A proprotor system for a ducted aircraft convertible between a vertical takeoff and landing flight mode and a forward flight mode includes a plurality of proprotor blades and a duct surrounding the proprotor blades. The duct includes an adaptive geometry device movable into various positions including a hover position and a cruise position. One or more actuators coupled to the adaptive geometry device are configured to move the adaptive geometry device between the hover position and the cruise position based on the flight mode of the ducted aircraft, thereby improving flight performance of the ducted aircraft.