THE ERA OF THE STRUT-AND-WIRE BIPLANE

In this and the subsequent sections, specific examples of important advances in aerodynamics will be examined, primarily from the point of view of the relative roles of science, engineering science, and engineering. The state of the art in aerodynamics has grown exponentially since the turn of the century; we can not do justice to the whole story in this limited paper. Instead, only a few specific examples from each era will be considered. In the present section, we examine some developments in aerodynamics contemporary with the hey-day of the strut-and-wire biplane exemplified by the British S. E.5 from World War I (Figure 3).

With Wilbur Wright’s flying demonstrations in Europe, which began on August 8, 1908, the world truly discovered the existence of the successful airplane. With this “discovery”, the attitudes surrounding the value of scientific and engineering work in aerodynamics changed radically. Almost overnight it became fashionable, indeed critical, to learn more about the laws of nature that sustained these flying machines in the air, and to develop engineering techniques that could lead to improved aerodynamic design. We have already discussed how academic science met the flying machine at the turn of the century. Now, with Wilbur’s dramatic demonstration that the airplane was indeed an established reality, the world of professional engineering suddenly had a new and very exciting discipline to develop.

This technical awakening was accompanied by the sound of new wind tunnels revving up throughout Europe. Nowhere was this as dramatic as in the shadow of the Eiffel Tower in Paris. In 1909, Gustav Eiffel designed and built a large wind tunnel on the Champ de Mars adjacent to the famous tower he had erected 20 years earlier. Indeed, Eiffel was soon to become France’s first great aerodynamicist on the strength of his wind tunnel experiments. Today, the name of Eiffel rarely crosses the lips of practitioners and students of aerodynamics. In fact, most people in general do not associate Eiffel’s name with aerodynamics at all. However, his contributions to

experimental aerodynamics were as important in the history of technology as were his structural innovations embodied in the design and construction of the Eiffel tower. At the beginning of the twentieth century, Eiffel pioneered some of the experimental techniques which we still use today, and in the process he was the first to quantitatively measure some of the most basic aerodynamic aspects of a complete airplane configuration.

The wind tunnel experiments at the Champ-de-Mars laboratory conducted during 1909 and 1910 led to five substantial contributions:

1. First, there was the wind tunnel itself, an innovative design using a free jet in a hermetically-sealed chamber. This was pure engineering.

2. Eiffel found that drag measurements made in his wind tunnel agreed with earlier measurements he made by dropping aerodynamic shapes from his Tower. Eiffel finally proved experimentally once-and-for-all the basic principle of the wind tunnel that had been first stated by Leonardo da Vinci more than four centuries earlier, namely that “the same force as is made by the thing against the air, is made by air against the thing.” Some doubt about this persisted until the twentieth century, even though wind tunnels had been in use since their invention by Francis Wenham in 1871. Eiffel proved the validity of the wind tunnel principle. This was science.

3. He was the first to make detailed measurements of the distribution of pressure over the surface of an aerodynamic body, proving conclusively that the aerodynamic lift on a wing was due to the presence of lower pressure on the top surface and higher pressure on the bottom surface. Moreover, he proved conclusively that the majority of the lift on a wing is derived not from the higher pressure exerted on the bottom of the wing, but rather from the lower pressure exerted on the top of the wing. This was engineering science.

4. Eiffel pioneered the general principle that the net resultant aerodynamic lift on a body is due to the integrated effect of the pressure distribution exerted over the surface, and he was the first to prove it. Using his measured pressure distributions on one hand, and his direct measurements of lift using a force balance on the other, he was able to state: “The direct measurement of pressure has given us a result to which we attach great importance; viz., the summation of the observed pressures was equal in every case to the reaction weighed on the balance.9 This was engineering science.

5. Eiffel was the first to conduct wind tunnel tests using models of complete airplanes, and to show conclusively the correspondence between such tests and the performance of the real airplane in actual flight. This was pure aeronautical engineering.

Perhaps the most important contribution Eiffel made to aerodynamics was as follows. Over the course of his earlier aerodynamic work, Eiffel had measured the drag of spheres. However, in experiments at Prandtl’s laboratory at Gottingen, the data showed sphere drag to be more than twice as large as Eiffel’s measurement. Eiffel was angered by a German insinuation that he had made a mistake, and in 1914 he carried out a definitive series of drag measurements on spheres in a new laboratory at Auteuil, a suburb of Paris. Testing spheres of various sizes, he found out that for each size, there was a velocity above which the drag decreased markedly – by slightly more than a factor of two. Every student of aerodynamics today recognizes this variation, and knows that the sudden decrease in drag is associated with a transition of the flow inside the boundary layer from laminar to turbulent at a value of the Reynolds number of about 300,000. Prandtl was the person who eventually explained why this phenomenon occurs. But Eiffel was the person to first observe and publish the phenomenon. This was a purely scientific contribution.

The period during and just after World War I saw major advances in the theoretical calculation of airfoil and wing aerodynamics. Based on the circulation theory of lift, Prandtl conceived a theoretical model of the aerodynamic properties for a finite wing (a real wing with wing tips, in contrast to the two-dimensional aspects of an airfoil shape). Labeled “Prandtl’s lifting line theory”, this model allowed the calculation of lift and induced drag (a pressure drag due to the influence of vortices generated at the wind tips and trailing edge of the wing). This is a rational, engineering-oriented theory that could be applied to the wings of real airplanes. It is still used today. In a similar vein, one of Prandtl’s colleagues, Max Munk, after immigrating to the United States after the war and going to work for the National Advisory Committee for Aeronautics (the NACA), derived the first practical theory for the calculation of the lift of airfoils of any arbitrary shape, as long as the airfoils are relatively thin. Prandtl’s lifting line theory for finite wings, combined with Munk’s thin airfoil theory, represented major contributions to applied aerodynamics. This was one of the first important examples of engineering science in the twentieth century. However, again emphasis is made that these theories were after-the-fact; airfoils and wings, albeit not optimum, were being designed successfully based on empirical experience and routine wind tunnel testing long before these theoretical tools became available.