Artist's impression of Sikorsky TRAC (Telescoping Rotor Aircraft)
Rotor system in which the rotor diameter could be decreased up to 60%. TRAC system intended to be applied to any VTOL that uses rotors for lift (i.e. compound helicopter and tiltrotor). In this design, the rotors would tilt for horizontal flight and then fold into nacelles. Turbofans would take over for cruise.