The Quest for Refinement

By the end of the 1960s, the "classic” era of aircraft design was argu­ably at an end. As exemplars of the highest state of aviation technology, the piston engine had given way to the gas turbine, the wood-and-fabric aircraft to the all-metal, the straight wing had given way to the swept and delta. Aircraft flight speeds had risen from a mere 40 mph at the time of the Wright brothers to over 100 times as fast, as the X-15A-2 dem­onstrated when it streaked to Mach 6.70 (4,520 mph) in October 1967, piloted by Maj. William J. Knight. Fighters, by that time, had been flying on a Mach 2 plateau for a decade and transports on a Mach 0.82 plateau for roughly the same amount of time. In space, Americans were basking in the glow of the recent Apollo triumph, where a team of astronauts, led by former NACA-NASA research pilot Neil Armstrong—a Round One and Round Two veteran whose experience included both the X-1 and the X-15—journeyed to the Moon, landed two of their number upon it, and then returned to Earth.

Such accomplishments hardly meant that the frontiers of the sky were closing, or that NASA had little to do. Indeed, in some respects, it was facing even greater challenges: conducting comprehensive aeronautical research at a time when, increasingly, more people identified it with space than aeronautics and when, in the aftermath of the Apollo success, mon­ies were increasingly tight. Added to this was a dramatically transforming world situation: increasing tension in the Middle East, a growing Soviet threat, rising oil prices, open concern over environmental stewardship,
and a national turning away from the reflexive perception that limitless technological progress was both a given and a good thing.

Within this framework, NASA work increasingly turned to achieving efficiencies: more fuel-efficient and energy-efficient civilian flight, and more efficient military systems. It was not NASA’s business to, per se, design new aircraft, but, as NACA-NASA history amply demonstrated, the Agency’s mark could be found on many aircraft and their innova­tions. Little things counted for much. When, for example, NACA High­Speed Flight Research Station pilots flew a Douglas D-558-1 Skystreak modified with a row of small vortex generators (little rectangular fins of 0.5-inch chord standing vertically like a row of razor blades) on its upper wing surface, they hardly expected that such a small energy – imparting modification would so dramatically improve its transonic handling qualities that rows of vortex generators would become a com­monly recognized feature on many aircraft, including such "classics” as the B-52, the 707, and the A-4.[121] In the post-1970 period, NASA assidu­ously pursued three concepts related to swept wing and delta flight, in hopes that each would pay great dividends: the supercritical wing, the winglet, and the arrow wing.[122] All had roots embedded and nourished in the earliest days of the supersonic and swept/delta revolution. Each reflected Whitcomb’s passion—indeed obsession, in its most positive sense—with minimizing interference effects and achieving the greatest possible aerodynamic efficiency without incurring performance-robbing complexity. Many had researched configurations approaching the purity of the arrow wing, but it was Whitcomb who first actually achieved such a configuration, as part of Langley’s Supersonic Transport study effort.

Long a subject of individual research and thought, Langley’s institu­tional SST studies had begun in 1958, when the ever-enthusiastic John Stack formed a Supersonic Transport Research Committee (STRC). It evaluated the maturity of various disciplines—particularly the "classics” of aerodynamics, structures, propulsion, and controls—and then fore­cast the overall feasibility of a Supersonic Transport. The Stack team presented the results of their studies to the head of the Federal Aviation

Administration (FAA), Elwood Quesada, a retired Air Force general, in December 1959. Their report, issued the following year, concluded: "the state of the art appears sufficiently advanced to permit the design of an airplane at least marginally capable of performing the super­sonic transport mission.”[123] NASA swiftly ramped up to match growing interest in the FAA in such aircraft; within a decade, SST-focused research would constitute over a quarter of all NASA aeronautics research undertaken at the Langley, Ames, and Lewis Centers.[124]

Given that the British and French subsequently designed the Mach 2+ Concorde, and the Soviets the Tupolev Tu-144, NASA Langley’s tech­nological optimism in 1959-1960 was, within limits, technically well justified, and such optimism infused Washington’s political community as well. In March 1966, President Lyndon Johnson announced that the first American SST, designed to cruise at Mach 2.7, would fly at decade’s end and enter commercial service in 1974.[125] But such expectations would prove overly optimistic. As Mach number rose, so too did a number of daunting technical challenges encountered by the more ambitious air­craft American SST proponents favored. Assessing the technology alone did not address the serious questions—research and development invest­ment, production costs, operating economics, and environmental con­cerns, for example—such aircraft would pose and would limit the airline acceptance (and, hence, market success) of even the "modest” Concorde and Tu-144. Air transport constitutes a system of systems, and excellence in some does not guarantee or imply excellence overall. Political support, strongly bipartisan over the Kennedy-Johnson era, withered in the Nixon

years as technical and other challenges arose, and a re-action against the SST set in, fueled by questions over the value of high technology and reaction to the long and costly war in Southeast Asia.[126]

From the standpoint of aircraft design, from Langley’s interest emerged a series of Supersonic Commercial Air Transport (SCAT) design studies, most of which incorporated variable-geometry planforms reflecting a growing popular wisdom that future military or civilian supersonic cruise designs would necessarily incorporate such wings. Whitcomb, focused on simplicity and efficiency, demurred, preferring instead a sharply swept arrow configuration, the SCAT-4, which he had derived. It drew upon a two-decade tradition of Langley swept and delta studies running through those of Clinton E. Brown and F. Edward McLean in the 1950s, back to the thin swept and delta research manifested in Robert T. Jones’s origi­nal concepts in 1944-1945. Though he was not successful at the time at selling his vision of what such an aircraft should be (and, in fact, left the Stack SST study effort as a result), in time the fixed wing predominated. In 1964, a Langley team comprised of Harry Carlson, Roy Harris, Ed McLean, Wilbur Middleton, and A. Warner Robins derived a fixed wing variant of the variable-sweep SCAT-15, generating an elegant slender arrow wing called the SCAT-15F. SCAT-15F had an incredible lift-to-drag ratio of 9.3 at Mach 2.6, well beyond what previous analysis and thought had deemed possible, though it also had serious low-speed pitch-up and deep-stall tendencies that triggered intensive investigations by research­ers using the Langley Full-Scale Tunnel.[127] Out of this came a revised SCAT-15F configuration, with leading edge flaps, wing notches, area-and – camber-increasing Fowler flaps, and a small, horizontal tail, all of which worked to make it a much more acceptable planform. The development

of the high supersonic L/D fixed wing eventually led Boeing (winner of the Government’s SST design competition) to abandon variable-sweep in favor of a highly refined small-tailed delta, for its final SST proposal, though congressional refusal to furnish needed developmental monies brought the American SST development effort to a sorry end.[128] It did not, however, end interest in similar configurations for a range of other mis­sions. Today, in an era of vastly different technology, with much higher­performing engines, better structures, and better means of modeling and simulating the aerodynamic and propulsive performance of such designs, tailored fixed arrow wing configurations are commonplace for future advanced high-speed civil and military aircraft applications.

As the American SST program, plagued by controversy and numerous wounds (many self-inflicted), died amid performance and environmental concerns, Whitcomb increasingly turned his attention to the transonic, thereby giving to aviation one of its most compelling images, that of the graceful supercritical wing and, of less aesthetic appeal but no less significance, the wingtip winglet. Both, in various forms, became standard design elements of future civil and military transport design and are examined elsewhere (by historian Jeremy Kinney) in this work.

As for the arrow wing, military exigency and the Cold War com­bined to ensure that studies of this most promising configuration spawned the "cranked arrow wing” of the late 1970s. Following cancel­lation of the national SST effort, NASA researchers continued study­ing supersonic cruise for both military and civil applications, under the guise of a new study effort, the Advanced Supersonic Technology (AST) effort. AST was succeeded by another Langley-run cruise-focused effort, the Supersonic Cruise Aircraft Research (SCAR, later shortened to SCR) program. SCR lasted until 1982, when NASA terminated it to focus more attention and resources on the already troubled Shuttle program. But meantime, it had spawned the Supersonic Cruise and Maneuver Prototype (SCAMP), a derivative of the F-16 designed to cruise at supersonic speeds. Its "cranked arrow” wing, blending a 70-degree swept inboard leading edge and a 50-degree swept outboard leading edge, looked deceptively simple but embodied sophisticated shaping and camber (reflecting the long legacy of SCAT studies, particularly the refinement of the SCAT – 15F), with leading edge vortex flaps to improve both transonic and low – speed performance. General Dynamics’ F-16 designer Harry J. Hillaker adopted the planform for a proposed strike fighter version of the F-16 because it reduced supersonic wave drag, increasing the F-16’s potential combat mission radius by as much as 65 percent and more than doubling its permissible angle-of-attack range as well. In the early 1980s, SCAMP, now designated the F-16XL, competed with the prototype F-15E Strike Eagle at Edwards Air Force Base for an Air Force deep-strike fighter con­tract. But the F-16XL was too small an airplane to win the completion; with greater internal fuel and volume, the larger Strike Eagle offered more growth potential and versatility. The two F-16XL aircraft, among the most beautiful ever flown, remained at Edwards, where they flew a variety of research missions at NASA Dryden, refining understanding of the complex flows around cranked arrow profiles and addressing such technical issues as the possibility of supersonic laminar flow control by using active suction. Interest in the cranked arrow has persisted, as it remains a most attractive design option for future supersonic cruise air­craft, whether piloted or not, both civil and military.[129]

By the end of the 1980s, for military aircraft, concern over aero­dynamic shaping of aircraft was beginning to take second place behind concern over their electromagnetic signature. Where something such as the blended wing-body delta SR-71 possessed an innate purity and beauty of form, inherent when aerodynamics is given the position of primacy in aircraft design, something such as the swept wing, V-tail F-117 stealth fighter did not: all angles and panels, it hardly looked aero­dynamic, and, indeed, it had numerous deficits cured only by its being birthed in the electronic fly-by-wire and composites era. But in other aspects it performed with equal brilliance: not the brilliance of Mach 3+, but the quiet brilliance of penetrating a high-threat integrated air defense network, attacking a key target, and escaping without detection.

For the future of the swept surface, one had to look elsewhere, back to the transonic, where it could be glimpsed in the boldly imagi­native lines of the Blended Wing-Body (BWB) transport. Conceived by Robert H. Liebeck, a gifted Boeing designer who had begun his career at Douglas, where he worked with the legendary A. M.O. Smith, the BWB represented a conception of pure aerodynamic efficiency predating NASA, the NACA that had preceded it, and even, indeed, Jack Northrop and the Horten brothers. It hearkened back to the earliest concepts for Nurflugeln (flying wings) by Hugo Junkers before the First World War, the first designer to appreciate how one could insightfully incorporate the cantilever all-metal structure to achieve a pure lifting surface.[130] Conceived while Liebeck worked for McDonnell-Douglas in the latter years before its own merger with Boeing, the graceful BWB was not strictly a flying wing but, rather, a hybrid wing-body combination whose elegant high aspect ratio wing blended smoothly into a wide, flat-bottom fuselage, the wings sprouting tall winglets at their tips for lateral con­trol, thus differing significantly from earlier concepts such as the Boeing "Spanloader” and the Horten, Armstrong-Whitworth, and Northrop fly­ing wings. Early design conceptions envisioned upward of 800 passen-

gers flying in a three-engine, double-deck, 823,000-pound, manta-shaped BWB (spanning 289 feet with a length of 161 feet), cruising across the globe at Mach 0.85. Subsequent analysis resulted in a smaller design sized for 450 passengers, the BWB-450, which served as the baseline for later research and evaluation, which concluded that the most suit­able role for the BWB might be for a range of global heavy-lift multi­purpose military missions rather than passenger-carrying.[131] Extensive studies by NASA Langley and Lewis researchers; McDonnell-Douglas (now Boeing) BWB team members; and academic researchers from Stanford University, the University of Southern California, Clark Atlanta University, and the University of Florida confirmed the aerodynamic and propulsive promise inherent in the BWB, particularly its poten­tial to carry great loads at transonic speeds over global distances with unprecedented aerodynamic and energy efficiency, resulting in poten­tially 30-percent better fuel economy than that achievable by traditional "tube and wing” airlifters.[132]

These and many other studies, including tests by Boeing and the United States Air Force, encouraged the next logical step: developing a subscale unmanned aerial vehicle (UAV) to assess the low-speed flight – control characteristics of the BWB in actual flight. This became the X-48B, a 21-foot span, 8.5-percent scale UAV testbed of the BWB-450 configuration, powered by three 240-pound thrust Williams turbojets. Boeing had Cranfield Aerospace, Ltd., in Great Britain build two X-48Bs for the company’s Phantom Works. After completion, the first X-48B com­pleted 250 hours of tunnel tests in the Langley Full-Scale Tunnel (run by Old Dominion University) in May 2006. Readying the BWB for flight

The NASA F-16XL cranked-arrow research aircraft aloft over the Dryden Flight Research Center on December 16, 1997. NASA.

consumed another year until, on July 20, 2007, the second example took to the air at Dryden, becoming the first of the X-48B testbeds to fly. By the end of the year, it had completed five research flights. Subsequent testing explored its stability and control at increasing angles of attack (to as great as 16-degree AoA), pointing to possible ways of furnishing improved controllability at even higher angles of attack.[133] Time will tell if the world’s skies will fill with blended wing-body shapes. But to those who follow the technology of the sky, if seemingly fantastic, it is well within the realm of the possible, given the history of the swept and delta wings—and NACA-NASA’s role in furthering them.

In conclusion, the invention of the swept and delta wing blended creative and imaginative analysis and insight, great risk, and steadfast research. If in introspect their story has a clarity and a cohesiveness that was not necessarily visible to those at the time, it is because time has stripped the story to its essence. It is unfortunate that the percep­tion that America was "given” (or "took”) the swept and delta wing in full-blown maturity from the laboratories of the Third Reich possesses

such persistency, for it obscures the complex roots of the swept and delta wing in both Europe and America, the role of the NACA and NASA in maturing them, and, at heart, the accomplishments of successive gener­ations of Americans within the NACA-NASA and elsewhere who worked to take what were, in most cases, very immature concepts and turn them into practical reality. Doing so required achieving many other things, among which were securing a practical means of effective longitudi­nal control at transonic speeds (the low, all-moving, and powered tail), reducing transonic drag rise, developing stability augmentation sys­tems, and refining aircraft handling qualities. Defeating the transonic drag "hump”; reducing pitch-up to nuance, not nuisance; and overcom­ing the danger of inertial coupling were all crucial to ensuring that the swept and delta wing could fulfill their transforming promise. Once achieved, that gave to the world the means to fulfill the promise of the jet engine. As a result, international security and global transportation patterns were dramatically altered and a new transnational global con­sciousness born. It is something that workers of the NACA past, and NASA past, present, and future, can look back upon with a sense of both pride and accomplishment.