In real flight, the pusher motor control airspeed and maintain it constantly at trim airspeed with some low oscillations and ground speed changes more because of wind speed. also, the throttle oscillates around the cruise throttle.
but in the gazebo SITL test with the standard plane, thrust increases to 100 % but can not increase airspeed to trim airspeed in the front wind. thrust decrease to 0% but can not decrease airspeed to trim airspeed in a tailwind.
I think may drag calculated by ground speed, not airspeed.