Copter: tell motors library when to use acro gyro gain

ArduCopter/flight_mode.cpp

@@ -241,6 +241,7 @@ void Copter::exit_mode(uint8_t old_control_mode, uint8_t new_control_mode)
     // firmly reset the flybar passthrough to false when exiting acro mode.
     if (old_control_mode == ACRO) {
         attitude_control.use_flybar_passthrough(false, false);
+        motors.set_acro_tail(false);
     }
 
     // reset RC Passthrough to motors

ArduCopter/heli_control_acro.cpp

@@ -13,6 +13,8 @@ bool Copter::heli_acro_init(bool ignore_checks)
     // if heli is equipped with a flybar, then tell the attitude controller to pass through controls directly to servos
     attitude_control.use_flybar_passthrough(motors.has_flybar(), motors.supports_yaw_passthrough());
 
+    motors.set_acro_tail(true);
+    
     // always successfully enter acro
     return true;
 }