NF-15B "Agile Eagle"

Last revised March 4, 2000


On October 3, 1984, the Flight Dynamics Laboratory of the Air Force's Aeronatical Systems Division awarded a contract to McDonnell Douglas for an advanced development Short Take-Off and Landing (STOL) Maneuvering Technology Demonstrator (MTD) experimental aircraft. The basic idea behind the program was to develop an aircraft that could land and take off from sections of wet, bomb-damaged runway under bad-weather conditions and under severe crosswinds and without active ground-based navigational assistance. The first two-seat F-15B (serial number 71-290) was selected for modification as the STOL/MTD demonstrator. The project was given the name Agile Eagle.

For the first phase of the program, the F-15 STOL/MTD aircraft was fitted with movable canard aerodynamic surfaces attached to the upper edges of the forward air intakes. The canards were mounted at 20 degree dihedral and could be operated either symmetrically or differentially. It was provided with a four-channel fly-by-wire integrated flight/propulsion control (IFPC) system, and an improved set of cockpit controls and displays (similar to those intended for the F-15E). In addition, the aircraft was fitted with a beefed-up undercarriage capable of handling rough field landings at high descent speeds. The APG-70 radar of the F-15B was provided with a high-resolution ground-mapping mode to enable the pilot to locate the airfield from a great distance, and a LANTIRN set was fitted to the aircraft to project an image of the airfield onto the HUD during the final landing approach.

The integrated flight/propulsion control (IFPC) system was developed by McDonnell Douglas and produced by General Electric. The IFPC manages all control parameters for the aircraft. It uses a new computer chip and employs high-level computer languages such as Ada. The system is intended to relieve the pilot of some of the more routine tasks of handling the aircraft. There are five modes of operation--conventional, short takeoff/approach, short landing, cruise and combat. Position sensors linked to the throttle, control stick and rudder pedals feed electrical signals to the IFPC system, and the computer converts them into a set of commands to the quadruply-redundant fly-by-wire activators to set the controls for the maneuvers required.

The modified 71-0290 flew for the first time on September 7, 1988, company test pilot Larry Walker being at the controls The aircraft was assigned the designation NF-15B, the N meaning that the modifications were sufficiently drastic that the aircraft was not expected to be returned to its original configuration. 43 test flights were carried out in this configuration.

In the second phase of the program, the standard circular dilating engine nozzles were replaced with rectangular two-dimensional thrust-vectoring, thrust-reversing nozzles. These nozzles were built by Pratt & Whitney out of chemically-milled, welded titanium honeycomb. They had flat upper and lower flaps that were independently driven and capable of adjusting the exhaust upwards or downwards by angles as much as 20 degrees. There were a set of vanes above and below the nozzle that made it possible for the thrust to be reversed.

The first flight with the thrust-vectoring nozzles took place on May 16, 1989. The plane was transferred to Edwards AFB for joint flight tests by the Air Force and McDonnell Douglas. The two-dimensional nozzles were first tested in flight on March 23, 1990. Test flights demonstrated that the thrust-vectoring features of the new nozzles worked as anticipated and validated the changes that had been made to the IFPC system software in order to accommodate the new thrust-vectoring nozzles. It turned out that thrust-vectoring resulted in a 25 percent reduction in takeoff roll. The thrust-reversing feature made it possible for the F-15 to land on just 1650 feet of runway. In addition, it was found possible to use thrust reversal during actual flight to produce rapid decelerations, a useful feature to have during close-in air-to-air combat. During its flying life, the F-15 STOL/MTD made numerous vectored takeoffs with rotation demonstrated at speeds as low as 42 mph. The shortest landing made by the plane took only 1366 feet, with a basic F-15 needing 7500 feet to land.

The program ended on August 15, 1991, having accomplished all of its flight objectives. The vectored thrust nozzles were returned to Pratt & Whitney.

Following the completion of the vectored thrust tests, the NF-15B was fitted with new thrust-vectoring nozzles. They are known as pitch/yaw balanced beam nozzles, and can deflect thrust up to 20 degrees in any direction. F100-PW-229 IPE engines. The program is known as ACTIVE (Advanced Control Technology for Integrated VEhicles). The goal of the program is to explore thrust vectoring in the supersonic regime, as well as to explore the possibility of using thrust vectoring to replace vertical tailfins. Initial flight trials begain in February of 1996. The first thrust vectoring flights took place shortly thereafter. The first supersonic thrust vectoring took place on April 24, 1996. On October 31, 1996, the aircraft demonstrated thrust vectoring at Mach 1.95.

The NF-15B has now received a NASA number of 837.

Sources:


  1. Agile Eagle, World Airpower Journal, Volume 1, Spring 1990.

  2. McDonnell Douglas Aircraft Since 1920, Volume II, Rene J. Francillon, Naval Institute Press, 1990.

  3. Combat Aircraft F-15, Michael J. Gething and Paul Crickmore, Crescent Books, 1992.

  4. The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.

  5. The World's Great Interceptor Aircraft, Gallery Books, 1989.

  6. F-15 Eagle, Robert F. Dorr, World Airpower Journal, Volume 9, Summer 1992.

  7. Boeing/McDonnell Douglas F-15 Eagle Variant Briefing, John D. Gresham, World Air Power Journal, Vol 33, Summer 1998.