Changes to make the default autopilot heading hold work much more like a C172

DG heading hold (i.e. it will follow the gyro drift.)

src/Autopilot/newauto.cxx

@@ -33,6 +33,7 @@
 #include <simgear/debug/logstream.hxx>
 #include <simgear/math/sg_geodesy.hxx>
 
+#include <Cockpit/steam.hxx>
 #include <Cockpit/radiostack.hxx>
 #include <Controls/controls.hxx>
 #include <FDM/flight.hxx>
@@ -365,8 +366,15 @@ int FGAutopilot::run() {
 	
     // heading hold
     if ( heading_hold == true ) {
+	if ( heading_mode == FG_DG_HEADING_LOCK ) {
+	    // cout << "DG heading = " << FGSteam::get_DG_deg()
+	    //      << " DG error = " << FGSteam::get_DG_err() << endl;
 
-	if ( heading_mode == FG_HEADING_LOCK ) {
+	    TargetHeading = DGTargetHeading + FGSteam::get_DG_err();
+	    while ( TargetHeading <   0.0 ) { TargetHeading += 360.0; }
+	    while ( TargetHeading > 360.0 ) { TargetHeading -= 360.0; }
+	    MakeTargetHeadingStr( TargetHeading );
+	} else if ( heading_mode == FG_HEADING_LOCK ) {
 	    // leave target heading alone
 	} else if ( heading_mode == FG_HEADING_NAV1 ) {
 	    double tgt_radial;
@@ -376,7 +384,7 @@ int FGAutopilot::run() {
 		tgt_radial = current_radiostack->get_nav1_radial();
 	    } else {
 		tgt_radial = current_radiostack->get_nav1_radial() +
-		    current_radiostack->get_nav1_magvar(); 
+		    current_radiostack->get_nav1_magvar();
 	    }
 	    cur_radial = current_radiostack->get_nav1_heading() +
 		    current_radiostack->get_nav1_magvar();
@@ -718,7 +726,10 @@ int FGAutopilot::run() {
 void FGAutopilot::set_HeadingMode( fgAutoHeadingMode mode ) {
     heading_mode = mode;
 
-    if ( heading_mode == FG_HEADING_LOCK ) {
+    if ( heading_mode == FG_DG_HEADING_LOCK ) {
+	// set heading hold to current heading (as read from DG)
+	DGTargetHeading = FGSteam::get_DG_deg();
+    } else if ( heading_mode == FG_HEADING_LOCK ) {
 	// set heading hold to current heading
 	TargetHeading = FGBFI::getHeading();
     } else if ( heading_mode == FG_HEADING_WAYPOINT ) {
@@ -923,14 +934,19 @@ void FGAutopilot::AltitudeAdjust( double inc )
 
 
 void FGAutopilot::HeadingAdjust( double inc ) {
-    heading_mode = FG_HEADING_LOCK;
-	
-    double target = ( int ) ( TargetHeading / inc ) * inc + inc;
+    if ( heading_mode != FG_DG_HEADING_LOCK && heading_mode != FG_HEADING_LOCK )
+    {
+	heading_mode = FG_DG_HEADING_LOCK;
+    }
+
+    if ( heading_mode == FG_DG_HEADING_LOCK ) {
+	double target = ( int ) ( DGTargetHeading / inc ) * inc + inc;
+	DGTargetHeading = NormalizeDegrees( target );
+    } else {
+	double target = ( int ) ( TargetHeading / inc ) * inc + inc;
+	TargetHeading = NormalizeDegrees( target );
+    }
 
-    TargetHeading = NormalizeDegrees( target );
-    // following cast needed ambiguous plib
-    // ApHeadingDialogInput -> setValue ((float)TargetHeading );
-    MakeTargetHeadingStr( TargetHeading );			
     update_old_control_values();
 }
 

src/Autopilot/newauto.hxx

@@ -36,10 +36,11 @@ class FGAutopilot {
 public:
 
     enum fgAutoHeadingMode {
-	FG_HEADING_LOCK = 0,
-	FG_HEADING_NAV1 = 1,
-	FG_HEADING_NAV2 = 2,
-        FG_HEADING_WAYPOINT = 3
+	FG_DG_HEADING_LOCK = 0,
+	FG_HEADING_LOCK = 1,
+	FG_HEADING_NAV1 = 2,
+	FG_HEADING_NAV2 = 3,
+        FG_HEADING_WAYPOINT = 4
     };
 
     enum fgAutoAltitudeMode {
@@ -63,7 +64,8 @@ private:
     // double TargetLatitude;	// the latitude the AP should steer to.
     // double TargetLongitude;	// the longitude the AP should steer to.
     double TargetDistance;	// the distance to Target.
-    double TargetHeading;	// the heading the AP should steer to.
+    double DGTargetHeading;     // the apparent DG heading to steer towards.
+    double TargetHeading;	// the true heading the AP should steer to.
     double TargetAltitude;	// altitude to hold
     double TargetAGL;		// the terrain separation
     double TargetClimbRate;	// climb rate to shoot for
@@ -151,6 +153,8 @@ public:
     inline void set_old_lon( double val ) { old_lon = val; }
     inline double get_TargetHeading() const { return TargetHeading; }
     inline void set_TargetHeading( double val ) { TargetHeading = val; }
+    inline double get_DGTargetHeading() const { return DGTargetHeading; }
+    inline void set_DGTargetHeading( double val ) { DGTargetHeading = val; }
     inline double get_TargetDistance() const { return TargetDistance; }
     inline void set_TargetDistance( double val ) { TargetDistance = val; }
     inline double get_TargetAltitude() const { return TargetAltitude; }

src/Cockpit/steam.cxx

@@ -123,8 +123,8 @@ void FGSteam::update ( int timesteps )
 	  fgTie("/steam/glidescope1", FGSteam::get_HackGS_deg);
 	  fgTie("/steam/adf", FGSteam::get_HackADF_deg);
 	  fgTie("/steam/gyro-compass-error",
-		FGSteam::get_DG_err,
-		FGSteam::set_DG_err);
+		FGSteam::get_DG_err, FGSteam::set_DG_err,
+		false);  /* don't modify the value */
 	  fgTie("/steam/mag-compass", FGSteam::get_MH_deg);
 	}
 	_UpdatesPending += timesteps;

src/Main/bfi.cxx

@@ -197,6 +197,7 @@ FGBFI::init ()
   fgTie("/autopilot/settings/altitude", getAPAltitude, setAPAltitude);
   fgTie("/autopilot/locks/heading", getAPHeadingLock, setAPHeadingLock);
   fgTie("/autopilot/settings/heading", getAPHeading, setAPHeading);
+  fgTie("/autopilot/settings/heading-dg", getAPHeadingDG, setAPHeadingDG);
   fgTie("/autopilot/settings/heading-magnetic",
              getAPHeadingMag, setAPHeadingMag);
   fgTie("/autopilot/locks/nav1", getAPNAV1Lock, setAPNAV1Lock);
@@ -1049,6 +1050,26 @@ FGBFI::setAPHeading (double heading)
 }
 
 
+/**
+ * Get the autopilot DG target heading in degrees.
+ */
+double
+FGBFI::getAPHeadingDG ()
+{
+  return current_autopilot->get_DGTargetHeading();
+}
+
+
+/**
+ * Set the autopilot DG target heading in degrees.
+ */
+void
+FGBFI::setAPHeadingDG (double heading)
+{
+  current_autopilot->set_DGTargetHeading( heading );
+}
+
+
 /**
  * Get the autopilot target heading in degrees.
  */

src/Main/bfi.hxx

@@ -137,6 +137,9 @@ public:
   static double getAPHeading (); // degrees
   static void setAPHeading (double heading); // degrees
 
+  static double getAPHeadingDG (); // degrees
+  static void setAPHeadingDG (double heading); // degrees
+
   static double getAPHeadingMag (); // degrees
   static void setAPHeadingMag (double heading);	// degrees
 

src/Main/keyboard.cxx

@@ -108,8 +108,8 @@ void GLUTkey(unsigned char k, int x, int y) {
 	        );
 	    return;
 	case 8: // Ctrl-H key
-	    current_autopilot->set_HeadingMode( 
-                  FGAutopilot::FG_HEADING_LOCK );
+	    current_autopilot->set_HeadingMode(
+		  FGAutopilot::FG_DG_HEADING_LOCK );
 	    current_autopilot->set_HeadingEnabled(
 		  ! current_autopilot->get_HeadingEnabled()
 	        );