Runback ice accretions present a unique situation in iced-airfoil aerodynamics in that the airfoil typically has a clean leading edge before the ice accretion. To investigate the aerodynamic effects of runback ice accretions, simulations were scaled from accretions obtained in the NASA Glenn Icing Research Tunnel for testing in the Illinois subsonic wind tunnel. Simple geometric scaling, based on airfoil chord, as well as boundary-layer scaling, based on estimated boundary-layer thickness, was used. The NACA 3415 and the NACA 23012 airfoils were tested at a Reynolds number of 1.8 x 10⁶ and Mach number of 0.18. Simple two-dimensional simulations were tested as well as three-dimensional simulations that more accurately simulated the features of the full-scale ice accretion. Significant aerodynamic penalties due to runback accretions were identified. In the worst case these penalties included a loss of over 0.75 in C_l,max and 7 deg in stalling angle of attack. In certain cases scaled runback accretions were found to increase the stalling angle of attack and maximum-lift coefficient. This phenomenon was investigated using boundary-layer measurements and fluorescent-oil flow visualization. It was concluded that the interaction between the boundary layer and the simulation was responsible for the phenomenon.