An integrally bladed fan (IBF) rotor of a gas turbine engine. The IBF rotor includes a hub and a plurality of fan blades extending radially outwardly from the hub and integral therewith. The hub has a fan attachment flange disposed at an end of the hub on a trailing edge side thereof for mounting a booster rotor to a trailing edge side of the fan. The fan attachment flange is disposed at a radial distance from a longitudinal center axis of the integrally bladed fan rotor. The hub has an outer hub surface disposed radially inward from the radial distance of the fan attachment flange.