Datos globales obtenidos mediante secuenciación dirigida

The rapid accelerations in aircraft speed and altitude capabilities that occurred at the end of World War II precipitated the beginning of serious interest in the possibility of piloted vehicles for access to and recovery from space for military and civil missions. Having conquered the transonic and supersonic flight regimes, researchers turned their attention to hypersonic flight and its challenges, which included propulsion sys-tems, structural heat loads, mission operational issues, and handling qualities of the unconventional vehi-cles under consideration. The NACA and military visionaries initiated early efforts for the X-15 hypersonic research aircraft, in-house design studies for hypersonic vehicles were started at Langley and Ames, and the Air Force began its X-20 Dyna-Soar space plane program. The evolution of long, slender configurations and others with highly swept lifting surfaces were yet another perturbation of new and unusual vehicles with unconventional aerodynamic, stability, and control characteristics requiring free-flight models for assess-ments of flight dynamics.

In addition to the high-speed studies of the X-15 in the Ames supersonic free-flight facility previously dis-cussed, the X-15 program sponsored low-speed investigations of free-flight models at Langley in the Full-Scale Tunnel, the Spin Tunnel, and an outdoor drop model launched from a helicopter.⁶² The Full-Scale Tunnel study concentrated on assessments of the high-angle-of-attack behavior of the early X-15 configu-ration (known as configuconfigu-ration 1), which included a tall vertical tail and fuselage side fairings that extended to the nose of the airplane. Key X-15 features under study were the use of an all-moving vertical tail for yaw control and differential deflection of the all-moving horizontal tail surfaces for roll control. The flight tests were conducted for angles of attack as high as 30 degrees, where the longitudinal and directional stability were noticeably degraded; however, the effectiveness and control harmony provided by the vertical tail and differential tail were satisfactory. Additional free-flight studies were made in the Full-Scale Tunnel for the final X-15 design (configuration 3), which did not have extended fuselage side fairings and used a sym-metrical vertical tail configuration with a jettisonable lower rudder for landing. The results were similar to those previously obtained and provided confidence in the design.

The most significant contribution of the NASA free-flight tests of the X-15 was confirmation of the effec-tiveness of the differential tail for control. North American had followed pioneering research at Langley on

CHAPTER 4: DYNAMIC STABILITY AND CONTROL

the use of the tail for roll control and had used such a design in its YF-107A aircraft. It opted to use the concept for the X-15 to avoid ailerons that would have complicated wing design for the hypersonic aircraft.

Nonetheless, skepticism existed over the potential effectiveness of the application until the free-flight tests at Langley dramatically demonstrated its success.⁶³

Along with the X-15 program, NASA activities in the late 1950s included a broad research program on hypersonic glider designs. At Langley, many concepts were conceived by performance-oriented organiza-tions and explored with free-flight models in the Full-Scale Tunnel, including half cones, pyramid-shaped vehicles, lenticular shapes, variable-geometry designs with foldout wings, and flat-bottom configurations with high wing sweep (on the order of 80 degrees).⁶⁴ Throughout these studies, results were compared to experiences with the X-15 to establish the feasibility of the unorthodox shapes for operational missions. On the basis of lift-to-drag ratio and wing loadings, the aerodynamic gliding performance of the concepts was acceptable, and the longitudinal characteristics of the designs appeared satisfactory, but the lateral-directional behavior of the highly swept winged designs was unsatisfactory without artificial roll stabilization.

Specifically, the models exhibited large unacceptable roll oscillations at moderate and high angles of attack.

As the existence of the oscillations was known, it was possible to design lateral control systems that damped the motions and produced satisfactory characteristics. In addition to the in-house exploratory studies, Lang-ley organizations contributed to the development of the short-lived Air Force Dyna-Soar project, including free-flight model tests in the Langley Full-Scale Tunnel.⁶⁵

The most important contribu-tions of free-flight models to reentry gliders occurred while the full-scale X-15 was being flight-tested. These historic efforts were produced dur-ing the conception and develop-ment of wingless lifting bodies. In the late 1950s, scientists at NASA Ames conducted in-depth studies of the aerodynamic and aerother-mal challenges of hypersonic reen-try and concluded that blunted half-cone shapes could provide adequate thermal protection for vehicle structures while also pro-ducing a significant expansion in operational range and landing options. As interest in the concept intensified after a major conference in 1958, a series of half-cone free-flight models provided proof that such vehicles exhibited satisfac-tory flight behavior.

Photograph of the Air Force Dyna-Soar configuration in free flight in the Full-Scale Tunnel.

The project contributed critical information on lateral-directional stability as part of a broad research program within NASA on the low-speed dynamic stability and control character-istics of hypersonic vehicles.

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At Langley, researchers had followed the development of an Ames reentry concept known as the M1 configuration, which resembled the nose of a blunted rocket. Intrigued by the issues concerning stability and control of this radical shape, the staff of the Full-Scale Tunnel conducted free-flight tests of a one-third-scale M1 model in 1959 to develop control surface concepts and to assess the dynamic stability and control of the configuration. After major control deficiencies were encountered in the test program, a modified con-figuration produced positive results that further fueled the interest in lifting bodies.⁶⁶

The most famous free-flight model activity in support of lifting body develop-ment was stimulated by the advocacy and leadership of Dale Reed of the Dryden Flight Research Center. In 1962, Reed became fascinated with the lifting body concept and proposed that a piloted research vehicle be used to validate the potential of lifting bodies.⁶⁷ He was par-ticularly interested in the flight character-istics of a second-generation Ames lifting body design known as the M2-F1 con-cept. After Reed’s convincing flights of radio-controlled models of the M2-F1, ranging from kite-like tows to launches from a larger radio-controlled mother ship, demonstrated the satisfactory flight characteristics of the M2-F1, Reed obtained approval for the construction and flight-testing of his vision of a low-cost piloted unpowered glider. The motion-picture films of Reed’s free-flight model flight tests had an overwhelming effect on skeptics, and management’s support led to a decade of successful lift-ing body flight research at Dryden.

Photograph of the Ames M1 reentry concept in free-flight during evaluations in 1959. The model was powered by compressed air through a nozzle at the rear of the model and used two pairs of control surfaces.

Photograph of Dryden free-flight research models of reentry lifting bodies. Dale Reed, second from left, and his test team pose with the mother ship and models of the M2-F2 and the Hyper III configurations.

At Langley, support for the M2-F1 flight program included free-flight tow tests of a model in the Full-Scale Tunnel, and the emergence of Langley’s own lifting body design, known as the HL-10, resulted in wind tunnel tests in nearly every facility at Langley.⁶⁸ Free-flight testing of a dynamic model of the HL-10 in the Full-Scale Tun-nel demonstrated outstanding dynamic

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stability and control to angles of attack as high as 45 degrees, and rolling oscillations that had been exhib-ited by the earlier highly swept reentry bodies were completely damped for the HL-10 with three vertical fins.⁶⁹ Within its lifting body activities, Langley also responded to an Air Force request to evaluate the stabil-ity of the Precision Recovery Including Maneuvering Entry (PRIME) SV-5 unpiloted reentry vehicle on tow during retrieval using a C-130 aircraft.⁷⁰

In the early 1970s, a new class of lifting body dubbed “racehorses” by Dale Reed emerged.⁷¹ Character-ized by high fineness ratios, long pointed noses, and flat bottoms, these configurations were much more efficient at hypersonic speeds than had been the earlier “flying bathtubs.” Reed and his team evaluated one Langley-developed configuration, known as the Hyper III, using free-flight models and the mother ship test technique. Although the Hyper III was efficient at high speeds, it exhibited a low lift-to-drag ratio at low speeds, requiring some form of variable geometry such as a pivot wing, flexible wing, or gliding parachute.

Reed successfully advocated for a low-cost, 32-foot-long helicopter-launched demonstration vehicle of the Hyper III with a pop-out wing, which made its first flight in 1969. Flown from a ground-based cockpit, the Hyper III flight was launched from a helicopter at an altitude of 10,000 feet. After being flown in research maneuvers by a research pilot using instruments, the vehicle was handed off to a safety pilot, who landed it. Unfortunately, funding for a low-cost piloted project similar to the earlier M2-F1 activity was not forthcom-ing for the Hyper III.

Ultimately, the NASA Space Shuttle configuration was selected as a winged vehicle in lieu of derivative lifting body options. Nonetheless, the successful lifting body concept has been revisited many times by NASA and international space programs. The NASA lifting body follow-on programs have included the HL-20 lifting body space ferry, the X-30 National Aero-Space Plane, the X-33 Shuttle replacement, and the X-38 Crew Return Vehicle, which used a parafoil gliding parachute for landings.

The full-scale version of the Hyper III was 32 feet long, launched from a helicopter at 10,000 feet, and controlled by a NASA research pilot from a ground-based cockpit.

During 1992, a Dryden team including Reed dem-onstrated the viability of an autonomous landing concept proposed for the X-38, using Global Posi-tioning System (GPS) for navigation and maneuver-ing to a landmaneuver-ing site. In the Spacecraft Autoland proj-ect, a 4-foot-long flattened biconical generic airframe called Spacewedge was used with a ram-air parafoil and controlled by a small onboard computer for autonomous flight.⁷² Using the vehicle shape of the Air Force–NASA X-24 lifting body, the X-38 team subsequently conducted radio-controlled drop tests of a 4-foot-long model equipped with a ram-air para-foil launched from a general aviation airplane in 1995. In 2000, a 0.80-scale X-38 vehicle was released from a B-52 at an altitude of 39,000 feet and completed a successful landing, but the X-38 program was canceled in 2002.

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