Airspeed auto tune problem

am using pixhawk cube (black). aircraft model is flyingwing . am given cruise speed 20 at the same time i change the parameter arspd_autocal =1 for auto tuning purpose. am instially tack off in FBWA mode and at 65 meters altitude given auto tune then autotune completd the airspeed sensor ratio is 3.56 like that, then after am given full throttle the plane is not going that much of speed . what is the proble . please help me
log file link: