Click on the picture below to watch the video:
Click on the picture below to watch the video:
(Text and photos: NASA courtesy)
Two SR-71 aircraft were used by NASA as testbeds for high-speed, high-altitude aeronautical research. The aircraft, an SR-71A and an SR-71B pilot trainer aircraft were based at NASA’s Dryden Flight Research Center, Edwards, Calif. They have been loaned to NASA by the U.S. Air Force. Developed for the USAF as reconnaissance aircraft more than 30 years ago, SR-71s are still the world’s fastest and highest-flying production aircraft.
The aircraft can fly more than 2200 miles per hour (Mach 3+ or more than three times the speed of sound) and at altitudes of over 85,000 feet. This operating environment makes the aircraft excellent platforms to carry out research and experiments in a variety of areas – aerodynamics, propulsion, structures, thermal protection materials, high-speed and high-temperature instrumentation, atmospheric studies and sonic boom characterization.
Data from the SR-71 high-speed research program may be used to aid designers of future supersonic/hypersonic aircraft and propulsion systems, including a high-speed civil transport. The SR-71 program at Dryden was part of NASA’s overall high-speed aeronautical research program, and projects involve other NASA research centers, other government agencies, universities and commercial firms.
One of the first major experiments to be flown in the NASA SR-71 program was a laser air-data collection system. It used laser light instead of air pressure to produce airspeed and attitude reference data such as angle of attack and sideslip normally obtained with small tubes and vanes extending into the air stream or from tubes with flush openings on an aircraft’s outer skin. The flights provided information on the presence of atmospheric particles at altitudes of 80,000 feet and above where future hypersonic aircraft will be operating. The system used six sheets of laser light projected from the bottom of the « A » model. As microscopic-size atmospheric particles passed between the two beams, direction and speed were measured and processed into standard speed and attitude references. An earlier laser air data collection system was successfully tested at Dryden on an F-l04 testbed.
The first of a series of flights using the SR-71 as a science camera platform for NASA’s Jet Propulsion Laboratory, Pasadena, Calif., was flown in March 1993. From the nosebay of the aircraft, an upward-looking ultraviolet video camera studied a variety of celestial objects in wavelengths that are blocked to ground-based astronomers. The SR-71 has also been used in a project for researchers at the University of California-Los Angeles (UCLA) who were investigating the use of charged chlorine atoms to protect and rebuild the ozone layer.
In addition to observing celestial objects in the various wavelengths, future missions could include « downward » looking instruments to study rocket engine exhaust plumes, volcano plumes and the Earth’s atmosphere, as part of the scientific effort to reduce pollution and protect the ozone layer.
The SR-71, operating as a testbed, also has been used to assist in the development of a commercial satellite-based, instant wireless personal comunications network, called the IRIDIUM system, under NASA’s commercialization assistance program. The IRIDIUM system was being developed by Motorola’s Satellite Communications Division. During the development tests, the SR-71 acted as a « surrogate satellite » for transmitters and receivers on the ground. The SR-71 also has been used in a program to study ways of reducing sonic boom overpressures that are heard on the ground much like sharp thunderclaps when an aircraft exceeds the speed of sound. Data from the study could eventually lead to aircraft designs that would reduce the « peak » of sonic booms and minimize the startle affect they produce on the ground.
Instruments at precise locations on the ground record the sonic booms as the aircraft passes overhead at known altitudes and speeds. An F-16XL aircraft was also used in the study. It was flown behind the SR-71, probing the near-field shockwave while instrumentation recorded the pressures and other atmospheric parameters.
In November 1998 the SR-71 completed the NASA/Lockheed Martin Linear Aerospike SR-71 experiment (LASRE). LASRE was a small, half-span model of a lifting body with eight thrust cells of an aerospike engine, mounted on the back of an SR-71 aircraft and operating like a kind of « flying wind tunnel. » During seven flights, the experiment gained information that may help Lockheed Martin predict how operation of aerospike engines at altitude will affect vehicle aerodynamics of a future reusable launch vehicle.
Dryden has a decade of past experience at sustained speeds above Mach 3. Two YF-12 aircraft were flown at the facility between December 1969 and November 1979 in a joint NASA/USAF program to learn more about the capabilities and limitations of high speed, high-altitude flight. The YF-12s were prototypes of a planned interceptor aircraft based on a design that later evolved into the SR-71 reconnaissance aircraft.
Research information from the YF-12 program was used to validate analytical theories and wind-tunnel test techniques to help improve the design and performance of future military and civil aircraft. The American supersonic transport project of the late 1960s and early 1970s would have benefited greatly from YF-12 research data. The aircraft were a YF-12A (tail #935) and a YF-12C (tail #937). Tail number 937 was actually an SR-71 that was called a YF-12C for security reasons. These aircraft logged a combined total of 242 flights during the program. A third aircraft, a YF-12A (tail #936), was flown by Air Force crews early in the program. It was lost because of an inflight fire in June l971. The crew was not hurt.
The YF-12s were used for a wide range of experiments and research. Among the areas investigated were aerodynamic loads, aerodynamic drag and skin friction, heat transfer, thermal stresses, airframe and propulsion system interactions, inlet control systems, high-altitude turbulence, boundary layer flow, landing gear dynamics, measurement of engine effluents for pollution studies, noise measurements and evaluation of a maintenance monitoring and recording system. On many YF-12 flights medical researchers obtained information on the physiological and biomedical aspects of crews flying at sustained high speeds.
From February 1972 until July 1973, a YF-12A was used for heat loads testing in Dryden’s High Temperature Loads Laboratory (now the Thermostructures Research Facility). The data helped improve theoretical prediction methods and computer models of that era dealing with structural loads, materials and heat distribution at up to 800 degrees (F), the same surface temperatures reached during sustained speeds of Mach 3.
The SR-71 was designed and built by the Lockheed Skunk Works, now the Lockheed Martin Skunk Works. SR-71s are powered by two Pratt and Whitney J-58 axial-flow turbojets with afterburners, each producing 32,500 pounds of thrust. Studies have shown that less than 20 percent of the total thrust used to fly at Mach 3 is produced by the basic engine itself. The balance of the total thrust is produced by the unique design of the engine inlet and « moveable spike » system at the front of the engine nacelles and by the ejector nozzles at the exhaust which burn air compressed in the engine bypass system.
Speed of the aircraft is announced as Mach 3.2 – more than 2000 miles per hour (3218.68 kilometers per hour). They have an unrefueled range of more than 2000 miles (3218.68 kilometers) and fly at altitudes of over 85,000 feet (25908 meters).
As research platforms, the aircraft can cruise at Mach 3 for more than one hour. For thermal experiments, this can produce heat soak temperatures of over 600 degrees (F). The aircraft are 107.4 feet (32.73 meters) long, have a wing span of 55.6 feet (16.94 meters, and are l8.5 feet (5.63 meters) high (ground to the top of the rudders when parked). Gross takeoff weight is about 140,000 pounds (52253.83 kilograms), including a fuel weight of 80,000 pounds (29859.33 kilograms).
The airframes are built almost entirely of titanium and titanium alloys to withstand heat generated by sustained Mach 3 flight. Aerodynamic control surfaces consist of all-moving vertical tail surfaces above each engine nacelle, ailerons on the outer wings and elevators on the trailing edges between the engine exhaust nozzles.
The two SR-71s at Dryden have been assigned the following NASA tail numbers: NASA 844 (A model), military serial 64-17980, manufactured in July 1967, and NASA 831 (B model), military serial 64-17956, manufactured in September 1965. From 1991 through 1994, Dryden also had another « A » model, NASA 832, military serial 64-17971, manufactured in October 1966. This aircraft was returned to the USAF inventory and was the first aircraft reactivated for USAF reconnaissance purposes in 1995.
The SR-71 last flight took place in October 1999.
The SR-71 was designed by a team of Lockheed personnel led by Clarence « Kelly » Johnson, at that time vice president of the Lockheed’s Advanced Development Company, commonly known as the « Skunk Works. »
The basic design of the SR-71 and YF-12 aircraft originated in secrecy in the late l950s with the aircraft designation of A-11. Its existence was publicly announced by President Lyndon Johnson on Feb. 29, 1964, when he announced that an A-11 had flown at sustained speeds of over 2000 miles per hour during tests at Edwards Air Force Base, Calif.
Development of the SR-71s from the A-11 design, as strategic reconnaissance aircraft, began in February 1963. First flight of an SR-71 was on Dec. 22, 1964.
(Text, photo, and sketches: NASA courtesy)
Two X-29 aircraft, featuring one of the most unusual designs in aviation history, were flown at the NASA Ames-Dryden Flight Research Facility (now the Dryden Flight Research Center), Edwards, Calif., as technology demonstrators to investigate advanced concepts and technologies. The multi-phased program was conducted from 1984 to 1992 and provided an engineering data base that is available in the design and development of future aircraft.
The X-29 almost looked like it was flying backward. Its forward swept wings were mounted well back on the fuselage, while its canards – horizontal stabilizers to control pitch – were in front of the wings instead of on the tail. The complex geometries of the wings and canards combined to provide exceptional maneuverability, supersonic performance, and a light structure. Air moving over the forward-swept wings tended to flow inward toward the root of the wing instead of outward toward the wing tip as occurs on an aft swept wing. This reverse air flow did not allow the wing tips and their ailerons to stall (lose lift) at high angles of attack (direction of the fuselage relative to the air flow).
The concepts and technologies the fighter-size X-29 explored were the use of advanced composites in aircraft construction; variable camber wing surfaces; the unique forward-swept wing and its thin supercritical airfoil; strake flaps; close-coupled canards; and a computerized fly-by-wire flight control system to maintain control of the otherwise unstable aircraft.
Research results showed that the configuration of forward swept wings, coupled with movable canards, gave pilots excellent control response at up to 45 degrees angle of attack. During its flight history, the X-29s were flown on 422 research missions – 242 by aircraft No. 1 in the Phase 1 portion of the program; 120 flights by aircraft No. 2 in Phase 2; and 60 flights in a follow-on « vortex control » phase. An additional 12 non-research flights with X-29 No. 1 and 2 non-research flights with X-29 No. 2 raised the total number of flights with the two aircraft to 436.
Before World War II, there were some gliders with forward-swept wings, and the NACA Langley Memorial Aeronautical Laboratory, Hampton, Va., did some wind-tunnel work on the concept in 1931. Germany developed a motor-driven aircraft with forward-swept wings during the war known as the Ju-287. The concept, however, was not successful because the technology and materials did not exist then to construct the wing rigid enough to overcome bending and twisting forces without making the aircraft too heavy.
The introduction of composite materials in the 1970s opened a new field of aircraft construction, making it possible to design rugged airframes and structures stronger than those made of conventional materials, yet lightweight and able to withstand tremendous aerodynamic forces.
Construction of the X-29’s thin supercritical wing was made possible because of its composite construction. State-of-the-art composites permit aeroelastic tailoring, which allows the wing some bending but limits twisting and eliminates structural divergence within the flight envelope (i.e., deformation of the wing or breaking off in flight).
In 1977, the Defense Advanced Research Projects Agency (DARPA) and the Air Force Flight Dynamics Laboratory (now the Wright Laboratory), Wright-Patterson Air Force Base, Ohio, issued proposals for a research aircraft designed to explore the forward swept wing concept. The aircraft was also intended to validate studies that said it should provide better control and lift qualities in extreme maneuvers, and possibly reduce aerodynamic drag as well as fly more efficiently at cruise speeds.
From several proposals, Grumman Aircraft Corporation was chosen in December 1981 to receive an $87 million contract to build two X-29 aircraft. They were to become the first new X-series aircraft in more than a decade. First flight of the No. 1 X-29 was Dec. 14, 1984, while the No. 2 aircraft first flew on May 23, 1989. Both first flights were from the NASA Ames-Dryden Flight Research Facility, later renamed the Dryden Flight Research Center.
The flight control surfaces on the X-29 were the forward-mounted canards, which shared the lifting load with the wings and provided primary pitch control; the wing flaperons (combination flaps and ailerons), used to change wing camber and function as ailerons for roll control when used asymmetrically; and the strake flaps on each side of the rudder that augmented the canards with pitch control. The control surfaces were linked electronically to a triple-redundant digital fly-by-wire flight control system (with analog back up) that provided an artificial stability.
The particular forward swept wing, close-coupled canard design used on the X-29 was unstable. The X-29’s flight control system compensated for this instability by sensing flight conditions such as attitude and speed, and through computer processing, continually adjusted the control surfaces with up to 40 commands each second. This arrangement was made to reduce drag. Conventionally configured aircraft achieved stability by balancing lift loads on the wing with opposing downward loads on the tail at the cost of drag. The X-29 avoided this drag penalty through its relaxed static stability.
Each of the three digital flight control computers had an analog backup. If one of the digital computers failed, the remaining two took over. If two of the digital computers failed, the flight control system switched to the analog mode. If one of the analog computers failed, the two remaining analog computers took over. The risk of total systems failure was equivalent in the X-29 to the risk of mechanical failure in a conventional system.
The No. 1 aircraft demonstrated in 242 research flights that, because the air moving over the forward-swept wing flowed inward, rather than outward as it does on a rearward-swept wing, the wing tips remained unstalled at the moderate angles of attack flown by X-29 No. 1. Phase 1 flights also demonstrated that the aeroelastic tailored wing did, in fact, prevent structural divergence of the wing within the flight envelope, and that the control laws and control surface effectiveness were adequate to provide artificial stability for this otherwise extremely unstable aircraft and provided good handling qualities for the pilots.
The aircraft’s supercritical airfoil also enhanced maneuvering and cruise capabilities in the transonic regime. Developed by NASA and originally tested on an F-8 at Dryden in the 1970s, supercritical airfoils – flatter on the upper wing surface than conventional airfoils – delayed and softened the onset of shock waves on the upper wing surface, reducing drag. The phase 1 flights also demonstrated that the aircraft could fly safely and reliably, even in tight turns.
The No. 2 X-29 investigated the aircraft’s high angle of attack characteristics and the military utility of its forward-swept wing/canard configuration during 120 research flights. In Phase 2, flying at up to 67 degrees angle of attack (also called high alpha), the aircraft demonstrated much better control and maneuvering qualities than computational methods and simulation models had predicted. The No. 1 X-29 was limited to 21 degrees angle of attack maneuvering.
During Phase 2 flights, NASA, Air Force, and Grumman project pilots reported the X-29 aircraft had excellent control response to 45 degrees angle of attack and still had limited controllability at 67 degrees angle of attack. This controllability at high angles of attack can be attributed to the aircraft’s unique forward-swept wing- canard design. The NASA/Air Force-designed high-gain flight control laws also contributed to the good flying qualities.
Flight control law concepts used in the program were developed from radio-controlled flight tests of a 22-percent X-29 drop model at NASA’s Langley Research Center, Hampton, Va. The detail design was performed by engineers at Dryden and the Air Force Flight Test Center at Edwards Air Force Base. The X-29 achieved its high alpha controllability without leading edge flaps on the wings for additional lift, and without moveable vanes on the engine’s exhaust nozzle to change or « vector » the direction of thrust, such as those used on the X-31 and the F-18 High Angle-of-Attack Research Vehicle. Researchers documented the aerodynamic characteristics of the aircraft at high angles of attack during this phase using a combination of pressure measurements and flow visualization. Flight test data from the high-angle-of-attack/military-utility phase of the X-29 program satisfied the primary objective of the X-29 program – to evaluate the ability of X-29 technologies to improve future fighter aircraft mission performance.
In 1992 the U.S. Air Force initiated a program to study the use of vortex flow control as a means of providing increased aircraft control at high angles of attack when the normal flight control systems are ineffective.
The No. 2 X-29 was modified with the installation of two high-pressure nitrogen tanks and control valves with two small nozzle jets located on the forward upper portion of the nose. The purpose of the modifications was to inject air into the vortices that flow off the nose of the aircraft at high angles of attack.
Wind tunnel tests at the Air Force’s Wright Laboratory and at the Grumman Corporation showed that injection of air into the vortices would change the direction of vortex flow and create corresponding forces on the nose of the aircraft to change or control the nose heading.
From May to August 1992, 60 flights successfully demonstrated vortex flow control (VFC). VFC was more effective than expected in generating yaw (left-to-right) forces, especially at higher angles of attack where the rudder loses effectiveness. VFC was less successful in providing control when sideslip (relative wind pushing on the side of the aircraft) was present, and it did little to decrease rocking oscillation of the aircraft.
Overall, VFC, like the forward-swept wings, showed promise for the future of aircraft design. The X-29 did not demonstrate the overall reduction in aerodynamic drag that earlier studies had suggested, but this discovery should not be interpreted to mean that a more optimized
design with forward-swept wings could not yield a reduction in drag. Overall, the X-29 program demonstrated several new technologies as well as new uses of proven technologies. These included: aeroelastic tailoring to control structural divergence; use of a relatively large, close-coupled canard for longitudinal control; control of an aircraft with extreme instability while still providing good handling qualities; use of three-surface longitudinal control; use of a double-hinged trailing-edge flaperon at supersonic speeds; control effectiveness at high angle of attack; vortex control; and military utility of the overall design.
The X-29 is a single-engine aircraft 48.1 feet long. Its forward-swept wing has a span of 27.2 feet. Each X-29 was powered by a General Electric F404-GE-400 engine producing 16,000 pounds of thrust. Empty weight was 13,600 pounds, while takeoff weight was 17,600 pounds.
The aircraft had a maximum operating altitude of 50,000 feet, a maximum speed of Mach 1.6, and a flight endurance time of approximately one hour. The only significant difference between the two aircraft was an emergency spin chute deployment system mounted at the base of the rudder on aircraft No. 2. External wing structure is primarily composite materials incorporated into precise patterns to develop strength and avoid structural divergence. The wing substructure and the basic airframe itself is aluminum and titanium. Wing trailing edge actuators controlling camber are mounted externally in streamlined fairings because of the thinness of the supercritical airfoil.
A program conducted between 1979 and 1982 at the NASA Dryden Flight Research Center, Edwards, Calif., successfully demonstrated an aircraft wing that could be pivoted obliquely from zero to 60 degrees during flight. The unique wing was demonstrated on a small, subsonic jet-powered research aircraft called the AD-1 (Ames Dryden -1). The aircraft was flown 79 times during the research program, which evaluated the basic pivot-wing concept and gathered information on handling qualities and aerodynamics at various speeds and degrees of pivot.
The oblique wing concept originated with Robert T. Jones, an aeronautical
engineer at NASA’s Ames Research Center, Moffett Field, Calif.
Analytical and wind tunnel studies Jones initiated at Ames indicated that a transport-size oblique-wing aircraft, flying at speeds up to Mach 1.4 (1.4 times the speed of sound), would have substantially better aerodynamic performance than aircraft with more conventional wings. At high speeds, both subsonic and supersonic, the wing would be pivoted at up to 60 degrees to the aircraft’s fuselage for better high-speed performance. The studies showed these angles would decrease aerodynamic drag, permitting increased speed and longer range with the same fuel expenditure. At lower speeds, during takeoffs and landings, the wing would be perpendicular to the fuselage like a conventional wing to provide maximum lift and control qualities. As the aircraft gained speed, the wing would be pivoted to increase the oblique angle, thereby reducing the drag and decreasing fuel consumption. The wing could only be swept in one direction, with the right wingtip moving forward.
The AD-1 aircraft was delivered to Dryden in February 1979. The Ames Industrial Co., Bohemia, N.Y., constructed it, under a $240,000 fixed-price contract. NASA specified the overall vehicle design using a geometric configuration studied by the Boeing Commercial Airplane Company, Seattle, Wash. The Rutan Aircraft Factory, Mojave, Calif., provided the detailed design and load analysis for the intentionally low-speed, low-cost airplane. The low speed and cost of course limited the complexity of the vehicle and the scope of its technical objectives.
Piloting the aircraft on its first flight Dec. 21, 1979, was NASA research pilot Thomas C. McMurtry, who was also the pilot on the final flight Aug. 7, 1982. Powered by two small turbojet engines, each producing 220 pounds of static thrust at sea level, the aircraft was limited for reasons of safety to a speed of about 170 mph. The AD-1 was 38.8 feet in length and had a wingspan of 32.3 feet unswept. It was constructed of plastic reinforced with fiberglass, in a sandwich with the skin separated by a rigid foam core. It had a gross weight of 2,145 pounds, and an empty weight of 1,450 pounds. A fixed tricycle landing gear, mounted close to the fuselage to lessen aerodynamic drag, gave the aircraft a very « squatty » appearance on the ground. It was only 6.75 feet high. The wing was pivoted by an electrically driven gear mechanism located inside the fuselage, just forward of the engines.
Read full article on the NASA (www.nasa.gov) website: NASA Dryden Past Projects: AD-1 Oblique Wing – updated August 12, 2009