F-15 Highly Integrated Digital Electronic Control (HIDEC) and F/A-18 aircraft
NASA F-15 #835 was a unique one-of-a-kind aircraft called the F-15 Flight Research Facility. It was highly instrumented and equipped with an integrated digital electronic flight control system. It could be flown over a broad flight envelope to carry out complex and sophisticated research projects. NASA obtained the F-15, an "A" model, from the U.S. Air Force on Jan. 5, 1976, and used it for more than 25 advanced research projects involving aerodynamics, performance, propulsion control, control integration, instrumentation development, human factors, and flight test techniques.
Highly Integrated Digital Electronic Control (HIDEC)
The system that was developed on the F-15 to investigate and demonstrate methods of obtaining optimum aircraft performance was called HIDEC - Highly Integrated Digital Electronic Control.
The major elements of HIDEC were a Digital Electronic Flight Control System (DEFCS), a Digital Electronic Engine Control (DEEC), an on-board, general purpose computer, and an integrated architecture to allow all components to "talk to each other."
Unlike standard F-15's, which have a mechanical and an analog electronic flight control system, the F-15 HIDEC also had a dual-channel, fail-safe digital flight control system that could be programmed in such high-order computer languages as Pascal, Ada, and FORTRAN. It was linked to the H009 and Military Standard 1553B data buses, which tied all other electronic systems together.
Adaptive Engine Control System (ADECS)
ADECS - Adaptive Engine Control System - traded excess engine stall margin for improved performance. This was achieved through the integrated and computerized flight and engine control systems. The engine stall margin - the amount that engine operating pressures must be reduced to provide a margin of safety against an engine stall from excessive pressure - was continually monitored and adjusted by the integrated system, based on the flight profile and real-time performance needs.
Initial ADECS engineering work began in 1983. Research and demonstration flights with the ADECS system, which began in 1986, displayed increases in engine thrust of from 8 to 10.5 percent (depending on altitude), and up to 16 percent lower fuel consumption at 30,000 feet and constant thrust. Overall, engine performance improvements (rate of climb and specific excess power) were in the range of 10 to 25 percent at maximum afterburning power.
No stalls were encountered during even aggressive maneuvering, although intentional stalls were induced to validate ADECS methodology.
Self-Repairing and Self-Diagnostic Flight Control System
During late 1989 and early 1990 the F-15 research aircraft investigated what could become a major breakthrough in airborne flight control capability. It was the first aircraft to demonstrate a Self-Repairing Flight Control System (SRFCS).
The program, sponsored by the U.S. Air Force, demonstrated the ability of a flight control system to identify the failure of a control surface and reconfigure commands to other control devices such as ailerons, rudders, elevators, and flaps to continue the aircraft's mission or allow it to be landed safely.
The SRFCS also had the capability of identifying failures in electrical, hydraulic, and mechanical systems. When a failure in a normal flight control system occurred, ground maintenance diagnostic tests had to be conducted to identify the origin of the failure so that appropriate corrective actions could be taken. Ground maintenance crews often spend up to 60 percent of their time attempting to duplicate flight failures and correcting them. In many cases, the failure cannot be identified on the ground because actual flight conditions cannot be duplicated. This can be costly and time-consuming. System malfunctions on an aircraft with a SRFCS can be identified and isolated at the time they occur and then repaired as soon as the aircraft is on the ground.
Among the participants in the SRFCS research program, along with NASA Dryden, were the Air Force Wright Research and Development Center, Wright-Patterson AFB, OH; McDonnell Aircraft Co., St. Louis, MO; and General Electric Aircraft Control Systems Division, Binghamton, NY.
Performance Seeking Control
Research flights with the F-15 HIDEC began in the summer of 1990 on a program to optimize total aircraft engine performance during steady-state engine operation. The project was called Performance Seeking Control (PSC).
Previous modes used on the HIDEC aircraft employed stored schedules of optimum engine pressure ratios for an average engine on a normal day. Using digital flight control, inlet control, and engine control systems, PSC employed integrated control laws to assure that peak engine and maneuvering performance was available to the pilot at all times, regardless of the mission or immediate needs.
Among the functions of PSC were reduction of fuel usage at cruise conditions; maximization of excess thrust during accelerations and climbs; and extending of engine life by reducing the fan turbine inlet temperature. PSC also included developing methods within the digital engine control system to detect degradation of components. This type of information, coupled with normal preventative maintenance, could help assure fail-safe propulsion systems in high performance aircraft of the future.
Propulsion Controlled Aircraft System
Several accidents in which part or all of an aircraft's flight control system was lost prompted Dryden to establish a research program to investigate the capability of a "propulsion controlled aircraft" (PCA), using only engine thrust for flight control. The NASA F-15 was modified to serve as the first-ever aircraft to intentionally demonstrate this PCA capability.
Initial flight studies with the pilot manually controlling the throttles and all F-15 flight controls locked showed that it was possible to maintain gross control. Altitude could be maintained within a few hundred feet using both throttles together. To climb, thrust would be added; to descend, thrust would be reduced. Heading could be controlled to within a few degrees, using differential throttle to generate yaw, which resulted in roll.
These initial flights also showed there was not enough precise flight control capability to land on a runway. This was due to the small control forces and moments of engine thrust, difficulty in controlling the airplane's shallow dive and climb motion, and difficulty in compensating for the lag in engine response.
The NASA F-15 was an ideal testbed for this research. It incorporated digital engine controls and digital flight controls, and it had a general purpose computer and data bus architecture to permit these digital systems to communicate with each other. There was also a cockpit computer panel for the pilot to make control system inputs so as to select options and vary system gains.
Tests of the PCA with the F-15 occurred at speeds of 150 knots with the flaps down and at 170 to 190 knots with the flaps up. Initial flights tested the "up and away" control capability, with landing approaches down to less than 10 feet above the ground.
Flight tests of PCA by Dryden with the F-15 concluded with a successful landing on April 21, 1993, using only engine power to turn, climb, and descend. A successful follow-on program with an MD-11 transport aircraft conclusively demonstrated the success of the technology in 1995.