Subsonic Airfoil Design -- page 1

Historical Background

From its very beginning, the National Advisory Committee for Aeronautics (NACA) recognized the importance of airfoils as a cornerstone of aeronautical research and development. In its first Annual Report to the Congress of the United States, the NACA called for "the evolution of more efficient wing sections of practical form, embodying suitable dimensions for an economical structure, with moderate travel of the center of pressure and still affording a large range of angle of attack combined with efficient action" (ref. 1). By 1920, the Committee had published a compendium of experimental results from various sources (ref. 2). Shortly thereafter, the development of airfoils by the NACA was initiated at the Langley Memorial Aeronautical Laboratory (ref. 3). The first series of airfoils, designated "M sections" for Max M. Munk, was tested in the Langley Variable-Density Tunnel (ref. 4). This series was significant because it represented a systematic approach to airfoil development as opposed to earlier, random, cut-and-try approaches. This empirical approach, which involved modifying the geometry of an existing airfoil, culminated in the development of the four- and five-digit-series airfoils in the mid 1930's (refs. 5-7).

Concurrently, Eastman N. Jacobs began work on laminar-flow airfoils. Inspired by discussions with B. Melvill Jones and G. I. Taylor in England, Jacobs inverted the airfoil analysis method of Theodore Theodorsen (ref. 8) to determine the airfoil shape that would produce the pressure distribution he desired (decreasing pressure with distance from the leading edge over the forward portion of the airfoil). This pressure distribution, it was felt, would sustain laminar flow.

Thus, the basic idea behind modern airfoil design was conceived: the desired boundary-layer characteristics result from the pressure distribution which results from the airfoil shape. The inverse method mathematically transforms the pressure distribution into an airfoil shape whereas the designer intuitively/empirically transforms the boundary-layer characteristics into the pressure distribution.

The resulting 2- through 7-series airfoils, the most notable of which are the 6-series, were tested in the Langley Low-Turbulence Tunnel and the Langley Low-Turbulence Pressure Tunnel (LTPT) in the late 1930's and early 1940's (refs. 9 and 10). To concentrate on high-speed aerodynamics, the NACA got out of the airfoil business in the 1950's, leaving the world with a large number of systematically designed and experimentally tested airfoils (ref. 11). The four- and five-digit-series, turbulent-flow airfoils produced relatively high maximum lift coefficients although their drag coefficients were not particularly low whereas the 6-series, laminar-flow airfoils offered the possibility of low drag coefficients although their maximum lift coefficients were not especially high. The quandary faced by the aircraft designers of the day over the type of airfoil to select, laminar- or turbulent-flow, was solved by the available construction techniques, which produced surfaces that were insufficiently smooth and rigid to support extensive laminar flow.

The airfoil scene then shifted to Germany where F. X. Wortmann and Richard Eppler were engaged in laminar-flow airfoil design. Wortmann employed singularity and integral boundary-layer methods (refs. 12-14) to develop a catalog of airfoils intended primarily for sailplanes (ref. 15). Because the theoretical methods he used were relatively crude, however, final evaluation of the airfoils was performed in a low-turbulence wind tunnel. Eppler, on the other hand, pursued the development of more accurate theoretical methods (refs. 16 and 17.)

The successor to the NACA, the National Aeronautics and Space Administration (NASA), reentered the airfoil field in the 1960's with the design of the supercritical airfoils by Richard T. Whitcomb (ref. 18). The lessons learned during the development of these transonic airfoils were transferred to the design of a series of turbulent-flow airfoils for low-speed aircraft. The basic objective of this series of airfoils was to achieve higher maximum lift coefficients than the earlier NACA airfoils. It was assumed that the flow over these airfoils would be turbulent because of the construction techniques then in use by general aviation manufacturers. While these NASA, turbulent-flow airfoils (ref. 19) did achieve higher maximum lift coefficients, the cruise drag coefficients were no lower than those of the NACA four- and five-digit-series airfoils. Emphasis was therefore shifted toward natural-laminar-flow (NLF) airfoils in an attempt to combine the low-drag characteristics of the NACA 6-series airfoils with the high-lift characteristics of the NASA low-speed airfoils. In this context, the term `natural-laminar-flow airfoil' refers to an airfoil that can achieve significant extents of laminar flow (at least 30-percent chord) on both the upper and lower surfaces simultaneously, solely through favorable pressure gradients (no boundary-layer suction or cooling).

The advent of composite structures (ref. 20) has also fueled the resurgence in NLF research. This construction technique allows NLF airfoils to achieve, in practice, the low-drag characteristics measured in low-turbulence wind tunnels (ref. 21).

Today, airfoils are being designed for an ever-widening range of applications (ref. 22). Examples include unmanned aerial vehicles (ref. 23), cooling-tower fans (ref. 24), sailplanes (refs. 25 and 26), wind turbines (refs. 27 and 28), rotorcraft (ref. 29), and general aviation (ref. 30), commuter (ref. 31), and transport aircraft (ref. 32).