Swept Wing Challenges

The NACA so rapidly focused its attention on swept planforms that, within 2 years of the end of the Second World War, George Gray, author of a popular yet surprisingly detailed study of the Agency, could already write: "Just how far the sweepback principle can be applied with result­ing advantage is a question. . . . At about 90 percent of the speed of sound both sweepback and low aspect ratio begin to be of value, and wings that combine the two features seem to offer a promising choice. At about Mach number 1.50, a sweepback of 60 degrees seems neces­sary to escape the backward flare of the Mach angle. . . . At Mach num­ber 2.00, the angle is so acute that it is impossible to avoid it and still preserve the wings. It may be that designers preparing for flight at this speed will return to wings of low angles of sweep, and place their main dependence for drag reduction on thinning the profiles, lowering the aspect ratio, and sharpening the edges of wings.”[31] By 1950, this grow­
ing confidence in the old-new swept planform had resulted in transonic and supersonic research airplanes, a variety of military prototypes, and two operational jet fighters that would shortly clash over North Korea: the American F-86 Sabre (first flight in October 1947) and, in the Soviet Union, the MiG-15 (first flight in December 1947).[32]

Swept Wing ChallengesSwept wing aircraft, for all their high-speed advantages, posed daunt­ing stability, control, and handling qualities challenges. Foremost of these was pitch-up at low and high speeds, resulting from deteriorating longitudinal stability.[33] A swept wing airplane’s lateral-directional sta­bility was compromised as well by so-called "dihedral effects.” Swept wing aircraft with excessive dihedral experienced pronounced combined rolling and yawing "Dutch roll” motions, which would be unacceptable on both production civil and military designs.[34] Such motions would induce airsickness in passengers on large aircraft and, on bomber, fighter, and attack aircraft, prevent accurate tracking of a maneuvering target or accurate bomb release. (Indeed, it was largely because of this kind of behavior that the U. S. Air Force did not proceed with production of Northrop’s YB-49 flying wing jet bomber.) Adverse yaw posed another problem. At higher speeds, as a swept wing plane rolled from aileron

deflection, it experienced higher drag and loss of lift involving the low­ered wing, generating a tendency of the airplane to turn (reverse) into the direction of the raised wing, effectively doing the opposite of what the pilot intended. Adverse yaw could be caused by aeroelastic effects as well. That swept wing aircraft would possess behavior characteris­tics significantly different than conventional straight wing designs did not come as a surprise to the NACA or other aerodynamic researchers in America and overseas. But all recognized the need to complement theory and ground-test methodologies with flight research.

Swept Wing ChallengesThe peculiarities of swept wing aircraft, at a time when early jet air­craft lacked the power-to-weight advantages of later designs, could—and often did—prove fatal. For example, Boeing designed the B-47, America’s first large swept wing aircraft, with pod-mounted engines and a broad, highly tapered, thin swept wing. During flight-testing at higher speeds, test pilots found aileron input to roll the aircraft would twist the wing, the aileron effectively acting as a trim-tab does on a control surface. The twisted wing would overcome the rolling moment produced by the aile­ron, rolling the aircraft in the opposite direction. Aeroelastic structural divergence caused several accidents of the B-47 during its flight-testing and service introduction, forcing the Air Force to limit its permissible airspeed to 425 knots, as high as it could be safely flown if roll reversal were to be avoided. As a result, Boeing built its successors, the XB-52 and the Model 367-80 (prototype for the KC-135 family and inspiration for the civil 707), with much thicker wing roots and structures that were torsion resistant but that could still flex vertically to absorb structural loads and gust-induced loads during flight.[35]

Confronting Pitch-Up

But the most serious swept wing problem in the early jet era was pitch – up, a condition affecting both low – and high-speed flight, reflecting stall onset either from decreasing speed (low-speed pitch-up) or from trim changes during high-speed flight, particularly during accelerated
maneuvers, such as "wind-up” turns that rapidly increased g-loading and angle of attack. Pitch-up occurred at the breakpoint in a lift curve, immediately beyond the peak point where the airplane’s wing was oper­ating at its highest lift-producing angle of attack, with its lift coefficient at maximum value. At the breakpoint, the wing would begin stalling, with flow separation from the airfoil, breaking the circulatory flow pat­tern around the wing. In ideal circumstances with a straight wing air­craft, the change in lift would occur simultaneously spanwise across the wing and would typically trigger a nose drop. But in a swept wing aircraft, the stall would first begin at the tips and progress inward, the center of lift shifting forward. As the plane’s longitudinal (nose-up nose – down) stability decreased, the shifting center of lift would abruptly rotate the nose upward (hence the use of the expression "pitch-up”), even at a rate of onset beyond the capabilities of its elevator control surfaces to correct. As well, of course, since the ailerons that governed lateral con­trol (roll control) were typically located outboard on a wing, a swept wing airplane could lose its lateral control authority precisely at a point when the pilot needed as much control capability and reserve as possi­ble. Because stall onset is not always triggered uniformly, a swept wing airplane nearing the pitch-up point could experience sudden loss of lift on one wing, inducing abrupt rolling motions (called "wing dropping”), complicating its already dangerous low-speed behavior.

Confronting Pitch-UpThere was, of course, the possibility of overcoming such problems by sweeping a wing forward, not aft. A forward-swept wing (FSW) had both desirable high – and low-speed aerodynamic characteristics. Since the spanwise flow would run from the tips to the fuselage, the outer portions of the wing would stall last, thus preserving lateral control. As well, it would have more desirable pitching characteristics. Already, in the midst of the Second World War, the Germans had flown an exper­imental bomber, the Junkers Ju 287, featuring a forward swept wing, and a number of aircraft and missile projects were forecast for such planforms as well. The forward swept wing, and combined-sweep "M,” "W,” and even "X” planforms, received a great deal of postwar atten­tion, both in America and abroad. Researchers at Langley modified wind tunnel and configuration models of both the XS-1 and D-558 to employ forward-swept wing planforms, and tested conceptual planforms with both aft and forward sweep to develop comparison data. But while the FSW undoubtedly had better low-speed behavior, it had higher profile drag and posed difficult structural problems for designers. In

the precomposite structure era, an FSW had to be necessarily heavier than an aft-swept wing to avoid aeroelastic flexing that could inhibit both good flight performance and even flight safety. Further, the struc­tural and weight limitations also limited the sweep angles that an FSW could then have; even as late as the 1960s, when Germany produced a business aircraft (the Hamburger Flugzeugbau HFB-320 Hansa Jet), it possessed only modest forward sweep and, though flown suc­cessfully and built in small numbers, was not a commercial success. It would take over three decades before the advent of computerized flight control, composite structures, and a more radical vision of for­ward sweep application would result in experimental planforms like the NASA-Rockwell HiMAT remotely piloted vehicle, the piloted Air Force – DARPA-NASA Grumman X-29 (and, in Russia, an X-29-like experimen­tal aircraft, the Sukhoi Su-37). Even so, and even though forward sweep would be applied to some weapon systems (for example, the AGM-129 stealthy cruise missile, where it contributed to its low radar reflectiv­ity), forward wing sweeping would remain the exception to "normative” aft-swept wing design practice.[36]

Confronting Pitch-UpPitch-up was profoundly dangerous. At low speeds in proximity to the ground, it could—and often did—trigger a disastrous departure and crash. The recognition of such problems had caused the U. S. Navy to procure two modified Bell P-63 Kingcobra fighters (designated L-39), which had their wing panels replaced with 35-degree swept wing sec­tions, and a fuselage extension to accommodate their now-changed cen­ter of lift. Not intended for high speeds, these two low-speed swept wing research aircraft were extensively flown by various contractor, Navy, and NACA research pilots to assess the basic behavior of the swept wing, with and without lift-and-control-augmenting devices such as wing slats and flaps. They quickly encountered its limitations. On one flight with the plane in "clean” (i. e., slat-free) configuration, Bell Company test pilot A. M. "Tex” Johnston gradually raised the nose of the plane while retarding power. After just "a slight tremor” indicating the onset of asymmetric tip stall, it "instantaneously rolled to an almost inverted

position.”[37] Grumman test pilot Corwin "Corky” Meyer recalled that while the L-39 was "docile” with leading edge slats, without them it "cavorted like a cat on catnip.”[38] The two L-39 aircraft furnished vital insight into the low-speed performance and limitations of swept wing aircraft, but they also clearly demonstrated that such aircraft could, in fact, be safely flown if their wings incorporated careful design and safety devices such as fixed leading edge slots or movable slats.[39]

Confronting Pitch-UpIn military aircraft, pitch-up could prevent a pilot from maneuvering effectively against a foe, could lead to loss of control of the airplane, and could result in such excessive airframe loadings that an airplane would break up. It was no respecter of designs, even outstanding ones such as North American’s evocative F-86 Sabre, generally considered the finest jet fighter of its time by both American and foreign test pilots. First flown in October 1947, the Sabre quickly became an internationally recognized symbol of aeronautical excellence and advancement. When British test pilot Roland Beamont, a distinguished Royal Air Force fighter ace of the Second World War, evaluated the Sabre at Muroc Dry Lake in May 1948

(a month after it had dived past the speed of sound, becoming the world’s first supersonic turbojet airplane), he likewise dived it through Mach 1, thus becoming the first supersonic British pilot. Afterward, he noted approvingly in his test report, "The P-86 is an outstanding aircraft.”[40] The Sabre’s reputation was such that British authorities (frustrated by the slow development pace of Albion’s own swept wing aircraft) tell­ingly referred to it simply as "That Aircraft.” Vickers-Supermarine test pilot David Morgan recalled, "No British fighter of the day could match the handling of the North American F-86.”[41] Indeed, designers from his company, frustrated by their slow progress turning the experimen­tal Swift into a decent airplane, even resorted to crude subterfuge in an effort to unlock the Sabre’s secrets. When a pair of Canadian pilots landed at the Supermarine plant in their Canadair-built Sabres, com­pany officials, with apparent generosity, laid on a fancy lunch, driving them off to a local hotel. While the visiting airmen dined and chatted with solicitous Supermarine representatives, another team of engineers "swarmed over the Sabres to study their construction,” marveling at "this splendid aircraft.”[42]

Confronting Pitch-UpYet however "splendid” "That Aircraft” might otherwise have been, the Sabre killed unwary pilots by the dozens in accidents triggered by its low-speed pitch-up tendencies. Apollo 11 astronaut Michael Collins recalled his introduction to the F-86 at Nellis Air Force Base as "a bru­tal process. . . . In the eleven weeks I was there, twenty-two people were killed. In retrospect it seems preposterous to endure such casualty rates without help from the enemy, but at the time the risk appeared perfectly acceptable. . . . I’m surprised to have survived. I have never felt quite so threatened since.”[43] In over a decade of tests with various Sabre variants

to improve their low-speed handling qualities, NACA Ames research­ers assessed a variety of technical "fixes.” The most beneficial was the combination of artificial feel (to give the pilot more reassuring higher maneuvering control forces during the approach-to-landing, combined with greater inherent stability than possible with a non-artificial-feel sys­tem), coupled with leading-edge suction to draw off the boundary layer airflow.[44] First evaluated on a test rig installed in the Ames 40-foot by 80-foot full-scale wind tunnel, the boundary layer control (BLC) exper­iment on the F-86 proved most valuable. Ames researchers concluded: "Leading edge boundary-layer control was most effective in providing a large reduction in both stalling speed and approach speed together with an increased margin of lift for flare and maneuvering during the [land­ing] approach,” an important point, particularly for swept wing naval aircraft, which had to be controllable down to a landing on the con­fined deck of an aircraft carrier.[45] The trials benefitted not only future swept wing studies but, more generally, studies of BLC applications for Vertical/Short Take-Off and Landing (V/STOL) aircraft systems as well.[46]

Confronting Pitch-UpNor were the Sabre’s high-speed pitch-up characteristics innocuous. The NACA flew extensive Sabre evaluations at its High­Speed Flight Research Station and at Ames to refine understanding of

its transonic pitch-up behavior, which test pilot A. Scott Crossfield recalled as "violent and dangerous.”[47] It could easily exceed its design load factors, sometimes pitching as high as 10 g. At 25,000 feet, in the very midst of its combat operating envelope (and at lift coefficients less than its maximum attainable lift) the Sabre’s pitch-up onset was so severe that g forces once momentarily "blacked out” the test pilot. Overall, after extensive Ames tests, the early slatted F-86A with a conventional fixed horizontal stabilizer and movable elevator was judged "unsatisfactory” by a group of highly experienced fighter test pilots, thanks to its "severe pitch – up tendencies.” The same group found the later slat-less "6-3” F-86F (so – called because its wing extended forward 6 inches at the root and 3 inches at the tip, a modification made by North American based on Korean war experience) had "moderate” pitch-up tendencies. Because of this, and because it had an adjustable (not fixed) horizontal stabilizer in addition to its elevator, the pilots judged the F-86F’s pitch-up behavior "unsatisfactory but acceptable.”[48]

Confronting Pitch-UpWorse swept wing problems plagued the Sabre’s great adversary, the Soviet MiG-15. Unlike the Sabre, the MiG-15 had a less aerodynamically pleasing configuration, and its fixed horizontal stabilizer and elevator combination, located midway up the vertical fin, made it more suscep­tible to aerodynamic "blanketing” of the tail by the wing and, hence severe pitch-up problems, as well as limiting its transonic maneuver­ability (to the Sabre’s advantage). During the Korean war, Sabre pilots frequently saw MiG pilots eject from otherwise perfectly sound aircraft that had pitched up during turns, stalled, and entered flat, unrecoverable spins. Nearly five decades later, Soviet pilot Stepan Mikoyan (nephew of Anushavan "Artem” Mikoyan, cofounder of the MiG design bureau) con­ceded that high-speed accelerated stalls often triggered unrecoverable spins, leading to "a number of ejections and fatal accidents.”[49] Postwar American testing of a MiG-15 delivered by a defecting North Korean

pilot confirmed the MiG’s marked vulnerability to pitch-up-induced stalls and spins; indeed, the defector’s own instructor had been lost in one such accident. Not surprisingly, when Mikoyan produced the MiG – 17—the lineal successor to the MiG-15—it had a very different outer wing configuration giving it more benign behavior.[50]

Confronting Pitch-UpWestern European swept wing aircraft exhibited similar problems as their American and Soviet counterparts. For a brief while, influenced by the Messerschmitt Me 163 Komet and a variety of other German proj­ects, designers were enthralled with the swept wing tailless configura­tion, believing it could resolve both the challenges of high-speed flight and also furnish inherent stability.[51] Then, in September 1946, British test pilot Geoffrey de Havilland perished in an experimental tailless transonic research aircraft, the de Havilland D. H. 108 Swallow, when it began an undamped violently divergent longitudinal pitching oscilla­tion at Mach 0.875, breaking up over the Thames estuary and proving that the "sound barrier” could bite.[52] Subsequently the NACA evaluated the Northrop X-4, a generally similar American configuration. Tested at high altitude (and hence, at low dynamic pressure), the X-4 fortu­nately never "diverged” as violently as the ill-fated D. H. 108. Instead, as NACA pilot A. Scott Crossfield remembered, at Mach 0.88 "it broke

into a steady porpoising motion, like an automobile cushioning over a washboard road.”[53] Conventional tailed European swept wing designs followed the same steep learning curve as American ones. Britain’s Supermarine Swift, a much-touted design from the builder of the legendary Spitfire, had a "vicious” transonic pitch-up. By the time it entered service, it was years late, obsolescent, and useless for any other role save low-level tactical reconnaissance.[54]

The Skyrocket: The NACA’s Pitch-Up Platform

Pitch-up afflicted a wide range of early transonic and supersonic jet fighters, and the NACA was fortunate in having an available research airplane that could study swept wing behavior across the transonic regime. This aircraft was the Douglas D-558-2 Skyrocket, "Phase II” of the larger D-558 research aircraft program, a Douglas Company ven­ture begun in 1945 and sponsored by the U. S. Navy and the NACA. The D-558 program had begun as a companion to the XS-1 effort and repre­sented a different design approach. Where the XS-1 was rocket powered, the D-558 Skystreak used a turbojet; where the XS-1 employed an ogi­val projectile shape with a midwing of 8-percent thickness-chord ratio, the D-558 used a constant-diameter tube wrapped around an axial-flow turbojet engine and a low wing of 10-percent thickness-chord ratio; and where the XS-1 was air launched, the D-558 took off from the ground as a conventional airplane. Both were straight wing designs, with their adjustable stabilizers and movable elevators placed midway up their ver­tical fins. All together, the Navy ordered six D-558 aircraft from the firm.[55]

Originally, swept wings had not featured in the D-558 program. Then the discovery by Douglas engineers of a plethora of German tech­nical reports (coupled with the work of Jones and others in the United States) caused the Navy, the NACA, and Douglas to modify the D-558
program.[56] The last three aircraft were completed as a new swept wing design. Initially, the planned modification seemed straightforward: replace the straight wing and tail surfaces with swept ones. In antic­ipation, Langley tested models of the D-558 with a variety of swept wings. But the possibility of giving the swept wing D-558 supersonic performance—something the D-558 straight wing lacked—resulted in a more radical redesign. Gone was the simple Pitot intake inlet. Instead, designer Edward "Ed” Heinemann and his team chose an ogival body shape resembling the XS-1. The new 35-degree slat-equipped swept wing was relocated to midfuselage position and given anhedral (droop), with the landing gear relocated into the fuselage. In contrast to the original single-engine D-558s, the new swept wing design featured both a 6,000- pound thrust rocket engine and a small turbojet. Thus recast, it received the designation D-558-2 and the name Skyrocket, to distinguish it from the straight wing Skystreak, itself redesignated D-558-1. The result was one of the most elegant and significant aircraft of all time.

The Skyrocket: The NACA's Pitch-Up PlatformThe first D-558-2 flew in February 1948, though initial flight tests gave little hint of how remarkably versatile and successful it would prove. At max takeoff weight, it was so underpowered (and thus so slug­gish) that it needed four solid-fuel jettisonable assistance takeoff (JATO) rockets to help kick it aloft. Eventually, the Navy and the NACA would arrange to take the second and third Skyrockets and modify them for air launch from a modified PB2-1S (Navy B-29) Superfortress, dramatically improving both their safety and high-speed performance; fuel previously

spent climbing aloft could now be more profitably expended exploring the transonic and supersonic regimes. While the third aircraft retained its jet and rocket engine, the second had its jet engine removed and additional tanks for rocket propellant and oxidizer installed. Thus modified, the second aircraft reached Mach 2.01 in November 1953, flown by Scott Crossfield, the first piloted Mach 2 flight, having earlier attained an alti­tude of 83,235 feet, piloted by Lt. Col. Marion Carl, a noted Marine aviator. Eventually, the NACA received the first D-558-2 as well (which Douglas had employed for contractor testing). The Agency modified it as an all-rocket aircraft, though it only completed a single check flight before being retired.

The Skyrocket: The NACA's Pitch-Up PlatformBefore all-rocket modification, the second Skyrocket introduced Agency pilots to the hazards of pitch-up. On August 8, 1949, during its seventh flight, pilot Robert Champine banked into a 4 g turn at Mach 0.6, and the Skyrocket violently pitched up, reaching 6 g. It responded rapidly to full-down elevator, and Champine made an uneventful (if pru­dently precautionary) landing. Thereafter, until returning the airplane to Douglas for all-rocket modification in 1950, the NACA flew extensive pitch-up investigations with it. In November, pilot John Griffith repli­cated the 4 g and Mach 0.6 pitch-up that Champine had experienced earlier. This time, however, he attempted to continue flying to more fully assess the Skyrocket’s behavior. Thus challenged, it snap-rolled on its back. After recovering, Griffith probed its low-speed behavior, gradually slowing, with flaps and gear extended and wing slats closed. At 14,000 feet and 130 mph, the Skyrocket pitched up, rolling into a spin, and los­ing 7,000 feet of altitude before its pilot could recover.[57] Clearly its ugly behavior did not match its alluring form.

Focused on extending the Skyrocket’s performance into the super­sonic regime by modifying the second aircraft as a pure rocket plane, the NACA turned to the third aircraft, which retained its jet engine as well as its rocket, for future pitch-up research. Air-launched, the jet-and- rocket Skyrocket had tremendous research productivity; it could accelerate

Подпись: Leading-edge wing chord extensions tested on the third D-558-2 Skyrocket, one of many combinations of flaps, slats, fences, and extensions evaluated in the NACA's 6-year-long study of the Skyrocket's pitch-up behavior. NASA.

into the supersonic regime, above Mach 1.1, and its jet engine enabled it to "loiter” in the transonic region, making repeated data-gathering runs. Its comprehensive instrumentation package enabled assessment of loads, pressure distributions, and accelerations, evaluated against background data on flight conditions, aircraft attitude, and control surface position and forces. Between the end of 1950 and the fall of 1956, it completed 66 research flights on pitch-up and associated transonic phenomena, includ­ing the evaluation of the effects external wing stores—tanks and bomb shapes—had on aircraft performance. It evaluated a variety of proposed aerodynamic solutions and fixes to resolve the pitch-up problem, includ­ing various wing fence designs to "channel” airflow and inhibit the char­acteristic spanwise-flow (flow toward the wingtips) found with swept wing planforms, various combinations of slat and flap position, changes to lead­ing edge shape, and "sawtooth” leading edge extensions on its outer wing panels. All of this testing reinforced what engineers suspected, namely that no one overall technical fix existed that could resolve the pitch-up challenge. Rather, swept wing aircraft design was clearly situational, and, depending on the mission of the aircraft and its resulting design, combina­tions of approaches worked best, chief among them being low placement of

the horizontal tail, below the chord-line of the wing, coupled with provision of stability augmentation and pitch-damping flight control technology.[58]

Ensuring Longitudinal Control: Transforming the Horizontal Tail

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]

Ensuring Longitudinal Control: Transforming the Horizontal TailMcDonnell’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]

Ensuring Longitudinal Control: Transforming the Horizontal TailBetter 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]

Ensuring Longitudinal Control: Transforming the Horizontal Tail

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.

Ensuring Longitudinal Control: Transforming the Horizontal TailFlight 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.

Inertial Coupling: Dangerous Byproduct of High-Speed Design

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.

Inertial Coupling: Dangerous Byproduct of High-Speed DesignWilliam 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]

Inertial Coupling: Dangerous Byproduct of High-Speed DesignIn 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]

Inertial Coupling: Dangerous Byproduct of High-Speed DesignTwo 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.

Inertial Coupling: Dangerous Byproduct of High-Speed DesignBy 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.

Inertial Coupling: Dangerous Byproduct of High-Speed DesignThe 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: Dangerous Byproduct of High-Speed DesignInertial 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]

Inertial Coupling: Dangerous Byproduct of High-Speed DesignThe 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.