Plane: Add use of airbrakes, used when throttle is negative OR via RC channel option.

ArduPlane/Plane.h

@@ -1034,6 +1034,7 @@ private:
     void set_servos_flaps(void);
     void set_landing_gear(void);
     void dspoiler_update(void);
+    void airbrake_update(void);
     void servo_output_mixers(void);
     void servos_output(void);
     void servos_auto_trim(void);

ArduPlane/servos.cpp

@@ -289,6 +289,46 @@ void Plane::dspoiler_update(void)
     SRV_Channels::set_output_scaled(SRV_Channel::k_dspoilerRight2, dspoiler_inner_right);
 }
 
+/*
+ set airbrakes based on reverse thrust and/or manual input RC channel
+ */
+void Plane::airbrake_update(void)
+{
+    // Calculate any manual airbrake input from RC channel option.
+    int8_t manual_airbrake_percent = 0;
+    
+    RC_Channel *airbrake_in_ch = rc().find_channel_for_option(RC_Channel::AUX_FUNC::AIRBRAKE);
+    
+    if (airbrake_in_ch != nullptr && !failsafe.rc_failsafe && failsafe.throttle_counter == 0) {
+        manual_airbrake_percent = airbrake_in_ch->percent_input();
+    }
+
+    // Calculate auto airbrake from negative throttle.
+    float throttle_min = aparm.throttle_min.get();
+    int16_t airbrake_pc = 0;
+
+    int throttle_pc = SRV_Channels::get_output_scaled(SRV_Channel::k_throttle);
+
+    if (throttle_min < 0) {
+        if (landing.is_flaring()) {
+            // Full airbrakes during the flare.
+            airbrake_pc = 100;
+        }
+        else {
+            // Determine fraction between zero and full negative throttle.
+            airbrake_pc = constrain_int16(-throttle_pc, 0, 100);
+        }
+    }
+
+    // Manual overrides auto airbrake setting.
+    if (airbrake_pc < manual_airbrake_percent) {
+        airbrake_pc = manual_airbrake_percent;
+    }
+
+    // Output to airbrake servo types.
+    SRV_Channels::set_output_scaled(SRV_Channel::k_airbrake, airbrake_pc);
+}
+
 /*
   setup servos for idle mode
   Idle mode is used during balloon launch to keep servos still, apart
@@ -771,6 +811,9 @@ void Plane::set_servos(void)
     set_landing_gear();
 #endif
 
+    // set airbrake outputs
+    airbrake_update();
+    
     if (auto_throttle_mode ||
         quadplane.in_assisted_flight() ||
         quadplane.in_vtol_mode()) {

libraries/RC_Channel/RC_Channel.h

@@ -204,6 +204,7 @@ public:
         MAINSAIL =           207, // mainsail input
         FLAP =               208, // flap input
         FWD_THR =            209, // VTOL manual forward throttle
+        AIRBRAKE =           210, // manual airbrake control
 
         // inputs for the use of onboard lua scripting
         SCRIPTING_1 =        300,

libraries/SRV_Channel/SRV_Channel.h

@@ -143,6 +143,7 @@ public:
         k_scripting14           = 107,
         k_scripting15           = 108,
         k_scripting16           = 109,
+        k_airbrake              = 110,
         k_LED_neopixel1         = 120,
         k_LED_neopixel2         = 121,
         k_LED_neopixel3         = 122,

libraries/SRV_Channel/SRV_Channel_aux.cpp

@@ -148,6 +148,7 @@ void SRV_Channel::aux_servo_function_setup(void)
     case k_throttle:
     case k_throttleLeft:
     case k_throttleRight:
+    case k_airbrake:
         // fixed wing throttle
         set_range(100);
         break;