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An investigation was conducted in the Langley 6- by 28-Inch Transonic Tunnel to determine the two-dimensional aerodynamic characteristics of three helicopter rotor airfoils at Reynolds numbers from typical model scale to full scale at Mach numbers from about 0.35 to 0.90. The model-scale Reynolds numbers ranged from about 0.7 times 10 to the 6th power to 1.5 times 10 to the 6th power and the full-scale Reynolds numbers ranged from about 3.0 times 10 to the 6th power to 6.6 times 10 to the 6th power. The airfoils tested were the NACA 0012 (0 deg Tab), the SC 1095-R8, and the SC 1095. Both the SC 1095 and the SC 1095-R8 airfoils a had trailing-edge tabs. The results of this investigation indicate that Reynolds number effects can be significant on the maximum normal-force coefficient and all drag-related parameters; namely, drag at zero normal force, maximum normal-force--drag-ratio, and drag-divergence Mach number. In general, the increments in these parameters at a given Mach number owing to the model-scale to full-scale Reynolds number change are different for each of the airfoils. (Author)