The original Phase I flight-test program agreement between the Air Force and GD had called for test objectives to be completed by May 15, 1983, with a minimum of 240 missions accomplished. In reality, by the end of Phase I test-ing, F-16XL-1 had completed 200 flights and XL-2 had flown 169 times, with the two aircraft averaging 36 sorties per month. The CTF commander, Marty Bushnell, commented that the aircraft’s demonstrated reliability and main-tainability was about the same as that of the F-16, and F-16XL sortie rates in operational USAF service should also be similar to those of the F-16.14 During the 369 flights completed in Phase I testing, a number of key performance items were demonstrated. These included a maximum speed of Mach 1.95 in full afterburning power while carrying six air-to-air missiles demonstrated using F-16XL-1. In the air-to-ground configuration, F-16XL-1 reached Mach 1.4 using full engine thrust with afterburner with an external weapons payload of 12 Mk-82 bombs plus 4 AIM-120s and 2 AIM-9 missiles. In the same air-to-ground configuration, a maximum airspeed of 600 KCAS was demonstrated using Military Power thrust (defined as 100 engine rpm without use of after-burning) at an altitude of 500 feet. When loaded with two 370-gallon wing tanks, six Mk-82 bombs, and a full load of six air-to-air missiles, F-16XL-1 was 65 knots faster in Military Power than a comparably loaded F-16. In the air-to-air configuration loaded with six air-air-to-air missiles, F-16XL-1 demonstrated a sustained load factor of 3.6 g’s at Mach 1.4 at an altitude of 30,000 feet.
During maximum load factor testing, 9.1 g’s was achieved with the air-to-air
missile load of 6 missiles with 7.2 g’s demonstrated while carrying a load of 12 Mk-82 bombs and 6 missiles. The 9.1-g maximum load factor demonstrated in testing was slightly above the maximum design load factor of 9.0 g’s; the slightly higher load factor registered was due to the g-overshoot issue that is discussed subsequently. During the flight-test program, missions were flown up to the maximum takeoff gross weight (MTOGW) of 48,000 pounds.15
Neither F-16XL flight demonstrator proved to be capable of true super-cruise performance, as was later attained by the Lockheed YF-22A ATF proto-type and the production Lockheed Martin F-22A Raptor. As defined by the Air Force, supercruise provided a capability for sustained supersonic flight without the use of afterburner.16 However, the F-16XL’s low-drag design, along with its ability to carry 11,200 pounds of internal fuel, did provide significantly increased range compared to the standard F-16C. Ironically, when both aircraft were configured for the air-to-air mission, the F-16C actually had better sub-sonic cruise efficiency than the F-16XL. 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. The clean aerodynamic design of the F-16XL is clearly seen in photographs of the type.
On the air-to-air mission profile, the XL demonstrated a 53-percent-better range capability when carrying internal fuel only compared to the F-16C.
With external fuel tanks, the F-16XL’s range capability was 124 percent higher.
Similar results were obtained with air-to-ground payloads. For example, with twice the payload, 12 Mk-82 bombs on the F-16XL compared to 6 similar bombs on the F-16A, the XL’s range was 44 percent greater. The F-16XL dem-onstrated a ferry range of 2,245 nautical miles without aerial refueling when carrying two 600-gallon external fuel tanks.17
Shortly after the conclusion of Phase I testing, General Dynamics was quoted in the aviation press as saying that the F-16XL had demonstrated maneuverability that was better than expected, with its cranked-arrow wing planform displaying none of the unfavorable drag characteristics of the tail-less delta. At the same time, and perhaps defensively, GD claimed that there was no discernible difference in performance between the Pratt & Whitney F100–powered aircraft and the aircraft powered by the General Electric F110 engine, and it added that extra thrust was not needed.18 However, the Air Force reported that the F-16XL was purposely designed to have improved instantaneous turn rates rather than high sustained maneuvering capability.19 In this regard, the F-16XL demonstrated an instantaneous turn rate that was about 30 percent higher than that of the standard F-16 in the air-to-ground configuration, with both aircraft carrying the same payload at 30,000 feet. At the same altitude, the F-16XL’s instantaneous turn rate was 14 percent better than that of the standard F-16 when both aircraft were carrying their full
air-to-air missile payload. In contrast, there was a significant loss of sustained turn capability compared to that of the F-16. For example, at a Mach number of 0.9 at 30,000 feet, the F-16XL’s sustained turn rate was 30 percent lower than that of the F-16 in both the air-to-air and the air-to-ground configura-tions. This poor sustained turning performance compared to the F-16 was iden-tified by the Air Force as resulting from the high induced drag of the F-16XL’s cranked-arrow wing and its relatively low thrust-to-weight ratio. Commenting on this specific issue, Maj. Patrick K. Talty, the former deputy for engineering within the F-16E Combined Test Force at the Air Force Flight Test Center, later reported the following:
Pilot comments generally expressed concern about the loss of energy that resulted during high g maneuvers with the XL. Typically, in a 180 degree heading change turn the XL would lose about 180 knots. The operational utility of the gain in instantaneous turning capability was masked by this loss of energy. The T/W ratio of the F-16XL with half fuel in the air-to-air loading is 0.7. This is below the historical trend that US fighter aircraft have been following… . The exceptional rolling ability of the XL was used to offset this loss of sustained maneuver capability in simulated combat, but the lack of sustained maneuver capability was always raised as a primary concern by the pilots attached to the test program.20
The F-16XL-2 had a fake “canopy” and air refueling receptacle markings painted on its underside to visually disorient opposing fighter pilots as to the true orientation of the aircraft. The circular white markings were intended to represent pilots’ helmets when seen from a distance. (USAF)