AP_MotorsHeli: Overload update_throttle_filter()

Do not reset filter to zero when disarmed to allow collective pitch servo movement.

libraries/AP_Motors/AP_MotorsHeli.cpp

@@ -820,3 +820,12 @@ void AP_MotorsHeli::set_delta_phase_angle(int16_t angle)
     _delta_phase_angle = angle;
     calculate_roll_pitch_collective_factors();
 }
+
+// update the throttle input filter
+void AP_MotorsHeli::update_throttle_filter()
+{
+    _throttle_filter.apply(_throttle_in, 1.0f/_loop_rate);
+
+    // prevent _rc_throttle.servo_out from wrapping at int16 max or min
+    _rc_throttle.servo_out = constrain_float(_throttle_filter.get(),-32000,32000);
+}

libraries/AP_Motors/AP_MotorsHeli.h

@@ -215,6 +215,9 @@ protected:
     void                output_armed_zero_throttle();
     void                output_disarmed();
 
+    // update the throttle input filter
+    void                update_throttle_filter();
+
 private:
 
     // heli_move_swash - moves swash plate to attitude of parameters passed in

libraries/AP_Motors/AP_Motors_Class.h

@@ -223,7 +223,7 @@ protected:
     virtual void        output_disarmed()=0;
 
     // update the throttle input filter
-    void                update_throttle_filter();
+    virtual void        update_throttle_filter();
 
     // update_max_throttle - updates the limits on _max_throttle for slow_start and current limiting flag
     void                update_max_throttle();