NASA's Contributions to Aeronautics, Volume 1 by National Aeronautics & Space Administration. - HTML preview

CASE

2

Richard Whitcomb and the Quest for Aerodynamic Efficiency

Jeremy Kinney

Case-2 Cover Image: Whitcomb evaluates the shape of one of his area rule models in the 8-foot High Speed Tunnel. NASA.

On December 21, 1954, Convair test pilot Richard L. “Dick” Johnson flew the YF-102A Delta Dagger prototype to Mach 1, an achievement that marked the meeting of a challenge that had been facing the American aeronautical community. The Delta Dagger’s contoured fuselage, shaped by a new design concept, the area rule, enabled an efficient transition from subsonic to supersonic via the transonic regime. Seventeen years later, test pilot Thomas C. “Tom” McMurtry made the first flight in the F-8 Supercritical Wing flight research vehicle on March 9, 1971. The flying testbed featured a new wing designed to cruise at near-supersonic speeds for improved fuel economy. Another 17 years later, the Boeing Company announced the successful maiden flight of what would be the manufacturer’s best-selling airliner, the 747-400, on April 29, 1988. Incorporated into the design of the jumbo jet were winglets: small vertical surfaces that reduced drag by smoothing turbulent airflow at the wingtips to increase fuel efficiency.[1] All three of these revolutionary innovations originated with one person, Richard T. “Dick” Whitcomb, an aeronautical engineer working for the National Advisory Committee for Aeronautics (NACA) and its successor, the National Aeronautics and Space Administration (NASA).

A major aeronautical revolution was shaping the direction and use of the airplane during the latter half of the 20th century. The invention of the turbojet engine in Europe and its incorporation into the airplane transformed aviation. The aeronautical community followed a basic premise—to make the airplane fly higher, faster, farther, and cheaper than ever before—as national, military, industrial, and economic factors shaped requirements. As a researcher at the Langley Memorial Aeronautical Laboratory in Hampton, VA, Dick Whitcomb was part of this movement, which was central to the missions of both the NACA and NASA.[2] His three fundamental contributions, the area rule fuselage, the supercritical wing, and the winglet, each in their own aerodynamic ways offered an increase in speed and performance without an increase in power. Whitcomb was highly individualistic, visionary, creative, and practical, and his personality, engineering style, and the working environment nurtured at Langley facilitated his quest for aerodynamic efficiency.

The Making of an Engineer

Richard Travis Whitcomb was born on February 21, 1921, in Evanston, IL, and grew up in Worcester, MA. He was the eldest of four children in a family led by mathematician-engineer Kenneth F. Whitcomb.[3] Whitcomb was one of the many air-minded American children building and testing aircraft models throughout the 1920s and 1930s.[4] At the age of 12, he created an aeronautical laboratory in his family’s basement. Whitcomb spent the majority of his time there building, flying, and innovating rubberband-powered model airplanes, with the exception of reluctantly eating, sleeping, and going to school. He never had a desire to fly himself, but, in his words, he pursued aeronautics for the “fascination of making a model that would fly.” One innovation Whitcomb developed was a propeller that folded back when it stopped spinning to reduce aerodynamic drag. He won several model airplane contests and was a prizewinner in the Fisher Body Company automobile model competition; both were formative events for young American men who would become the aeronautical engineers of the 1940s. Even as a young man, Whitcomb exhibited an enthusiastic drive that could not be diverted until the challenge was overcome.[5]

A major influence on Whitcomb during his early years was his paternal grandfather, who had left farming in Illinois to become a manufacturer of mechanical vending machines. Independent and driven, the grandfather was also an acquaintance of Thomas A. Edison. Whitcomb listened attentively to his grandfather’s stories about Edison and soon came to idolize the inventor for his ideas as well as for his freethinking individuality.[6] The admiration for his grandfather and for Edison shaped Whitcomb’s approach to aeronautical engineering.

Whitcomb received a scholarship to nearby Worcester Polytechnic Institute and entered the prestigious school’s engineering program in 1939. He lived at home to save money and spent the majority of his time in the institute’s wind tunnel. Interested in helping with the war effort, Whitcomb’s senior project was the design of a guided bomb. He graduated with distinction with a bachelor’s of science degree in mechanical engineering. A 1943 Fortune magazine article on the NACA convinced Whitcomb to join the Government-civilian research facility at Hampton, VA.[7]

Airplanes ventured into a new aerodynamic regime, the so-called “transonic barrier,” as Whitcomb entered into his second year at Worcester. At speeds approaching Mach 1, aircraft experienced sudden changes in stability and control, extreme buffeting, and, most importantly, a dramatic increase in drag, which exposed three challenges to the aeronautical community, involving propulsion, research facilities, and aerodynamics. The first challenge involved the propeller and piston-engine propulsion system. The highly developed and reliable system was at a plateau and incapable of powering the airplane in the transonic regime. The turbojet revolution brought forth by the introduction of jet engines in Great Britain and Germany in the early 1940s provided the power needed for transonic flight. The latter two challenges directly involved the NACA and, to an extent, Dick Whitcomb, during the course of the 1940s. Bridging the gap between subsonic and supersonic speeds was a major aerodynamic challenge.[8]

Little was known about the transonic regime, which falls between Mach 0.8 and 1.2. Aeronautical engineers faced a daunting challenge rooted in developing new tools and concepts. The aerodynamicist’s primary tool, the wind tunnel, was unable to operate and generate data at transonic speeds. Four approaches were used in lieu of an available wind tunnel in the 1940s for transonic research. One way to generate data for speeds beyond 350 mph was through aircraft diving at terminal velocity, which was dangerous for test pilots and of limited value for aeronautical engineers. Moreover, a representative drag-weight ratio for a 1940-era airplane ensured that it was unable to exceed Mach 0.8. Another way was the use of a falling body, an instrumented missile dropped from the bomb bay of a Boeing B-29 Superfortress. A third method was the wing-flow model. NACA personnel mounted a small, instrumented airfoil on top of the wing of a North American P-51 Mustang fighter. The Mustang traveled at high subsonic speeds and provided a recoverable method in real-time conditions. Finally, the NACA launched small models mounted atop rockets from the Wallops Island facility on Virginia’s Eastern Shore.[9] The disadvantages for these three methods were that they only generated data for short periods of time and that there were many variables regarding conditions that could affect the tests.

Even if a wind tunnel existed that was capable of evaluating aircraft at transonic speeds, there was no concept that guaranteed a successful transonic aircraft design. A growing base of knowledge in supersonic aircraft design emerged in Europe beginning in the 1930s. Jakob Ackeret operated the first wind tunnel capable of generating Mach 2 in Zurich, Switzerland, and designed tunnels for other countries. The international high-speed aerodynamics community met at the Volta Conference held in Rome in 1935. A paper presented by German aerodynamicist Adolf Busemann argued that if aircraft designers swept the wing back from the fuselage, it would offset the increase in drag beyond speeds of Mach 1. Busemann offered a revolutionary answer to the problem of high-speed aerodynamics and the sound barrier. In retrospect, the Volta Conference proved to be a turning point in high-speed aerodynamics research, especially for Nazi Germany. In 1944, Dietrich Küchemann discovered that a contoured fuselage resembling the now-iconic Coca-Cola soft drink bottle was ideal when combined with Busemann’s swept wings. American researcher Robert T. Jones independently discovered the swept wing at NACA Langley almost a decade after the Volta Conference. Jones was a respected Langley aerodynamicist, and his five-page 1945 report provided a standard definition of the aerodynamics of a swept wing. The report appeared at the same time that high-speed aerodynamic information from Nazi Germany was reaching the United States.[10]

As the German and American high-speed traditions merged after World War II, the American aeronautical community realized that there were still many questions to be answered regarding high-speed flight. Three NACA programs in the late 1940s and early 1950s overcame the remaining aerodynamic and facility “barriers” in what John Becker characterized as “one of the most effective team efforts in the annals of aeronautics.” The National Aeronautics Association recognized these NACA achievements three times through aviation’s highest award, the Collier Trophy, for 1947, 1951, and 1954. The first award, for the achievement of supersonic flight by the X-1, was presented jointly to John Stack of the NACA, manufacturer Lawrence D. Bell, and Air Force test pilot Capt. Charles E. “Chuck” Yeager. The second award in 1952 recognized the slotted transonic tunnel development pioneered by John Stack and his associates at NACA Langley.[11] The third award recognized the direct byproduct of the development of a wind tunnel in which the visionary mind of Dick Whitcomb developed the design concept that would enable aircraft to efficiently transition from subsonic to supersonic speeds through the transonic regime.

Dick Whitcomb and the Transonic-Supersonic Breakthrough

Whitcomb joined the research community at Langley in 1943 as a member of Stack’s Transonic Aerodynamics Branch working in the 8-foot High-Speed Tunnel (HST). Initially, NACA managers placed him in the Flight Instrument Research Division, but Whitcomb’s force of personality ensured that he would be working directly on problems related to aircraft design. As many of his colleagues and historians would attest, Whitcomb quickly became known for an analytical ability rooted in mathematics, instinct, and aesthetics.[12]

In 1945, Langley increased the power of the 8-foot HST to generate Mach 0.95 speeds, and Whitcomb was becoming increasingly familiar with transonic aerodynamics, which helped him in his developing investigation into the design of supersonic aircraft. The onset of drag created by shock waves at transonic speeds was the primary challenge. John Stack, Ezra Kotcher, and Lawrence D. Bell proved that breaking the sound barrier was possible when Chuck Yeager flew the Bell X-1 to Mach 1.06 (700 mph) on October 14, 1947. Designed in the style of a .50- caliber bullet with straight wings, the Bell X-1 was a successful supersonic airplane, but it was a rocket-powered research airplane designed specifically for and limited to that purpose. The X-1 would not offer designers the shape of future supersonic airplanes. Operational turbojet-powered aircraft designed for military missions were much heavier and would use up much of their fuel gradually accelerating toward Mach 1 to lessen transonic drag.[13] The key was to get operation aircraft through the transonic regime, which ranged from Mach 0.9 to Mach 1.1.

A very small body of transonic research existed when Whitcomb undertook his investigation. British researchers W.T. Lord of the Royal Aeronautical Establishment and G.N. Ward of the University of Manchester and American Wallace D. Hayes attempted to solve the problem of transonic drag through mathematical analyses shortly after World War II in 1946. These studies generated mathematical symbols that did not lend themselves to the design and shape of transonic and supersonic aircraft.[14]

Whitcomb’s analysis of available data generated by the NACA in ground and free-flight tests led him to submit a proposal for testing swept wing and fuselage combinations in the 8-foot HST in July 1948. There had been some success in delaying transonic drag by addressing the relationship between wing sweep and fuselage shape. Whitcomb believed that careful attention to arrangement and shape of the wing and fuselage would result in their counteracting each other. His goal was to reach a milestone in supersonic aircraft design. The tests, conducted from late 1949 to early 1950, revealed no significant decrease in drag at high subsonic (Mach 0.95) and low supersonic (Mach 1.2) speeds. The wing-fuselage combinations actually generated higher drag than their individual values combined. Whitcomb was at an impasse and realized he needed to refocus on learning more about the fundamental nature of transonic airflow.[15]

Just before Whitcomb had submitted his proposal for his wind tunnel tests, John Stack ordered the conversion of the 8-foot HST in the spring of 1948 to a slotted throat to enable research in the transonic regime. In theory, slots in the tunnel’s test section, or throat, would enable smooth operation at very high subsonic speeds and at low supersonic speeds. The initial conversion was not satisfactory because of uneven flow. Whitcomb and his colleagues, physicist Ray Wright and engineer Virgil S. Ritchie, hand-shaped the slots based on their visualization of smooth transonic flow. They also worked directly with Langley woodworkers to design and fabricate a channel at the downstream end of the test section that reintroduced air that traveled through the slots. Their painstaking work led to the inauguration of transonic operations within the 8-foot HST 7 months later, on October 6, 1950.[16] Whitcomb, as a young engineer, was helping to refine a tunnel configuration that was going to allow him to realize his potential as a visionary experimental aeronautical engineer.

The NACA distributed a confidential report on the new tunnel during the fall of 1948, which was distributed to the military services and select manufacturers. By the following spring, rumors had been circulating about the new tunnel throughout the industry. Initially, the call for secrecy evolved into outright public acknowledgement of the NACA’s new transonic tunnels (including the 16-foot HST) with the awarding of the 1951 Collier Trophy to John Stack and 19 of his associates at Langley for the slotted wall. The Collier Trophy specifically recognized the importance of a research tool, which was a first in the 40-year history of the award. The NACA claimed that its slotted-throat transonic tunnels gave the United States a 2-year lead in the design of supersonic military aircraft.[17]

With the availability of the 8-foot HST and its slotted throat, the combined use of previously available wind tunnel components—the tunnel balance, pressure orifice, tuft surveys, and schlieren photographs—resulted in a new theoretical understanding of transonic drag. The schlieren photographs revealed three shock waves at transonic speeds. One was the familiar shock wave that formed at the nose of an aircraft as it pushed forward through the air. The other two were, according to Whitcomb, “fascinating new types” of shock waves never before observed, in which the fuselage and wings met and at the trailing edge of the wing. These shocks contributed to a new understanding that transonic drag was much larger in proportion to the size of the fuselage and wing than previously believed. Whitcomb speculated that these new shock waves were the cause of transonic drag.[18]

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 cross-sectional 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.[19]

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.[20]

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 aerodynamicists 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.[21]

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.”[22] His relied upon his mind’s eye—the non- verbal thinking necessary for engineering—to visualize the aerodynamic process, specifically transonic airflow.[23] 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.”[24] 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.[25] 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.[26]

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.[27] 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.”[28] The name “area rule” came from the combination of “cross-sectional area” with “rule of thumb.”[29]

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.[30]

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.[31] A designer that followed those four steps would produce a successful design with minimum transonic drag.

Validation in Flight

As Whitcomb was discovering the area rule, Convair in San Diego, CA, was finalizing its design of a new supersonic all-weather fighter-interceptor, began in 1951, for a substantial Air Force contract. The YF-102 Delta Dagger combined Mach’s ideal high-speed bullet-shaped fuselage and delta wings pioneered on the Air Force’s Convair XF-92A research airplane with the new Pratt & Whitney J57 turbojet, the world’s most powerful at 10,000 pounds thrust. Armed entirely with air-to-air and forward-firing missiles, the YF-102 was to be the prototype for America’s first piloted air defense weapon’s system.[32] Convair heard of the NACA’s transonic research at Langley and feared that its investment in the YF-102 and the payoff with the Air Force would come to naught if the new airplane could not fly supersonic.[33] Convair’s reputation and a considerable Department of Defense contract were at stake.

A delegation of Convair engineers visited Langley in mid-August 1952, where the engineers witnessed a disappointing test of an YF-102 model in the 8-foot HST. The data indicated, according to the NACA at least, that the YF-102 was unable to reach Mach 1 in level flight. The transonic drag exhibited near Mach 1 simply counteracted the ability of the J57 to push the YF-102 through the sound barrier. They asked Whitcomb what could be done, and he unveiled his new rule of thumb for the design of supersonic aircraft. The data, Whitcomb’s solution, and what was perceived as the continued skepticism on the part of his boss, John Stack, left the Convair engineers unconvinced as they went back to San Diego with their model.[34] They did not yet see the area rule as the solution to their perceived problem.

Nevertheless, Whitcomb worked with Convair’s aerodynamicists to incorporate the area rule into the YF-102. New wind tunnel evaluations in May 1953 revealed a nominal decrease in transonic drag. He traveled to San Diego in August to assist Convair in reshaping the YF-102 fuselage. The NACA notified Convair that the modified design, soon be designated the YF-102A, was capable of supersonic flight in October.[35]

Despite the fruitful collaboration with Whitcomb, Convair was hedging its bets when it continued the production of the prototype YF-102 in the hope that it was a supersonic airplane. The new delta wing fighter with a straight fuselage was unable to reach its designed supersonic speeds during its full-scale flight evaluation and tests by the Air Force in January 1954. The disappointing performance of the YF-102 to reach only Mach 0.98 in level flight confirmed the NACA’s wind tunnel findings and validated Whitcomb’s research that led to his area rule. The Air Force realistically shifted the focus toward production of the YF-102A after NACA Director Hugh Dryden guaranteed that Chief of Staff of the Air Force Gen. Nathan F. Twining developed a solution to the problem and that the information had been made available to Convair and the rest of the aviation industry. The Air Force ordered Convair to stop production of the YF-102 and retool to manufacture the improved area rule design.[36]

It took Convair only 7 months to prepare the prototype YF-102A, thanks to the collaboration with Whitcomb. Overall, the new fighter-interceptor was much more refined than its predecessor was, with sharper features at the redesigned nose and canopy. An even more powerful version of the J57 turbojet engine produced 17,000 pounds thrust with afterburner. The primary difference was the contoured fuselage that resembled a wasp’s waist and obvious fairings that expanded the circumference of the tail. With an area rule fuselage, the newly re-designed YF-102A easily went supersonic. Convair test pilot Pete Everest undertook the second flight test on December 21, 1954, during which the YF-102A climbed away from Lindbergh Field, San Diego, and “slipped easily past the sound barrier and kept right on going.” More importantly, the YF-102A’s top speed was 25 percent faster, at Mach 1.2.[37]

The Air Force resumed the contract with Convair, and the manufacturer delivered 975 production F-102A air defense interceptors, with the first entering active service in mid-1956. The fighter- interceptors equipped Air Defense Command and United States Air Force in Europe squadrons during the critical period of the late 1950s and 1960s. The increase in performance was dramatic. The F-102A could cruise at 1,000 mph and at a ceiling of over 50,000 feet. It replaced three subsonic interceptor aircraft in the Air Force inventory—the North American F-86D Sabre, F-89 Scorpion, and F-94 Starfire—which were 600–650 mph aircraft with a 45,000-foot ceiling range. Besides speed and altitude, the F-102A was better equipped to face the Soviet Myasishchev Bison, Tupolev Bear, and Ilyushin Badger nuclear-armed bombers with a full complement of Hughes Falcon guided missiles and Mighty Mouse rockets. Convair incorporated the F-102A’s