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.
There 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.
Pitch-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.” 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.” 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.
In 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.” 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.” 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.”
Yet 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.” 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. 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. 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.
Nor 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.” 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.”
Worse 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.” 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.
Western 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. 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. 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.” 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.