APM: switch back to old nav_roll calculation

the old nav_roll will be used for the 2.50 release to prevent the need
for re-tuning. For the release after that we will use the new
calculation

ArduPlane/ArduPlane.pde

@@ -394,8 +394,6 @@ static int32_t	target_bearing;
 //This is the direction from the last waypoint to the next waypoint
 // deg * 100 : 0 to 360
 static int32_t	crosstrack_bearing;
-// A gain scaler to account for ground speed/headwind/tailwind
-static float	nav_gain_scaler 		= 1;
 // Direction held during phases of takeoff and landing
 // deg * 100 dir of plane,  A value of -1 indicates the course has not been set/is not in use
 static int32_t    hold_course       	 	= -1;		// deg * 100 dir of plane

ArduPlane/Attitude.pde

@@ -200,6 +200,8 @@ static void calc_nav_pitch()
 
 static void calc_nav_roll()
 {
+#define NAV_ROLL_BY_RATE 0
+#if NAV_ROLL_BY_RATE
 	// Scale from centidegrees (PID input) to radians per second. A P gain of 1.0 should result in a
 	// desired rate of 1 degree per second per degree of error - if you're 15 degrees off, you'll try
 	// to turn at 15 degrees per second.
@@ -221,6 +223,14 @@ static void calc_nav_roll()
 	// Bank angle = V*R/g, where V is airspeed, R is turn rate, and g is gravity.
 	nav_roll = ToDeg(atan(speed*turn_rate/9.81)*100);
 
+#else
+    // this is the old nav_roll calculation. We will use this for 2.50
+    // then remove for a future release
+    float nav_gain_scaler = (float)g_gps->ground_speed / (STANDARD_SPEED * 100.0);
+    nav_gain_scaler = constrain(nav_gain_scaler, 0.2, 1.4);
+    nav_roll = g.pidNavRoll.get_pid(bearing_error, nav_gain_scaler); //returns desired bank angle in degrees*100
+#endif
+
 	nav_roll = constrain(nav_roll, -g.roll_limit.get(), g.roll_limit.get());
 }
 

ArduPlane/Log.pde

@@ -315,7 +315,7 @@ static void Log_Write_Nav_Tuning()
 	DataFlash.WriteInt((uint16_t)nav_bearing);
 	DataFlash.WriteInt(altitude_error);
 	DataFlash.WriteInt((int16_t)airspeed.get_airspeed_cm());
-	DataFlash.WriteInt((int16_t)(nav_gain_scaler*1000));
+	DataFlash.WriteInt(0); // was nav_gain_scaler
 	DataFlash.WriteByte(END_BYTE);
 }