With its afterburners roaring, NASA research pilot Jim Smolka pulls NASA's highly modified NF-15B research aircraft into a steep climb after takeoff from Edwards Air Force Base on its final flight. NASA Photo / Tony Landis
NASA's NF-15B research aircraft, tail number 837, was flown by NASA's Dryden Flight Research Center from 1993 through early 2009. Originally designated a TF-15 trainer version, the aircraft was the first two-seat F-15 Eagle built by McDonnell Douglas, the sixth F-15 off the assembly line, and was the oldest F-15 flying up to its retirement. First flown in July 1973, the aircraft was used initially for F-15 developmental testing and evaluation as part of the F-15 combined test force at Edwards Air Force Base in the 1970s.
In the 1980's, the aircraft was extensively modified for the U.S. Air Force's Short Takeoff and Landing Maneuver Technology Demonstrator (S/MTD) program. Those modifications included equipping the aircraft with a digital fly-by-wire control system, canards consisting of modified F-18 horizontal stabilators mounted on the engine inlets ahead of the wings, and two-dimensional thrust-vectoring, thrust-reversing nozzles which could redirect engine exhaust either up or down, giving the aircraft greater pitch control and aerodynamic braking capability. The aircraft, now sporting the "N" before the F-15 designation to reflect its status as a highly modified research aircraft, was used in the S/MTD program from 1988 until 1993. It was during this period that the aircraft was given its distinctive red, white, and blue paint scheme, which made it one of the most recognizable test aircraft ever to fly in the skies above Edwards.
Advanced Control Technology for Integrated Vehicles
When the Air Force completed its S/MTD program with the aircraft, it was transferred on loan to NASA Dryden for NASA's Advanced Control Technology for Integrated Vehicles or "ACTIVE" research project in 1994. ACTIVE was a multi-year flight research effort to enhance the performance and maneuverability of future civil and military aircraft. For this project, advanced digital flight control systems and omni-directional thrust vectoring of engine exhaust were integrated into the highly-modified F-15 research aircraft. It was also given NASA tail number 837 at this time.
NASA's NF-15B research aircraft is shown during an early yaw-vectoring mission in the ACTIVE flight research project. NASA Photo The project was a collaborative effort by NASA, the Air Force Research Laboratory, Pratt & Whitney, and Boeing (formerly McDonnell Douglas) Phantom Works. The ACTIVE project supported the Revolutionary Technology Leaps pillar of NASA's former Aeronautics and Space Transportation Technology Enterprise. The effort was intended to revolutionize the way in which aircraft are designed and built by providing the design tools to increase design confidence and cut design time for next-generation aircraft in half.
After being loaned to NASA for the ACTIVE project, the twin-engine F-15 was equipped with a powerful research flight control computer, higher-thrust versions of the Pratt & Whitney F-100 engine and newly developed axisymmetric thrust-vectoring engine exhaust nozzles that were capable of redirecting the engine exhaust in any direction, not just in the pitch (up and down) axis.
The new nozzles, also developed by Pratt & Whitney, could deflect engine exhaust up to 20 degrees off center line, giving the aircraft thrust control in pitch and yaw (left and right), or any combination of the two axes. This research sought to discover if this vectored thrust could be used to reduce drag and increase fuel economy or range compared with conventional aerodynamic controls. The nozzles were a production design that could be incorporated into other aircraft.
In addition, an integrated system to control its aerodynamic control surfaces and its engines was installed in the NF-15B along with cockpit controls and electronics from later-model F-15E aircraft.
Several flight research milestones were achieved during the ACTIVE project. The first supersonic yaw-vectoring flight was flown in early 1996, and pitch and yaw thrust vectoring at speeds up to Mach 2, twice the speed of sound, was evaluated during several flights late that year. On subsequent flights, Dryden research pilots flew the unique research aircraft at angles of attack up to 30 degrees while employing yaw vectoring.
An adaptive performance software program was developed and successfully tested. The performance-optimization program installed in the aircraft's flight control computer automatically determined the optimal setting or trim for the thrust-vectoring nozzles and aerodynamic controls to minimize aircraft drag. On the last flight of 1996, the F-15 ACTIVE demonstrated the software's effectiveness by gaining a speed increase of Mach 0.1 with no increase in engine power while in level flight at 30,000 ft altitude and a speed of approximately Mach 1.3.
The NF-15B continued to expand the limits of its thrust-vectoring capabilities during 1997 and 1998, including an experiment that combined thrust vectoring with its regular aerodynamic controls to improve the performance of the F-15E tactical fighter on ground attack missions.
Test Bed Experiments
The NF-15B aircraft's unique propulsion control systems and flight test instrumentation allowed it to be used as a test bed for several research experiments following the ACTIVE project. Each experiment contributed to NASA's aeronautics program goals.
High Stability Engine Control (HISTEC)
This experiment, developed and managed by NASA's Lewis (now Glenn) Research Center, evaluated a computerized system that could sense and respond to high levels of engine inlet airflow turbulence to prevent sudden in-flight engine compressor stalls and potential engine failures. The system used a high-speed processor to evaluate the airflow data coming from sensors on the left engine, and it in turn directed the aircraft's engine control computer to automatically command engine trim changes to accommodate for changing turbulence levels. The system could enhance engine stability when the inlet airflow is turbulent, and increased engine performance when the airflow is stable or smooth. Approximately a dozen flights were flown in the summer of 1997 to validate the HISTEC concept.
High-Speed Research Acoustics
The unique ability of the thrust-vectoring nozzles to change the area ratio, or the difference in the geometric area between the nozzles' throat and exit, led to the NF-15B being used for research in the fall of 1997 for engine noise reduction tests. Conducted on behalf of Langley Research Center's High-Speed Research program, this flight experiment focused on validating noise prediction data that could be applied to reducing noise generated during takeoffs and landings of the High-Speed Civil Transport, a proposed second-generation American supersonic jetliner. By fully expanding the nozzles' exit area, noise generated by the hot jet exhaust entering the surrounding cooler air was reduced. The acoustics research involved flying the NF-15B in precise patterns over an array of 30 microphones spread out over more than a mile along the northeast side of Rogers Dry Lake on Edwards Air Force Base.
Graphic depicting use of propulsion controls in place of aerodynamic controls. NASA Illustration Intelligent Flight Control System (IFCS)
Following the acoustics research performed on the NF-15B, the aircraft became the Intelligent Flight Control System, or IFCS, project's primary research aircraft in 1999. This project was established by NASA to exploit a potentially revolutionary technological breakthrough in aircraft flight controls that could efficiently optimize aircraft performance in both normal and failure conditions. The goal of the project was to continue developing adaptive and fault-tolerant flight control systems leading to unprecedented levels of safety and survivability for both civil and military aircraft.
The IFCS team integrated innovative neural network technologies with state-of-the-art control algorithms to correctly identify and respond to changes in aircraft stability and control characteristics, and immediately adjust to maintain the best possible flight performance during an unexpected failure. The adaptive neural network software "learns" the new flight characteristics, onboard and in real-time, thereby helping the pilot to maintain or regain control and prevent a potentially catastrophic aircraft accident.
The final research project conducted with the NF-15B research aircraft was the Lift and Nozzle Change Effects on Tail Shock, or LaNCETS, project. The goal of the project was to develop and validate computational prediction tools to be used in the design of civilian supersonic aircraft that could fly overland without generating unacceptable sonic booms.
The flight portion of the LaNCETS project consisted of measuring the aft-shockwave structure of the NF-15B test aircraft using another instrumented NASA F-15 as a probing aircraft. The aft-shockwave included those shockwaves emanating from the N F-15B's tail surfaces, exhaust plume, and the aircraft's wake.
After an illustrious career as a test and research aircraft with the McDonnell Douglas Co., the U.S. Air Force, and NASA, NF-15B No. 837 was retired after a final mission on Jan. 30, 2009 at NASA Dryden. During the course of its 14 years at NASA Dryden, it had flown 251 missions, adding to the total it had amassed over its entire 35-year history as a flight test aircraft. Plans are under way to place the unique NF-15B with a group of other retired research aircraft that are on permanent public display outside the Center.
- Designation: NF-15B, originally TF-15A
- Manufacturer: McDonnell Douglas, 1972-73
- USAF Registration: 71-0290
- NASA registration: tail number 837
- NASA role: Integrated controls/propulsion research/research testbed
- Maximum altitude: 60,000 ft
- Max. speed: Mach 2.0
- Engines: Two Pratt & Whitney F100-PW-229
- Max. thrust: 29,000 each in full afterburner
- Weight: 47,000 lb takeoff; 35,000 lb empty
- Wingspan: 42.8 ft
- Length: 63.7 ft, excluding flight test nose boom
- Horizontal tail span: 28.2 ft
- Canard span: 25.6 ft