Complex sentence: The "contract" of the book title refers to the Wright-Smithsonian agreement, which prohibits the Smithsonian from officially recognizing any manned, powered, controlled airplane flight before that of the Wright brothers on Dec.17,1903.
Split into simpler sentences: The "contract" of the book title refers to the Wright-Smithsonian agreement. It prohibits the Smithsonian from officially recognizing any manned, powered, controlled airplane flight before that of the Wright brothers on Dec.17,1903.