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From 1972 to 1985 the NASA Dryden Flight Research Center conducted flight research with an F-8C employing the first digital fly-by-wire flight control system without a mechanical back up. The decision to replace all mechanical control linkages to rudder, ailerons, and other flight control surfaces was made for two reasons. First, it forced the research engineers to focus on the technology and issues that were truly critical for a production fly-by-wire aircraft. Secondly, it would give industry the confidence it needed to apply the technology--confidence it would not have had if the experimental system relied on a mechanical back up.
In the first few decades of flight, pilots had controlled aircraft through direct force--moving control sticks and rudder pedals linked to cables and push rods 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 grew in the 1960s, so did the idea of aircraft with electronic flight-control systems. Wires replacing mechanical devices 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 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 without any mechanical backup. 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 analog 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 triply redundant digital system. It would provide backup computer capabilities if a failure occurred.
The DFBW program lasted 13 years. The final research 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 in a variety of airplanes ranging from the F/A-18 to the Boeing 777. 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 second U.S. high performance aircraft with a DFBW system.
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 series of PIO suppression filters was developed and tested on the aircraft for the shuttle program office.
DFBW research carried out with NASA 802 at Dryden is now considered one of the most significant and successful aeronautical programs in NASA history.