APM: added RUDDER_STEER option

when enabled, this uses only the rudder to steer during takeoff and
landing, using aileron only for levelling

ArduPlane/ArduPlane.pde

@@ -1076,7 +1076,7 @@ static void update_current_flight_mode(void)
 
         switch(nav_command_ID) {
         case MAV_CMD_NAV_TAKEOFF:
-            if (hold_course != -1) {
+            if (hold_course != -1 && g.rudder_steer == 0) {
                 calc_nav_roll();
             } else {
                 nav_roll_cd = 0;
@@ -1106,14 +1106,19 @@ static void update_current_flight_mode(void)
             break;
 
         case MAV_CMD_NAV_LAND:
-            calc_nav_roll();
+            if (g.rudder_steer == 0) {
+                calc_nav_roll();
+            } else {
+                nav_roll_cd = 0;
+            }
 
-            calc_nav_pitch();
-            calc_throttle();
             if (!alt_control_airspeed() || land_complete) {
                 // hold pitch constant in final approach
                 nav_pitch_cd = g.land_pitch_cd;
+            } else {
+                calc_nav_pitch();
             }
+            calc_throttle();
 
             if (land_complete) {
                 // we are in the final stage of a landing - force

ArduPlane/Parameters.h

@@ -74,6 +74,7 @@ public:
         k_param_land_flare_alt,
         k_param_land_flare_sec,
         k_param_crosstrack_min_distance,
+        k_param_rudder_steer,
 
         // 110: Telemetry control
         //
@@ -331,6 +332,7 @@ public:
     AP_Int8 inverted_flight_ch;             // 0=disabled, 1-8 is channel for inverted flight trigger
     AP_Int8 sonar_enabled;
     AP_Int8 stick_mixing;
+    AP_Int8 rudder_steer;
 
     // Camera
 #if CAMERA == ENABLED

ArduPlane/Parameters.pde

@@ -81,6 +81,13 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Advanced
     GSCALAR(stick_mixing,           "STICK_MIXING",   1),
 
+    // @Param: RUDDER_STEER
+    // @DisplayName: Rudder steering on takeoff and landing
+    // @Description: When enabled, only rudder will be used for steering during takeoff and landing, with the ailerons used to hold the plane level
+    // @Values: 0:Disabled,1:Enabled
+    // @User: User
+    GSCALAR(rudder_steer,           "RUDDER_STEER",   0),
+
     // @Param: land_pitch_cd
     // @DisplayName: Landing Pitch
     // @Description: Used in autoland for planes without airspeed sensors in hundredths of a degree