A gas turbine engine includes a turbomachinery core operable to generate a flow of pressurized combustion gases at a variable flow rate, while maintaining a substantially constant core pressure ratio; a rotating fan disposed upstream of the core, the fan adapted to extract energy from the core and generate a first flow of air which is compressed at a first pressure ratio; and at least a first bypass duct surrounding the core downstream of the fan adapted to selectively receive at least a first selected portion of the first flow which is compressed at a second pressure ratio lower than the first pressure ratio, and to bypass the first selected portion around the core, thereby varying a bypass ratio of the engine. The fan is adapted to maintain a flow rate of the first flow substantially constant, independent of the bypass ratio.