Tweaks to add an altitude indicator adjustment.

diff --git a/src/Cockpit/steam.cxx b/src/Cockpit/steam.cxx
index 3d01ff43f0a0a5846d6a34e6914ba16b62bdedea..bd58a1bc54a7f7b117b55fae3ef864a528536fed 100644 (file)
--- a/src/Cockpit/steam.cxx
+++ b/src/Cockpit/steam.cxx
@@ -53,9 +53,16 @@ static bool isTied = false;
 
 
 double FGSteam::the_STATIC_inhg = 29.92;
-double FGSteam::the_ALT_ft = 0.0;
+double FGSteam::the_ALT_ft = 0.0;  // Indicated altitude
 double FGSteam::get_ALT_ft() { _CatchUp(); return the_ALT_ft; }
 
+double FGSteam::the_ALT_datum_mb = 1013.0;
+double FGSteam::get_ALT_datum_mb() { return the_ALT_datum_mb; }
+
+void FGSteam::set_ALT_datum_mb ( double datum_mb ) {
+    the_ALT_datum_mb = datum_mb;
+}
+
 double FGSteam::get_ASI_kias() { return FGBFI::getAirspeed(); }
 
 double FGSteam::the_VSI_case = 29.92;
@@ -104,6 +111,9 @@ void FGSteam::update ( int timesteps )
          isTied = true;
          fgTie("/steam/airspeed", FGSteam::get_ASI_kias);
          fgTie("/steam/altitude", FGSteam::get_ALT_ft);
+         fgTie("/steam/altimeter-datum-mb",
+               FGSteam::get_ALT_datum_mb, FGSteam::set_ALT_datum_mb,
+               false);  /* don't modify the value */
          fgTie("/steam/turn-rate", FGSteam::get_TC_std);
          fgTie("/steam/slip-skid", FGSteam::get_TC_rad);
          fgTie("/steam/vertical-speed", FGSteam::get_VSI_fps);
@@ -124,6 +134,7 @@ void FGSteam::update ( int timesteps )
 #undef DF2
 
 
+/* tc should be (elapsed_time_between_updates / desired_smoothing_time) */
 void FGSteam::set_lowpass ( double *outthe, double inthe, double tc )
 {
        if ( tc < 0.0 )
@@ -135,7 +146,7 @@ void FGSteam::set_lowpass ( double *outthe, double inthe, double tc )
                }
        } else
        if ( tc < 0.2 )
-       {       /* Normal mode of operation */
+       {       /* Normal mode of operation; fast approximation to exp(-tc) */
                (*outthe) = (*outthe) * ( 1.0 - tc )
                          +    inthe  * tc;
        } else
@@ -152,6 +163,24 @@ void FGSteam::set_lowpass ( double *outthe, double inthe, double tc )
 }
 
 
+#define INHG_TO_MB 33.86388  /* Inches_of_mercury * INHG_TO_MB == millibars. */
+
+// Convert air pressure to altitude by ICAO Standard Atmosphere
+double pressInHgToAltFt(double p_inhg)
+{
+    // Ref. Aviation Formulary, Ed Williams, www.best.com/~williams/avform.htm
+    const double P_0 = 29.92126;  // Std. MSL pressure, inHg. (=10135.25 mb)
+    const double p_Tr = 0.2233609 * P_0;  // Pressure at tropopause, same units.
+    const double h_Tr = 36089.24;  // Alt of tropopause, ft. (=11.0 km)
+
+    // return (P_0 - p_inhg) * 1000.0;  // ### crude approx. for low alt's
+    if (p_inhg > p_Tr)  // 0.0 to 11.0 km
+       return (1.0 - pow((p_inhg / P_0), 1.0 / 5.2558797)) / 6.8755856e-6;
+    return h_Tr + log10(p_inhg / p_Tr) / -4.806346e-5;  // 11.0 to 20.0 km
+    // We could put more code for higher altitudes here.
+}
+
+
 \f
 ////////////////////////////////////////////////////////////////////////
 // Here the fun really begins
@@ -287,24 +316,20 @@ void FGSteam::_CatchUp()
        */
        the_ENGINE_rpm = controls.get_throttle(0) * 26.0;
 
-       /**************************
-       This is just temporary, until the static source works,
-       so we just filter the actual value by one second to
-       account for the line impedance of the plumbing.
-       */
-       set_lowpass ( & the_ALT_ft, FGBFI::getAltitude(), dt );
-
        /**************************
        First, we need to know what the static line is reporting,
        which is a whole simulation area in itself.  For now, we cheat.
+       We filter the actual value by one second to
+       account for the line impedance of the plumbing.
        */
-       the_STATIC_inhg = 29.92; 
-       i = (int) the_ALT_ft;
+       double static_inhg = 29.92; 
+       i = (int) FGBFI::getAltitude();
        while ( i > 9000 )
-       {       the_STATIC_inhg *= 0.707;
+       {       static_inhg *= 0.707;
                i -= 9000;
        }
-       the_STATIC_inhg *= ( 1.0 - 0.293 * i / 9000.0 );
+       static_inhg *= ( 1.0 - 0.293 * i / 9000.0 );
+       set_lowpass ( & the_STATIC_inhg, static_inhg, dt ); 
 
        /*
        NO alternate static source error (student feature), 
@@ -312,6 +337,21 @@ void FGSteam::_CatchUp()
        NO slip-induced error, important for C172 for example.
        */
 
+       /**************************
+       Altimeter.
+       ICAO standard atmosphere MSL pressure is 1013.25 mb, and pressure
+       gradient is about 28 ft per mb at MSL increasing to about 32 at
+       5000 and 38 at 10000 ft.
+       Standard altimeters apply the subscale offset to the output altitude,
+       not to the input pressure; I don't know exactly what pressure gradient
+       they assume for this.  I choose to make it accurate at low altitudes.
+       Remember, we are trying to simulate a real altimeter, not an ideal one.
+       */
+       set_lowpass ( & the_ALT_ft,
+           pressInHgToAltFt(the_STATIC_inhg) +
+           (the_ALT_datum_mb - 1013.25) * 28.0, /* accurate at low alt. */
+           dt * 10 ); /* smoothing time 0.1 s */
+
        /**************************
        The VSI case is a low-pass filter of the static line pressure.
        The instrument reports the difference, scaled to approx ft.

diff --git a/src/Cockpit/steam.hxx b/src/Cockpit/steam.hxx
index 69911e1aef2be9e232786b58f4cd133af8449cf0..ed31f803bd936adb9facbca7b4aeb6fc817853b6 100644 (file)
--- a/src/Cockpit/steam.hxx
+++ b/src/Cockpit/steam.hxx
@@ -72,6 +72,10 @@ public:
                                // Engine Gauges
   static double get_VACUUM_inhg ();
 
+                               // Atmosphere
+  static double get_ALT_datum_mb ();
+  static void set_ALT_datum_mb(double datum_mb);
+
                                // Hacks ... temporary stuff
   static double get_HackVOR1_deg ();
   static double get_HackOBS1_deg ();
@@ -83,6 +87,7 @@ public:
 
 private:
        static double   the_ALT_ft;
+        static double  the_ALT_datum_mb;
         static double   the_TC_rad, the_TC_std;
        static double   the_STATIC_inhg, the_VACUUM_inhg;
        static double   the_VSI_fps, the_VSI_case;

diff --git a/src/Main/main.cxx b/src/Main/main.cxx
index 1d4b266f67123382a2fe896990cf1ab815114f63..0cddd59dc531156b758b561d04e21761d734cf34 100644 (file)
--- a/src/Main/main.cxx
+++ b/src/Main/main.cxx
@@ -1058,18 +1058,18 @@ static void fgMainLoop( void ) {
            double mp_factor;
            if ( cur_fdm_state->get_engine(0) != NULL ) {
                mp_factor = 
-                   cur_fdm_state->get_engine(0)->get_Manifold_Pressure() / 28;
+                   cur_fdm_state->get_engine(0)->get_Manifold_Pressure() / 100;
            } else {
-               mp_factor = 1.0;
+               mp_factor = 0.3;
            }
-           // cout << "mp = " 
-           //      << cur_fdm_state->get_engine(0)->get_Manifold_Pressure()
-           //      << endl;
+           /* cout << "mp = " 
+                << cur_fdm_state->get_engine(0)->get_Manifold_Pressure()
+                << endl; */
 
-           double volume = mp_factor;
+           double volume = 0.3 + mp_factor;
 
            if ( volume < 0.3 ) { volume = 0.3; }
-           if ( volume > 2.0 ) { volume = 2.0; }
+           if ( volume > 1.0 ) { volume = 1.0; }
            // cout << "volume = " << volume << endl;
 
            pitch_envelope.setStep  ( 0, 0.01, pitch );