Complex sentence: The S.510 was doomed to obsolescence before it even flew, although when it was designed many pilots and experts strongly believed that biplanes would prove better fighters than monoplanes because of their tighter turning circles.
Split into simpler sentences: The S.510 was doomed to obsolescence before it even flew. Though at the time it was designed, many pilots and experts strongly believed that biplanes would prove better fighters than monoplanes because of their tighter turning circles.