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Load Description WT

Launchers Symmetry 1 Clean Aircraft—Mid cg (x/c = 0.460)

(x/c = the location of the aircraft’s cg on the wing chord line)

Off Symmetric

2 4 AMRAAM (2 pylon, 2 wingtip) On Symmetric

2a 1 AMRAAM (pylon mounted) On Asymmetric

2b 1 AMRAAM (wingtip mounted) On Asymmetric

2c 2 AMRAAM (1 wingtip, 1 pylon on same side) On Asymmetric 3 6 AGM-65 missiles (3 on each wing, single

pylons)

On Symmetric

3a 5 AGM-65 missiles (3 on one wing, 2 on the other)

On Asymmetric

3b 1 AGM-65 missile on outboard pylon) On Asymmetric

4 6 AGM-65 missiles (on 2 TER racks) On Symmetric

4a 3 AGM-65 missiles (on 1 TER rack) On Asymmetric

5 10 SUU-65 stores on 10 single pylons On Symmetric 5a 9 SUU-65 stores (outboard rear pylon empty) On Asymmetric 6 Clean Aircraft—Forward cg (x/c = 0.430) Off Symmetric 7 Clean Aircraft—Aft cg (x/c = 0.491) Off Symmetric

8 2,370-gallon drop tanks On Symmetric

8a 1,370-gallon drop tank On Asymmetric

During the NASA Langley spin test program, the center of gravity of the F-16XL model was varied between its allowable forward and aft locations to determine the effect of cg location on spin recovery. The allowable cg range as a percentage of the mean aerodynamic chord for the F-16XL aircraft was determined to lie between 46 percent (forward cg) to slightly over 49 percent (aft cg). Different spin recovery flight control approaches were evaluated for their relative effectiveness during both erect and inverted spins. Spin test results showed that an automatic spin prevention system, integrated into the F-16XL flight control system, could eliminate the development of erect spins. Without an automatic spin prevention system, two flat erect spin modes—one fast and steady, one slower and oscillatory—were shown to be possible. In addition, testing showed that the potential existed for a moderately steep, relatively slow inverted spin mode. Recovery from this inverted spin mode was easily achieved by neutralization of the flight control surfaces.

The NASA spin tests determined the appropriate piloting techniques for acceptable recoveries from the different spin modes of the aircraft. The F-16XL’s spin prevention system, as integrated into its fly-by-wire flight control system, was refined as a result of the spin tunnel testing. The ability of this spin prevention system to automatically prevent spins from fully developing was completely validated through a rigorous combination of test techniques.

These included the spin tunnel tests, NASA Langley drop model testing, and follow-on Air Force flight testing with the full-scale aircraft. Additionally, a considerable effort was made in the development of the antispin system using the Langley Differential Maneuvering Simulator.²⁶ The NASA drop model tests are described in the following section while the Air Force flight-test effort is described in detail in a separate chapter.

The emergency spin recovery parachute installation, designed for use on the F-16XL during high-AoA maneuvering, was evaluated in conjunction with tests of the spin recovery system. The aerodynamic effects of the so-called

“Quadra Pod” spin recovery chute installation on the F-16XL’s flight characteristics were determined using the 0.18-scale captive free-flight model. During these tests, sizing requirements for the anti-spin parachute were investigated using a variety of miniature spin recovery parachutes with different towline lengths. The spin chutes were deployed by remote com-mand from a Quadra Pod spin

F-16XL model fitted with the “Quadra Pod” spin recovery parachute installation during wind tunnel testing at NASA Langley in May 1982. (NASA)

chute installation mounted on the 1/25-scale spin model during tests in the Langley 20-foot Vertical Spin Tunnel. Testing showed that for the full-scale F-16XL, a 34.2-foot-diameter parachute with a drag coefficient of 0.50 on a 100-foot towline, deployed in conjunction with spin recovery rudder and aile-ron control deflections, produced the best results on the full-scale F-16XL. This spin chute–towline combination was designed to provide emergency recovery from all potential spin modes, even with very large lateral weight asymme-tries. Based on the NASA recommendation, this spin chute configuration was adopted for the flight demonstration program, but it never had to be used to recover the aircraft from an inadvertent out-of-control or spin situation.

In addition to the tests described above, other potential flight control sur-face deflection combinations and optional/alternative design approaches were evaluated in the NASA Langley spin tunnel using the appropriately modified F-16XL spin model. These included all-moving wingtips, an all-moving verti-cal tail, and inboard-mounted leading-edge vortex flaps. These flight control approaches had been included as possible options in the early SCAMP concep-tual layout, and they were evaluated by NASA Langley at GD’s request. NASA testing showed that these alternative control surface approaches had limited to negligible effects on F-16XL spin recovery characteristics.²⁷

High-Angle-of-Attack Drop Model Tests

A large, radio-controlled 0.18-scale drop model of the single-seat F-16XL was used in remotely controlled high-AoA investigations conducted by NASA Langley. The configuration of this model was similar to that of the 18-percent captive free-flight wind tunnel model used in earlier Langley testing. It was constructed mainly of composite materials in order to better repre-sent the actual flight characteris-tics of the full-scale aircraft, with appropriate beefing up to with-stand landing impact loads. The drop model tests were conducted at NASA Langley’s Plum Tree Island satellite test facility, located about 5 miles from the Langley AFB runway complex. The 0.18-scale model was released from a specially configured launch rack mounted on the right-hand side of a NASA-operated Bell UH-1 A radio-controlled 0.18-scale drop model is

pre-pared for release from a NASA Bell HU-1 helicopter at the Plum Tree Island test facility in 1983. (NASA)

helicopter. The NASA Langley lead researcher for the drop model test effort was Mark A. Croom.²⁸

After checkout and release, the drop model was maneuvered to high-AoA flight conditions to test its aerodynamic response to a variety of potential out-of-control situations. A high-resolution video camera was used to track the model after release. Telemetry uplink and downlink capabilities were provided via a flight operations computer located in the control center. Graphic displays, including images of the model in flight and its location within the geographic confines of the test range, were presented at the remote pilot control station at the Plum Tree Island test complex. A video image of the view from the model was presented to the pilot along with digital displays that included parameters of interest such as angle of attack, sideslip angle, altitude, yaw rate, and normal acceleration level. A ground-based flight control computer located at the Plum Tree Island test complex was capable of being reprogrammed with revised flight control laws between

test missions. Proposed control system refinements could be programmed into the ground-based computer using the drop model as a validation tool. An all-terrain vehicle was used to retrieve the drop model from the soft marshy terrain on Plum Tree Island follow-ing its parachute landfollow-ing.²⁹

Weapons Carriage and Separation Tests

General Dynamics conducted an F-16XL weapons carriage study in conjunc-tion with the USAF Armament Division at Eglin AFB, FL. Results from the study supported the development of the conformal weapons carriage system that was successfully used on the F-16XL. Additional wind tunnel testing was dedicated to examining both air-to-air and air-to-ground weapons in vari-ous combinations using low-drag conformal carriage concepts that emerged from the study. F-16XL conformal weapons carriage wind tunnel tests were conducted at NASA Langley in the Unitary Plan Wind Tunnel with detailed weapons separation tests accomplished at the Air Force’s Arnold Engineering Development Center (AEDC) in Tullahoma, TN. During weapons carriage testing at NASA Langley, the 4-foot wind tunnel was used to investigate supersonic F-16XL conformal carriage weapons integration issues. These tests

The 0.18-scale drop model being recovered after a successful drop model test at the Plum Tree Island test facility in Virginia. (NASA)

determined the F-16XL’s aerodynamic drag character-istics with various conformal weapons carriage configu-rations over the supersonic speed range from Mach 1.6 to Mach 2.16. The F-16XL wind tunnel model carried various combinations of external wing tanks and con-formally carried munitions during wind tunnel testing at NASA Langley.³⁰

Wind tunnel testing at the AEDC addressed the aero-dynamic characteristics of a

number of air-to-air missiles and air-to-ground weapons during release and separation from the F-16XL. These separation tests were conducted over the speed range from moderate subsonic to high supersonic. A team of engineers representing the Air Force and General Dynamics played key roles in the F-16XL weapons separation test effort. These included AEDC’s Alex Money, Bob Bigi from the F-16 Systems Program Office, Bruce Frantz from GD, and R.A. Paulk of Calspan Engineering Services. A 1/15-scale model of the F-16XL—

along with an array of 1/15-scale models of air-to-air and air-to-ground weapons;

external stores, such as drop tanks and sensor pods; and weapons launchers and pylons—were specially fabricated for the weapons separation tests at AEDC.

The subscale stores models were both geometrically and dynamically scaled to represent their full-scale counterparts and included the AGM-65 Maverick air-to-ground guided missile (AGM), the AIM-9L Sidewinder and AIM-120 advanced medium-range air-to-air missiles (AMRAAM), the 370-gallon exter-nal fuel tank, CBU-58/B and SUU-65 sub-munitions dispensers, and Mk-82 500-pound and Mk-84 2,000-pound bombs in both conventional unguided free-fall and laser-guided configurations.³¹

Subscale weapons and stores separation tests were conducted in Aerodynamic Wind Tunnel (4T), located in the Propulsion Wind Tunnel (PWT) facility at AEDC, from May 21 through June 8, 1982.³² Separation trajectories were obtained at a variety of Mach numbers, angles of attack, simulated altitudes, and simulated load factors. During the subscale-model separation tests, aero-dynamic loads on the stores were obtained using the captive trajectory sup-port system. This technique involved attaching the subscale weapon or store model to a stinger that then was moved through the aerodynamic flow field

F-16XL wind tunnel test model fitted with six CBU-58 sub-munitions dispensers, two 370-gallon fuel tanks, and two wingtip-mounted AIM-9L missiles in the Langley Unitary Plan Wind Tunnel in 1982. (NASA)

surrounding the F-16XL model as aerodynamic force data were collected.

Postrelease trajectory data were collected using both captive trajectory and dynamic drop techniques. During dynamic drop testing, the scale weapons and external stores models were photographed with two high-speed cameras (400 frames per second) as they were released from the F-16XL model using scaled ejection forces produced by springs. The cameras were installed such that their sight lines intersected orthogonally at the wind tunnel centerline.

Special screens installed in the wind tunnel for this purpose were used to catch the models for subsequent reuse. Data from the motion picture cameras were reduced using a film reader that projected each frame onto a screen. Positions of the stores’ reference points, relative to the same point on each frame of data film, were measured along two orthogonal axes by manually positioning horizontal and vertical crosshairs located on the surface of the screen. Digital output from the film reader was input into a computer that then calculated full-trajectory positions and attitudes. Following the separation tests, force, moment, and trajectory data for each weapon evaluated at AEDC were pro-vided to General Dynamics for use in refining the design and developing the weapons carriage and release system and the stores management system.³³