Plane: added RUDDER_ONLY parameter

this gives much easier setup for rudder only aircraft.

ArduPlane/Attitude.pde

@@ -968,7 +968,9 @@ static void set_servos(void)
 
     // send values to the PWM timers for output
     // ----------------------------------------
-    channel_roll->output();
+    if (g.rudder_only == 0) {
+        channel_roll->output();
+    }
     channel_pitch->output();
     channel_throttle->output();
     channel_rudder->output();

ArduPlane/Parameters.h

@@ -136,6 +136,7 @@ public:
         k_param_hil_mode,
         k_param_land_disarm_delay,
         k_param_glide_slope_threshold,
+        k_param_rudder_only,
 
         // 100: Arming parameters
         k_param_arming = 100,
@@ -428,6 +429,7 @@ public:
     AP_Int8 mix_mode;
     AP_Int8 vtail_output;
     AP_Int8 elevon_output;
+    AP_Int8 rudder_only;
     AP_Float mixing_gain;
     AP_Int8 reverse_elevons;
     AP_Int8 reverse_ch1_elevon;

ArduPlane/Parameters.pde

@@ -757,6 +757,13 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: User
     GSCALAR(mixing_gain,            "MIXING_GAIN",    0.5f),
 
+    // @Param: RUDDER_ONLY
+    // @DisplayName: Rudder only aircraft
+    // @Description: Enable rudder only mode. The rudder will control attitude in attitude controlled modes (such as FBWA). You should setup your transmitter to send roll stick inputs to the RCMAP_YAW channel (normally channel 4). The rudder servo should be attached to the RCMAP_YAW channel as well. Note that automatic ground steering will be disabled for rudder only aircraft. You should also set KFF_RDDRMIX to 1.0. You will also need to setup the YAW2SRV_DAMP yaw damping appropriately for your aircraft. A value of 0.5 for YAW2SRV_DAMP is a good starting point.
+    // @Values: 0:Disabled,1:Enabled
+    // @User: User
+    GSCALAR(rudder_only,             "RUDDER_ONLY",  0),
+
     // @Param: SYS_NUM_RESETS
     // @DisplayName: Num Resets
     // @Description: Number of APM board resets

ArduPlane/failsafe.pde

@@ -90,7 +90,9 @@ void failsafe_check(void)
             channel_pitch->output();
         }
         channel_throttle->output();
-        channel_rudder->output();
+        if (g.rudder_only == 0) {
+            channel_rudder->output();
+        }
 
         // setup secondary output channels that do have
         // corresponding input channels

ArduPlane/radio.pde

@@ -8,7 +8,11 @@
  */
 static void set_control_channels(void)
 {
-    channel_roll     = RC_Channel::rc_channel(rcmap.roll()-1);
+    if (g.rudder_only) {
+        channel_roll     = RC_Channel::rc_channel(rcmap.yaw()-1);
+    } else {
+        channel_roll     = RC_Channel::rc_channel(rcmap.roll()-1);
+    }
     channel_pitch    = RC_Channel::rc_channel(rcmap.pitch()-1);
     channel_throttle = RC_Channel::rc_channel(rcmap.throttle()-1);
     channel_rudder   = RC_Channel::rc_channel(rcmap.yaw()-1);