Category AERONAUTICS

While swept wing adaptation in Europe, Russia, and America followed a generally similar pattern, the delta wing underwent markedly differ­ent international development. Generally, European designers initially emulated the Lippisch approach, resulting in designs with relatively thick wing sections (exemplified by the Avro Vulcan bomber and the "tailed” Gloster Javelin interceptor) that inhibited their ability to operate beyond the transonic. Only after the practical demonstration of Convair’s emerging family of thin-wing delta designs—the XF-92A research air­craft, the F-102 interceptor, the XF2Y-1 experimental naval fighter, the B-58 supersonic bomber, and the F-106 interceptor—did they conceptu­alize more "supersonic friendly” designs, typified by the Swedish Saab J35 Draken ("Dragon”), the British Fairey F. D.2 research airplane, the French Dassault Mirage I (progenitor of the Mirage fighter and bomber family). By the late 1950s, British and French aerodynamicists had so completely "closed” any "delta gap” that might have existed between Europe and America that they were already conceptualizing development of a Mach 2 supersonic transatlantic transport using a shapely "ogee” reflexive delta planform, a study effort that would, a decade later, spawn the Anglo-French Concorde.[88] Not so taken with the pure delta, Soviet designers joined American-like thin delta wings to the low-placed hori­zontal tail, generating advanced MiG and Sukhoi fighters and intercep­tors. These "tailed deltas” (particularly the MiG-21) possessed far better transonic and supersonic turning performance than could be attained by a conventional delta with its high induced drag onset at the increas­ing angles of attack characteristic of hard-maneuvering. (An American equivalent was the Douglas Company’s superlative A4D-1 Skyhawk,
a light attack bomber with maneuvering performance better than most fighters.)

Although it is commonly accepted that American delta aircraft owe their inspiration to the work of Lippisch—Convair’s delta aircraft repeatedly being cited as the products of his influence—in fact, they do not.[89] Unlike, say, the swept wing F-86 and B-47, which directly reflected German aerodynamic thought and example, America’s delta wing air­craft reflected indigenous, not foreign, research and inspiration. By the time that Lippisch first met with Allied technical intelligence experts, American aerodynamicists were already advancing along a very differ­ent path than the one he had followed. Jones had already enunciated his thin, sharply swept delta theory and undertaken his first tunnel tests of it. In June 1946—a full year after the German collapse—Convair engi­neers developing the experimental delta XP-92 interceptor had their chance to meet with Lippisch at Wright Field. By then, however, they had already independently decided upon a thin delta planform. "We had heard about Dr. Lippisch’s work and this gave us some moral support,” Convair designer Adolph Burstein recalled, adding: "but not much else. . . . We did not go along with many of his ideas, such as a very thick airfoil.”[90] Burstein and his colleagues arrived at their delta shape by beginning with a 45-degree swept wing, gradually increasing its sweepback angle, and then "filling in” the ever-closing trailing edges, until they arrived at the classic 60-degree triangular delta planform the company incorporated on all its subsequent delta aircraft. With a 6.5 thickness-chord ratio— less than half that of Lippisch’s DM-1—it was an altogether different – looking airplane.[91] Nor was Convair alone in going its own way; Douglas naval aircraft designer Edward Heinemann acknowledged that "At the close of World War II the work with delta planforms accomplished by

Dr. Lippisch in Germany became generally known and appreciated,” but that "Extensive wind tunnel tests showed there was no special merit to an equilateral triangle planform—especially those designed with thicker airfoils.”[92]

The chronology of American delta development, and the technical choices and paths followed by American engineers, supports both state­ments. At war’s end, advancing ground forces at Prien, Austria, had dis­covered a thick-wing wooden delta glider, the DM-1, which Lippisch had intended as a low-speed testbed for a proposed supersonic fighter, the P 13. At Army Air Forces’ request, it was shipped back to America in January 1946 for comprehensive testing in the Full-Scale Tunnel at the NACA’s Langley Aeronautical Laboratory. Had the tests gone well,
the possibility existed as that, as the Germans had intended, it might be flown as a glider. But the tunnel tests quickly disabused delta enthu­siasts of these hopes. As the AAF’s Langley liaison officer subsequently reported, the "Initial test results were very disappointing; the lift coef­ficient was low, the drag was high, the directional stability was unsatis­factory, and the craft was considered unsafe for flight tests.”[93]

Afterward, Langley engineers undertook a comprehensive study of the DM-1 configuration, not in the spirit of emulation but rather attempt­ing to find a way to fix it. After giving its wings sharp leading edges, seal­ing all slots and gaps around control surfaces, and removing the thick vertical fin and replacing it with a thin one (relocating the pilot under a streamlined bubble canopy), they had markedly improved its perfor­mance, doubling its lift coefficient, from 0.6 to over 1.2. But it remained an unsatisfactory design, proof enough that the Lippisch concept of del­tas was hardly one that could serve—or did serve—as a veritable template (as has been so often alleged) for the supersonic American, Swedish, and French delta fighters and bombers that flew over the next decade.[94] Subsequently, NACA engineers looked to far thinner and more streamlined configurations that, if not yet as extreme as Robert T. Jones’s original daggerlike concept, were even more amenable to the rigors of transonic and supersonic flight than the generously rounded contours of Lippisch’s thick wings and awkward pilot-enclosing vertical fins. By the beginning of 1947, they were already examining the technical requirements of slender, low aspect ratio delta configurations

to meet emerging military specifications for a Mach 1.5, 60,000-foot bomber interceptor.[95]

Out of this mutually reinforcing climate of thought emerged the world’s first delta jet airplane, the Convair XF-92A, first flown in September 1948. This technology explorer (for despite its "fighter” designation, it was always intended for research purposes) demonstrated the poten­tial of the delta wing and encouraged Convair and Air Force authorities to pursue a delta planform for a future interceptor design. Originally, that design had been the "XP-92,” an impractical barrel-shaped rocket – boosted ramjet with the pilot sitting in a conical nose within the ramjet’s
circular inlet, similar to Rene Leduc’s straight wing air-launched French ramjet designs of the same period. Following its cancellation, work on the XF-92A continued, supporting the Air Force’s "1954 Interceptor” ini­tiative, which Convair hoped to win with, essentially, a bigger and more powerful version of the XF-92A. Aside from greater power, the intercep­tor would have to have a nose radar and thus "cheek” inlets rather than the simple Pitot nose inlet of the smaller testbed. The "1954 Interceptor” eventually became two: the "interim” Mach 1+ F-102 Delta Dagger and the "ultimate” Mach 2+ F-106 Delta Dart.

The XF-92A contributed markedly to delta understanding but was far from a trouble-free design. Deltas evinced a variety of quirks and per­formance deficiencies, some of which they shared with their swept wing brethren. Deltas manifested the same tendency to persistent combined lat­eral-directional Dutch roll motions, as well as pitch-up, from Mach num­ber effects as they entered further into the transonic regime. The extreme sweep of their wings accentuated spanwise flow tendencies, making wing fences almost mandatory in all cases. Their high angle-of-attack ("hi AoA”) landing approaches highlighted potentially serious control deficiencies, for, unlike a conventional fighter, the delta lacked separate elevators and aile­rons. It relied instead on elevons—combined elevator-ailerons—for pitch and roll control. Thus, with the stick pulled back on final approach, the nose would rise, and if the plane encountered a sudden gust that induced a rolling motion, the pilot might lack sufficient remaining reserve "travel” from the deflected elevon to correct for the rolling motion. Further com­plicating landing approaches and turn performance was the delta’s inher­ently high-induced drag as it turned or was at higher angles of attack. Deltas needed lots of power. The high-induced drag of the delta led to a rapid bleeding off of airspeed during turns and thus inhibited its holding altitude during turning maneuvers. Tests with the little XF-92A in 1953 by NACA research pilot Scott Crossfield indicated that as much as 3,000 feet of altitude could be lost trying to maintain constant speed in a turn­ing maneuver—and this was after it had been modified to incorporate an afterburner for greater power. "Every time I took off in that plane I held my brief until I reached sufficient altitude to use the ejection seat,” Crossfield recollected later. "The pilot never really flew that airplane, he corralled it.”[96]

All together, the NACA completed 25 flights in the XF-92A before a land­ing gear collapse brought its research career to an end.

Tests of the XF-92A foreshadowed similar challenges with the next Convair delta, the prototype YF-102 interceptor. The YF-102 is infamous for having suffered from such high transonic drag rise that it could not accelerate through the speed of sound, a discovery that led, as Air Force test pilot Lt. Col. Frank K. "Pete” Everest recalled, to "surprise and con­cern. . . . The National Advisory Committee for Aeronautics had claimed all along that the airplane would not go supersonic, and now their predic­tions came true.”[97] (How the YF-102 was transformed from embarrass­ing failure to operational success, thanks to Richard Whitcomb’s "area rule” theory and its practical application to the F-102 design, is covered elsewhere in this volume in a case study on Whitcomb’s contributions to aeronautics, by historian Jeremy Kinney.) But more than reshaping of its fuselage was required before the F-102 became a success. Instead, its wing underwent fundamental aerodynamic redesign reflecting the second stage in American delta development and its third stage overall.

Having preceded the explication of the swept wing in Jones’s original research, the roots of the delta’s redesign now lay, somewhat ironically, in his expanding upon the slender swept wing research he had first begun at Langley. After the war, Jones had left Virginia’s Tidewater region for the equally pleasant Bay area environment of Sunnyvale, CA, and there had continued his swept wing studies. By 1947, he had evolved a sharply swept symmetrical airfoil planform he considered suitable for a supersonic jet transport. Such a planform, with the leading edges of the wings within the shock cone formed around the vehicle and thus in a region of subsonic flow, could perhaps have a lift-to-drag (L/D) ratio as high as 10, though at the price of much higher landing speeds.[98] Tests of a small model in the Ames 1-foot by 3-foot supersonic tun­nel and a larger one in the Ames 40-foot by 80-foot tunnel encouraged Jones and inspired fellow Ames researchers Charles F. Hall and John C. Heitmeyer to build upon his work. Hall and Heitmeyer considered the behavior of the combined wing-body, with the wing twisted and
given camber (curvature) to evenly distribute the flight loads, deriv­ing a sharply swept and tapered wing configuration that demonstrated an L/D of 8.9 during tunnel tests to Mach 1.53.[99] In the refinement of its planform, it called to mind the shape (though, of course, not the airfoil section) of Whitcomb’s later supercritical transonic transport wing conceptualizations.[100]

Hall and Heitmeyer next broadened their research to examine slen­der deltas likewise featuring aerodynamic twist and camber. In 2 years, 1951-1952, they coauthored a dozen reports, culminating in the issu­ance of a seminal study by Hall in the spring of 1953 that summarized the lift, drag, pitching moment, and load distribution data on a variety of thin delta wings of varying aspect ratios operating from Mach 0.25 (touchdown velocity) to Mach 1.9. Out of this came the concept of lead­ing edge "conical camber”: twisting and rounding the leading edge of a delta wing to minimize performance-robbing drag generated by the wing’s own lifting force. The modified delta had minimal camber at the wing root and maximum camber at the tip, the lineal development of the camber along the leading edge effectively representing the surface of a steadily expanding cone nestled under the leading edge of the wing.[101]

Hall’s conical camber, like Whitcomb’s area rule, came just in time to save the F-102 program. Both were necessary to make it a success: Whitcomb’s area rule to get it through the sound barrier, and Hall’s to give it acceptable transonic and supersonic flying qualities. If over­shadowed by Whitcomb’s achievement—which resulted in the young Langley aerodynamicist receiving the Robert J. Collier Trophy, American aviation’s most prestigious award, in 1954—Hall’s conical camber con­cept was nevertheless a critical one. Comparative flight-testing of the YF-102 at the NACA High-Speed Flight Station at Edwards from late
1954 to mid-1955 with and without conical camber indicated that con­ical camber gave it lower drag and increased its maximum lift-to-drag ratio by approximately 20 percent over a test Mach number range of 0.6 to 1.17, at altitudes of 25,000, 40,000, and 50,000 feet. Transonic sta­bility of the cambered versus symmetrical YF-102 more than doubled, and "no severe pitch-up tendencies were exhibited, except when accel­erating or decelerating through the trim-change region.”[102]

With the advent of conical camber, the age of the practical transonic – supersonic delta wing had arrived. By mid-decade, the F-102’s aero­dynamic deficiencies had been cured, and it was well on its way to ser­vice use.[103] Convair designers were refining the delta planform to generate the F-102’s successor, the superlative F-106, and a four-engine Mach 2+ bomber, the delta wing B-58 Hustler. Overseas, Britain’s Fairey Company had under test a delta of its own, the F. D.2, which would shortly estab­lish an international speed record, while, in France, Dassault engineers were conceptualizing a design that would spawn the Mirage family and be responsible, in 1967, for one of the most remarkable aerial victories of all time. Jones’s supersonic delta vision from over a decade previously had become reality, thanks in part to Whitcomb’s interference studies (which Jones himself would expand at Ames) and Hall’s conceptualiza­tion of conical camber.

The next stage in delta development took it from the realm of the tran­sonic and supersonic into the hypersonic, again thanks to a healthy rivalry and differing technical perspective between those two great research centers, Ames and Langley. The area was hypersonics: flight at speeds higher than Mach 5, an area of intense inquiry in the mid – 1950s following upon the success of the supersonic Round One research

aircraft. Already a Round Two hypersonic test vehicle, the soon-to-emerge North American X-15 was underway. But what of high-hypersonics, the hypersonics of flight at Mach 10 to orbital velocity?

Hypersonics constituted a natural application for the low aspect ratio delta planform. Before the Second World War, Austrian engineer Eugen Sanger and his mathematician wife, Irene Sanger-Bredt, had concep­tualized the Silbervogel ("Silver Bird”), a flat-bottom, half ogive body shape as a potential Earth-girdling hypersonic boost-glider. It had, for its time, a remarkable advanced aerodynamic profile, introducing the flat bottom and ogival configuration that did, in fact, come to charac­terize hypersonic aerothermodynamic design. But in one respect it did not: Sanger-Bredt’s "antipodal aircraft” had a conventional wing (though of low aspect planform and with supersonic wedge airfoils). Although it proved very influential on the course of postwar hypersonics, by the mid-1950s, as high-speed aerodynamic thinking advanced beyond the supersonic and into the hypersonic realm, attention increasingly turned toward the sharply swept delta planform.

In 1951, Ames researchers H. Julian Allen and Alfred Eggers, Jr., had postulated the blunt-body reentry theory that led to the advent of the practical reentry shape used subsequently both for missile warheads and the first human presence in space.[104] (Their work, and the emer­gence of the hypersonics field generally, are discussed in greater detail in T. A. Heppenheimer’s accompanying essay on transatmospherics.) While blunt-body theory enabled safely transiting the atmosphere, it did not furnish the flexibility of a large landing "footprint”; indeed, in practice, blunt-body reentry was limited to "throwaway” reentry shapes and pro­grams such as Mercury, Gemini, and Apollo that necessitated a large and cumbersome investment in oceanic recovery of returning space­craft. Some sort of lifting vehicle that could fly at hypersonic velocities would have far greater flexibility.

Related to the problem of hypersonic flight was the challenge of increasing lift-to-drag ratios at high supersonic speeds. Eggers, work­ing with Ames researcher Clarence A. Syvertson, now turned away from blunt-body theory to examine thin, slender deltas. The two rec­ognized that "the components of the aircraft should be individually
and collectively arranged to impart the maximum downward and the minimum forward momentum to the surrounding air.”[105] Out of this emerged the hypersonic "flattop” delta, a high-wing concept having the wing perched above the body (in this case, surmounting the classic half-ogive hypersonic shape), incongruously much like a general aviation light airplane such as a Cessna 152. At mid-span, its tips would angle sharply downward, capturing the momentum of flow imparted laterally outward from the body and deflecting it into downward momentum, thus greatly increasing lift. The tips as well furnished directional stability. This flattop concept, which Eggers and Syvertson enunciated in 1956, spawned an Ames concept for a hypersonic "beyond X-15” Round Three research vehicle that could be air-launched from a modified Convair B-36 bomber for ini­tial trials to Mach 6 and, once proven, could then be launched vertically as the second stage of a two-stage system capable of reach­ing Mach 10 and transiting the United States. The Ames vehicle, with an overall length of 70 feet and a span of just 25 feet, represented a bold concept that seemed likely to spawn the anticipated Round Three hypersonic boost-glider.[106]

But the flattop delta was swiftly undone by a rival Round Three Langley concept that echoed more the earlier work of Sanger-Bredt. A 1957 study by Peter Korycinski and John Becker demonstrated that a flat-bottom (that is, low-wing) delta boost-glider would have bet­ter cooling characteristics (a vital concern at hypersonic velocities) and thus require less weight for thermal protection systems. Any lift-to-drag advantages of the Ames flattop high-wing concept were thus nullified. Round Three went forward, evolving into the abortive Air Force-NASA X-20 Dyna-Soar program, which employed the Langley

flat-bottom approach, not the high-wing flattop delta of Ames.[107] Ames and Langley contested a decade later, this time in rival lifting bodies, with the Ames half-cone flattop M-2 (the product of Allen, Eggers, Syvertson, George Edwards, and George Kenyon) competing against Langley’s HL-10 fattened flat-bottom delta (by Eugene S. Love). Again, it was the flat-bottom delta that proved superior, confirmed by tests in the mid-1970s with an even more refined flat-bottom Air Force-derived slender delta body shape, the Martin X-24B.[108]

When orbital cross range proved even of greater significance, Shuttle proponents from the National Aeronautics and Space Administration (NASA) and the Air Force in the 1970s looked away from flattop and lift­ing body approaches and more toward blended bodies, modified delta planforms, and exotic delta "wave riders.” Though NASA’s Spacecraft Design Division briefly considered a conventionally tailed, straight and swept wing Shuttle concepts, reflecting an influential study by Johnson’s Maxime Faget, it moved rapidly toward deltas after analysis indicated such designs had a tendency of hypersonic spins, suspect aero – thermal survivability, and too small a cross range during return from orbit. Between mid-1971 and the late summer of 1972, the Spacecraft Design Division evaluated no less than 37 separate delta configurations, ranging from simple triangular shapes echoing the early days of Jones to much more complex ogee shape reflecting the refinement of the delta as exemplified by the Anglo-French Concorde. Aside from continuous review by the Manned Spacecraft Center (MSC; subsequently the NASA Lyndon B. Johnson Space Center), these evaluations benefitted greatly from aerodynamic analysis by NASA’s Ames and Langley hypersonic

communities, the practical low lift-to-drag-ratio flight-test experience of researchers at the NASA Flight Research Center, and the rocketry and space flight expertise of the Marshall Space Flight Center, whose experts assessed each proposal from the standpoint of technical feasi­bility and launch vehicle practicality. This multi-Center review strongly endorsed development of a modified delta planform, in part because the delta had inherently better stability characteristics during the high angle-of-attack reentry profile that any returning Shuttle would have to experience. Two families emerged as finalists: The 036 series, with small payload bays and three engines, and the 040 family, of similar planform but with larger payload bays and four engines. Then, in late January 1972, MSC engineers evolved the 040C configuration: a three-engine design using new high-pressure engines. The 040C design became the baseline for subsequent Orbiter studies. While many questions remained over the final form that Shuttle’s launch system would take, with the 040C study, the shape of the orbiter, and its all-important wing, was essentially fixed. Again, the flat-bottom delta had carried the day.[109]

S THIS BOOK GOES TO PRESS, the National Aeronautics and Space Administration (NASA) has passed beyond the half cen­tury mark, its longevity a tribute to how essential successive Presidential administrations—and the American people whom they serve—have come to regard its scientific and technological expertise. In that half century, flight has advanced from supersonic to orbital veloc­ities, the jetliner has become the dominant means of intercontinental mobility, astronauts have landed on the Moon, and robotic spacecraft developed by the Agency have explored the remote corners of the solar system and even passed into interstellar space.

Born of a crisis—the chaotic aftermath of the Soviet Union’s space triumph with Sputnik—NASA rose magnificently to the challenge of the emergent space age. Within a decade of NASA’s establishment, teams of astronauts would be planning for the lunar landings, first accom­plished with Neil Armstrong’s "one small step” on July 20, 1969. Few events have been so emotionally charged, and none so publicly visible or fraught with import, as his cautious descent from the spindly lit­tle Lunar Module Eagle to leave his historic boot-print upon the dusty plain of Tranquillity Base.

In the wake of Apollo, NASA embarked on a series of space initia­tives that, if they might have lacked the emotional and attention-getting impact of Apollo, were nevertheless remarkable for their accomplish­ment and daring. The Space Shuttle, the International Space Station, the Hubble Space Telescope, and various planetary probes, landers, rov­ers, and flybys speak to the creativity of the Agency, the excellence of its technical personnel, and its dedication to space science and exploration.

But there is another aspect to NASA, one that is too often hidden in an age when the Agency is popularly known as America’s space agency and when its most visible employees are the astronauts who courageously

rocket into space, continuing humanity’s quest into the unknown. That hidden aspect is aeronautics: lift-borne flight within the atmosphere, as distinct from the ballistic flight of astronautics, out into space. It is the first "A” in the Agency’s name and the oldest-rooted of the Agency’s technical competencies, dating to the formation, in 1915, of NASA’s lineal predecessor, the National Advisory Committee for Aeronautics (NACA). It was the NACA that largely restored America’s aeronautical primacy in the interwar years after 1918, deriving the airfoil profiles and configuration concepts that defined successive generations of ever-more-capable aircraft as America progressed from the subsonic piston era into the transonic and supersonic jet age. NASA, succeed­ing the NACA after the shock of Sputnik, took American aeronautics across the hypersonic frontier and onward into the era of composite structures, electronic flight controls, and energy-efficient flight.

This volume, the first of a two-volume set, traces contribu­tions by NASA and the post-Second World War NACA to the field of aeronautics. It was that work that enabled the exploitation of the turbojet and high-speed aerodynamic revolution that led to the gas – turbine-powered jet age that followed, within which we still live. The subjects covered in this first volume are an eclectic mix of sur­veys, case studies, and biographical examinations ranging across multiple disciplines and technical competencies residing within the National Aeronautics and Space Administration. The topics are indic­ative of the range of Agency work and the capabilities of its staff. They include:

• The advent of the sharply swept-back wing, which enabled taking fullest advantage of the turbojet revolution and thereby launched the era of high-speed global mass mobility, becoming itself the iconic symbol of the jet age.

• The contributions and influence of Richard T. Whitcomb, a legendary NACA-NASA researcher who gave to aero­nautics some of the key methods of reducing drag and improving flight efficiencies in the challenging transonic region, between subsonic and supersonic flight.

• The work of the NACA and NASA in furthering the rotary wing revolution via research programs on a range of rotorcraft from autogiros through helicopters, conver – tiplanes, ducted fan, tilt wing, and tilt rotor craft.

• How NASA worked from the earliest days of the super­sonic revolution to mitigate the shock and disturb­ing effects of the sonic boom, developing creative test approaches to evaluate boom noise and overpressures, and then methods to alleviate boom formation and impingement, leading to novel aircraft shaping and methods that are today promising to revolutionize the design of transonic and supersonic civil and military aircraft.

• How the NACA and NASA, having mastered the tran­sonic and supersonic regions, took on the challenge of extending lift-borne flight into the hypersonic region and thence into space, using exotic "transatmospheric” vehicles such as the legendary X-15, various lifting bodies, and the Space Shuttle, and extending the fron­tiers of air-breathing propulsion with the Mach 9+ scramjet-powered X-43.

• The physical problems and challenges that forced NASA and other researchers to study and find pragmatic solu­tions for such thorny issues as aeroelasticity, oscillatory instabilities forcing development of increasingly sophis­ticated artificial stability systems, flight simulation for high-performance aerospace vehicles, and aerothermo – dynamic structural deformation and heating.

• NASA’s role in advancing and maturing computational fluid dynamics (CFD) and applying this new tool to aero­nautical research and aerospace vehicle design.

• The exploitation of materials science and development of high-temperature structures to enable design of prac­tical high-speed military and civil aircraft and spacecraft.

• The advent of computerized structural loads prediction, modeling, and simulation, which, like CFD, revolution­ized aerospace design practices, enhancing both safety and efficiency.

• NASA’s pioneering of electronic flight control ("fly-by­wire”), from rudimentary testbeds evolved from Apollo – era computer architectures and software, to increasingly sophisticated systems integrating aerodynamic and pro­pulsion controls.

• How the NACA and NASA advanced the gas turbine revolution, producing more efficient engine concepts and technology for application to new generations of military and civilian aircraft.

• How NASA has contributed to the quest for fuel-efficient and environmentally friendly aircraft technology, study­ing combustion processes, alternative fuels, and pollut­ant transfer into the upper atmosphere, searching for appropriate technological solutions, and resulting in less polluting, less wasteful, and more efficient aircraft designs.

• The Agency’s work in promoting global environmental good stewardship by applying its scientific and technical competencies to wind and solar energy, resulting in more efficient energy-producing wind turbines and high-altitude solar-powered long-endurance unpiloted aerial vehicles.

The record of NACA-NASA accomplishments in aeronautics dem­onstrates the value of consistent investment in aeronautical research as a means of maintaining the health and stability of America’s aerospace industrial base. That base has generated an American predominance in both civil and military aeronautics, but one that is far from assured as the Nation enters the second century of winged flight. It is hoped that these studies, offering a glimpse at the inner workings of the Agency and its personnel, will prove of value to the men and women of NASA, to those who benefit across the United States and overseas from their ded­icated work, and to students of aeronautics and members of the larger aerospace community. It is to the personnel of NASA, and the NACA before them, that this volume is dedicated, with affection and respect.

Dr. Richard P. Hallion

August 4, 2010

Richard P. Hallion

The development of the swept and delta wing planform enabled practical attainment of the high speeds promised by the invention of the turbojet engine and the solid-and-liquid-fueled rocket. Refining the swept and delta planforms from theoretical constructs to practical reali­ties involved many challenges and problems requiring creative analysis and study by NACA and NASA researchers. Their insight and persever­ance led to the swept wing becoming the iconic symbol of the jet age.

HE PROGRESSIVE EVOLUTION OF AIRCRAFT DESIGN HAS WITNESSED continuous configuration changes, adaptations, and reinterpreta­tions. The canard wood-and-fabric biplane launched the powered flight revolution and gave way to the tractor biplane and monoplane, and both gave way to the all-metal monoplane of the interwar era. The tur­bojet engine set aside the piston engine as the primary motive power for long-range commercial and military aircraft, and it has been continually refined to generate the sophisticated bypass turbofans of the present era, some with afterburning as well. The increasing airspeed of aircraft drove its own transformation of configuration, measurable in the changed rela­tionship between aspect and fineness ratios. Across the primacy of the propeller-driven era, from the beginning of the 20th century to the end of the interwar era, wingspan generally far exceeded fuselage length. That changed early in the jet and rocket era. By the time military and test pilots from the National Advisory Committee for Aeronautics (NACA) first probed the speed of sound with the Bell XS-1 and Douglas D-558-1 Skystreak, wingspan and fuselage length were roughly equal. Within a decade, as aircraft speed extended into the supersonic regime, the ratio of wingspan to fuselage length dramatically reversed, evidenced by aircraft such as the Douglas X-3, the Lockheed F-104 Starfighter, and the Anglo-French Concorde Supersonic Transport (SST). Nicknames handily captured the

transformation: the rakish X-3 was known informally as the "Stiletto” and the only slightly less sleek F-104 as the "Missile with a Man in It.”

There was as well another manifestation of profound design transfor­mation, one that gave to the airplane a new identity that swiftly became a global icon: the advent of the swept wing. If the biplane constituted the normative airplane of the first quarter century of flight and the straight wing cantilever monoplane that of the next quarter century, by the time of the golden anniversary of Kitty Hawk, the swept wing airplane had sup­planted both, its futuristic predominance embodied by the elegant North American F-86 Sabre that did battle in "MiG Alley,” high over North Korea’s blue-gray hills bordering the Yalu River. In the post-Korean era, as swept wing Boeing 707 and Douglas DC-8 jet airliners replaced what historian Peter Brooks termed the "DC-4 generation” of straight wing propeller-driven transports, the swept wing became the iconic embodiment of the entire jet age.[1] Today, 75 years since its enunciation at an international conference, the high-speed swept wing is the commonly accepted global highway sym­bol for airports, whether an intercontinental center such as Los Angeles, Frankfurt, or Heathrow; regional hubs such as Dallas, Copenhagen, or Charlotte; or any of the myriad general aviation and business aviation air­fields around the world, even those still primarily populated, ironically, by small, straight wing propeller-and-piston-driven airplanes.

The Tailless Imperative: The Early History of Swept and Delta Wings

The high-speed swept wing first appeared in the mid-1930s and, like most elements in aircraft design, was European by birth. But this did not mark the swept wing’s first appearance in the world’s skies. The swept wing dated to before the First World War, when John Dunne had developed a series of tailless flying wing biplanes using the swept planform as a means of ensuring inherent longitudinal stability, imparting "self-correcting” res­toration of any gust-induced pitching motions. Dunne’s aircraft, while freakish, did enjoy some commercial success. He sold manufacturing
rights to the Burgess Company in the United States, which subsequently produced two "Burgess-Dunne” seaplanes for the U. S. Navy. Lt. Holden C. Richardson, subsequently one of the first members of the NACA, had urged their purchase "so that the[ir] advantages and limitations can be thoroughly determined. . . as it appears to be only the beginning of an important development in aeronautical design.”[2]

That it was, though not in the fashion Richardson expected. The swept wing remained an international staple of tailless self-stabilizing design, typified in the interwar years by the various Westland Pterodactyl aircraft designed by Britain’s G. T.R. Hill, the tailless aircraft of Boris Ivanovich Cheranovskiy, Waldo Waterman’s Arrowplane, and a series of increas­ingly sophisticated sailplanes and powered aircraft designed by Germany’s Alexander Lippisch. However, it would not become the "mainstream” ele­ment of aircraft design its proponents hoped until applied to a very dif­ferent purpose: reducing transonic aerodynamic effects.[3] The transonic swept wing effectively increased a wing’s critical Mach number (the "drag divergence Mach number”), delaying the onset of transonic drag rise and enabling an airplane to fly at higher transonic and supersonic speeds for the same energy expenditure and drag penalty that a straight wing airplane would expend and experience at much lower subsonic speeds.

In 1935, leading aerodynamicists gathered in Rome for the Volta Congress on High Speeds in Aviation, held to coincide with the opening of Italy’s impressive new Guidonia laboratory complex. There, a young German fluid dynamicist, Adolf Busemann, unveiled the concept of using the swept wing as a means of attaining supersonic flight.[4] In his presentation, he
demonstrated the circulation pattern around a swept wing that, essen­tially, "fooled” it into "believing” it was flying at lower velocities. As well, he presented a sketch of an aircraft with such a "Pfielformiges Tragwerk” ("Arrow-Shaped Lifting Surface”), though one that had, by the standards of subsequent design, very modest sweep and very high aspect ratio.[5]

Theodore von Karman recalled not quite two decades later that after­ward, at the conference banquet, "General [Arturo] Crocco, the orga­nizer of the congress and a man of far-reaching vision, went further while doodling on the back of the menu card, drawing a plane with swept – back wings and tail, and even swept propeller blades, laughingly calling it ‘Busemann’s airplane.’”[6] Evidence exists that Crocco took the concept beyond mere dinner conversation, for afterward, Guidonia researchers evaluated a design blending modestly swept wings with a "push-pull” twin-engine fuselage configuration. However, Guidonia soon returned to the more conventional, reflecting the Italian air ministry’s increas­ing emphasis upon building a large and powerful air arm incorporating already proven and dependable technology.[7]

Delegates from other nations present at Busemann’s briefing missed its significance altogether, perhaps because his gently swept configuration—in the era of the DC-2 and DC-3, which had pronounced leading edge taper— looked far less radical than the theory and purpose behind it implied. NACA Langley Memorial Aeronautical Laboratory researchers had already evaluated far more sharply swept planforms at Langley for a seminal wing taper study the laboratory issued the next year.[8] Thus, at first glance, Busemann’s design certainly did not look like a shape that would trans­form aviation from the firmly subsonic to the transonic, making possible the potential of the jet engine, and the jet age (with its jet set) that followed.

Therefore, for the United States and most other nations, over the next decade, the normative airplane remained one having straight (if tapered) wings and piston propulsion. For Germany, however, the future belonged to increasingly sharply swept and delta wings, and jet and rocket propulsion as well. Within 5 years of the Volta conference, with Europe engulfed in a new war, its engineers had already flown their first jet and rocket-powered aircraft, had expanded beyond Busemann’s initial conception to derive shapes more closely anticipating subsequent high-speed aircraft and missile designs, and were busily testing models of swept wing transonic airplanes and supersonic missiles. Lippisch’s swept wing sailplanes had presaged a new Messerschmitt rocket – propelled interceptor, the Me 163 Komet ("Comet”), and his broad, high aspect ratio deltas had given way to a rounded triangular planform that he envisioned as meeting the needs for transonic and supersonic flight. While many of these concepts by Lippisch and other German designers were impracticable, or unrelated to Germany’s more imme­diate military needs, others possessed significant military or research potential. Only flawed decisions by the Third Reich’s own leadership and the Allies’ overrunning of Germany would prevent them from being developed and employed before the collapse of the Hitler regime in May 1945.[9]

The extent to which the swept wing permeated German aeronauti­cal thought understandably engendered tremendous postwar interest

A sampling of various design concepts for Lippisch swept wing and delta aircraft. These orig­inal Lippisch sketches were incorporated in "German Aircraft: New and Projected Types,” a 1946 Allied technical intelligence summary. USAF.

in the benefits of swept planforms for transonic and supersonic flight within the American, European, and Soviet aeronautical communi­ties.[10] However, for America, uncovering German swept wing research and development furnished the confirmation of its value, not its discovery, for Robert T Jones, an aerodynamicist at the Langley Memorial Aeronautical Laboratory, had independently discovered its benefits in 1944, a year before the Allies first entered Germany’s shattered and shut­tered research laboratories and design shops.[11]

In 1936, Michael E. Gluhareff, an emigree Russian engineer who was chief of design for the Vought-Sikorsky Aircraft Division of United

Aircraft Corporation, began examining various tailless aircraft con­figurations. By July 1941, his study had spawned a proposed intercep­tor fighter powered by a piston engine driving a contra-rotating pusher propeller. It had a rounded delta planform resembling an arrowhead, with leading edges swept aft at 56 degrees. It featured a tricycle retract­able landing gear, twin ventral vertical fins, an extremely streamlined and rounded configuration, provisions for six heavy machine guns, and elevons (combined ailerons and elevators) for roll and pitch control. Gluhareff informed company founder Igor I. Sikorsky that its sharp sweep would delay the onset of transonic compressibility, noting, "The general shape and form of the aircraft is, therefore, outstandingly adapt­able for extremely high speeds.”[12]

In retrospect, Gluhareff’s design was a remarkable achievement, conceived at just the right time to have been completed with turbojet propulsion (for which its configuration and internal layout was emi­nently suited) though circumstances conspired against its development. Sikorsky was then perfecting the first practical helicopter—the VS-300, another revolutionary development, of course—and chose understand­ably to concentrate on rotary wing flight. He did authorize Gluhareff to solicit support from inventor-entrepreneur Roger W. Griswold, presi­dent of the Ludington-Griswold Company, about building a wind tunnel model of the configuration.[13] Tests by United Aircraft proved so encour­aging that Griswold approached the engineering staff of the Army Air Forces (AAF) at its Wright Field Aircraft Laboratory about sponsor­ing what was now called the "Dart.”[14] But having their fill of visitors

bringing a series of the weird and unconventional, and charged with ensuring that the AAF acquired large numbers of aircraft, and quickly, the AAF’s engineers did not pursue the project.[15]

So the Gluhareff-Griswold Dart never reached the hardware stage, the failure to build it counting as a loss to American midcentury aeronautics. As for Gluhareff, though he had made notable contributions to Sikorsky’s large flying boats (and would, as well, to his helicopters), he continued
to explore the basic design of his intriguing if abortive configuration, proposing a variety of derivatives, including in 1959 a Mach 2+ super­sonic transport with a small canard wing and double-deck fuselage.[16] If the Dart never saw development, its configuration nevertheless proved significant. In 1944, Griswold resurrected the Dart shape for a proposed 2,000-pound guided glide bomb, or "glomb.” The Army Air Forces recom­mended he obtain the NACA’s opinion of its aerodynamics, and for this, Griswold turned to Langley Memorial Aeronautical Laboratory. There, on August 19, he met with the NACA’s resident aerodynamic expert on "pilotless missiles,” Robert T. Jones. Out of that contact would emerge both the American delta and swept wing.

"R. T.” Jones was a brilliant, flight-obsessed, and largely self-taught fluid dynamicist, having dropped out of the University of Missouri to join a flying circus, then working as a designer for Nicholas-Beazley, a small Missouri aircraft company. When the Great Depression collapsed the firm, his father used political connections as Chairman of the local Democratic Party to secure Jones a job running elevators in the U. S. Capitol. In his spare time and evenings, he studied mathematics and aerodynamics with Albert Zahm, the aeronautics Chair at the Library of Congress, and with Max Munk at Catholic University. Despite his lack of a formal engineering degree, through the efforts of Representative David Lewis (a homespun Maryland progressive with a strong interest in self-improvement who had taken math instruction from the young elevator operator), Jones received a temporary appointment as a "sci­entific aide” to the NACA. There, he quickly proved such a gifted and insightful researcher that he soon secured a coveted permanent posi­tion at Langley, consorting with the likes of John Stack, Eastman Jacobs, and Theodore Theodorsen.[17]

As he considered Griswold’s "glomb,” Jones recognized that its extremely low aspect ratio shape (that is, a shape having a very long

wing root in relation to its total wingspan) could not be adequately ana­lyzed using conventional Prandtl-rooted "lifting line” theory. Instead, Jones drew on the work of his mentor Munk, using papers that Munk had written on the flow of air around inclined airship hulls and swept wings, and one by the Guggenheim Aeronautical Laboratory’s Hsue-shen Tsien, a von Karman associate at the California Institute of Technology (Caltech), on airflow around inclined bodies of revolution. He analyzed it using linear equations governing two-dimensional incompressible flow, con­sidering his results of little practical value, recalling three decades later, "I thought, well, this is so crude, nobody would be interested. So I just hid it in my desk.”[18]

But it sparked his curiosity, and in January 1945, by which time he was busy thinking about nonlinear compressible flows, he had a rev­elation: the equations he had developed months earlier for the glomb analysis could be applied to a low aspect triangular wing operating in supersonic flow, one whose wing-leading edges were so sharply swept as to place them within the shock cone formed around the vehicle and hence operating in subsonic flow. In these conditions, the wing was essentially "fooled” into behaving as if it were operating at a much lower Mach number. As Jones recalled, "It finally dawned on me that the slen­der wing theory would hold for compressible flow and even at supersonic speed if it were near the center of the Mach cone. So, I immediately got the paper out and I added the compressible flow parts to it, which was really the important part, and then I wondered well, why is it that this slender wing doesn’t have an effect on compressibility? Then I realized that it was because the obliquity of the edge and that this is the sim­ple sweep theory and would work in spite of the compressibility effect. So, I wrote a paper which incorporated the slender wing theory and also sweep theory.”[19] Jones then moved from considering a slender triangular delta [Д] to the sharply sweptback wing [л], the reverse of

Germany, where the high-speed swept wing had preceded, not followed, the delta.[20]

Jones’s delta and swept wing utilized, for their time, very thin airfoil sec­tions, ones typical of supersonic aircraft to come. In contrast, German swept and delta wing developer Alexander Lippisch had employed much thicker sections that proved unsuitable for transonic flight. His tailless rocket – propelled swept wing Me 163 Komet ("Comet”) interceptor, for example, essentially became uncontrollable at speeds slightly above Mach 0.82 thanks to stability changes induced by shock wave formation on its relatively thick wing. His design for a rocket-boosted, ramjet-powered delta fighter, the P 13, had such thick wing and tail sections—the pilot actually sat within the leading edge of the vertical fin—that it could never have achieved its desired transonic performance. As discussed subsequently, postwar NACA tests of a captured glider configuration of this design, the DFS DM-1, confirmed that transonic delta wings should be far thinner, with sharper leading edges. As a consequence, NACA researchers rejected the Lippisch approach, and, though some of them tried extrapolations of his designs (but with lower thickness-chord ratios and sharper leading edges), the NACA (and industry as well) adapted instead the thin slender delta, a la Jones.[21]

In February 1945, Jones showed his notes on sweep to Jean Roche, the Army Air Forces technical liaison at Langley, and informed others as well, including Maj. Ezra Kotcher of the AAF’s Air Technical Service

Command, and NACA colleagues Arthur Kantrowitz and Hartley A. Soule.[22] Kotcher passed it along to von Karman and Tsien—then work­ing as scientific advisers to Gen. Henry H. "Hap” Arnold, the Army Air Forces’ Chief of Staff—and Soule and Kantrowitz urged Jones to inform the Agency’s Director of Research, George W. Lewis, of his discovery.[23] Accordingly, on March 5, 1945, Jones informed Lewis, "I have recently made a theoretical analysis which indicates that a V-shaped wing travel­ling point foremost would be less affected by compressibility than other planforms. In fact, if the angle of the V is kept small relative to the Mach angle [the angle of the shockwave], the lift and center of pressure remain the same at speeds both above and below the speed of sound.”[24] Jones subsequently undertook tests in the Langley 9-inch supersonic tunnel of a small, 4-inch-long daggerlike sheet-steel triangular wing with rounded leading edges and a span of only 1.5 inches, tests that complemented other trials at Aberdeen, MD, arranged by von Karman and Tsien.

The Langley tests, through the transonic region and up to Mach 1.75, confirmed his expectations, and Jones published his first test results May 11, 1945, noting, "The lift distribution of a pointed airfoil travelling point-foremost is relatively unaffected by the compressibility of the air below or above the speed of sound.”[25] This was almost 2 weeks before Lippisch informed von Karman, then leading an AAF European study team, of his high-speed delta concepts (during a technical intelligence interrogation at St. Germain, France, on May 23), not quite a month before von Karman assistant Clark Millikan visited the Messerschmitt advanced projects group at Oberammergau on June 9-10 and inter­rogated Waldemar Voigt about his swept wing fighter concepts, and well over a month before Millikan journeyed to Volkenrode to inter-

rogate German swept wing inventor Adolf Busemann, on June 20-21.[26]

Langley’s peer reviewers and senior Agency official Theodore Theodorsen did not immediately accept Jones’s assumption that a unified slender wing theory could apply to both compressible and incompressible flows and even questioned the evidence of sweep’s benefits. Fortunately, Jones was greatly assisted in confounding skeptics by the timely results of NACA tunnel tests and falling body experiments, which left little doubt that sweep worked. As well, an associate of Jones made a most helpful discov­ery: locating a 1942 British translation of Busemann’s 1935 paper. Evidence of an enemy’s interest coincident with one’s own work always heightens its perceived value, and undoubtedly, the Busemann paper, however dated, now strongly bolstered Jones’s case. When it became time to assemble a bibliography for his swept wing report, Jones added Busemann’s paper and other German sources by Albert Betz, H. G. Kussner, Ludwig Prandtl, and Hermann Schlichting, though it is unclear whether this reflected a collegial respect across the chasm of war or simply a shrewd apprecia­tion of their persuasive value.[27]

Langley released his report in late June 1945.[28] In it, Jones noted: "the attachment of plane waves to the airfoil at near-sonic or supersonic speeds (Ackeret theory) may be avoided and the pressure drag may be reduced by the use of planforms in which the angle of sweepback is greater than the Mach angle. The analysis indicates that for aerodynamic efficiency, wings designed for flight at supersonic speeds should be swept back at an

angle greater than the Mach angle and the angle of sweepback should be such that the component of velocity normal to the leading edge is less than the critical speed of the airfoil sections. This principle may also be applied to wings designed for subsonic speeds near the speed of sound, for which the induced velocities resulting from the thickness might otherwise be suf­ficiently great to cause shock waves.”[29] Such marked the effective birth of
the high-speed swept wing airplane in the United States, as his report weeks earlier had marked the birth of the American high-speed delta.

By the time Jones’s report appeared, Germany’s aeronautical estab­lishment was already under the microscope of Allied technical intelli­gence, whose teams swiftly focused on its intensive investment in swept wing aerodynamics for its missiles and aircraft. Replicating reaction to the earlier "discovery” of "Gottingen aerodynamics” after the First World War, the post-Second World War influence of German example and practice was even more profound. Indeed, it affected the entire post­war course of European, Soviet, and American high-speed aerodynamic research, development, test, evaluation, and acquisition. In the increas­ingly tense national security environment of the burgeoning Cold War, the national intelligence services of the various advanced aeronauti­cal nations understandably maintained very active technical collection efforts to learn what they did not already know.[30]