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Abstract
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 Universität
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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