Philosophy

At this point, it is important to emphasize that the goal of the effort at NASA Langley Research Center was not the design of a series of natural-laminar-flow airfoils for low-speed aircraft. Rather the philosophy of Richard Eppler was adopted, which is to develop a (theoretical) method and verify it such that others can use the method to design airfoils for their own specific applications. The key to the success of this philosophy is the verification of the method. The objective then was to accomplish this verification through selective testing of various airfoil concepts in the Langley Low-Turbulence Pressure Tunnel.

Note that this philosophy is contrary to the approach taken by the NACA and F. X. Wortmann. They developed catalogs from which aircraft designers could select airfoils for their proposed vehicles. This approach was necessary for the NACA because the theoretical methods of the day were too primitive to predict accurately the aerodynamic characteristics of an airfoil. The use of catalogs has been successful, however, because the applications for which the airfoils were used were indeed those for which the airfoils were intended. In addition, the section characteristics were painstakingly measured in good, low-turbulence wind tunnels at the appropriate Reynolds numbers. Thus, the airfoils, their measured characteristics, and their applications coincided well. As applications have become more diverse, however, the older airfoils and the measured characteristics have become less appropriate. Today, with applications ranging from fans to transport aircraft, the use of airfoils designed for aircraft having Reynolds numbers of 3 to 9 million, low Mach numbers, and relatively low lift coefficients is unacceptable. For some applications, the use of such airfoils is particularly unsuitable because the design requirements for these low-speed aircraft airfoils are significantly different from those for the airfoils for the other applications; wind turbines are a good example (ref. 33).

The catalogs also suffer from a lack of coverage. Each application requires a specific performance from the airfoil. If this performance falls within the range of characteristics contained in a catalog, an airfoil can be selected from that catalog for the given application. More than likely, however, this airfoil will still represent a compromise because its characteristics do not match exactly those of the application.

A related advantage of the theoretical airfoil design method is that it allows many different concepts to be explored economically. Such efforts are generally impractical in wind tunnels because of time and money constraints.

Thus, the need for a theoretical airfoil design method is threefold: first, for the design of airfoils that fall outside the range of applicability of existing catalogs; second, for the design of airfoils that more exactly match the requirements of the intended application; and third, for the economic exploration of many airfoil concepts.

The ultimate acceptance of this philosophy faces one final hurdle that can be summed up by the following saying:

This hurdle can be overcome by a rigorous verification of the method.

© 2000 Airfoils, Incorporated.