Plane: support REVERSE_THROTTLE rc option

this provides a more convenient way to setup for reverse throttle

ArduPlane/Plane.h

@@ -1065,6 +1065,8 @@ private:
 
     void read_aux_all();
 
+    bool reversed_throttle;
+    bool have_reverse_throttle_rc_option;
     bool allow_reverse_thrust(void) const;
     bool have_reverse_thrust(void) const;
     int16_t get_throttle_input(bool no_deadzone=false) const;

ArduPlane/RC_Channel.cpp

@@ -35,7 +35,20 @@ void RC_Channel_Plane::init_aux_function(const RC_Channel::aux_func_t ch_option,
     case ARMDISARM:
     case INVERTED:
         break;
+
+    case REVERSE_THROTTLE:
+        plane.have_reverse_throttle_rc_option = true;
+        // setup input throttle as a range. This is needed as init_aux_function is called
+        // after set_control_channels()
+        if (plane.channel_throttle) {
+            plane.channel_throttle->set_range(100);
+        }
+        // note that we don't call do_aux_function() here as we don't
+        // want to startup with reverse thrust
+        break;
+
     default:
+        // handle in parent class
         RC_Channel::init_aux_function(ch_option, ch_flag);
         break;
 }
@@ -63,6 +76,11 @@ void RC_Channel_Plane::do_aux_function(const aux_func_t ch_option, const aux_swi
         plane.inverted_flight = (ch_flag == HIGH);
         break;
 
+    case REVERSE_THROTTLE:
+        plane.reversed_throttle = (ch_flag == HIGH);
+        gcs().send_text(MAV_SEVERITY_INFO, "RevThrottle: %s", plane.reversed_throttle?"ENABLE":"DISABLE");
+        break;
+        
     default:
         RC_Channel::do_aux_function(ch_option, ch_flag);
         break;

ArduPlane/radio.cpp

@@ -28,7 +28,13 @@ void Plane::set_control_channels(void)
         channel_throttle->set_range(100);
     } else {
         // reverse thrust
-        channel_throttle->set_angle(100);
+        if (have_reverse_throttle_rc_option) {
+            // when we have a reverse throttle RC option setup we use throttle
+            // as a range, and rely on the RC switch to get reverse thrust
+            channel_throttle->set_range(100);
+        } else {
+            channel_throttle->set_angle(100);
+        }
         SRV_Channels::set_angle(SRV_Channel::k_throttle, 100);
         SRV_Channels::set_angle(SRV_Channel::k_throttleLeft, 100);
         SRV_Channels::set_angle(SRV_Channel::k_throttleRight, 100);

ArduPlane/reverse_thrust.cpp

@@ -106,8 +106,15 @@ bool Plane::have_reverse_thrust(void) const
  */
 int16_t Plane::get_throttle_input(bool no_deadzone) const
 {
+    int16_t ret;
     if (no_deadzone) {
-        return channel_throttle->get_control_in_zero_dz();
+        ret = channel_throttle->get_control_in_zero_dz();
+    } else {
+        ret = channel_throttle->get_control_in();
     }
-    return channel_throttle->get_control_in();
+    if (reversed_throttle) {
+        // RC option for reverse throttle has been set
+        ret = -ret;
+    }
+    return ret;
 }