When estimating the skin friction drag on bodies of revolution at the project design stage, it is normal practice to use an “equivalent flat-plate” method. This calculation is simple and requires little time to perform. However, in the final design stage, when more accurate estimates of the skin friction drag should be made the “equivalent flat-plate” method is still used. More exact methods, which include pressure gradient effects, are very laborious if they are evaluated by hand calculators. Even if a digital computor is used to evaluate the more exact methods, the time required to perform the calculations is considerably longer than that taken to calculate the other forms of drag. For these reasons, the simple “flat-plate” method has long held favour with designers.