Publisher for ardrone

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Hi there, i have a problem with publishing sth. to the ardrone. I am using ROS Hydro and this is a part of my code for one of the publishers i am using:

(in the header-file)

ros::Publisher land_pub;

(in cpp-file)

ros::NodeHandle nh; land_pub = nh.advertise<std_msgs::empty>("ardrone/land", 1);

(with a button-klick of my joypad i want to publish this message:)

std_msgs::Empty emptymsg; land_pub.publish(emptymsg);

But when i am using "rostopic echo ardrone/land" (or takeoff) nothing is published and my ardrone does nothing... but if i am publishing under this topic via terminal it works... could anybody help me solving this, please?