F-16E/F/XL Strike 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. It was not possible to convert the F-16 into an aviation platform for long-range strikes deep behind enemy lines due to lack of funds and changing command priorities — the project was canceled at the final stage. This page in the history of the "War Falcon" was an example of how a technical breakthrough can be stopped not by a lack of opportunities, but by changing political and military priorities. The aircraft were developed from basic F-16 airframes, with the most notable difference was 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. Following the success of the basic multi-role F-16 fighter, General Dynamics engineers led by Harry Hilleker began the SCAMP (Supersonic Cruise and Maneuver Prototype) project in 1977 with the aim of increasing the range and strike capabilities of the Fighting Falcon without losing the aircraft's maneuverability. The result of the project was the F-16XL prototype with an elongated fuselage and a completely new wing of a special shape. Its features were to preserve the maximum elements of the basic F-16 while radically increasing its combat potential to transform the light single-engine fighter into a multi-role combat aircraft with a range and combat load on the level of heavier machines.
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. The Enhanced Tactical Fighter (ETF) to replace the F-111 bombers for so-called deep strike missions - strikes deep behind enemy lines without fighter escort or jammers. General Dynamics entered the F-16XL, codenamed "Strike Falcon", 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). The first flight of the F-16XL-1 took place on July 3, 1982. The new product demonstrated such impressive capabilities that it turned from a conceptual demonstrator into a real contender in the ETF program, becoming a direct competitor to the twin-engine F-15E Strike Eagle. The test program for the two F-16XLs continued throughout 1982–1983, after which they were submitted to a face-to-face competition against a modified two-seat F-15 (the future F-15E) as part of the Enhanced Tactical Fighter. The concepts differed significantly: the F-15E was an evolutionary modification of an existing fighter, while the F-16XL required more extensive design changes and systems refinement. This meant greater initial investment and risk for launching the Strike Falcon into production. On the other hand, using the ready-made F-15 platform gave the Air Force the opportunity to preserve the Eagle production line and ensure unification with the existing F-15 fleet. Despite obvious advantages, the fate of the F-16XL depended on comparison with its competitor, the twin-engine F-15E Strike Eagle. The F-16XL Strike Falcon was inferior to its rival in maximum speed (Mach 2.0 versus Mach 2.5 for the F-15E) and altitude (ceiling about 15 km versus about 18 km). However, by other criteria, the F-16XL had advantages: it carried more outboard weapons (up to 27 missiles and bombs of various calibers on 17 suspension nodes versus 15 nodes in the F-15E), was characterized by better efficiency (one engine required less fuel and maintenance) and a smaller radar silhouette. In contrast, the twin-engine F-15E had an advantage in survivability over enemy territory, as if one engine was damaged it could continue flying on the other. Whereas damage to the F-16XL's single engine was a critical survival factor. On the other hand, for extreme range strike missions, where every kilogram of fuel counts, the advantage was on the side of the lighter F-16XL. Some experts later noted that the ideal solution would be to adopt both platforms to complement each other's capabilities. In February 1984, after a detailed analysis, the US Air Force announced the winner of the competition project, the McDonnell Douglas design - the future F-15E Strike Eagle. The main reasons were two circumstances: engine configuration and price. The F-15E, with its twin engines, had greater payload and reliability, which was considered critical for long-range strikes, and its development was cheaper due to minimal changes compared to the base model. The F-15E Strike Eagle became the backbone of US tactical aviation for several decades. The F-16XL remained an interesting prototype with no serial future. After losing the Enhanced Tactical Fighter competition, both F-16XLs were placed in storage. They were retired at Edwards Air Force Base in the summer of 1985, having flown a total of 798 test flights.
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
F-16E/F/XL Strike Falcon - Design
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. 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. Due to its long range and versatility, the F-16XL was considered not only by the US Air Force. Japan was also interested in it as a replacement for its strike aircraft, and the US Navy was studying its suitability for coastal anti-ship defense. 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 was 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 was 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. The new F-16 variant had a 1.4 m (56 in) longer fuselage and a larger wing area, allowing for significantly more fuel and weapons points. This allowed the F-16XL to carry approximately twice as much ammunition as the standard F-16A and deliver it 40–50% further. The main technical breakthrough of the F-16XL project was a new wing with increased area and a special shape, known as the "cranked-arrow wing". This was a delta-shaped wing with a characteristic "crank" along the leading edge. Its inner fuselage section has a moderate sweep with an angle of about 50°, while the outer section was swept at an angle of about 70°. The inner wing section creates the main lift, improves takeoff and landing performance, and houses a significant portion of the internal fuel tanks. The outer section was optimized to reduce drag at supersonic speeds and stabilize the aircraft during maneuvers. This design retained the advantages of the delta wing at high speeds, but eliminated its typical disadvantages at low speeds. The less swept part of the wing provided good stability and controllability at landing speeds. The designers actually turned the F-16 into a tailless aircraft: the standard horizontal tailplanes were removed, and the role of elevators and ailerons was performed by four sections of movable flaps along the trailing edge of the large wing. This scheme retained the advantages of the delta wing at high speeds, but eliminated its typical disadvantages at low speeds - the less swept rear part of the wing provided good stability and controllability at landing speeds. Thanks to the carefully calculated aerodynamic profile, drag was reduced, not increased, which gave an approximately 25% gain in aerodynamic efficiency in supersonic flight without compromising subsonic flight characteristics. Pilots noted the XL's surprisingly smooth flight at high speeds and low altitudes, where a conventional F-16 would experience turbulence and shaking. The wing area has increased to 58.8 m², more than double the standard F-16, which has a wing area of about 28 m². This has allowed the aircraft to dramatically increase its range and combat load. The new wing was partly made of composites, which has saved about 270 kg of weight. Due to internal cavities in the wing and inserts in the fuselage, the volume of fuel tanks increased by 65–80% - in fact, the F-16XL carried almost as much fuel as twin-engine fighters, without needing outboard tanks for most missions. The wing area also allowed for optimal placement of a huge arsenal: up to 17 suspension nodes for bombs and missiles. The total combat load of the F-16XL reached about 6.8 tons (15 thousand pounds) - twice as much as that of the standard F-16. The armed F-16XL could maintain high speed: it was capable of performing a supersonic dive even with a full bomb load at high and low altitudes. The 500-pound (227 kg) bombs were hung in rows of 3–4 and partially “sunk” into the streamlined pylons, which minimized additional drag. In addition, the new wing fuselage provided a smaller effective radar area and greater angles of permissible overloads even with suspended bombs. In essence, engineers created a “wing of the future,” which turned a light fighter into a compact “flying truck” without losing flight characteristics. Importantly, the F-16XL retained all the positive features of the regular Fighting Falcon. It had a high thrust reserve, excellent maneuverability and modern avionics. Even with bombs hanging, the aircraft could perform sharp maneuvers with overloads up to 9g, almost like an empty F-16A. The F-16XL could also carry a full complement of air-to-air guided weapons for self-defense (4 AIM-120 AMRAAM missiles and 2 AIM-9 Sidewinder missiles) along with its bomb load. This meant that if intercepted, it was not an easy target, but could hold its own in a dogfight. General Dynamics engineers also envisioned a two-seat configuration for the F-16XL. Having a weapons operator alongside the pilot was valuable for complex strike missions, similar to how a navigator/weapons officer assists on the F-111 or F-15E. It was also planned to equip the Strike Falcon with a LANTIRN targeting and navigation pod for effective strikes at night and in bad weather. The suspension units under the air intake were adapted for it. Thus, the "Strike Falcon" promised to become a multi-role aircraft that would combine the roles of an air superiority fighter, a tactical bomber, and even a long-range interceptor. The lessons learned from the F-16XL were not in vain: a number of engineering solutions were used in subsequent versions of the F-16. For example, the enlarged air intake, tested on the F-16XL-2 prototype for the new engine, later became standard on the later series of F-16C/D. This was the so-called "Big Mouth" for GE engines. The task of increasing the Fighting Falcon's range was solved in other ways - by installing conformal fuel tanks and improved engines in export versions. Thus, the F-16E/F Block 60 Desert Falcon received integrated expansion fuel tanks on the upper fuselage, which gave it a range almost as good as the F-16XL, albeit at the cost of increased mass. The Israeli modification of the F-16I Sufa was also equipped with additional fuel tanks and performs long-range raids. These machines were involved in strikes on distant targets in the Middle East. The media occasionally mentions the F-16XL in the context of an unused chance for the United States to have its own Strike Falcon in service alongside the Strike Eagle. Today, as the F-15E fleet gradually ages, the story of the Super Falcon reminds us of an alternative path for tactical aviation. The F-16XL was ahead of its time and proved that a proven platform could be evolved to produce revolutionary results. However, the change in strategic priorities proved fatal for the Strike Falcon, leaving it an experiment that was ahead of its time but didn't fit in.
F-16E/F/XL Strike Falcon - NASA
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. Although the F-16XL did not enter service, it played a significant role in aeronautical science and indirectly influenced subsequent aircraft development. In 1988, both prototypes were transferred to NASA for use in research programs. Having received new NASA tail numbers 849 and 848, the aircraft were involved in experiments with Supersonic Laminar Flow Control (SLFC) - the control of laminar flow at supersonic speeds. The aerospace agency considered the F-16XL an ideal carrier for such research because of its large wingspan and ability to fly for long periods at high speeds and altitudes.
NASA's single-seat F-16XL (ship #1), tail number 849, was 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 was 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. After completion of research in 1996, both F-16XLs were sent to the Dryden Flight Research Center at Edwards AFB for storage. They were finally retired in 2009. One example was later transferred to the Edwards AFB museum.
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