AP_Motors: added MOT_BOOST_SCALE

this is to support boost motors for copters, such as having a petrol
motor for extra lift on a multicopter

The scaling factor allows the user to control how much of the lift is
generated from the booster and how much from the main multicopter
motors

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -162,6 +162,14 @@ const AP_Param::GroupInfo AP_MotorsMulticopter::var_info[] = {
     // @Increment: 0.1
     // @User: Advanced
     AP_GROUPINFO("SPOOL_TIME",   36, AP_MotorsMulticopter,  _spool_up_time, AP_MOTORS_SPOOL_UP_TIME_DEFAULT),
+
+    // @Param: BOOST_SCALE
+    // @DisplayName: Motor boost scale
+    // @Description: This is a scaling factor for vehicles with a vertical booster motor used for extra lift. It is used with electric multicopters that have an internal combusion booster motor for longer endurance. The output to the BoostThrottle servo function is set to the current motor thottle times this scaling factor. A higher scaling factor will put more of the load on the booster motor. A value of 1 will set the BoostThrottle equal to the main throttle.
+    // @Range: 0 5
+    // @Increment: 0.1
+    // @User: Advanced
+    AP_GROUPINFO("BOOST_SCALE",  37, AP_MotorsMulticopter,  _boost_scale, 0),
     
     AP_GROUPEND
 };
@@ -217,8 +225,21 @@ void AP_MotorsMulticopter::output()
     
     // convert rpy_thrust values to pwm
     output_to_motors();
+
+    // output any booster throttle
+    output_boost_throttle();
 };
 
+// output booster throttle, if any
+void AP_MotorsMulticopter::output_boost_throttle(void)
+{
+    if (_boost_scale > 0) {
+        float throttle = constrain_float(get_throttle() * _boost_scale, 0, 1);
+        SRV_Channels::set_output_scaled(SRV_Channel::k_boost_throttle, throttle*1000);        
+    }
+}
+    
+
 // sends minimum values out to the motors
 void AP_MotorsMulticopter::output_min()
 {

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -137,6 +137,9 @@ protected:
     // apply any thrust compensation for the frame
     virtual void        thrust_compensation(void) {}
 
+    // output booster throttle, if any
+    virtual void        output_boost_throttle(void);
+    
     // save parameters as part of disarming
     void save_params_on_disarm();
 
@@ -168,6 +171,9 @@ protected:
 
     // time to spool motors to min throttle
     AP_Float            _spool_up_time;
+
+    // scaling for booster motor throttle
+    AP_Float            _boost_scale;
     
     // motor output variables
     bool                motor_enabled[AP_MOTORS_MAX_NUM_MOTORS];    // true if motor is enabled