Plane: added support for elevon and vtail servo functions

these make trimming and limiting servo movement easier

ArduPlane/Plane.h

@@ -1058,6 +1058,8 @@ private:
     bool suppress_throttle(void);
     void channel_output_mixer_pwm(uint8_t mixing_type, uint16_t & chan1, uint16_t & chan2)const;
     void channel_output_mixer(uint8_t mixing_type, SRV_Channel::Aux_servo_function_t servo1, SRV_Channel::Aux_servo_function_t servo2);
+    void channel_function_mixer(SRV_Channel::Aux_servo_function_t func1_in, SRV_Channel::Aux_servo_function_t func2_in,
+                                SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out);
     void flaperon_update(int8_t flap_percent);
     bool start_command(const AP_Mission::Mission_Command& cmd);
     bool verify_command(const AP_Mission::Mission_Command& cmd);

ArduPlane/servos.cpp

@@ -229,6 +229,27 @@ void Plane::channel_output_mixer(uint8_t mixing_type, SRV_Channel::Aux_servo_fun
 }
 
 
+/*
+  mixer for elevon and vtail channels setup using designated servo
+  function values. This mixer operates purely on scaled values,
+  allowing the user to trim and limit individual servos using the
+  SERVOn_* parameters
+ */
+void Plane::channel_function_mixer(SRV_Channel::Aux_servo_function_t func1_in, SRV_Channel::Aux_servo_function_t func2_in,
+                                   SRV_Channel::Aux_servo_function_t func1_out, SRV_Channel::Aux_servo_function_t func2_out)
+{
+    // the order is setup so that non-reversed servos go "up", and
+    // func1 is the "left" channel. Users can adjust with channel
+    // reversal as needed
+    float in1 = SRV_Channels::get_output_scaled(func1_in);
+    float in2 = SRV_Channels::get_output_scaled(func2_in);
+    float out1 = constrain_float((in2 - in1) * g.mixing_gain, -4500, 4500);
+    float out2 = constrain_float((in2 + in1) * g.mixing_gain, -4500, 4500);
+    SRV_Channels::set_output_scaled(func1_out, out1);
+    SRV_Channels::set_output_scaled(func2_out, out2);
+}
+
+
 /*
   setup flaperon output channels
  */
@@ -614,6 +635,11 @@ void Plane::servo_output_mixers(void)
                                                 (int16_t(ch4)-1500) * (int16_t)(SERVO_MAX/300) / (int16_t)2);
         }
     }
+
+
+    // allow for extra elevon and vtail channels
+    channel_function_mixer(SRV_Channel::k_aileron, SRV_Channel::k_elevator, SRV_Channel::k_elevon_left, SRV_Channel::k_elevon_right);
+    channel_function_mixer(SRV_Channel::k_rudder,  SRV_Channel::k_elevator, SRV_Channel::k_vtail_right, SRV_Channel::k_vtail_left);
 }
 
 /*