Complex sentence: Rotorcraft is a category of heavier-than-air flying machines that use lift generated by rotors but may also include the use of static lifting surfaces, the primary distinguishing feature being lift provided by rotating lift structures.
Split into simpler sentences: Rotorcraft is a category of heavier-than-air flying machines that use lift generated by rotors. They may also include the use of static lifting surfaces, but the primary distinguishing feature being lift provided by rotating lift structures.