The Digital Fly-By-Wire (DFBW) concept uses an electronic flight-control system coupled with a digital computer to replace conventional mechanical flight controls.
The first test of a DFBW system in an aircraft was in l972 on a modified F-8 Crusader at the Flight Research Center, Edwards, Calif. (now Dryden Flight Research Center). It was the forerunner of the fly-by-wire flight control systems now used on the space shuttles and on today’s military and civil aircraft to make them safer, more maneuverable and more efficient. It was safer because of its redundancies and because, for military aircraft, wires were less vulnerable to battle damage than the hydraulic lines they replaced. It was more maneuverable because computers could command more frequent adjustments than a human pilot and designers could do away with features that made the plane more stable and thus harder to maneuver. For airliners, computerized flight control could also ensure a smoother ride than a human pilot alone could provide. Finally, digital fly-by-wire was more efficient because it was lighter and took up less volume than hydraulic controls and thus either reduced the fuel required to fly with the extra weight and/or permitted carrying more passengers or cargo. It also required less maintenance than older systems.
In the first few decades of flight, pilots controlled aircraft through direct force – moving control sticks and rudder pedals linked to cables and pushrods that pivoted control surfaces on the wings and tails.
As engine power and speeds increased, more force was needed and hydraulically boosted controls emerged. Soon, all high performance and large aircraft had hydraulic-mechanical flight-control systems. These conventional flight-control systems restricted designers in the configuration and design of aircraft because of the need for flight stability.
As the electronic era evolved in the 1960s, so did the idea of aircraft with electronic flight-control systems. Wires replacing cables and pushrods would give designers greater flexibility in configuration and in the size and placement of components such as tail surfaces and wings. A fly-by-wire system also would be smaller, more reliable, and in military aircraft, much less vulnerable to battle damage. A fly-by-wire aircraft would also be much more responsive to pilot control inputs. The result would be more efficient, safer aircraft with improved performance and design.
By the late 1960s, engineers at Dryden began discussing how to modify an aircraft and create a digital fly-by-wire testbed.
Support for the concept at NASA Headquarters came from Neil Armstrong, former research pilot at Dryden. He served in the Office of Advanced Research and Technology following his historic Apollo 11 lunar landing and knew electronic control systems from his days training in and operating the lunar module. Armstrong supported the proposed Dryden project and backed the transfer of an F-8C Crusader from the U.S. Navy to NASA to become the Digital Fly-By-Wire (DFBW) research aircraft. It was given the tail number « NASA 802. »
Wires from the control stick in the cockpit to the control surfaces on the wings and tail surfaces replaced the entire mechanical flight-control system in the F-8. The heart of the system was an off-the-shelf backup Apollo digital flight-control computer and inertial sensing unit which transmitted pilot inputs to the actuators on the control surfaces.
On May 25, 1972, the highly modified F-8 became the first aircraft to fly completely dependent upon an electronic flight-control system. The pilot was Gary Krier.
The first phase of the DFBW program validated the fly-by-wire concept and quickly showed that a refined system – especially in large aircraft – would greatly enhance flying qualities by sensing motion changes and applying pilot inputs instantaneously.
The Phase 1 system had a backup fly-by-wire system in the event of a failure in the Apollo computer unit, but it was never necessary to use the system in flight.
In a joint program carried out with the Langley Research Center in the second phase of research, the original Apollo system was replaced with a triple redundant digital system. It would provide backup computer capabilities if a failure occurred.
The DFBW program lasted 13 years. The final flight – the 210th of the program – was made April 2, 1985, with Dryden Research Pilot Ed Schneider at the controls.
The F-8 DFBW validated the principal concepts of the all-electric flight control systems now used on nearly all modern high performance aircraft and on military and civilian transports. A DFBW flight-control system also is used on the space shuttles.
NASA 802 was the testbed for the sidestick-controller used in the F-16 fighter, the first U.S. high-performance aircraft with a DFBW system.
Among other electronic devices flown on the DFBW F-8 were an angle-of-attack limiter and maneuver leading- and trailing-edge flaps, features commonly used on today’s new generation of aircraft.
In addition to pioneering the Space Shuttle’s fly-by-wire flight-control system, NASA 802 was the testbed that explored pilot induced oscillations (PIO) and validated methods to suppress them. PIOs occur when a pilot over-controls an aircraft and a sustained oscillation results. On the last of five free flights of the prototype Space Shuttle Enterprise during approach and landing tests in 1977, a PIO developed as the vehicle settled onto the runway. The problem was duplicated with the F-8 DFBW and a PIO suppression filter was developed and tested on the aircraft for the Shuttle program office.
The aircraft was used to develop a concept called Analytic Redundancy Management, in which dynamic and kinematic relations between various dissimilar sensors and measurements are used to detect and isolate sensor failures.
In another series of successful tests, a software back-up system (Resident Backup System) was demonstrated as a means to survive common software faults that could cause all three channels to fail. This system has been subsequently used on many experimental and production aircraft systems.
The Dryden project also worked with the British Royal Aircraft Establishment using the DFBW F-8 to produce ground-based software to use when researchers are investigating flight controls in high-risk flight environments. During contingencies, pilots can disengage the ground control software and switch to backup on-board controls. DFBW research carried out with NASA 802 at Dryden is now considered one of the most significant and successful aeronautical programs in NASA history.
Digital fly-by-wire is now used in a variety of airplanes ranging from the F/A-18 to the Boeing 777 and the space shuttles.
The F-8 aircraft was originally built by LTV Aerospace, Dallas, Texas, for the U.S. Navy, which made it available to Dryden as a test vehicle.
(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.
I was reading a gripping blog in French called “Objets du ciel » (broken link) when I bumped into an amazing article written by Carl Conrad. I first thought that this post was unbelievable. I daresay that all the articles he writes are amazing. I am going to report hereafter what I have read about this topic – nuclear-powered aircraft – from different sources, but Carl Conrad’s article is the one that inspired me most.
As a major oil crisis is looming, airlines are cancelling some less financially viable air links of theirs. The future of aviation as we currently know it, seems to be in jeopardy. Nothing seems to be used as a substitute for any current kind of energy, not even electricity. What about nuclear-powered engines?
Nowadays, nobody would bear any nuclear-powered test flights. However those tests did occur within a USAF-carried-out weapons system (WS 125-A) nuclear-powered bomber aircraft programme. Those tests were performed with a 1,000-kilowatt-nuclear jet engine airborne on a Convair NB-36H. This aircraft named « The Crusader », took-off 47 times during the 50s. The engine was not used for propelling. It only worked at an altitude which was deemed sensible. Those tests allowed to assess the nuclear engine drive performance. Every flight would involve troops deployment in the area to prevent as soon as possible from any accident fallout spreading. The aircraft was modified in order to enhance the five crew member’s safety. The USAF considered the concept not realistic and gave the programme up in late 1956.
However, this technology might be coming back to fly some drones for long-lasting flights. People might be relunctant to see nuclear-powered drones taking-off and flying past over their heads. Who knows? Maybe some day.
Another project to mention: Project Orion should have become a 4,000-ton, long-range spacecraft powered by controlled nuclear pulses, or explosions. For this purpose, a small test vehicle was built. It was dubbed « Hot Rod », and was conventional-explosive-powered craft. Finally, Orion was cancelled in 1965 because it would not have been politically correct and because of technical challenges.
I have not found a piece of information about nuclear-powered craft after the year 2004. By the way, if someone knows further information about nuclear-powered aircraft, they will be welcome if they want to add some comments.
Span: 230 ft. 0 in.
Length: 162 ft. 1 in. (as B-36H, the NB-36H was slightly shorter)
Height: 46 ft. 8 in.
Weight: 357,500 lbs. (max. gross weight)
Engines: Six Pratt & Whitney R-4360-53 radials of 3,800 hp each (takeoff power) and four General Electric J47-GE-19 turbojets of 5,200 lbs. thrust each
Crew: Five ( pilot, copilot, flight engineer and two nuclear engineers)
Maximum speed: Approx. 420 mph at 47,000 ft.
Cruising speed: 235 mph
Service ceiling: Approx. 47,000 ft.