F-16XL – The Crank Wing Experiment
The F-16XL is a derivative of the iconic F-16, and was superior in many aspects to its predecessor despite being lesser known. Crafted with a unique cranked arrow delta wing, this unusually long prototype was originally commissioned to explore the possibilities of supersonic cruise speed, and would be later converted into a fighter and then a test-bed for the US Air Force and NASA respectively.
In February 1980, General Dynamics proposed to USAF that a modified F-16 fighter with a radically different wing configuration could be the key to achieving supersonic cruise speed for the first time.
As a result the Supersonic Cruise and Maneuvering Program, abbreviated as ‘SCAMP,’ was commenced, the aim being to examine whether a cranked-arrow-shaped delta wing, which was to have more than twice the surface area of an F-16 wing at 633 square feet, could provide low drag at supersonic velocities without limiting manoeuvrability at low speeds.
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In addition, another objective was to experiment with different wing planform and camber designs to see whether they improved or degraded performance and agility at the supersonic level. Following the signing of an agreement with the USAF in late 1980, General Dynamic procured two FSD F-16As while the Air Force provided another 2 for modification.
The F-16XL was 54 feet and 2 inches long, 17 feet and 7 inches high, and had an empty weight of 22,000 pounds and a maximum take-off weight of 48,000 pounds. There were two editions, namely the single-seat F-16XL-1 and the twin-seat F-16XL-2.
The F-16XL had a longer fuselage than the F-16 at 54 feet and 1.86 inches and was lengthened by the attachment of two new fuselage sections, with a 26 inch part added to the rear and another 30 inches extension welded at the front, enabling the tail to be tilted up 3 degrees so that the engine nozzle didn’t hit the runway during take-off and landing.
With a wingspan of 34 feet and an enormous wing area of 633 square feet, the cranked-arrow delta wing, made from carbon composite material to reduce its weight by 272 kilograms, was 120% larger than that of an F-16 and possessed swept angles ranging from 50 degrees to 70 degrees.
By enlarging the fuselage and the wing the F-16XL had 82% more fuel capacity for its single Pratt & Whitney F100-PW-200 turbofan engine or afterburning General Electric F110-GE-129 turbofan, the former providing a maximum velocity of 1,260 mph or Mach 1.8, while the latter ensured a top speed of 1,400 miles or Mach 2. Elsewhere, the F-16XL had a maximum range of 2850 miles and an altitude ceiling of 50,000 feet.
Later considered as a combat aircraft by USAF, the F-16XL could be armed with a 20-mm M61A1 cannon and an AIM-9 Sidewinder infrared-homing air-to-air missile at each wingtip, while possessing an external ordnance load of up to 15,000 pounds that could be carried by 17 external hard points.
Surprisingly, the ‘XL’ designation did not stand for ‘extra large’ and was instead coined by the inventor of the F-16 Harry Hillaker during a round of golf. Whilst discussing what he should call the new project with a friend, Hillaker looked down to see he was using a brand of gold balls named Top Flite XL, promptly deciding that XL was the perfect moniker for a craft that was intended to fly further than the original F-16.
Following an extended developmental period which witnessed over 3,600 hours of wind tunnel testing, the first model to fly was the single-seat F-16XL-1 installed with the F100-PW-200 turbofan, taking to the skies with pilot James McKinney at the controls on July 3rd 1982 without incident.
The other version, namely the F-16XL-2, was propelled by the 110-GE-129 engine and made its maiden voyage on October 29th 1982 with Alex Wolf and Jim McKinney at the helm.
This second prototype had been converted from the remains of the third F-16XL, which in August 1980 at Edwards Air Force Base was severely damaged after crashing into the lake bed because of a malfunction with the nose tire, which burst during take-off and snapped off when it hit the ground, with the equipment bay and radar nose also wrecked in the process.
Arriving at Fort Worth, the front end of the downed plane was completely mangled and had to be entirely reconstructed as it was prepared for the F-16XL program.
Advanced Tactical Fighter Program
In March 1981 USAF announced they were looking for an advanced tactic fighter to replace the aging F-111. For the competition, General Dynamics entered both F-16XLs, which were to compete against a modified version of McDonnell Douglas’ F-15B Eagle.
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With the F-16XL able to carry double the payload of the F-16 and travel 40% further because of its increased fuel capacity, General Dynamics believed it had great potential as a fighter aircraft. If they successfully passed the test, both model were to be renamed by USAF as the F-16E and the F-16F.
Despite illustrating that the F-16XL had twice the range and payload of the F-16 and that it was more stable than its predecessor, in February 1984 USAF revealed that the winner was the F-15B Eagle, later more widely known as the F-15E Strike Eagle which, fitted with two engines was favored for its superior thrust and payload qualities as well as its lower production cost.
Moreover, in contrast to the single-engined F-16XL, the F-15B was preferred because it had a higher rate of survivability in a combat situation, owing to the fact that if one of its engines was neutralized it could still fly away from danger with its other one.
Consequently, both F-16XLs were placed into storage at Fort Worth during the summer of 1985 after making a combined total of 798 flights.
John G. Williams, the chief engineer of the F-16XL, would later opine that USAF’s decision to choose the F-15B Eagle was largely political than practical:
“The XL is a marvelous airplane, but was a victim of the USAF wanting to continue to produce the F-15, which is understandable. Sometimes you win these political games, sometimes not. In most ways, the XL was superior to the F-15 as a ground attack airplane, but the F-15 was good enough.”
Rebranded with the serials #848 and #849, the F-16XLs were taken out of storage in late 1988 by NASA, who planned to use them as test beds for a program exploring different aerodynamic concepts at supersonic speeds.
After being flown to the Ames-Dryden Flight Research Facility on March 9th 1989, the F-16XL-1 was fitted with an experimental titanium segment on the left wing, called a ‘glove,’ a component designed by the Rockwell International North American Aircraft Division perforated with millions of tiny holes, more precisely 2500 per square inch.
Launching a laminar-flow study, engineers envisioned that the minuscule incisions of the ‘glove’ could funnel away the surface air of the wing, a phenomenon that tended to increase drag as well as fuel consumption.
This experimental prototype, commanded by aviator Steve Ishmael, was first flown on May 3rd 1990, later completing a string of supersonic evaluations in January 1995 alongside NASA’s flagship reconnaissance aircraft, the SR-71 Blackbird, where it was used to research how sonic booms are affected by atmospheric conditions as part of NASA’s high-speed civil aviation program.
After registering speeds between Mach 1.25 to Mach 1.8, the F-16XL-1 was next assigned to the NASA facility in Langley, Vermont where, repainted black and yellow to ensure high visibility, it was involved in a number of tests investigating take-off performance and methods to reduce engine noise.
Between 1991 and 1992 the F-16XL-2 was retrofitted with a General Electric F110-129 engine and installed with what was termed a ‘passive glove’ on its right wing, a device made of foam and fibreglass that would shed light on the aerodynamic fluid mechanics on the leading edge, while on the left wing an ‘active glove,’ twice the size of the other model and also fabricated with foam and fibreglass casing, was installed.
Invented by a titan-studded team of leading aircraft manufacturers working in conjunction with the Langley Research Centre including Rockwell International, Boeing, and McDonnell Douglas, this particular ‘glove’ had a maximum thickness of 2.5 inches and covered 75% of the wing’s surface and 60% of the leading edge. Between October 1995 and November 1996 the F-16XL-2 has flown a total of 45 times, experiencing only minor issues with the suction components.
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After the project concluded in 1999 both F-16XLs were placed in storage until 2007 when they were entered into a flight program by NASA, who hoped to return at least one of them to operational status. Interest however was short-lived and in 2008 the program was discontinued. Presently however only one remains in storage, with the other now exhibited at the Air Force Flight Center Museum at Edwards.
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- Crew: One (XL #1) or Two (XL #2)
- Length: 54 ft 2 in (16.51 m)
- Wingspan: 34 ft 3 in (10.44 m)
- Height: 17 ft 7 in (5.36 m)
- Empty weight: 22,000 lb (9,980 kg)
- Max takeoff weight: 48,000 lb (21,800 kg)
- Powerplant: 1 × General Electric F110-GE-100 turbofan, 17,100 lbf (76 kN) thrust dry, 28,900 lbf (129 kN) with afterburner
- Maximum speed: 1,350 mph (2,170 km/h, 1,170 kn)
- Range: 2,850 mi (4,590 km, 2,480 nmi)
- Service ceiling: 50,000 ft (15,000 m)
- Rate of climb: 62,000 ft/min (320 m/s)