General Dynamics F-16XL

F-16XL
F-16XL in flight
Role	Experimental fighter
Manufacturer	General Dynamics
First flight	3 July 1982
Primary users	United States Air Force NASA
Number built	2
Developed from	F-16 Fighting Falcon

The General Dynamics F-16XL is a derivative of the F-16 Fighting Falcon, with a cranked arrow delta wing, similar in appearance to the wing planform of the earlier Saab 35 Draken from Sweden, that is over twice the size of that of the standard F-16 wing. It was entered in the United States Air Force's Enhanced Tactical Fighter (ETF) competition but ultimately lost to the F-15E Strike Eagle. Several years after the prototypes were shelved, they were turned over to NASA for aeronautical study.

 

Contents 1 Development 1.1 SCAMP 1.2 Enhanced Tactical Fighter competition 1.3 NASA 1.4 Project Management 2 Design 3 Specifications (F-16XL number 2)

Development

SCAMP

Model 400 design team, starting left: Harry Hillaker, Andrew Lewis, Kenny Barnes, Jim Gordon.

In 1977, the F-16XL started out as the F-16 SCAMP (Supersonic Cruise and Maneuver Prototype) at General Dynamics Fort Worth. Under the leadership of Harry Hillaker (father of the original F-16), the original goal of the program was to demonstrate the applicability of supersonic transport technologies to military aircraft.^[1] That year, under the supervision of Hillaker and Jim Gordon, a young engineer, Andrew Lewis, was given what was supposed to be a quick project to study the applicability of supersonic transport technologies to military aircraft. The big wing generated a lot of lift, and typical aerodynamic limitations of delta wings were overcome by the F-16's relaxed static stability. Kenny Barnes led the work into tweaking the F-16s electronic flight control system to allow control at high angles of attack. The wing was also a big fuel tank which boosted range considerably. The study went on for two years with important contributions from many other engineers, but Lewis was able to stay out in front and designed the wing's plan form, airfoil, twist and camber. The goal of the cranked arrow was to have a high sweep inboard panel for low drag at supersonic speeds, and a low sweep outboard panel that would provide better handling and maneuverability at subsonic speeds.
Working closely with NASA's Langley Research Center, the company invested significant Internal Research and Development (IRAD) funds for wind tunnel testing and that led to the Model 400. It featured all moving wing tips for roll control and an all moving vertical tail. These surfaces were actually the horizontal tail surfaces from the F-16A. These surfaces were later dropped as they did not provide adequate control at low speed, high angle of attack. Also, there would have been no provision for wing-tip mounted missiles. The main wing incorporated forebody strakes to enhance vortex generation for high angle of attack maneuverability, negative stability for improved subsonic lift and reduced supersonic drag. It was built around a 40-inch fuselage stretch. Both the large wing and fuselage stretch yielded a dramatic increase in range at all speeds. In 1979, with a strong positive response by the USAF, GD released the Model 400 for a company funded preliminary design effort.^[2]

Enhanced Tactical Fighter competition

F-16XL compared with a conventional F-16.

In 1980, the USAF signed on as a partner, providing two early model F-16s for conversion (the third and fifth production airframes). These two airframes became the only extant examples of the F-16XL.
In March 1981, the USAF announced the Enhanced Tactical Fighter program to procure a replacement for the F-111 'Aardvark'. The concept envisioned an aircraft capable of launching deep interdiction missions without requiring additional support in the form of fighter escort 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 were fairly different in design approach: the F-15E is basically an F-15D two-seat trainer with the back-seat station modified to support ground-attack instruments, while the F-16XL has major structural and aerodynamic differences from the original F-16. As such, the XL would have required much more effort, time and money to put into full production. Additionally, the Strike Eagle has two engines, which gives it more thrust and thus the capacity to carry more weapons and/or armor into combat. Furthermore, engine redundancy can be very useful for an airplane whose mission involves operating within the reach of anti-aircraft artillery and Surface-to-Air Missiles, in addition to the standard threats of fighter aircraft and interceptors.
In February 1984, the USAF awarded the ETF contract to McDonnell Douglas. Both F-16XL aircraft were returned to the Air Force and placed in storage.

NASA

Lasers illuminate airflow over a model F-16XL in a NASA wind tunnel

In 1988, the two airplanes were taken out of storage and turned over to NASA for research. The first aircraft was fitted with an active suction titanium glove encasing the left wing and delivered to the Ames-Dryden Flight Research Facility at Edwards AFB. Designed and built by North American Aviation (a division of Rockwell International), the glove had laser-cut holes that were nominally 0.0025 in (0.0635 mm) diameter. Distance between holes varied between 0.010 and 0.055 in (0.25 and 1.40 mm) depending on the suction. The glove covered over 5 ft² (0.5 m²) of the wing. It was intended to suck away turbulent airflow over the wing, restoring laminar flow and reducing drag. The aircraft was also involved in testing sonic boom characteristics, takeoff performance, and engine noise, for NASA's civil transport program.
The second aircraft (a two seater) had its experimental engine replaced with a General Electric F110-129. It accidentally achieved supercruise, a design goal of the F-16XL that was never attained in ETF testing, when it reached Mach 1.1 at 20,000 ft (6,096 m) on full military power. It was mounted with a passive fiberglass and foam glove on the right wing to examine supersonic flow, and an active glove on the left wing. This second glove was composed of fiberglass and foam over a titanium skin, and covers 75% of the wing's surface and 60% of its leading edge. The active portion consists the middle two-thirds of the glove, with laser-drilled holes leading to cavities beneath the wing. It was designed collaboratively by Langley research center, Dryden, Rockwell, Boeing, and McDonnell Douglas. The glove is intended as a testbed for supersonic laminar flow.
At the conclusion of their test programs in 1999, both F-16XLs were placed into storage at NASA Dryden. In 2007, NASA approached Lockheed Martin to request a study into the feasibility and cost of returning F-16XL #1 to flight status and upgrading it with many of the improvements found in the USAF's Block 40 F-16s. As of July, 2007, the study is still underway; however, in the meantime F-16XL #1 has been taxi tested at Dryden and given systems checks.

Project Management

The F-16XL flight project office was located at the NASA Dryden Flight Research Center, Edwards AFB, CA. The NASA Langley Research Center, developed and coordinated F-16XL experiments. Project managers at Dryden were Marta Bohn-Meyer and Carol Reukauf.^[3]

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 were replaced with a cranked-arrow delta wing 120% larger than the original wing. Extensive use of carbon fiber composites allowed the savings of 600 lb (270 kg) of weight but the F-16XL was still 2,800 lb (1300 kg) heavier than the original F-16A.
Less noticeable is that the fuselage was lengthened by 56 in (1.4 m) by the addition of 2 sections at the joints of the main fuselage sub-assemblies. With the new wing design, the tail section had to be canted up 3°, and the ventral fins removed, to prevent them from striking the pavement during takeoff and landing. However, as the F-16XL exhibits greater stability than the native F-16, these changes were not detrimental to the handling of the aircraft.
These changes resulted in a 25% improvement in maximum lift-to-drag ratio in supersonic flight and 11% in subsonic flight, and a plane that reportedly handled much smoother at high speeds and low altitudes. The enlargements increased fuel capacity by 82%. The F-16XL could carry twice the ordnance of the F-16 and deliver it 40% further. The enlarged wing allowed a total of 27 hardpoints on the plane:

• 16 wing stations of capacity 750 lb (340 kg) each
• 4 semi-recessed AIM-120 AMRAAM stations under fuselage
• 2 wingtip stations
• 1 centerline station
• 2 wing "heavy/wet" stations
• 2 chin LANTIRN stations

However, the "heavy/wet" stations interfered with up to 4 wing stations

Specifications (F-16XL number 2)

Data from Darling,^[4] F-16.net^[5]

General characteristics

• 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)
• Wing area: 646 ft² (60.0 m²)
• Empty weight: 22,000 lb (9,980 kg)
• Loaded weight: 48,000 lb (21,800 kg)
• Max takeoff weight: 48,000 lb (21,800 kg)
• Powerplant: 1× General Electric F110-GE-100 turbofan
  • Dry thrust: 17,100 lbf (76.3 kN)
  • Thrust with afterburner: 28,900 lbf (125 kN)

Performance

• Maximum speed: Mach 2.05 (1,400 mph)
• Cruise speed: 600 mph
• Range: 2,480 nmi (2,850 mi, 4,590 km)
• Service ceiling: 50,000 ft (15,000 m)
• Rate of climb: 62,000 ft/min (320 m/s)

Armament

• Guns: 1× 20 mm (0.79 in) M61 Vulcan (Gatling) gun
• Hardpoints: 17 pylons with a capacity of up to 15,000 lb (6,800 kg) of payload