Turbulent Skin Friction by the Reference Temperature Method of Sommer and Short. The calculation of skin friction on an airplane from first principles is an incredibly complex task and requires powerful methods of computational fluid mechanics. Engineering estimates are usually made from formulas that were developed to provide an empirical fit to extensive sets of wind tunnel data on flat plates with no external pressure gradient. From the vast collection of measurements of skin friction, there have been many attempts to make a simple formula which fits the observed data. All of these formulas are in some sense successful, and the student of aerodynamics may choose from among them. The formula developed by Sommer and Short has one feature that is not found in most other equations. Sommer and Short were primarily concerned with the large amount of scatter in the data at low hypersonic Mach numbers, and Reynolds numbers of about one million or less. They found that better correlations could be obtained if the temperature of the flow is included as a variable. As a result, the function supplied here has three arguments, Reynolds number, Mach number and freestream temperature. From a few numerical experiments, the user can see that for low Mach numbers (say < 3) the value of skin friction coefficient returned is quite independent of temperature. Temperature is of little importance at high Reynolds number, regardless of Mach number. But, if you are designing a trans- atmospheric vehicle for high altitudes at hypersonic speeds, you may want this added variable