Incompressible Aerodynamics: Summary

It is now becoming clear that it is also mistaken to assume that computers could pro­duce optimum designs in an empirical manner: it cannot be carried out in practice.

D. Kuchemann “The Aerodynamic Design of Aircraft” Pergamon Press, 1978

9.1 Introduction

The preceding eight chapters take wholesale advantage of the assumption that the flow field for low-speed flight is incompressible. This allows considerable sim­plification in the formulation of the governing equations and in the solution of key aerodynamic problems. However, results of the calculations are limited in an important way that is emphasized in this summary chapter. What we attempt to do here is:

0. Summarize the most important elements of the first eight chapters.

1. Demonstrate how the results are incorporated in actual vehicle design.

2. Define the limits of application of the results.

Modeling of Airflows

What is accomplished to this point is the application of basic fluid mechanics in con­structing detailed models for the airflow over aerodynamic surfaces (e. g., wings and bodies) at speeds low enough that compressibility effects do not seriously affect the results. These models are intended to provide accurate estimates of the aerodynamic forces and moments needed in solving the basic problem of aerodynamics as it was defined in Chapter 1. Although there is much discussion centered on the application of modern computational tools, for the most part, we rely on simplified mathemat­ical representations. We try to emphasize the role of valid, simplifying assumptions in arriving at useful representations for the airflow. As Kuchemann described the pro­cess in his famous book on the aerodynamic design of aircraft (Kuchemann, 1978), “. . . the most drastic simplifying assumptions must be made before we can even
think about the flow of gases and arrive at equations which are amenable to treat­ment. Our whole science lives on highly idealized concepts and ingenious abstrac­tions and approximations.” First-class examples of this approach are demonstrated in this book, including Prandtl’s elegant models describing the creation of lift by an airfoil, three-dimensional wing theory, and boundary-layer flows. These provide the backbone of the subject of aerodynamics.