Translations:KAVAN SLING Mini 616mm - Instruction manual/111/en
From KAVAN RC Wiki
Remove the servo arms from the servos and move carefully the elevator and rudder up&down resp. left&right with your hand repeatedly (10–20 times) to soften the hinge.
If this does not help or does not help enough, you can cut the elevator and rudder with a sharp modeller’s knife completely and hinge them using clear sticky tape. Model hobby shops usually offer special hinge tapes; but the 3M/Scotch® “Crystal” clear sticky tape (width 20 mm) from your local stationery will work well too.
