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F-16E/F/XL Fighting Falcon

The F-16XL aircraft were built by General Dynamics as prototypes for a derivative fighter evaluation program conducted by the Air Force between 1982 and 1985. The aircraft were developed from basic F-16 airframes, with the most notable difference is the delta (cranked arrow) wing which give the aircraft a greater range because of increased fuel capacity in the wing tanks, and a larger load capability due to increased wing area. The F-16XL was able to take off and land in two thirds of the distance required by the F-16A. It was capable of speeds of 90 knots greater than the F-16A at sea level and had a 125% greater range than an F-16A with the same payload.

In the mid-1970's the U.S. Air Force became interested in a fighter aircraft capable of "supercruise"-the ability to cruise supersonically without an afterburner while retaining respectable maneuver, takeoff, and landing characteristics. The supercruise requirement drove aircraft configurations to highly swept wing platforms. LMTAS appreciated the fact that the modular construction of the YF-16 allowed for relatively simple replacement of the outer wing panels and that a supercruiser demonstrator aircraft with a highly swept wing would undoubtedly attract considerable interest within the Air Force. In 1977 NASA Langley and LMTAS agreed to a cooperative study to design a new cranked-arrow wing for the F-16 to permit supersonic cruise capability.

By the early 1980s the day of the classical dogfight was almost over, since the first aircraft to acquire its opponent would be first to fire and most likely to win the engagement. A new philosophy for air combat tactics was thus developed by the USAF, who envisaged long range medium to high altitude penetration of hostile airspace by supersonic cruise capable fighters with all aspect fire and forget missile armament. A key element in the new strategy was the AIM-120 Amraam missile. The first aircraft to embody this new approach was the ill-fated F-16XL.

In February 1980 General Dynamics proposed the Supersonic Cruise and Maneuvering Program (SCAMP). The final configuration became known as the F-16XL (later designated the F-16E), which displayed an excellent combination of reduced supersonic wave drag, utilization of vortex lift for transonic and low-speed maneuvers, low structural weight, and good transonic performance. In March 1981 the US Air Force announced an effort to develope a new multi-role strike strike fighter. General Dynamics entered the F-16XL in the competition, with McDonnell Douglas submitting an adaptation of the two-seat F-15B Eagle [which eventually entered production as the F-15E Strike Eagle]. Had the F-16XL won the competition, production aircraft would have been designated F-16E (single-seat) and F-16F (two-seat).

A radical redesign of the F-16A, the XL was a supersonic cruise demonstrator with a cranked arrow delta wing optimised for that flight regime. The aircraft was a major technical success, with two demonstrators eventually flying. Although supersonic cruise without afterburner was an original goal of the F-16XL program, the aircraft did never achieved this feat. The highly swept inboard wing section of this aircraft produced substantial vortex lift at supersonic speeds, while also improving instantaneous turn rate and extending the 9G manoeuvre envelope well above Mach 1. An additional benefit of the new configuration was a substantial increase in internal fuel capacity, providing a 120% improvement in combat radius performance.

The single-seat F-16XL aircraft is powered by a Pratt and Whitney 100-PW-100 engine (with afterburner), rated at 23,830 pounds thrust, and features an analog fly-by-wire electronic flight control system. The delta (cranked arrow) wings on both aircraft provide strength for high wing loads during flight. The aircraft's dimensions are; length, 54.2 feet (16.52 m); wingspan, 34.3 feet (10.45 m); height at vertical tail, 17.7 feet (5.39 m). The aircraft's maximum weight is 48,000 pounds (17915.60 kg), has a design load of 9 "Gs" (In the research configuration, 3 "Gs"), and has a top design speed Mach 1.8.

In the mid-1970's the U.S. Air Force became interested in a fighter aircraft capable of "supercruise"-the ability to cruise supersonically without an afterburner while retaining respectable maneuver, takeoff, and landing characteristics. The supercruise requirement drove aircraft configurations to highly swept wing platforms. LMTAS appreciated the fact that the modular construction of the YF-16 allowed for relatively simple replacement of the outer wing panels and that a supercruiser demonstrator aircraft with a highly swept wing would undoubtedly attract considerable interest within the Air Force.

NASA Langley staff had developed a research program known as the Supersonic Cruise Integrated Fighter (SCIF) Program under the leadership of Roy V. Harris, Jr. As participants in previous national and NASA civil supersonic transport programs (SST), the Langley staff were leaders in the development of databases and design methods for efficient SST configurations. Several in-house supercruiser fighters were designed and tested across the speed ranges at Langley. Subsequent to the SCIF program, Langley joined several industry partners in cooperative, nonproprietary studies of supercruiser configurations.

In 1977 Langley and LMTAS agreed to a cooperative study to design a new cranked-arrow wing for the F-16 to permit supersonic cruise capability. Personnel from LMTAS worked alongside the NASA researchers under the direction of Charles M. Jackson at Langley during the studies. The project leader for supersonic design was David S. Miller. The results of the wind-tunnel and analytical studies indicated that a viable wing could be designed to satisfy the supersonic and transonic requirements. With these results, LMTAS initiated a company funded development of an F-16 derivative with supersonic cruise capability. Following the spirit of the previous wing design cooperative venture with NASA, a cooperative agreement was signed for mutual efforts on the new demonstrator, which was called the Supersonic Cruise and Maneuver Prototype (SCAMP).

Extensive tests for SCAMP took place in Langley facilities, including the Unitary Plan Wind Tunnel, the 7- by 10-Foot High-Speed Tunnel, the 16-Foot Transonic Dynamics Tunnel, the Full-Scale Tunnel, the DMS, the Spin Tunnel, and a helicopter drop model. During these tests, a team led by researcher Joseph L. Johnson, Jr. identified low-speed stability and control issues that required modifying the wing apex with a rounded planform. Research on the SCAMP configuration by Langley researchers identified numerous advanced concepts for improved performance, including the application of vortex flaps on the highly swept leading edge for improved low-speed and transonic performance, automatic spin prevention concepts, and optimized wings for supersonic cruise. The final configuration became known as the F-16XL (later designated the F-16E), which displayed an excellent combination of reduced supersonic wave drag, utilization of vortex lift for transonic and low-speed maneuvers, low structural weight, and good transonic performance. The F-16XL flutter envelope was cleared in the 16-Foot Transonic Dynamics Tunnel by Charles L. Ruhlin without significant problems.

Two (a one-seat and a two-seat) F-16XL demonstrator aircraft were subsequently built and entered flight tests in mid-1982. In recognition of Langley's many contributions to the F-16XL, LMTAS management sent letters of recognition to Langley and senior NASA management. Marilyn E. Ogburn of Johnson's group was an invited participant at flight-test evaluations of the F-16XL at Edwards Air Force Base. The results of flight tests validated the accuracy of Langley wing design procedures, wind-tunnel predictions, and control system designs based on DMS tests. Unfortunately, the interest in supersonic cruise was replaced by an urgency to develop a dual role fighter with ground strike capability.

The F-16XL suffered the fate of many pioneering aircraft before their time. The F-16E dual role lost out in a flyoff against MDC's bigger and more capable F-15E Strike Eagle, thus ending all prospects for its eventual production. Although the relatively large wing of the F-16XL carried a significant amount of weapons, the Air Force ultimately selected the F-15E in 1983 for developmental funding and terminated interest in the F-16XL. Many observers attributed its demise to a political strategy played by the USAF, to prevent an older generation airframe derivative from being used by legislators as an excuse to kill off or postpone the ATF program. Equipped with Amraam, higher thrust engines and new radar, the F-16XL could cover a large part of the role envisaged for the ATF at substantially lower unit and program costs. As an older generation airframe however its infrared and radar signatures are substantial and this would greatly reduce its effectiveness

NASA's single-seat F-16XL (ship #1), tail number 849, is stationed at Dryden Flight Research Center, Edwards, California. It arrived at Dryden on March 10, 1989, from General Dynamics in Fort Worth, TX. The aircraft was most recently used in the Cranked-Arrow Wing Aerodynamics Project (CAWAP) to test boundary layer pressures and distribution. The modified airplane featured a delta "cranked-arrow" wing with strips of tubing along the leading edge to the trailing edge to sense static on the wing and obtain pressure distribution data. The right wing received data on pressure distribution and the left wing had three types of instrumentation - preston tubes to measure local skin friction, boundary layer rakes to measure boundary layer profiles (the layer where the air interacts with the surfaces of a moving aircraft), and hot films to determine boundary layer transition locations. The first flight of CAWAP occurred on November 21, 1995, and the test program ended in April 1996.

The NASA Dryden two-seat F-16XL Ship #2 aircraft was used by the Dryden Flight Research Center, Edwards, California, in a NASA-wide program to improve laminar airflow on aircraft flying at sustained supersonic speeds. It is the first program to look at laminar flow on swept wings at speeds representative of those at which a High Speed Civil Transport may fly. Technological data from the program will be available for the development of future high speed aircraft, including commercial transports.



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