AC_AttitudeControl: WeatherVane: add tail in option

libraries/AC_AttitudeControl/AC_WeatherVane.cpp

@@ -163,6 +163,17 @@ bool AC_WeatherVane::get_yaw_out(float &yaw_output, const int16_t pilot_yaw, con
             dir_string = "side in";
             break;
 
+        case Direction::TAIL_IN:
+            if (is_negative(pitch_cdeg)) {
+                output = fabsf(roll_cdeg) - pitch_cdeg;
+            } else {
+                output = MAX(fabsf(roll_cdeg) - deadzone_cdeg, 0.0);
+            }
+            if (is_positive(roll_cdeg)) {
+                output *= -1.0;
+            }
+            dir_string = "tail in";
+            break;
     }
 
     if (!active_msg_sent) {

libraries/AC_AttitudeControl/AC_WeatherVane.h

@@ -28,6 +28,7 @@ class AC_WeatherVane {
             NOSE_IN = 1, // Only nose into wind
             NOSE_OR_TAIL_IN = 2, // Nose in or tail into wind, which ever is closest
             SIDE_IN = 3, // Side into wind for copter tailsitters
+            TAIL_IN = 4, // backwards, for tailsitters, makes it easier to descend
         };
 
         // Paramaters