F-8 Digital Fly-By-Wire Aircraft

The Digital Fly-By-Wire (DFBW) concept uses an electronic flight-control system coupled with a digital computer to replace conventional mechanical flight controls.

F-8 DFBW in flight

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.

The Aircraft

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.

Pilot Gary Krier in front of F-8 DFBW

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.

Research Benefits

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.

F-8 DFBW Apollo computer interface box

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.

F-8 DFBW 3-view drawing

  • NASA 802: Navy Bureau #145546
  • Powerplant: Pratt and Whitney J57 turbojet
  • Wingspan: 35 feet 2 inches (350 square feet)
  • Overall length: 54 feet 6 inches and height is 15 feet 9 inches
  • Flown as the DFBW testbed by NASA from 1972 to 1985.
  • Fleet F-8s were the first carried based planes with speeds in excess of 1,000 mph. LTV won the Collier Trophy for its design and development. Total production was 1,261.

NASA courtesy (www.nasa.gov)



NASA Dryden flight

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.

NASA AD-1 X-plane

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