Is it possible to manually control plane servo and motor?

Hello
I am trying to control motor and servo manually with mavlink but still cannot figure it out and cannot find how to do it. I find a way to set parameter with service /mavros/param/set but still dont know which one that control motor and servo. I use basic plane model that comes with px4 firmware. Thanks.

Is this an extra servo, or part of the main mixer?

I mean servo in aileron, elevator and rudder

here what i have done so far

/*
Copyright Marcel StĆ¼ttgen 2015 <stuettgen@fh-aachen.de>
simple ros node that will publish ActuatorControl message to /mavros/actuator_controls
*/
#include <ros/ros.h>
#include <mavros/ActuatorControl.h>
int main(int argc, char *argv[])
{
    ros::init(argc, argv, "actuator_controls_publisher");
    ros::NodeHandle nh;
    ros::Publisher actuator_controls_pub = nh.advertise<mavros::ActuatorControl>("/mavros/actuator_controls", 1000);
    ros::Rate loop_rate(10);
    double ros_roll;
    double ros_pitch;
    double ros_yaw;
    double ros_throttle;
    while (ros::ok())
    {
        mavros::ActuatorControl actuator_control_msg;
        nh.param<double>("ros_roll", ros_roll, 0.0);
        nh.param<double>("ros_pitch", ros_pitch, 0.0);
        nh.param<double>("ros_yaw", ros_yaw, 0.0);
        nh.param<double>("ros_throttle", ros_throttle, 0.0);
        /*send to control group 0:
        0:roll
        1:pitch
        2:yaw
        3:throttle
        4:flaps
        5:spoilers
        6:airbrakes
        7:landing gear
        see https://pixhawk.ethz.ch/mavlink/#SET_ACTUATOR_CONTROL_TARGET
        and https://pixhawk.org/dev/mixing
        */
        actuator_control_msg.header.stamp = ros::Time::now();
        actuator_control_msg.group_mix = 0;
        actuator_control_msg.controls[0] = ros_roll;
        actuator_control_msg.controls[1] = ros_pitch;
        actuator_control_msg.controls[2] = ros_yaw;
        actuator_control_msg.controls[3] = ros_throttle;
        actuator_control_msg.controls[4] = 0.0;
        actuator_control_msg.controls[5] = 0.0;
        actuator_control_msg.controls[6] = 0.0;
        actuator_control_msg.controls[7] = 0.0;
        actuator_controls_pub.publish(actuator_control_msg);
        ros::spinOnce();
        loop_rate.sleep();
    }
    return 0;
}

I got that example for ground rover from here but I think it could be implemented for fixedwing. the problem is the system always fail to change mode to OFFBOARD. the response message is TIMED OUT.
then I try to combine offb_node.cpp from development web with that code, and still fail to switch to OFFBOARD mode. Got response success to switch to OFFBOARD mode but it change to AUTO.LOITER mode.

I suggest you to use dronekit python channel override.