The SCAMP design team in late 1977, starting left: Harry Hillaker, Andrew Lewis, Kenny Barnes, Jim Gordon
Shortly after winning the lightweight fighter program, General Dynamics Fort Worth began investigating possible F-16 derivatives with the goal of enhancing both air-to-air and air-to-ground mission capabilities while retaining parts commonality with the F-16A.[1] Under the leadership of Harry Hillaker (designer of the original F-16), the Supersonic Cruise and Maneuver Prototype (SCAMP) project was started. Several wing designs were considered, including one using a forward-swept wing, but the large "cranked-arrow" wing (similar to that of the Saab 35 Draken)[note 1] was pursued due to its much more efficient lift-to-drag ratio at supersonic speeds.[2]
The company worked closely with NASA's Langley Research Center[3] and invested significant R&D funds for wind tunnel testing. Over several years the design was refined which led to the final F-16XL design by late 1980.[4]
Enhanced Tactical Fighter competition
F-16XL and a conventional F-16
In 1980, the USAF signed on as a partner,[5] providing the fuselages of the third[note 2] and fifth[note 3] production F-16s for conversion. These two fuselages became the only examples of the F-16XL.[6]
In March 1981, the USAF announced the Enhanced Tactical Fighter (ETF) program to procure a replacement for the F-111 Aardvark.[7] The concept envisioned an aircraft capable of launching deep interdiction missions without requiring additional support in the form of fighter escorts or jamming support. General Dynamics submitted the F-16XL, while McDonnell Douglas submitted a variant of the F-15 Eagle. Though the two aircraft were competing for the same role, they had fairly different design approaches. The F-15E required very few alterations from its base F-15B or D, while the F-16XL had major structural and aerodynamic differences from the original F-16.[8] As such, the F-16XL would have required much more effort, time, and money to put into full production.[9] Additionally, the F-15E had two engines, which gave it a much higher maximum takeoff weight and redundancy in the case of engine failure.[9][note 4]
In February 1984, the USAF awarded the ETF contract to McDonnell Douglas.[10][11][12] The two F-16XLs were returned to the Air Force and placed in storage at Edwards Air Force Base.[13] Had General Dynamics won the competition, the F-16XL would have gone into production as the F-16E/F (E for single seat, F for two seats).[14]
Design
An air-to-air left underside view of an F-16XL aircraft. The aircraft is armed with two wingtip-mounted AIM-9 Sidewinder and four fuselage-mounted AIM-120 AMRAAM missiles along with 12 Mark 82 500-pound bombs.
The wing and rear horizontal control surfaces of the base F-16A were replaced with a cranked-arrow delta wing 115% larger than the original wing.[15] Extensive use of graphite-bismaleimide composites allowed the savings of 595 pounds (270 kg) of weight,[16] but the F-16XL-1 and XL-2 were 4,100 pounds (1,900 kg) and 5,600 pounds (2,500 kg) heavier respectively than the original F-16A.[17][note 5]
Less noticeable is that the fuselage was lengthened by 56 inches (140 cm) by the addition of two sections at the joints of the main fuselage sub-assemblies.[15] With the new wing design, the tail section had to be canted up 3.16°,[18] and the ventral fins removed, to prevent them from striking the pavement during takeoff and landing.[19] The F-16XL-2 also received a larger inlet which would go on to be included in later F-16 variants.[20]
These changes resulted in a 25% improvement in lift-to-drag ratio in supersonic flight[21] while remaining comparable in subsonic flight,[22] and a plane that reportedly handled smoothly at high speeds and low altitudes.[23] The enlargements increased internal fuel capacity by 4,350 pounds (1,970 kg), or about 65%.[15][note 6] The F-16XL could carry twice the ordnance of the F-16A and deliver it 50% farther.[26] The enlarged wing and strengthened hardpoints allowed for a highly configurable payload:[27]
NASA F-16XL #2 conducting laminar flow researchPassive and active aerodynamic gloves were used during NASA testingNASA F-16X: #1flying alongside an NASA SR-71A
In 1988, the two aircraft were turned over to NASA Ames-Dryden Flight Research Facility for supersonic laminar flow research for the High Speed Civil Transport (HSCT) program.[28] The F-16XL was considered ideal for these tests because of its cranked-arrow wing and high-speed, high-altitude capabilities.[29] The tests were carried out by a NASA and industry team[note 10] and were intended to achieve laminar flow over the wings, validate computational fluid dynamics (CFD) design methodology, and test active suction systems.[30] These tests involved the installation of either passive or active suction aerodynamic gloves. The active suction glove was intended to suck away turbulent airflow over the wings during supersonic flight, restoring laminar flow and reducing drag.[31][32][33] The NASA Langley Research Center developed and coordinated F-16XL experiments.[34]
F-16XL-1 was fitted with an active suction glove encasing the left wing.[35] Designed and built by North American Aviation, it had laser-cut holes that were nominally 0.0025 inches (0.064 mm) diameter at a uniform 2,500 per square inch (390/cm2) spacing.[35] The suction was provided by a Convair 880 air-conditioning turbocompressor where the 20mm cannon's ammunition had been.[31][35] The glove covered over 5 square feet (0.46 m2) of the wing. Overall, F-16XL-1 completed 31 test flights for these tests from May 1990 to September 1992.[32] Afterwards, it was used to test takeoff performance, engine noise, and sonic boom phenomena.[36]
F-16XL-2 had its engine replaced with the more powerful General Electric F110-129.[12][37] It achieved limited supercruise, a design goal of the F-16XL that was never attained in ETF testing, when it reached Mach 1.1 at 20,000 feet (6,100 m) on full military power.[38] It was mounted with a passive glove on the right wing and an active suction glove on the left wing.[32] The passive glove was fitted with instruments to measure the flow characteristics over the wing.[39] The active suction glove was designed and fabricated by Boeing; it was made of titanium and had over 12 million laser-cut holes, each 0.0025 inches (0.064 mm) in diameter, spaced 0.010 to 0.055 inches (0.025 to 0.140 cm) apart.[40][31][41] Suction was provided by a cabin-air pressurization turbocompressor from a Boeing 707, installed where the 20mm ammunition drum had been, which exhausted above the right wing.[42][32][33] Overall, F-16XL-2 performed 45 test flights from October 1995 to November 1996.[43][31]
While "significant progress" was made towards achieving laminar flow at supersonic speeds, neither aircraft achieved the requisite laminar flow characteristics at intended speeds and altitudes.[44][45][46] Nonetheless, NASA officials considered the test program to have been successful.[32] NASA briefly investigated using a Tupolev Tu-144 which would more closely resemble the high-speed civil transport aircraft to continue supersonic laminar flow research, but did not pursue the idea due to a limited budget.[47]
At the conclusion of their test programs in 1999, both F-16XLs were placed into storage at NASA Dryden.[12] In 2007, Boeing and NASA studied the feasibility of returning F-16XL-1 to flight status and upgrading it with many of the improvements found in the USAF's F-16 Block 40 in order to further test sonic boom mitigation technology.[48]F-16XL-1 was taxi tested at Dryden and given systems checks.[48] However, both F-16XLs were retired in 2009 and stored at Edwards AFB.[49]
Hardpoints: 17 pylons with a capacity of up to 15,000 pounds (6,800 kg) of payload (Note: stations 2–5 and 13–16 were split into groups, similar to the F-15E)
^Piccirillo 2014, p. 7: "These were oriented to extending range and payload, expanding basic missions, and developing advanced versions or derivative configurations of the aircraft. Importantly, these were intended to enhance both air-to-air and air-to-ground capabilities while retaining the maximum possible commonality with the basic F-16 design."
^Piccirillo 2014, p. 159: "...the F-16E required major changes to the basic F-16 airframe. ... Changes required for the F-15E were not considered by the GAO to be as great as those needed for the F-16E, and mainly consisted of structural modifications to the wings as well as a strengthened landing gear."
^Piccirillo 2014, p. 116: "As speed approached Mach 1.0, the F-16XL's comparative cruise efficiency improved, and at Mach 1.4, the F-16XL had a 25-percent-higher lift-to-drag ratio than that of the F-16C."
^Piccirillo 2014, p. 9: "...the L/D ratios of the cranked-arrow, canard-delta, and baseline F-16 were essentially equal at subsonic speeds..."
^Piccirillo 2014, p. 202: "F-16XL-2 was also able to demonstrate limited supercruise performance by maintaining Mach 1.1 at an altitude of 20,000 feet in full military power without resorting to the use of afterburner."
^Just under 11,300 pounds (5,100 kg),[24][25] up from the F-16A's 6,950 pounds (3,150 kg)[15]
^Dummy AIM-120s, fabricated from wood & sheet metal, were scabbed onto the undersurfaces of the F-16XL flight demonstrators because the AIM-120 missile had yet to be integrated onto the standard F-16; incorporation of the semisubmerged missile housing with its associated ejector launcher would have required a separate development and integration effort.[27]
^Mach 2.0 was only achieved during the supersonic laminar flow tests from around 1990–1992;[55] maximum speed prior had been limited to Mach 1.95, though faster speeds were likely possible.[56]
^Ammunition bay was removed in 1991–1992 and replaced with a turbocompressor to provide suction for the aerodynamic glove tests[31]
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