sending velocity commands to the robot throught cmd_vel topic

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Hi, I want to send velocity comands to move th bebop drone up and then forward. Here is my code:

#include<ros/ros.h>
#include<nav_msgs/Odometry.h>
#include<geometry_msgs/Twist.h>
#include<stdlib.h>
#include<math.h>

float tag_yaw,tag_x,tag_y,tag_z;
ros::Publisher pub;
ros::Subscriber sub;
geometry_msgs::Twist msg;
float err;


 void getpos(const nav_msgs::Odometry& posmsg){

 if (posmsg.pose.pose.position.z<= 1){

 ROS_INFO_STREAM( "Showing position : "
 << "x1 =" << posmsg.pose.pose.position.x
 << "y1 =" << posmsg.pose.pose.position.y
  << "z1 =" << posmsg.pose.pose.position.z);

   msg.linear.x=0.0;
   msg.linear.y=0.0;
   msg.linear.z=  0.1;   // exp ( 0.1);
   msg.angular.x= 0;
   msg.angular.y= 0;
   msg.angular.z=0.0;
    }
    else {

    ROS_INFO_STREAM( "Showing position : "
    << "x =" << posmsg.pose.pose.position.x
    << "y =" << posmsg.pose.pose.position.y
    << "z =" << posmsg.pose.pose.position.z);

     msg.linear.x=0.8*(tag_x-posmsg.pose.pose.position.x);
     msg.linear.y=0.0;
     msg.linear.z=  0.0;   // exp ( 0.1);
     msg.angular.x= 0;
     msg.angular.y= 0;
     msg.angular.z=0.0;
     }

      pub.publish(msg);
     }


   int main(int argc, char **argv){
   ros::init(argc, argv,"velBebop");
   ros::NodeHandle nh;

  tag_x= .5;
  tag_y= 0.0;
   tag_z= 0.0;
   sub = nh.subscribe("/bebop/odom",100, &getpos) ;
    pub = nh.advertise<geometry_msgs::Twist>("/bebop/cmd_vel" ,100);

    ros::spin();
    return 0;
     }

I can see that the velocities are being published through the rqt_graph here:

but the drone is not moving at all! it is not even rotating the helices. Any suggestion!!! Thanks