Effects of Surface Emittance on Turbulent Skin Friction at Supersonic and Low Hypersonic Speeds

A search of available literature indicates that the radiative properties of supersonic aircraft materials depend heavily upon the metal chosen for the aircraft skin and the degree of oxidation of the surface. A calculative study has been made to determine the effect of emittance upon wall temperature and skin friction over a range of supersonic and low hypersonic flight conditions. Calculations were made for Mach numbers up to 9, altitudes up to 80 000 feet, and vehicle lengths up to 200 feet. The results indicate that the skin-friction drag of a supersonic aircraft increases with increased surface emittance. Emittance increases with surface oxidation. Thus, for a new supersonic aircraft with skin made of a metal with low radiative properties in the polished, unoxidized state, the skin-friction drag will increase with time until the surface becomes stably oxidized. This effect of emittance on skin friction increases with Mach number and becomes substantial above Mach numbers of about 2.5. Hence, for specified flight conditions, the level of skin-friction drag at which the vehicle operates depends upon the m tal used for the skin of the aircraft and the degree of oxidation of the metal.