AP_Motors: support inverted flight for helicopters

libraries/AP_Motors/AP_MotorsHeli.h

@@ -92,6 +92,9 @@ public:
     // set_collective_for_landing - limits collective from going too low if we know we are landed
     void set_collective_for_landing(bool landing) { _heliflags.landing_collective = landing; }
 
+    // set_inverted_flight - enables/disables inverted flight
+    void set_inverted_flight(bool inverted) { _heliflags.inverted_flight = inverted; }
+    
     // get_rsc_mode - gets the rotor speed control method (AP_MOTORS_HELI_RSC_MODE_CH8_PASSTHROUGH or AP_MOTORS_HELI_RSC_MODE_SETPOINT)
     uint8_t get_rsc_mode() const { return _rsc_mode; }
 
@@ -185,6 +188,7 @@ protected:
     struct heliflags_type {
         uint8_t landing_collective      : 1;    // true if collective is setup for landing which has much higher minimum
         uint8_t rotor_runup_complete    : 1;    // true if the rotors have had enough time to wind up
+        uint8_t inverted_flight         : 1;    // true for inverted flight
     } _heliflags;
 
     // parameters

libraries/AP_Motors/AP_MotorsHeli_Dual.cpp

@@ -465,6 +465,9 @@ void AP_MotorsHeli_Dual::move_actuators(float roll_out, float pitch_out, float c
         }
     }
 
+    if (_heliflags.inverted_flight) {
+        collective_in = 1 - collective_in;
+    }
 
     float yaw_compensation = 0.0f;
 

libraries/AP_Motors/AP_MotorsHeli_Quad.cpp

@@ -239,12 +239,21 @@ void AP_MotorsHeli_Quad::move_actuators(float roll_out, float pitch_out, float c
         limit.throttle_lower = true;
     }
 
+    if (_heliflags.inverted_flight) {
+        collective_out = 1 - collective_out;
+    }
+    
     // feed power estimate into main rotor controller
     _rotor.set_motor_load(fabsf(collective_out - _collective_mid_pct));
 
     // scale collective to -1 to 1
     collective_out = collective_out*2-1;
 
+    if (collective_out < 0) {
+        // with negative collective yaw torque is reversed
+        yaw_out = -yaw_out;
+    }
+
     float out[AP_MOTORS_HELI_QUAD_NUM_MOTORS] {};
     for (uint8_t i=0; i<AP_MOTORS_HELI_QUAD_NUM_MOTORS; i++) {
         out[i] =

libraries/AP_Motors/AP_MotorsHeli_Single.cpp

@@ -382,6 +382,10 @@ void AP_MotorsHeli_Single::move_actuators(float roll_out, float pitch_out, float
     limit.throttle_lower = false;
     limit.throttle_upper = false;
 
+    if (_heliflags.inverted_flight) {
+        coll_in = 1 - coll_in;
+    }
+        
     // rescale roll_out and pitch_out into the min and max ranges to provide linear motion
     // across the input range instead of stopping when the input hits the constrain value
     // these calculations are based on an assumption of the user specified cyclic_max