The traditional approach to axial-flow compressor aerodynamic design was to use various families of airfoils as the basis for blade design. American practice was based on various families defined by the National Advisory Committee for Aeronautics (NACA), the most popular being the 65-series family. British practice often centered about the C-series families, using circular-arc or parabolic-arc camberlines. As design requirements began to favor transonic operation, double-circular-arc blades became popular. The performance characteristics of these airfoil families are well understood due to extensive experimental cascade testing, much of which is available in the literature.

In recent years, use of blades designed for a prescribed surface velocity distribution or blade loading style, instead of for predefined airfoil families has become popular. Often, inverse design methods that predict the blade shape required for the desired blade loading are used. As the relation between blade shape and preferred loading styles became better understood, it also became common to use conventional or direct analysis methods in a trial-and-error mode to arrive at the same result. These airfoils have been referred to as prescribed velocity distribution (PDF) blades (Cumpsty, 1989), even though the term controlled diffusion airfoils is probably more common today. Although the literature offers general guidelines for these designs, the actual airfoil designs in use are proprietary. In general, the performance characteristics of these airfoils are well known only to the organizations that developed them.

As discussed in the preface to this book, this situation posed a significant complication to the goal of providing a complete description of the working design and analysis system. It was quickly recognized that it is no longer possible to write a book that can be directly applied to all of the many proprietary designs in use today. But this is not considered to be a serious limitation. In this writer's experience, the process of adapting classical blade performance prediction models to a more modern controlled diffusion airfoil design is not particularly difficult, assuming the performance characteristics of the airfoil are known.