Reconstructing the tree of life requires deciphering major evolutionary transformations and the functional capacities of fossils with "transitional" morphologies. Some of the most iconic, well-studied fossils with transitional features are theropod dinosaurs, whose skeletons and feathered forelimbs record the origin and evolution of bird flight. However, in spite of over a century of discussion, the functions of forelimb feathers during the evolution of flight remain enigmatic. Both aerodynamic and non-aerodynamic roles have been proposed, but few of the form-function relationships assumed by these scenarios have been tested. Here, we use the developing wings of a typical extant ground bird (Chukar Partridge) as possible analogues/homologues of historical wing forms to provide the first empirical evaluation of aerodynamic potential in flapping theropod "protowings." Immature ground birds with underdeveloped, rudimentary wings generate useful aerodynamic forces for a variety of locomotor tasks. Feather development in these birds resembles feather evolution in theropod dinosaurs, and reveals a predictable relationship between wing morphology and aerodynamic performance that can be used to infer performance in extinct theropods. By spinning an ontogenetic series of spread-wing preparations on a rotating propeller apparatus across a range of flow conditions and measuring aerodynamic force, we explored how changes in wing size, feather structure, and angular velocity might have affected aerodynamic performance in dinosaurs choosing to flap their incipient wings. At slow angular velocities, wings produced aerodynamic forces similar in magnitude to those produced by immature birds during behaviors like wing-assisted incline running. At fast angular velocities, wings produced forces sufficient to support body weight during flight. These findings provide a quantitative, biologically relevant bracket for theropod performance and suggest that protowings could have provided useful aerodynamic function early in maniraptoran history, with improvements in aerodynamic performance attending the evolution of larger wings, more effective feather morphologies, and faster angular velocities.