History of Airfoil Development

The earliest serious work on the development of airfoil sections began in the late 1800's. Although it was known that flat plates would produce lift when set at an angle of incidence, some suspected that shapes with curvature, that more closely resembled bird wings would produce more lift or do so more efficiently. H.F. Phillips patented a series of airfoil shapes in 1884 after testing them in one of the earliest wind tunnels in which "artificial currents of air (were) produced from induction by a steam jet in a wooden trunk or conduit." Octave Chanute writes in 1893, "...it seems very desirable that further scientific experiments be be made on concavo-convex surfaces of varying shapes, for it is not impossible that the difference between success and failure of a proposed flying machine will depend upon the sustaining effect between a plane surface and one properly curved to get a maximum of 'lift'."

At nearly the same time Otto Lilienthal had similar ideas. After carefully measuring the shapes of bird wings, he tested the airfoils below (reproduced from his 1894 book, "Bird Flight as the Basis of Aviation") on a 7m diameter "whirling machine". Lilienthal believed that the key to successful flight was wing curvature or camber. He also experimented with different nose radii and thickness distributions.

Airfoils used by the Wright Brothers closely resembled Lilienthal's sections: thin and highly cambered. This was quite possibly because early tests of airfoil sections were done at extremely low Reynolds number, where such sections behave much better than thicker ones. The erroneous belief that efficient airfoils had to be thin and highly cambered was one reason that some of the first airplanes were biplanes.

The use of such sections gradually diminished over the next decade.

A wide range of airfoils were developed, based primarily on trial and error. Some of the more successful sections such as the Clark Y and Gottingen 398 were used as the basis for a family of sections tested by the NACA in the early 1920's.

In 1939, Eastman Jacobs at the NACA in Langley, designed and tested the first laminar flow airfoil sections. These shapes had extremely low drag and the section shown here achieved a lift to drag ratio of about 300.

A modern laminar flow section, used on sailplanes, illustrates that the concept is practical for some applications. It was not thought to be practical for many years after Jacobs demonstrated it in the wind tunnel. Even now, the utility of the concept is not wholly accepted and the "Laminar Flow True-Believers Club" meets each year at the homebuilt aircraft fly-in.

One of the reasons that modern airfoils look quite different from one another and designers have not settled on the one best airfoil is that the flow conditions and design goals change from one application to the next. On the right are some airfoils designed for low Reynolds numbers.

At very low Reynolds numbers (<10,000 based on chord length) efficient airfoil sections can look rather peculiar as suggested by the sketch of a dragonfly wing. The thin, highly cambered pigeon wing is similar to Lilienthal's designs. The Eppler 193 is a good section for model airplanes. The Lissaman 7769 was designed for human-powered aircraft.

Unusual airfoil design constraints can sometimes arise, leading to some unconventional shapes. The airfoil here was designed for an ultralight sailplane requiring very high maximum lift coefficients with small pitching moments at high speed. One possible solution: a variable geometry airfoil with flexible lower surface.

The airfoil used on the Solar Challenger, an aircraft that flew across the English Channel on solar power, was designed with an totally flat upper surface so that solar cells could be easily mounted.

The wide range of operating conditions and constraints, generally makes the use of an existing, "catalog" section, not best. These days airfoils are usually designed especially for their intended application. The remaining parts of this chapter describe the basic ideas behind how this is done.