The focus of this investigation is on the development and validation of structural dynamic reduced order models for the geometrically nonlinear response of flat cantilevered structures, e.g. beams and plates. The specificities of cantilevered structures, as compared to those supported all around, are first highlighted. On this basis, extensions of existing reduced order modeling strategies are presented which provide a complete representation of the structural response, i.e., including both transverse and inplane displacement fields. Next, this methodology is successfully applied, first to a simple beam model and then to a flat wing both of which subjected to transverse loads. Finally, the response of a beam under combined transverse and inplane loads as well as its postbuckling behavior are also shown to be also accurately predicted by the reduced order model.