # define KTTOFPS 1.68781
 #endif
@@ -52,11 +56,11 @@ PitotSystem::update (double dt)
     if (_serviceable_node->getBoolValue()) {
                                 // The pitot tube sees the forward
                                 // velocity in the body axis.
-        double p = _pressure_node->getDoubleValue(); // static
+        double p = _pressure_node->getDoubleValue() * INHGTOPSF;
         double r = _density_node->getDoubleValue();
         double v = _velocity_node->getDoubleValue() * KTTOFPS;
-        double q = 0.5 * r * v * v / INHGTOPSF; // dynamic
-        _total_pressure_node->setDoubleValue(p + q);
+        double q = 0.5 * r * v * v; // dynamic
+        _total_pressure_node->setDoubleValue((p + q) * PSFTOINHG);
     }
 }