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Subsonic Airfoil Design


Airfoil design has progressed considerably over the past century. The first airfoils were mere copies of birds's wings. These airfoils were followed by cut-and-try shapes, some of which were tested in simple, low-Reynolds-number wind tunnels. The National Advisory Committee for Aeronautics (NACA) systematized this approach by perturbing successful airfoil geometries to generate series of related airfoils. These airfoils were carefully tested in a more sophisticated wind tunnel that could replicate flight Reynolds numbers. Eastman Jacobs of NACA recognized the need for a theoretical method that would determine the airfoil shape that would produce a specified pressure distribution that would exhibit the desired boundary-layer characteristics. This idea represents the basis of modern airfoil design: the desired boundary-layer characteristics result from the pressure distribution, which results from the airfoil shape.

The inversion of an airfoil analysis method provided the means of transforming the pressure distribution into an airfoil shape. The transformation of the desired boundary-layer characteristics into a pressure distribution was left to the imagination of the airfoil designer. Since that time, over 60 years ago, Richard Eppler of Universitšt Stuttgart, through his computer code, has developed a much more direct connection between the boundary-layer development and the pressure distribution.

The National Aeronautics and Space Administration (NASA) adopted the philosophy of Eppler that a reliable theoretical airfoil design method should be developed instead of catalogs of experimental section characteristics. The method can then be used to explore many concepts with respect to each specific application. The success of this philosophy hinges on the verification of the method.

Several airfoils have been designed to test Eppler's method. By investigating the airfoils in low-turbulence wind tunnels, the range of applicability of the method has been established. Initially, the classical, low-speed Reynolds-number range of 3 to 9 million was investigated. From there, higher Reynolds numbers (~20 million) and Mach numbers (~0.7) were explored. More recently, lower Reynolds numbers (~0.5 million) have been investigated. The latest indications are that the method is also applicable at even lower Reynolds numbers (~0.1 million). The method has been steadily improved in response to inadequacies revealed during these experimental investigations.

In summary, an experimentally-verified, theoretical method has been developed that allows airfoils to be designed for almost all subcritical applications.

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