Previously we looked at the F-15 ACTIVE, an experimental program which set out to produce a workable thrust vectoring variant of the F-15B. The success of the ACTIVE program helped pave the way for future thrust vectoring technology. Beyond the F-15 ACTIVE program, there were many other large scale modification projects undertaken during this same period. Today, we take a look at one of the most promising jet fighter programs that never got off the ground – the F-16XL

 

BACKGROUND

From its inception, the F-16 had been intended to serve foremost as a lightweight dogfighter. Today, it looks nothing like this. Often equipped with two to three drop tanks for extended range, the Viper serves as a multirole aircraft, carrying bombs, air to ground missiles, rockets, and so on – yet the original design had none of this in mind. Thus, during its formative years, the F-16 program was to fulfil a very specific and very niche role; that of air superiority, with an emphasis on close range combat.

Before the first F-16s had rolled off their production line, designers and engineers were planning a major upgrade, which would overhaul almost every aspect of the original aircraft. This ambitious upgrade would improve multirole capability, increasing stores capacity, high altitude performance, speed, and even agility.

The first of these attempts would become known as the F-16 Supersonic Cruise and Manoeuvre Prototype, or SCAMP, which originated in the mid 70s. It would retain most parts from the original F-16, albeit with a redesigned wing. Several proposals were made, including one with forward swept wings. Ultimately it was decided that the most efficient design would be a cranked arrow delta wing, similar to that used in some European fighters. Prototyping for the SCAMP program would continue under the leadership of Harry Hillaker – who is often credited as being one of the primary designers of the original F-16.

With the new delta design, the wing shape itself would allow for 25% more lift, while also improving handling at both supersonic and low speeds. Having been born out of 3600 hours of wind tunnel testing, the wings had a number of special modifications. Near the root was a 50 degree angle, aimed at increasing handling at supersonic speeds, whilst further down the wing was a 70 degree angle for low speed performance.

The SCAMP F-16 was a working concept, and incredibly saw zero increase in drag over the original F-16. Yet it still had a number of issues. Namely, the wing had not been designed for weapons hard points, and it still suffered at low speeds.

 

SCAMP TO XL

The initial prototyping caught the attention of the US Air Force who weighed in to support the program in 1980. They would supply two F-16s, which would then be used to test the improved design. In 1981, the Enhanced Tactical Fighter program was announced, to find a replacement for the ageing fleet of F-1-11s. The goal was to find an aircraft capable of both air-to-air and air-to-ground tasking. It would need to be able to fly interdiction missions without a fighter escort, and have long-range capacity. The F-16XL would be thrown into the competition, up against a modified variant of the McDonnell Douglas F-15.

The team began work converting their two F-16s into workable prototypes. The fuselage length was increased by 140cm. The ventral fins were removed and the tail canted up at over 3 degrees to allow clearance on takeoff and landing. The delta wing was then assembled. Thanks to the enlarged fuselage and wing, the XL could not only carry 65% more fuel, but twice the payload of the F-16A. Fully loaded with fuel and weapons, the F-16XL could fly far further than its predecessor.  Made from graphite-bismaleimide composite, these wings were 270 kilograms lighter than a similar delta design using conventional materials from the F-16.

Unlike the SCAMP concept, the XL had 27 hard points on the wings, allowing for dozens of weapons to be stored. It was such an improvement over the original F-16 design that word of the new prototype soon spread through the Air Force, gaining much attention.

In a real-world scenario, it was noted that if the F-16XL was loaded up with the same armament as a standard F-16, it could fly twice as far. If the XL was loaded up with its own maximum payload – which would consist of ground ordinance, as well as four AMRAAMS and two Sidewinders – it could still travel 44% further than an F-16A. All of this could be done without external fuel tanks, which were increasingly necessary on the standard F-16.

The new design also retained the great performance the F-16 had become known for. Just like the F-16A, the XL was rated for 9G manoeuvres, the highest of any fighter at the time. However, when loaded up with ground ordinance, the F-16A was limited to 5.58Gs so as to not stress the wings. The XL could pull 7.2G with a full payload. In comparison, a clean F-15A was rated at 7.3Gs.

When it came to speed, it was a similar story. At military power at sea level, an XL, max-loaded with bombs, was still 83 knots faster than a clean F-16A. At altitude in afterburner, the same max-loaded XL was 300 knots faster than the F-16A. Those testing the aircraft noted that it had no issue climbing with a heavy payload, and could enter into supersonic flight at both low and high altitudes. Air Force veteran and write F Clifton Berry, who flew in the XL, recalls that in supersonic flight, a heavily loaded XL felt similar to a clean F-16A.

With interest in the design gaining momentum, the Air Force entered it into direct competition with the modified McDonnell Douglas F-15, which would become known as the F-15E Strike Eagle. Unlike the F-16, the F-15 Eagle had been the Air Force’s premiere fighter for almost a decade. It had a flawless record and was often preferred over other designs for a number of reasons.

When it came to the competition, the F-15E Strike Eagle was wanting in carrying capacity, having only 15 hardpoints in contrast to the XL’s 27. However, the F-15E held several advantages. For one, it was faster, rated at Mach 2.5, whilst the XL would reach only Mach 2. It could also fly at 60000 feet – 10000 feet higher than the XL. And of course, it had two engines, which was often seen as advantageous, since it not only allowed for a large maximum takeoff weight, but also reduced the possibility of an aircraft loss over enemy territory.

But perhaps the biggest difference between the F-16XL and the F-15E was cost. An F-15E shared much in common with other F-15 designs. In contrast, the XL required a massive overhaul from an original F-16. A lengthened fuselage, tail and intake redesign, and new wings meant that each F-16 conversion would take a lot of time and money. Ultimately, the competition would end with the F-15E on top. A tested airframe, with less modifications required was seen as a far more attractive option.

The two F-16XLs were sent back to Edwards Air Force Base, and then turned over to NASA Dryden. Here, they would go on to serve as testbeds for various aeronautical experiments, including further research into super cruise.

 

CONCLUSION

The modernised variants of the F-16 today are powerful and do their multi-role job well, despite building on a design originally intended for pure air combat. Yet, looking back, it seems many possibilities may have been lost with the cancellation of the XL program. Some consider the F-16XL to be one of the greatest jet fighters that was never given the opportunity to prove itself. Some argue that – just as with the original F-16 program – the competition between the F-15E and F-16XL was a misapplication of the technology, and that the XL should have been presented as a complimentary fighter to the F-15. As can be the case, designs with great potential don’t always receive support or extensive use, almost always as a result of budgeting considerations.

From our perspective, the F-16XL will live on as one of the most interesting designs that never saw the chance to prove itself.