Copter: add RTL_SPEED parameter

ArduCopter/Parameters.cpp

@@ -128,6 +128,15 @@ const AP_Param::Info Copter::var_info[] = {
     // @User: Standard
     GSCALAR(rtl_altitude,   "RTL_ALT",     RTL_ALT),
 
+    // @Param: RTL_SPEED
+    // @DisplayName: RTL speed
+    // @Description: Defines the speed in cm/s which the aircraft will attempt to maintain horizontally while flying home. If this is set to zero, WPNAV_SPEED will be used instead.
+    // @Units: cm/s
+    // @Range: 0 2000
+    // @Increment: 50
+    // @User: Standard
+    GSCALAR(rtl_speed_cms,   "RTL_SPEED",     0),
+
     // @Param: RNGFND_GAIN
     // @DisplayName: Rangefinder gain
     // @Description: Used to adjust the speed with which the target altitude is changed when objects are sensed below the copter

ArduCopter/Parameters.h

@@ -375,6 +375,7 @@ public:
     AP_Float        pilot_takeoff_alt;
 
     AP_Int16        rtl_altitude;
+    AP_Int16        rtl_speed_cms;
     AP_Float        sonar_gain;
 
     AP_Int8         failsafe_battery_enabled;   // battery failsafe enabled

ArduCopter/control_rtl.cpp

@@ -84,6 +84,11 @@ void Copter::rtl_climb_start()
     // initialise waypoint and spline controller
     wp_nav.wp_and_spline_init();
 
+    // RTL_SPEED == 0 means use WPNAV_SPEED
+    if (!is_zero(g.rtl_speed_cms)) {
+        wp_nav.set_speed_xy(g.rtl_speed_cms);
+    }
+
     // get horizontal stopping point
     Vector3f destination;
     wp_nav.get_wp_stopping_point_xy(destination);