Strange behavior of takeoff in AR Drone simulator if launch file is separated.

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I use tum_simulator for AR Drone from here

I tried to fly the drone. So I launched:

roslaunch cvg_sim_gazebo ardrone_testworld.launch

and I send empty message so the drone take off ( refer to wiki page )

rostopic pub -1 /ardrone/takeoff std_msgs/Empty

It worked fine : The drone take off and stay at a certain height..

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HOWEVER If I separated the ardrone_world.launch into 2 parts, it will make a strange take off behavior when I tried to take off: It kept flying higher and higher (not stay at a certain level)

The ardrone_world.launch is:

<?xml version="1.0"?>
<launch>
  <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find cvg_sim_gazebo)/worlds/ardrone_testworld.world"/>
  </include>


  <!-- Spawn simulated quadrotor uav -->
  <include file="$(find cvg_sim_gazebo)/launch/spawn_quadrotor.launch" >
    <arg name="model" value="$(find cvg_sim_gazebo)/urdf/quadrotor_sensors.urdf.xacro"/> 
  </include>
</launch>

The launch file is basically just launch a world, and a quadrotor.

So , I created a launch file (world.launch) that consist only the part to load/launch the world.

Then I launch the world.launch. And I launch the spawn_quadrotor.launch

Then I tried to take off, but the behavior is different with the previous one : It kept flying higher and higher.

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How can that be different? I meant It supposed to do the same thing.