Complex sentence: Its fuselage was very simple and based on four longerons, rectangular in section and ply-covered, tapering strongly in profile from the square section over the fuselage to a horizontal knife-edge at the tail.
Split into simpler sentences: Its fuselage was very simple and based on four longerons. It was ply-covered and rectangular in section, tapering strongly in profile from the square section over the fuselage to a horizontal knife-edge at the tail.