CONCEPTOS BÁSICOS

Initial follow-on testing evaluated F-16XL flutter and performance characteris-tics with various external store loadings. These payloads are listed in a follow-on section. They included six SUU-65s and wingtip-mounted AIM-9 Sidewinder missiles, flown both with and without 370-gallon fuel tanks. The full load of six air-to-air weapons was carried with two 600-gallon fuel tanks and six AGM-65 Maverick missiles. An AGM-65 was successfully launched from F-16XL-1 during its 375th flight on March 29, 1985. Additional flight-test efforts were focused on resolving the pitch gallop issue and obtaining comprehensive air-frame vibration data. Stability and control test maneuvers prepared the way for high-AoA testing at more extreme flight conditions than had been previously evaluated during Phase I. A major aspect of this phase of the flight-test effort was focused on obtaining stability and control and performance data with the leading-edge flaps locked at the zero deflection position. Approximately 40 test

flights were conducted to verify the aft cg limit, identify the effects of 370-gallon tanks on high-AoA handling qualities, and evaluate aircraft characteristics with locked leading-edge flaps.

During these tests, the F-16XL was determined to have a relatively unlim-ited maneuvering envelope for all store configurations tested. Handling quali-ties were not degraded with any of the external store configurations evaluated.

Longitudinal maneuvering characteristics were evaluated using constant Mach number turns and maximum-g slowdown turns. Static stability was assessed using 1-g accelerations and decelerations. In the air-to-air configuration, the cg location varied over a region that extended from 44.7 percent to 46.4 percent of the wing’s mean effective chord as fuel was burned. The aircraft was tested over the cg range from 44.7 to 47.5 percent; this allowed flight testing to be conducted with the cg both forward and aft of the neutral point of the F-16XL.

With the cg forward of the neutral point, the aircraft was statically stable. When the cg was aft of the neutral point, the aircraft was statically unstable, relying on continuous inputs from the fly-by-wire flight control system to maintain adequate control. At subsonic Mach numbers, external stores did not cause a shift in neutral point. An increase in pitch control surface deflection was required to maintain 1-g trim above 0.95 Mach number. This was due to a decrease in pitch surface control effectiveness and the aft shift of the neutral point as sonic speed was approached. Trim requirements without external tanks were similar with all weapons loads. When external tanks were carried, 2 to 3 degrees of additional trailing-edge-down trim were required for a given cg location. While the F-16XL had positive longitudinal static stability over all Mach numbers tested, its pitch stability was very low at higher angles of attach and low airspeeds.⁴³

High-Angle-of-Attack Testing

The high-AoA flying qualities of the F-16XL, both with and without external stores, were rated as outstanding during the Air Force flight-test evaluation.

No airspeed or AoA maneuver limitations were identified with the cg as far aft as 47 percent of the wing mean aerodynamic chord. When AoA excur-sions did occur, the aircraft rapidly recovered to controlled flight without any pilot actions or control inputs being required. Control response was excel-lent, even at the very low airspeeds that were encountered while the pilot was maneuvering the aircraft to lower angles of attack For follow-on high AoA and departure resistance flight testing conducted during the summer of 1983, F-16XL-1 was again equipped with the Quadra Pod spin recovery parachute installation. The Quadra Pod spin chute installation was mounted on the aft fuselage in the beginning of August 1984 and was removed in early September when the high-AoA portion of the flight-test program was com-pleted. High-AoA test maneuvers included pitch-yaw-roll doublets, sideslips,

1-g and maximum-g decelerations, maximum commanded rolls, and climb reversals. A number of extreme maneuvers were used to “trick” the aircraft into exceeding the programmed maximum AoA limit. Steep, high-pitch-attitude climbs, often reaching an attitude of 90 degrees (vertical to the horizon), were employed. Other techniques included a 180-degree roll followed by a full-aft pitch control input, forward push maneuvers, and maneuvering while holding a full-aft control input.⁴⁴

Using these techniques, uncommanded AoA excursions were obtained at angles of attack above those imposed by the AoA limiter installed in the F-16XL’s flight control system. This was achieved by accomplishing both upright and inverted constant pitch attitude climbs to minimum airspeed while conducting repeated roll reversals in the AoA range of known low pitch stability (16 to 24 degrees AoA) and with the aircraft cg near the allowable aft limit. In some cases, a 90-degree pitch attitude (vertical) climb was maintained to zero airspeed.

During the subsequent tail slide and recovery to a nose-down pitch attitude, extreme AoA excursions greater than +120 and –90 degrees were encountered.

Total recovery time from start of the tail slide to the point where the pilot was able to fly the aircraft was about 20 seconds. These AoA excursions were the only type of departure from controlled flight that was encountered during the F-16XL flight-test program during flight testing both with and without external stores. Pitch recovery maneuvers were illustrated, as adopted from Talty.⁴⁵ So-called “deep stall” tendencies (such as existed with both the standard F-16 and the Swedish J 35 Draken fighter at higher angles of attack) were not encountered during the F-16XL test program.⁴⁶ This result was in agreement with results from the Langley Differential Maneuvering Simulator, where the same NASA team that had worked the original F-16 deep stall problem had also worked the F-16XL. The aircraft demonstrated high resistance to inadvertent loss of control or spin entry. The F-16XL was tested with its center of gravity located as far aft as 47 percent of the MAC. The planned high-AoA stability investigation at the full-aft cg limit of 47.5 percent MAC was not completed at the time the F-16XL program was cancelled in mid-1985.⁴⁷

The lateral-directional static stability of the F-16XL was evaluated by wings-level sideslips to the maximum rudder deflection or by applying the maximum lateral control surface deflection required to maintain wings-level flight. Lateral-directional stability was not significantly affected by external stores. As angle of attack was increased above 20 degrees, directional stability began to reduce approaching zero at 24 degrees AoA; however, this reduction in directional stability was not noticeable to the pilot. The lateral control power and rolling performance of the aircraft were also rated as excellent, both with and without external stores. Roll accelerations and roll-time constants consistently were better than those of the standard F-16A. During rolling

maneuvers with external weapons loads, the automatic flight control system in the F-16XL effectively reduced the maximum allowable roll rate when the air-to-ground mode was selected by the pilot. With the cockpit selector switch in the air-to-ground mode, maximum allowable roll rate was reduced to 230 degrees per second from the 308 degrees per second available in the air-to-air configuration. During maneuvering flight, the actual allowable roll rate was automatically scheduled by the flight control computer as a function of aircraft angle of attack to prevent overstressing the aircraft or encountering potential out-of-control situations. This insured that the flight control computer main-tained precise control over roll and nose-pointing maneuvers when the aircraft was carrying heavy external loads. In addition, tendencies to overshoot or lose control during rapid rolls were minimized. CTF pilot comments were very positive when discussing the responsiveness and maneuverability of the aircraft during ground attack profiles.⁴⁸

Weapons Testing and Loading Configurations

A wide variety of weapons and external store combinations were evaluated using both the single-seat and two-seat F-16XL aircraft. The drag characteristics of the aircraft with these payloads and their effects on performance and handling qualities were determined. However, weapons release and separation testing was not accomplished with all of the configurations flown. In most regions of the flight envelope, flight-test results confirmed GD-provided performance predictions. However, in the transonic and supersonic regions of the flight envelope, the contractor’s predictions of

excess thrust with most store loadings were anywhere from 10 to 20 percent higher (poorer) than what was actually demon-strated during flight test. This discrepancy held true for both military thrust (100 per-cent power without the use of afterburner) and maximum (full afterburning) thrust.

When the aircraft was configured with SUU-65 Tactical Munitions Dispensers (TMDs), drag was as much as 30 percent higher than had been predicted at tran-sonic and supertran-sonic speeds. The payload configurations that were tested included the combinations of weapons and stores listed below. These weapon payloads were carried on the external store stations shown in the illustration.

The F-16XL could carry a range of exter-nal stores at 29 different locations on the airframe. (USAF)

• Standard air-to-air payload (four AIM-120s plus two AIM-9Ls on wingtip missile launchers)

• Limited air-to-air payload (four 120s only, no wingtip AIM-9Ls, no wingtip missile launchers)

• Standard air-to-air payload plus one 300-gallon centerline fuel tank

• Standard air-to-air payload plus two 370-gallon underwing drop tanks

• Standard air-to-air payload plus six 500-pound Mk-82 bombs

• Standard air-to-air payload plus 12 500-pound Mk-82 bombs

• Standard air-to-air payload plus 12 500-pound Mk-82 bombs and two 370-gallon drop tanks

• Standard air-to-air payload plus two 2,000-pound MK-84 bombs

• Standard air-to-air payload plus four 2,000-pound MK-84 bombs

• Standard air-to-air payload plus six SUU-65 Tactical Munitions Dispensers

• Standard air-to-air payload plus six SUU-65 TMDs and one 300-gallon centerline fuel tank

• Standard air-to-air payload plus six SUU-65 TMDs and two 370-gallon drop tanks

• Standard air-to-air payload plus two BDU-38s and two 370-gallon drop tanks

• Standard air-to-air payload plus six AGM-65 Maverick missiles Weapons delivery and separation characteristics were rated by the CTF pilots as excellent for those weapons released from the F-16XL during the flight-test program. These included a total of 218 Mk-82 500-pound bombs, dropped on 37 test flights; 18 Mk-84 2,000-pound bombs, released on 7 flights; and 6 800-pound CBU-58 tactical munitions dispensers, ripple released

on a single flight. Other ripple releases included 4 Mk-84s and 12 Mk-82s. As mentioned earlier, a single unguided AGM-65 Maverick air-to-surface mis-sile was successfully launched from F-16XL-1 on March 29, 1985. Over the course of the entire flight-test program, 233 F-16XL missions were flown with bombs. This represented about 29 percent of all F-16XL test missions.

AGM-65 Maverick missiles were carried on six flights. Fifteen flights were dedicated to investigating the effects of firing the M61 cannon. During these flights, more than 7,300 rounds of 20 mm ammunition were successfully fired.⁴⁹ The complete F-16XL Weapon Separation Log listed by specific flight and pilot is reproduced in Appendix B.