A number of studies relating to the use of Freon-12 as a substitute medium for air in aerodynamic testing have been made. The use of Freon-12 instead of air makes possible large savings in wind-tunnel drive power. Because of the fact that the ratio of specific heats is approximately 1.13 for Freon-12 as compared with 1.4 for air, some differences exist between data obtained in Freon-12 and air. Methods for predicting aerodynamic characteristics of bodies in air from data obtained in Freon-12, however, have been developed from the concept of similarity of the streamline pattern. These methods, derived from consideration of two-dimensional flows, provide substantial agreement in all cases for which comparative data are available. These data consist of measurements throughout a range of Mach number from approximately 0.4 to 1.2 of pressure distributions and hinge moments on swept and unswept wings having aspect ratios ranging from 4.0 to 9.0, including cases where a substantial part of the wing was stalled.