Category AERONAUTICS

Though not seemingly connected to the swept wing, the researching and documenting of the advantages of low-placed horizontal tail surfaces con­stituted one of the major NACA postwar contributions to flight, one dra­matically improving both the safety and flight performance of swept wing designs. As a consequence, the jet fighter and attack aircraft of 1958 looked very different than did the initial jet (and rocket) aircraft of the imme­diate postwar era. Then, high-speed aircraft designers had emphasized tailless planforms, or ones in which the horizontal tail was well up the vertical fin (for example, both the XS-1 and the D-558 families). A decade later, aircraft introduced into test or service—such as the Vought F8U-1 Crusader, the Republic F-105B Thunderchief, the Grumman F11F-1 Tiger, the McDonnell F4H-1 Phantom II, the North American A3J-1 Vigilante, and the Northrop N-156 (progenitor of both the T-38 supersonic trainer and the F-5 lightweight fighter)—shared common characteristics: irre­versible power-operated flight controls, stability augmentation, and damp­ing, large vertical fins for enhanced directional stability, area-ruling, and low-placed, all-moving tails. Foreign aircraft exhibited similar features: for example, the MiG-21, Folland Gnat, and English Electric Lightning.

Aircraft lacking such features manifested often-perilous behavior. The Douglas XF4D-1 Skyray, a graceful rounded delta, had a sudden transonic pitch change reflecting its legacy of Messerschmitt-inspired tailless aerodynamic design. During one test run to Mach 0.98, it pitched
up so violently that test pilot Robert Rahn blacked out, becoming one of the first pilots to experience sudden g-induced loss-of-consciousness (g-loc). Fortunately, he recovered and returned safely, the battered plane now marred by prominent stress-induced wrinkles, giving it a prunelike appearance.[59] When Grumman entered the transonic swept wing era, it did so by converting its conventional straight wing F9F-5 Panther into a swept wing design, spawning the F9F-6 Cougar. (The use of an identical prefix—"F9F”—indicates just how closely the two aircraft were related.) But the Cougar’s swept wing, midplaced horizontal tail, and thick wing section (inherited from the firmly subsonic Panther) were ill matched. The new Cougar had serious pitch-up and departure characteristics at low and high speeds, forcing redesign of its wing before it could be intro­duced into fleetwide service. Even afterward, however, it retained some unpleasant characteristics, particularly a restricted angle-of-attack range during carrier landing approaches that gave the pilot only a small maneu­ver margin before the Cougar would become unstable. Well aware of the likely outcome of stalling and pitching up in the last seconds of flight prior to "trapping” on a carrier, pilots opted to fly faster, though the safety they gained came at the price of less-precise approaches with greater risk of "wave-offs” (aborted landings) and "bolters” (touching down beyond the cables and having to accelerate back into the air).[60]

McDonnell’s XF-88, a beefy twin-engine jet fighter prototype from the late 1940s, was placed on hold while more powerful engines were developed. When finally ordered into development in the early 1950s as the F-101 Voodoo, it featured a T-tail, a most unwise choice. Acceptable on airliners and transports, the T-tail was anathema for high-perfor­mance jet fighters. The Voodoo experienced serious pitch-up problems, and the cure was less a "fix” than simply a "patch”: McDonnell installed

a stick-kicker that would automatically push the stick forward as angle of attack increased and the Voodoo approached the pitch-up point. Wisely, for their next fighter project (the superlative F4H-1 Phantom II), McDonnell designers lowered the horizontal tail location to the base of the fin, giving it a characteristically distinctive anhedral (droop).[61]

Better yet, however, was placing the horizontal tail below the line of the wing chord, which, in practical terms, typically meant at the base of the rear fuselage, and making it all-moving as well. In 1947, even before the first supersonic flights of the XS-1, NACA Langley researchers had evalu­ated a wind tunnel model of the proposed Bell XS-2 (later X-2) with a low – placed horizontal tail and a ventral fin, though (unfortunately, given its history as related subsequently) Bell completed it with a more conventional layout mirroring the XS-1’s midfin location.[62] The now-classic jet age low, all-moving "stabilator” tail was first incorporated on the North American YF-100A Super Sabre, the first of the "Century series” of American fighters.

The low all-moving tail reflected extensive NACA research dating to the midst of the Second World War. While the all-moving tail surface had been a standard feature of early airplanes such as the German Fokker Eindecker ("Monoplane”) and French Morane Bullet fighters of the "Great War,” the near constant high workload it made for a pilot caused it to fall
from grace, in favor of a fixed stabilizer and movable elevator surface. But by the early 1940s, NACA researchers recognized "its possible advantages as a longitudinal control for flight at high Mach numbers.”[63] Accordingly, researchers at the Langley Memorial Aeronautical Laboratory modi­fied an experimental Curtiss XP-46 fighter on loan from the Army Air Forces by removing its conventional horizontal tail surfaces and replac­ing them with an all-moving tail plane hinged at its aerodynamic cen­ter and controlled by a trailing edge servotab. Initial tests during turns at 200 mph proved disappointing, with pilots finding the all-moving sur­face too sensitive and its control forces too light (and thus dangerous, for they could easily subject the airplane to excessive maneuvering loads) and demanding continuous attention particularly in choppy air. So the XP-46 was modified yet again, this time with a geared, not servotab, con­trol mechanism. If not perfect, the results were much better and more encouraging, with pilots now having the kind of variation in stick force to give them feedback on how effectively they were controlling the air­plane.[64] Recognizing that the all-moving tail could substantially increase longitudinal control authority in the transonic region, NACA research­ers continued their study efforts into the postwar years, encouraged by initial flight-test results of the Bell XS-1, which began approaching higher transonic Mach numbers in the fall of 1946. Though its adjustable horizontal stabilizer with a movable elevator constituted an admittedly interim step on the path to an all-moving surface, the XS-1’s excursions through the speed of sound generated convincing proof that designers could dramatically increase transonic longitudinal control authority via an all-moving tail.[65]

Tail location—midfin (as in the XS-1 and D-558), at the base of the fin (as with the F-86 and most other jet aircraft), or high (as with the T-tail F-101)—was another significant issue. German wartime research had favored no tail surfaces or, at the other extreme, high T-tails—for example, the DFS 346 supersonic research aircraft under development at war’s end or the proposed Focke-Wulf Ta 183 swept wing jet fighter (which influenced the design of the MiG-15 and early Lavochkin swept wing jet fighters and a proposed British supersonic research aircraft). But the pitch-up problems encountered by the Skyrocket and even the F-86, as angle of attack increased, argued powerfully against such loca­tions. In 1949, coincident with the Air Force and North American begin­ning development of the Sabre 45, a 45-degree swept wing successor to the F-86, Jack D. Brewer and Jacob H. Lichtenstein, two researchers at Langley, undertook a series of studies of tail size, length, and vertical location using the Langley stability tunnel and a model having 45-degree swept wing and tail surfaces. Their research demonstrated that placing a tail well aft of the wing and along the fuselage centerline (as viewed from the side) improved longitudinal stability and control.[66] Building upon their work, Langley researchers William Alford, Jr., and Thomas Pasteur, Jr., ran an investigation in the Langley 7-foot by 10-foot high­speed tunnel to determine aspect ratio and location effects on the lon­gitudinal stability of a swept wing model across the transonic regime from Mach 0.80 to Mach 0.93. "The results,” they subsequently reported in 1953, "indicted that, within the range of variables considered, the most favorable pitching-moment characteristics at a Mach number of 0.90 were obtained by locating the tail below the wing-chord plane.”[67] Compared to this, other changes were inconsequential.

Flight tests at Ames in 1952 of a North American YF-86D (an inter­ceptor variant of the F-86) specially modified with a low-placed horizon­tal tail, confirmed the Langley test results. As researchers noted, "The

test airplane, while having essentially the same unstable airplane static pitching moments as another version of this airplane [the F-86A] with an uncontrollable pitch-up, had only a mild pitch-up which was easily con­trollable,” and had a nearly 40-percent increase in stabilizer and elevator effectiveness at transonic speeds.[68] The prototype YF-100 Super Sabre, first flown in May 1953, incorporated the fruits of this research. Next came the Vought XF8U-1 Crusader and the Republic YF-105 Thunderchief, and thereafter a plethora of other types. Aviation had returned full circle to the technology with which powered, controlled flight had begun: back to pivoted all-moving pitch-control surfaces of a kind the Wrights and other pioneers would have immediately recognized and appreciated.

The progression of aircraft flight speeds from subsonic to transonic and on into the supersonic changed the proportional relationship of wing to fuselage. As speed rose, the ratio of span to fuselage length decreased. At the onset of the subsonic era, the Wright Flyer had a wingspan-to – fuselage length ratio of 1.91. The SPAD XIII fighter of World War I was 1.30. The Second World War’s P-51D decreased to 1.14. Then came the supersonic era: the XS-1 was 0.90. In 1953, the F-100A, lowered the ratio to 0.80, and the F-104A of 1954 cut this in half, to 0.40. The radical X-3 had a remarkably slender wingspan-to-fuselage length ratio of just 0.34: not without reason was it nicknamed the "Stiletto.” But while the dra­matic increase in fuselage length at the expense of span spoke to the need to reduce wing-aspect ratio and increase fuselage fineness ratio to achieve idealized supersonic shaping, any resulting aerodynamic benefit came only at the price of significant performance limitations and risk.

Increasing fuselage length while reducing span dramatically changed the mass distribution of these new designs: whereas earlier airplanes had most of their mass concentrated along the span of their wings, as the wing-fuselage ratio changed from well above 1.0 to well below this figure, the distribution of mass shifted to along the fuselage. Since a long forward fuselage inherently reduces directional stability, and since the small low aspect ratio wings of these airplanes reduced their roll stability, a potentially deadly mix of technical circumstances existed to
produce a major crisis: the onset of transonic and supersonic inertial coupling, also termed roll-coupling.

William Hewitt Phillips of the NACA’s Langley laboratory had first forecast inertial coupling. His pronouncement sprang from a fortuitous experience while supervising tests of a large XS-1 "falling body” model in the summer of 1947. The model (dropped from a high-flying B-29 over a test range near Langley to assess XS-1 elevator control effectiveness as it approached Mach 1) incorporated a simple autopilot and was intended to rotate slowly as it fell, so as to maintain a "predictable trajectory.”[69] But after the drop, things went rapidly awry. The model experienced violent pitching and rapid rolling "well below” the speed of sound and fell so far from its planned impact point that it literally disappeared from history. But optical observations, coupled with telemetric data, led Phillips to conclude that "some kind of gyroscopic effect” had taken place. Intrigued, he drew upon coursework from Professors Manfred Rauscher and Charles Stark Draper of the Massachusetts Institute of Technology, using the analogy of the coupling dynamics of a rotating rod. He substituted the values obtained from the falling XS-1 model, discovering that "the results clearly showed the possibility of a diver­gent motion. . . . The instability was likely to occur when the values of longitudinal stability and directional stability were markedly differ­ent and when a large amount of the weight was distributed along the fuselage.”[70] Hewitt subsequently published a seminal NACA Technical Note in 1948, which presciently concluded: "Design trends of very high­speed aircraft, which include short wing spans, fuselages of high density, and flight at high altitude, all tend to increase the inertia forces due to rolling in comparison with the aerodynamic restoring forces provided by the longitudinal and directional stabilities. It is therefore desirable to investigate the effects of rolling on the longitudinal and directional stabilities of these aircraft. . . . The rolling motion introduces coupling between the longitudinal and lateral motion of the aircraft.”[71] Out of

this came the expression "inertial coupling” and its more descriptive equivalent, "roll-coupling.” Phillips continued his research on roll-cou­pling and rolling maneuvers in accelerated flight, noting in 1949 that high-speed rolls could generate "exceptionally large” sideslip loads on a vertical fin that might risk structural failure. He concluded: "The provision of adequate directional stability, especially at small angles of sideslip, in order to prevent excessive sideslipping in rolls at high speed is therefore important from structural considerations as well as from the standpoint of providing desirable flying qualities.”[72]

In the summer of 1952, as part of an investigation effort studying coupled lateral and longitudinal oscillations, researchers at the NACA’s Pilotless Aircraft Research Division at Wallops Island, VA, fired a series of large rocket-boosted swept wing model airplanes. Spanning over 3 feet, but with a length of nearly 6 feet, they had the general aero­dynamic shape of the D-558-2 as originally conceived: with a slightly shorter vertical fin. These models accelerated to supersonic speed and then, after rocket burnout and separation, glided onward while onboard telemetry instrumentation relayed a continuous stream of key performance and behavior parameters as they decelerated through the speed of sound before diving into the sea. On August 6, 1952, techni­cians launched one equipped with a small pulse rocket to deliberately destabilize it with a timely burst of rocket thrust. After booster burn­out, as the model decelerated below Mach 1, the small nose thruster fired, inducing combined yawing, sideslip, and rolling motions. But instead of damping out, the model swiftly went out of control, as if a replay of the XS-1 falling body test 5 years previously. It rolled, pitched, and yawed until it plunged into the Atlantic, its death throes caught by onboard instrumentation and radioed to a NACA ground station. If dry, the summary words of the resulting test report held ominous import for future flight-testing of full-size piloted aircraft: "From the flight time history of a rocket-propelled model of a representative 35° sweptback wing airplane, it is indicated that coupled longitudinal motions were excited and sustained by pure lateral oscillations. The resulting longitudinal motions had twice the frequency of the lateral oscillations and rapidly developed lift loads of appreciable magnitude. The longitudinal moments are attributed to two sources, aerodynamic

moments due to sideslip and inertial cross-coupling. The roll charac­teristics are indicated to be the predominating influence in the inertial cross-coupling terms.”[73]

Two model tests, 5 years apart, had shown that roll coupling was clearly more than a theoretical possibility. Shortly thereafter it turned into an alarming reality when the Bell X-1A, North American YF-100 Super Sabre, and Douglas X-3 entered flight-testing. Each of these encoun­tered it with varying degrees of severity. The first to do so was the Bell X-1A, a longer, more streamlined, and more powerful derivative of the original XS-1.[74] The X-1A arrived at Edwards in early 1953, flew a brief contractor program, and then entered Air Force evaluation in November. On December 12, 1953, test pilot Charles E. "Chuck” Yeager nearly died when it went out of control at Mach 2.44 at nearly 80,000 feet. In the low dynamic pressure ("low q” in engineering parlance) of the upper atmo­sphere, a slight engine thrust misalignment likely caused it to begin a slow left roll. As Yeager attempted to control it, the X-1A rolled rapidly to the right, then violently back to the left, tumbling completely out of control and falling over 50,000 feet before the badly battered Yeager man­aged to regain control. Gliding back to Edwards, he succinctly radioed: "You know, if I’d had a seat, you wouldn’t still see me in this thing.”[75] Afterward, NACA engineers concluded that "lateral stability difficulties were encountered which resulted in uncontrolled rolling motions of the airplane at Mach numbers near 2.0. Analysis indicates that this behav­ior apparently results from a combination of low directional stability

and damping in roll.”[76] The predictions made in Phillips’s 1948 NACA Technical Note had come to life, and even worse would soon follow.

By the time of Yeager’s harrowing X-1A flight, the prototype YF-100, having first flown in May 1953, was well into its flight-test program. North American and the Air Force were moving quickly to fulfill ambitious pro­duction plans for this new fighter. Yet all was not well. The prototype Super Sabre had sharply swept wings, a long fuselage, and a small ver­tical fin. While fighter pilots, entranced by its speed, were enthusiastic about the new plane, Air Force test pilots were far less sanguine, noting its lateral-directional stability was "unsatisfactory throughout the entire combat speed range,” with lateral-directional oscillations showing "no tendency to damp at all.”[77] Even so, in the interest of reducing weight and drag, North American actually shrank the size of the vertical fin for the production F-100A, lowering its height, reducing its area and aspect ratio, and increasing its taper ratio. The changes further cut the direc­tional stability of the F-100A, by some estimates as much as half, over the YF-100.[78] The first production F-100As entered service in the late sum­mer of 1954. Inertial coupling now struck with a vengeance. In October and November, two accidents claimed North American’s chief test pilot, George "Wheaties” Welch, and Royal Air Force Air Commodore Geoffrey Stephenson, commander of Britain’s Central Fighter Establishment. Others followed. The accidents resulted in an immediate grounding while the Air Force, North American, and the NACA crafted complementary research programs to analyze and fix the troubled program.[79]

Then, in the midst of the F-100’s travail, inertial coupling struck the Douglas X-3. First flown in October 1952, the X-3 had vestigial straight wings and tail surfaces joined to a missile-like fuselage. Though it was
the most highly streamlined airplane of its time, mediocre engines con­founded hopes it might achieve Mach 2 speeds, and it never flew faster than Mach 1.21, and that only in a dive. The NACA acquired it for research in December 1953, following contractor flights and a brief Air Force eval­uation. On October 27, 1954, during its 10th NACA flight, test pilot Joseph A. Walker initiated an abrupt left aileron roll at Mach 0.92 at 30,000 feet. The X-3 pitched up as it rolled, sideslipping as well. After it returned to stable flight, Walker initiated another left roll at Mach 1.05. This time, it responded even more violently. Sideslip angle exceeded 21 degrees, and it reached -6.7 g during a pitch-down, immediately pitching up to over 7 g. Fortunately, the wild motions subsided, and Walker, like Yeager before him, returned safely to Earth.[80] With the example of the X – 1A, the F-100A, and the X-3, researchers had conclusive proof of a newly emer­gent crisis imperiling the practical exploitation of the high-speed frontier.

The F-100A raised the most concern, for it was the first of an entire new class of supersonic fighter aircraft, the "Century series,” with which the United States Air Force and at least some of its allies hoped to reequip. Welch’s F-100A had sideslipped and promptly disintegrated during a diving left roll initiated at Mach 1.5 at 25,000 feet. As Phillips had predicted in 1949, the loads had proven too great for the fin to with­stand (afterward, North American engineers "admitted they had been naive in estimating the effects of reducing the aspect ratio and area of the YF-100 prototype tail”).[81] Curing the F-100’s inertial coupling prob­lems took months of extensive NACA and Air Force flight-testing, much of it very high-risk, coupled with analytical studies by Langley personnel using a Reeves Electronic Analogue Computer (REAC), an early form of a digital analyzer. During one roll at Mach 0.7 (and only using two – thirds of available aileron travel), NACA test pilot A. Scott Crossfield experienced "a large yaw divergence accompanied by a violent pitch – down. . . which subjected the airplane to approximately -4.4g vertical acceleration.”[82] Clearly the F-100A needed significant redesign: the Super

Sabre’s accidents and behavior (and that of the X-3 as well) highlighted that streamlined supersonic aircraft needed greatly increased tail area, coupled with artificial stability and motion damping, to keep sideslip from developing to dangerous values. North American subsequently dramatically increased the size of the F-100’s vertical fin, increased its wingspan by 2 feet (to shift the plane’s center of gravity forward), and incorporated a yaw damper to control sideslip. Though the F-100 sub­sequently became a reliable fighter-bomber (it flew in American service for almost a quarter century and longer in foreign air arms), it remained one that demanded the constant attention and respect of its pilots.[83]

Inertial coupling was not, of course, a byproduct of conceptualizing the swept and delta wings, nor was it limited (as the experience of the XS-1 falling model, X-1A, and X-3 indicated) just to aircraft possessing swept or delta planforms. Rather, it was a byproduct of the revolution in high-speed flight, reflecting the overall change in the parametric rela­tionship between span and length that characterized aircraft design in the jet age. Low aspect ratio straight wing aircraft like the X-3 and the later Lockheed F-104 were severely constrained by the threat of iner­tial coupling, even more than many swept wing aircraft were.[84] But for swept wing and delta designers, inertial coupling became a particular challenge they had to resolve, along with pitch-up. As the low-placed horizontal tail reflected the problem of pitch-up, the increasing size of vertical fins (and the addition of ventral fins and strakes as well) incor­porated on new aircraft such as the Navy’s F8U-1 and the Air Force’s

F-105B (and the twin-fins that followed in the 1970s on aircraft such as the F-14A, F-15A, and F/A-18A) spoke to the serious challenge the iner­tial coupling phenomenon posed to aircraft design. Not visible were such "under the skin” systems as yaw dampers and the strict limitations on abrupt transonic and supersonic rolling taught to pilots transitioning into these and many other first-generation supersonic designs.[85]

The story of the first encounters with inertial coupling is a salu­tary, cautionary tale. A key model test had resulted in analysis leading to the issuance of a seminal report but one recognized as such only in retrospect. A half decade after the report’s release, pilots died because the significance of the report for future aircraft design and behavior had been missed. Even within the NACA, recognition of seriousness of reduced transonic and supersonic lateral-directional stability had been slow. When, in August 1953, NACA engineers submitted thoughts for a tentative research plan for an F-100A that the Agency would receive, attention focused on longitudinal pitch-up, assessing its handling qual­ities (particularly its suitability as a gun platform, something seemingly more appropriately done by the Air Force Flight Test Center or the Air Proving Ground at Eglin), and the correlation of flight and wind tun­nel measurements.[86] Even after the experience of the X-1A, F-100A, and X-3, even after all the fixes and training, it is disturbing how inertial coupling stilled claimed the unwary.[87] Over time, the combination of refined design, advances in stability augmentation (and eventually the advent of computer-controlled fly-by-wire flight) would largely render

inertial coupling a curiosity. But for pilots of a certain age—those who remember aircraft such as the X-3, F-100, F-101, F-102, and F-104— the expression "inertial coupling,” like "pitch-up,” will always serve to remind that what is an analytical curiosity in the engineer’s laboratory is a harsh reality in the pilot’s cockpit.

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.