Change cg in mixer file for get good flight in xplane

hi… i am using px4 v1.13.3 for connecting px4 to xplane12… i am able to get the input and output from px4… however the flight was not that i expected… I modelled my vtol and during take-off the vtol shows pitch down motion and crash… i doubt about the cg position which is set in the px4… So how to change the cg in the mixer file or any other file… The motor positions are correctly set in mixer, but not sure about cg in px4… So, how to set or change the cg in px4 for xplane12?
thanks in advance…