In 1953, the Air Force had a problem: Its new jet-powered interceptor, Convair’s F-102, was a dud.
Unlike earlier airplanes that relied on brute strength to push past the speed of sound, the dart-shaped F-102 had a “delta” wing and an afterburning engine that was supposed to allow it to slip across the sonic frontier. Dubbed the “1954 Interceptor” (a reference to its planned in-service date), the new F-102 was to scramble from US bases, climb to high altitude, and intercept incoming Soviet bombers, dashing if necessary to Mach 1.25 before blasting them from the sky.
On paper, it seemed a winner. High over the Mojave, flight tests were starting to show otherwise.
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As the prototype YF-102 approached the speed of sound, it would begin to slow down, like a marathon runner hitting the 20 Mile wall. Even with its afterburner blasting away, the aircraft would top out at only Mach 0.98 before returning to Earth. It was clear that something was very wrong with Convair’s new delta. Could it be salvaged
The answer was yes. The F-102 would require extensive redesign, delaying its introduction into service by two years, but it would become a Cold War mainstay, with approximately 1,000 serving from 1956 through 1976 in Air Force and Air National Guard interceptor squadrons (and in the militaries of Greece and Turkey as well).
That the F-102 didn’t remain an embarrassing failure was thanks to the genius of a young government engineer, Richard T. Whitcomb. Whitcomb, who died last October in Hampton, Va., at age 88, has been rightly acclaimed as the most gifted and influential aeronautical researcher of his time.
Incredible Insight
An aerodynamicist with the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA), Whitcomb was a standout even in an agency known for the extraordinary quality of its scientific personnel.
“Dick Whitcomb wasn’t just another brilliant aerodynamicist,” notes former Air Force Chief Scientist and University of Maryland professor Mark J. Lewis, who knew him. “He had incredible intuition, giving him special insight into the physics of fluid flow. He initially used that intuition, not just quantitative analysis, when he derived his Area Rule theory. His intuition thus directly led to one of the most important design innovations in aviation history.”
Born in Illinois in 1921, Whitcomb came from a family attuned to technology and invention. His father, a mechanical engineer, had been a balloon pilot, and his grandfather was an inventor who had known Thomas Edison.
Like many boys of his generation, Whitcomb was fascinated with flight and earned an engineering scholarship to Massachusetts’ Worcester Polytechnic Institute. Most engineering students aspired to work in industry, but Whitcomb was different: “I knew,” he said later, “that I wanted to go into research.”
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Whitcomb, below left, with an early wind tunnel model featuring the “wasp-waist.” His flash of genius led to the Area Rule, which made it possible to increase the speed of the experimental YF-102 25 percent by the time it became the operational F-102A interceptor.(NACA photo)
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As he approached graduation, he read a Fortune magazine article extolling NACA’s Langley Memorial Aeronautical Laboratory at Langley Field, Va. (now the NASA Langley Research Center at Langley Air Force Base). The article convinced him it “was the place to come for applied research.” Accordingly, when a NACA recruiter visited Worcester Poly, Whitcomb recalled, “I was just waiting for the guy to hand me the papers.”
Whitcomb joined NACA in 1943, the year when America’s aeronautical community began planning its assault upon the then-fearsome “sound barrier.”
The rocket and the jet now made supersonic flight a possibility. Perhaps an airplane could fly faster than sound, if it could first survive the intermediate transonic region with its much-feared sound barrier.
The transonic region was between Mach 0.75 and Mach 1.25. In that band, airplanes encountered compressible flow, shock waves streaming from their wings and bodies, loss of lift and rising drag, dangerous buffeting, and sometimes loss of control and even structural failure.
Whitcomb would prove his mastery over this treacherous and daunting arena but almost missed the chance. When he arrived at Langley, personnel staffers initially assigned him to the instrumentation branch.
With a surprising outspokenness for someone so young, he refused.
“I said, ‘I don’t want to work in instruments,’” he recalled. “‘I want to work on aerodynamics.’” So they assigned him to the Langley eight-foot high-speed tunnel, run by John Stack. NACA’s most forceful high-speed research enthusiast, Stack was energetic, insightful, charismatic, and destined to twice share the Robert J. Collier Trophy—the most prestigious of all American aviation accolades.
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A Flash of Light
Chief among his many accomplishments was conceptualizing and developing the slotted throat wind tunnel, the most significant tunnel advance over the previous 25 years. Conventional wind tunnels “choked” near the speed of sound when shock waves streaming from models reflected back and forth across their test sections, producing misleading or incomprehensible results. The slotted throat tunnel removed this transonic “blind spot” via a ring of carefully placed longitudinal slots that dissipated flow-disturbing shock waves, enabling much more accurate measurements as airflows neared and exceeded the speed of sound.
Whitcomb made the slotted throat tunnel his personal laboratory. In the early years of the postwar era, NACA aerodynamicists focused on the behavior of the swept wing. Whitcomb looked beyond its singular purity, studying instead the aerodynamic interaction of the wing and fuselage. He discovered their combined drag was greater than the sum of their individual drags measured separately.
Using photographic imaging, Whitcomb noted strong shock waves emanating where the leading and trailing edges of wings joined the fuselage, producing a performance-robbing “bunching” of air. It was as if the wave patterns of two boats intersected, becoming just one large pattern.
“The theoreticians,” he later told one interviewer, “were at a loss as to how to handle this problem.” All except one—Adolf Busemann, a scientist who had invented the high-speed swept wing in Germany before World War II, and who came to Langley with Project Paperclip afterward. At a staff seminar, Busemann joked that transonic flows were so constricted that aerodynamicists were like pipe fitters, finding how to fit flows around the vehicle.
One day in 1951, while Whitcomb contemplated his tunnel results and pondered Busemann’s analogy, he had an insight like a flash of light: If a designer gave the bunching air a place to go, the flow would smooth out, and the drag would decrease. He later recalled the moment, saying, “I sat there at my desk, feet propped up, and suddenly it dawned on me, the basic idea of the Area Rule: Transonic drag is a function of the longitudinal development of the cross-sectional areas of the entire plane.”
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The winglet (seen here in testing on a KC-135) later became crucial to the C-17’s performance. Seemingly simple, the winglet adds lift far beyond its cost in weight, and has been adopted on many airliners. |
It was the latter insight—that drag was a function of the length of the entire airplane and not just the fuselage diameter at the wing-fuselage juncture—which separated Whitcomb’s work from others who focused their research more narrowly.
The ideal supersonic shape was long and slender, with its cross-sectional area expanding gradually, reaching a maximum diameter and then contracting equally smoothly. Designers would have to pinch the fuselage at the wing roots, and lengthen the fuselage before and behind the wing as well, increasing its fineness ratio—the ratio of its length to its width.
Whitcomb broached this “rule of thumb” at the next NACA Langley technical seminar. After he finished, Busemann rose to his feet. “Some people come up with half-baked ideas and call them theories,” he said. “Whitcomb comes up with a brilliant idea and calls it a rule of thumb.”
A landmark, confidential NACA research memorandum issued in September 1952 was the result of comprehensive wind tunnel testing that followed. The memo’s prosaic title—“A Study of the Zero-Lift Drag-Rise Characteristics of Wing-Body Combinations Near the Speed of Sound”—belied its revolutionary implications.
At just over 30, using a mix of intuition, experimentation, and borrowed theoretical analogy, Whitcomb had reshaped the airplane. Area Rule was—next to the swept and delta wings themselves—the most significant and most visible manifestation of transonic design.
Nicknamed “Coke Bottle” and “Wasp Waist,” Area Rule came just in time for the F-102. Weeks before its first flight, Convair engineers had learned from NACA tunnel tests that the F-102 likely would not exceed the speed of sound.
Whitcomb’s confidential memorandum came afterward, followed shortly by the first F-102’s disastrous performance in the skies over Edwards.
For Convair it was, literally, back to the drawing board. The F-102, redesigned to conform to Area Rule principles with a lengthened fuselage and also modified wing leading edges, returned to Edwards in 1954, handily accelerating through the speed of sound.
Whitcomb’s Area Rule had added 25 percent more speed to the design.
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Area ruling also reshaped the Navy’s F9F-9, which became the supersonic F11F-1 Tiger, and Vought’s F8U-1 Crusader. It added sinuous curves to Republic’s F-105 Thunderchief, McDonnell’s F4H-1 Phantom II, and Northrop’s tiny N-156, which spawned the T-38 Talon trainer and the F-5A Freedom Fighter.
Uncorking No. 2
Whitcomb received the 1954 Collier Trophy “for discovery and experimental verification of the Area Rule, a contribution to base knowledge yielding significantly higher airplane speed and greater range with same power.”
He had also redeemed the reputation of the NACA with the Air Force. Since 1941, when Gen. Henry H. “Hap” Arnold had been surprised to learn that Britain was more advanced in the field of jet propulsion, the agency had faced criticism for technological complacency. The appearance of German jets, the V-2 missile, and discovery of advanced German high-speed research by Theodore von Karman’s technical study team in 1945 had added to such beliefs.
Whitcomb’s discovery enabled the Air Force’s transonic force structure that was subsequently deployed for the Cold War and beyond. Proof that Area Rule also restored NACA to the service’s good graces was evident in Whitcomb’s receiving of the Air Force Exceptional Service Medal.
Whitcomb was not content to rest on this single laurel. Called into the early design conferences for a proposed supersonic civil air transport, he returned to transonics when SCAT studies grew ever more complex and improbable.
“I said, ‘I’m going to quit the field,’” he recalled in 1973. “‘I’m going back where I know I can make things pay off,’ and I went back to the region right near the speed of sound.” Shortly after, he uncorked his second great idea, the “supercritical wing.”
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Whitcomb with a wind tunnel model of a TF-8 configured with his supercritical wing, and NASA’s flying version. His innovation has saved billions of dollars in fuel costs.(NASA photo)
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The supercritical wing had a largely flattop airfoil cross-section with its camber (curvature) well aft. It raised the wing’s critical Mach number—the point where the accelerated airflow over the wing creates a shock wave. A jetliner cruising at Mach 0.82 with a conventional wing might, with a supercritical one, cruise at Mach 0.86 or even higher for the same expenditure of fuel. While this idea was exciting enough, it became one of vital strategic significance for both military and airline operators in the oil crises of the 1970s.
Early tests on a modified trainer encouraged NASA to move to higher-Mach trials using an ex-Navy Vought TF-8A Crusader jet fighter. Flight tests began in 1971, and quickly demonstrated that the anticipated benefits of the wing were correct. The modified F-8 had 15 percent better transonic performance than a standard Crusader.
Tests indicated commercial airliners with supercritical wings would be more profitable than ones with conventional wings, saving the airline industry $78 million per year in 1974 dollars—equivalent to $342 million today.
In 1969, Whitcomb received NASA’s Exceptional Scientific Achievement Medal. Four years later, President Nixon awarded him the National Medal of Science. In 1974, Whitcomb received a $25,000 cash award for inventing the supercritical wing, and later that year, the National Aeronautic Association awarded him the 1974 Wright Brothers Memorial Trophy.
The supercritical wing was very quickly applied to a range of commercial aircraft, beginning with the Rockwell Sabreliner 65, the Canadair Challenger, and the French Falcon 50 business jet aircraft.
It had clear military value, and so was incorporated on the Boeing and McDonnell Douglas YC-14 and YC-15 Advanced Medium Short Takeoff and Landing Transport test beds. It was evaluated for fighter and attack aircraft applications on a modified F-111A as part of the joint NASA-Air Force Transonic Aircraft Technology (TACT) program, as was a supercritical spin-off, the Mission Adaptive Wing.
The TACT F-111A had 50 percent more lift during maneuvering than a standard F-111A.
Building upon the earlier flight-test experience of the YC-15 program, and continued and lengthy wind tunnel studies at NASA and elsewhere, the McDonnell Douglas C-17A Globemaster III transport went a step further. It featured not only a supercritical wing, but another Whitcomb innovation—the wingtip winglet.
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A swing-wing Transonic Aircraft Technology (TACT) F-111A configured with Whitcomb’s supercritical wing. It delivered 50 percent better lift than the standard F-111A. (NASA photo)
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“I Shock People”
The winglet constituted the third of his innovations and was even more readily visible than area ruling. Aerodynamicists had long recognized that swirling wingtip vortices, a byproduct of the airflow’s circulation around the wing, constituted a source of performance-robbing turbulence and drag. While not the first to experiment with some sort of endplates located on wingtips, Whitcomb employed greater insight and intuition than his predecessors.
He developed a deceptively simple-looking fin that could reduce a transport’s drag by a fifth. But would it work
Tests in 1979 using a Boeing KC-135 Stratotanker loaned to NASA proved that winglets functioned better at reducing drag and increasing lift than merely extending an airplane’s wingspan. The winglets increased its range by seven percent and revealed the power of the invisible vortices: The KC-135 returned from one flight with a winglet bent in a half-circle, a testimony of the strength and energy contained within the small horizontal tornadoes.
Since the late 1980s, winglets have become ubiquitous, visible to anyone living near an airport offering commercial and business jet aircraft service.
The general aviation and business airplane communities were the first to incorporate winglets, just as they were the first to adopt his supercritical wing. In 1979, Learjet introduced the Longhorn, a winglet-equipped series of its famed business jet aircraft. Gulfstream added them to its own line of business aircraft, and later Boeing and McDonnell Douglas adapted them for the 747-400 series jumbo jet and the MD-11.
Winglets were incorporated on the C-17 airlifter, together with a supercritical wing. The combination gives the C-17 better range and speed than would have been possible had it used a larger, conventional planform.
Whitcomb retired from NASA in 1980, convinced that no large challenges in aviation remained. Thereafter, he turned his interests toward broader questions in the physical sciences.
Asked in 2003 whether he advised young people to follow his footsteps, he replied, “I shock people. I say if you want to make an impact or have an effect, don’t go into aeronautics. It’s pretty well stabilized. No big things have come up in aeronautics since my inventions, and it has been 20 years since I left.”
If many might disagree, most would concur that it will take an extraordinary breakthrough to match any one of his remarkable trio of transonic accomplishments.
Richard P. Hallion is an aerospace historian who served 11 years as the Air Force Historian and has written widely on aerospace technology and airpower topics. This is his first article for Air Force Magazine.