From 1c000fef961926b99eb15d421474a49ccbd83d94 Mon Sep 17 00:00:00 2001
From: curt <curt>
Date: Tue, 23 Jan 2001 07:17:47 +0000
Subject: [PATCH] Tweaks to add an altitude indicator adjustment.
---
src/Cockpit/steam.cxx | 66 ++++++++++++++++++++++++++++++++++---------
src/Cockpit/steam.hxx | 5 ++++
src/Main/main.cxx | 14 ++++-----
3 files changed, 65 insertions(+), 20 deletions(-)
diff --git a/src/Cockpit/steam.cxx b/src/Cockpit/steam.cxx
index 3d01ff43f..bd58a1bc5 100644
--- 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.
+}
+
+
////////////////////////////////////////////////////////////////////////
// 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 69911e1ae..ed31f803b 100644
--- 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 1d4b266f6..0cddd59dc 100644
--- 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 );