The Path to Area Rule

Conventional high-speed aircraft design emulated Ernst Mach’s finding that bullet shapes produced less drag. Aircraft designers started with a pointed nose and gradually thickened the fuselage to increase its cross­sectional area, added wings and a tail, and then decreased the diam­eter of the fuselage. The rule of thumb for an ideal streamlined body for supersonic flight was a function of the diameter of the fuselage. Understanding the incorporation of the wing and tail, which were added for practical purposes because airplanes need them to fly, into Mach’s ideal high-speed soon became the focus of Whitcomb’s investigation.[152]

The 8-foot HST team at Langley began a series of tests on various wing and body combinations in November 1951. The wind tunnel mod­els featured swept, straight, and delta wings, and fuselages with varying amounts of curvature. The objective was to evaluate the amount of drag generated by the interference of the two shapes at transonic speeds. The tests resulted in two important realizations for Whitcomb. First, vari­ations in fuselage shape led to marked changes in wing drag. Second, and most importantly, he learned that the combination of fuselage and wing drag had to be considered together as a synergistic aerodynamic system rather than separately, as they had been before.[153]

While Whitcomb was performing his tests, he took a break to attend a Langley technical symposium, where swept wing pioneer Adolf Busemann presented a helpful concept for imagining transonic flow. Busemann asserted that wind tunnel researchers should emulate aero – dynamicists and theoretical scientists in visualizing airflow as analogous to plumbing. In Busemann’s mind, an object surrounded by streamlines constituted a single stream tube. Visualizing "uniform pipes going over the surface of the configuration” assisted wind tunnel researchers in determining the nature of transonic flow.[154]

Whitcomb contemplated his findings in the 8-foot HST and Busemann’s analogy during one of his daily thinking sessions in December 1951. Since his days at Worcester, he dedicated a specific part of his day to thinking. At the core of Whitcomb’s success in solv­ing efficiency problems aerodynamically was the fact that, in the words of one NASA historian, he was the kind of "rare genius who can see things no one else can.”[155] His relied upon his mind’s eye—the non­verbal thinking necessary for engineering—to visualize the aerodynamic process, specifically transonic airflow.[156] Whitcomb’s ability to apply his findings to the design of aircraft was a clear indication that using his mind through intuitive reasoning was as much an analytical aerody­namic tool as a research airplane, wind tunnel, or slide rule.

With his feet propped up on his desk in his office a flash of inspira­tion—a "Eureka” moment, in the mythic tradition of his hero, Edison— led him to the solution of reducing transonic drag. Whitcomb realized that the total cross-sectional area of a fuselage, wing, and tail caused transonic drag or, in his words: "transonic drag is a function of the longitudinal development of the cross-sectional areas of the entire airplane.”[157] It was simply not just the result of shock waves forming at the nose of the airplane, but drag-inducing shock waves formed just behind the wings. Whitcomb visualized in his mind’s eye that if a designer narrowed the fuselage or reduced its cross section, where the wings attached, and enlarged the fuselage again at the trailing edge, then the fuselage would facilitate a smoother transition from subsonic to supersonic speeds. Pinching the fuselage to resemble a wasp’s waist allowed for smoother flow of the streamlines as they traveled from the nose and over the fuselage, wings, and tail. Even though the fuselage was shaped differently, the overall cross section was the same along the length of the fuselage. Without the pinch, the streamlines would bunch and form shock waves, which created the high energy losses that pre­vented supersonic flight.[158] The removal at the wing of those "aerody­namic anchors,” as historians Donald Baals and William Corliss called them, and the recognition of the sensitive balance between fuselage and wing volume were the key.[159]

Verification of the new idea involved the comparison of the data compiled in the 8-foot HST, all other available NACA-gathered transonic data, and Busemann’s plumbing concept. Whitcomb was convinced that his area rule made sense of the questions he had been investigat­ing. Interestingly enough, Whitcomb’s colleagues in the 8-foot HST, including John Stack, were skeptical of his findings. He presented his findings to the Langley community at its in-house technical seminar.[160] After Whitcomb’s 20-minute talk, Busemann remarked: "Some peo­ple come up with half-baked ideas and call them theories. Whitcomb comes up with a brilliant idea and calls it a rule of thumb.”[161] The name "area rule” came from the combination of "cross-sectional area” with "rule of thumb.”[162]

With Busemann’s endorsement, Whitcomb set out to validate the rule through the wind tunnel testing in the 8-foot HST. His models fea­tured fuselages narrowed at the waist. He had enough data by April 1952 indicating that pinching the fuselage resulted in a significant reduction in transonic drag. The resultant research memorandum, "A Study of the Zero Lift Drag Characteristics of Wing-Body Combinations near the Speed of Sound,” appeared the following September. The NACA immediately distributed it secretly to industry.[163]

The area rule provided a transonic solution to aircraft designers in four steps. First, the designer plotted the cross sections of the aircraft fuselage along its length. Second, a comparison was made between the design’s actual area distribution, which reflected outside considerations, such as engine diameter and the overall size dictated by an aircraft car­rier’s elevator deck, and the ideal area distribution that originated in previous NACA mathematical studies. The third step involved the recon­ciliation of the actual area distribution with the ideal area distribution. Once again, practical design considerations shaped this step. Finally, the designer converted the new area distribution back into cross sec­tions, which resulted in the narrowed fuselage that took into account the overall area of the fuselage and wing combination.[164] A designer that followed those four steps would produce a successful design with min­imum transonic drag.