A highly modified version of the F-16 Fighting Falcon
. Years after the F-16 originally went into service, General Dynamics
engineers revisited an early design concept for the F-16 and rediscovered its advantages. Starting with a basic F-16, the engineers lengthened the fuselage and replaced the wings and horizontal stabilizers with a "cranked arrow
" delta wing
. They also were able to remove the two ventral fins under the plane (which increase the difficulty of landing and taking off in a normal F-16 as they can be scraped against the runway while the nose is up).
A delta wing describes a type of wing that performs the functions of both the wing and the horizontal stabilizers, completely replacing the horizontal stabilizers. Typically, delta wings are so named because they, like the Greek letter of the same name, look more or less like triangles.
The cranked-arrow delta wing used on the F16XL, however, does not look very much like a triangle. Its contour changes throughout its length, blending in to the fuselage with a gentle curve near the aircraft's nose, continuing in a more or less straight but highly swept aspect towards the tail, and culminating in a much less dramatically swept section at the rear.
The much larger cranked-arrow wing offered several advantages. First, the internal fuel capacity was increased by roughly 80%. Second, the transonic drag (drag between Mach .8 and Mach 1.2) created by the cranked-arrow wing was substantially less than the basic F-16's wing. Third, the larger wing offered the ability to carry a much larger number of weapons than the standard F-16, and many of these weapons could be carried in a semi-recessed manner, meaning that the weapons were partially submerged into the aircraft's skin, thus further reducing drag.
As a result, the F-16XL was said to have twice the range of a standard F-16 while carrying the same weapons load, and to be able to carry twice the load of a standard F-16 50% farther.
General Dynamics submitted the F-16XL to the USAF for its new strike fighter program. The result of this program was the F-15E Strike Eagle. It has been suggested that the F-16XL (which would have gone into production as the single seat F-16E and two seat F-16F) was a victim of Air Force politics, as the top Air Force brass were not about to support an airplane that would result in the purchase of fewer F-15's.
In the end, only two F-16XL's were built, a single seat version and a two seat version. One plane was powered by the Pratt & Whittney F100, and the other by the General Electric F110. Following the selection of the F-15E, the two XL's were turned over to NASA, who have used them to conduct various high speed aerodynamic research projects at the Dryden Flight Research Center at Edwards Air Force Base, and at the NASA facility in Langley, Virginia.
It is noteworthy that one of the F-16XL goals was to achieve supercruise, that is, to fly supersonically without the use of a fuel guzzling afterburner. Ironically, this goal was never achieved during the USAF evaluation of the plane, but was achieved briefly by NASA many years later. The first planes to achieve supercruise, the Lockheed YF-22 and Northrop YF-23, were able to do so as a result of their very powerful, very advanced engines, while the F-16XL was able to achieve this in large part due to the reduced drag offered by its modified wing.