The term 'copter forerunner' can be broken down into four crossword solutions: GIRO (4 letters), ROTOR (5 letters), ROTORS (6 letters), and GIROS (5 letters). GIRO is a type of aircraft that was developed in the early 20th century, and it is considered to be the predecessor of the modern helicopter. ROTOR is the spinning part of a helicopter that provides lift and propulsion. ROTORS is the plural form of ROTOR, and GIROS is the plural form of giro. HELI (4 letters) is another word for helicopter. All of these words are related to the term 'copter forerunner, ' as they all refer to the development of the modern helicopter.