PRINCIPALES IMPACTOS SOBRE LA LAGUNA DEL MAR MENOR Y SU ENTORNO

Extensive wind tunnel testing was accomplished on a SCAMP model with an 8.6-degree trailing-edge sweep angle. But during testing in the Langley Unitary Plan Wind Tunnel during early 1978, this configuration exhibited a serious increase in nose-up pitching moment as angle of attack was increased. Pitching moment characteristics were determined to be nonlinear with increasing angle of attack—a sudden up was the result. Such an increased nose-up pitch-ing moment with increaspitch-ing angle of attack is a highly undesirable aerodynamic characteristic indicative of longitudinal instability. Such pitch axis instability can easily result in the nose suddenly pitching up during slow-speed or maneu-vering flight with a high potential for total loss of aircraft control.

A NASA Langley team led by Joseph L. Johnson, Jr., of the Dynamic Stability Branch identified low-speed stability and control issues with the SCAMP con-figuration. They worked closely with GD engineers on wing-planform revisions that led to a much more stable configuration. This intense work was accom-plished in the Langley Full-Scale Tunnel and tunnels at General Dynamics.

The forward wing shape was truncated and smoothly blended into the forward fuselage. Other modifications resulting from NASA wing camber/twist/reflex tradeoff studies were incorporated into the design. The wing trailing edge was also changed to improve pitch stability. This modification involved a rearward extension of the aft inner wing segments. Overall wing area increased from 600 to 643 square feet. The new aft inner trailing-edge wing segment was swept forward at an 8-degree angle. The rounded and blended forward wing and the extended trailing edge created the wing planform that would become familiar on the F-16XL aircraft. This configuration featured elevons mounted on the inner trailing edge of the wing and ailerons on the outboard wing segments.

Finally, the full-span leading-edge flaps on the inner wing segments, previously SCAMP matrix model seen during 1978 tests in the NASA Langley Unitary Plan Wind Tunnel. (NASA)

under consideration, were deleted, as were the movable wingtips. Leading-edge maneuvering flaps were fitted on the outboard wing segments of the cranked-arrow wing. These enhanced airflow over the ailerons and provided resistance to yaw divergence at higher angles of attack. As eventually implemented, the leading-edge flaps would be automatically scheduled by the F-16XL flight control system in response to actual flight conditions.¹⁸

In summation, the cooperative NASA Langley–General Dynamics research effort led to significant modifications in the final SCAMP configuration. This was especially the case in the important area of aircraft stability and control.

Pitch and yaw instability had been encountered during wind tunnel testing of earlier SCAMP configurations and became an area of special emphasis. This led to the apex of the wing, at the point where it merged into the forward fuselage, being reshaped to incorporate an S-curve blend. This S-shaped apex would subsequently become a distinctive feature of the F-16XL’s cranked-arrow wing. In addition, prominent fairings were incorporated at the trailing edge of the wing. Positioned at the intersection of the inner and outer wing seg-ments, these fairings housed the hydraulic actuators that activated the ailerons.

They also provided volume for aft-facing electronic sensors and chaff and flare dispensers—capabilities that would have been incorporated into a production version of the aircraft. In conjunction with what NASA termed “air dams,”

the wing fairings helped to control spanwise airflow over the outer portions of the cranked-arrow wing at higher angles of attack, significantly improv-ing lateral stability and enhancimprov-ing controllability.¹⁹ The penultimate SCAMP configuration incorporated the S-curved wing apex, the aft wing trailing-edge extension, the wing fairings, and the outer wing dams or fences, but it did not yet include the larger vertical fin and the large drag chute fairing that would be of the wind tunnel testing accomplished at Langley to resolve the low-speed, high AoA issues associated with earlier SCAMP configurations.²⁰ In the report, she noted that earlier NASA research efforts had determined that highly swept arrow wings typically A near-final SCAMP–F-16XL configuration mounted in

the LaRC Unitary Plan Wind Tunnel. The vertical tailfin was returned to the original F-16 production tail for improved lateral stability at higher angles of attack. (NASA)

experienced pitch-up tendencies along with both directional and roll instabil-ity at higher angles of attack. These tendencies were determined to be caused by leading-edge vortex breakdown over the wing that resulted in severe lateral and directional instabilities. The Langley wind tunnel investigation of SCAMP stability used a 0.15-scale wind tunnel model of the configuration that was intensively tested in the Langley 30- by 60-foot Full-Scale Tunnel. This rather large model, constructed mainly of wood and molded fiberglass, incorporated a flow-through inlet duct and had a length of 7.34 feet, a wingspan of 4.86 feet, an aspect ratio of 1.75, and a wing area of 13.5 square feet. Using the 15-per-cent scale model, aerodynamic forces were measured and the resultant pitching moments on the aircraft were determined for a baseline SCAMP configuration as well as for several alternative configurations. The baseline configuration had been modified to incorporate a notched-wing apex, an extended-wing trailing edge, wing fences, and combinations of all of these fixes. The wind tunnel test program covered the angle of attack range from –4 degrees to +41 degrees, with sideslip angles as high as 13.5 degrees being investigated. Smoke-flow investigations were used to assist in documenting wing vortex flow behavior and to establish the relationship between vortex flow breakdown as a function of angle of attack and angle of sideslip. Results for all tested configurations were comprehensively documented in the form of detailed graphs and tables that provided aerodynamic coefficients and stability factors related to pitch, yaw, and roll for each tested configuration as a function of variations in angle of attack and angle of sideslip.²¹

The successful F-16XL design evolution was heavily dependent on experi-mental aerodynamic research efforts conducted in Langley wind tunnel facili-ties. As it unfolded, the NASA-GD cooperative test program demonstrated that the earlier SCAMP configurations exhibited high levels of maximum lift but also displayed unstable longitudinal and lateral-directional stability character-istics at moderate to high angles of attack. Longitudinal- and lateral-directional stability characteristics were significantly improved by the combination of the aforementioned wing-apex notch, the wing trailing-edge extension, and the addition of wing fences. However, these features were also noted by Langley researchers as causing some reduction in achievable maximum lift coefficient, a factor that pales in relation to the fact that without these critical aerodynamic fixes, any practical fighter derived from the earlier SCAMP concepts would have been unacceptable to test pilots from an aircraft handling qualities per-spective. In addition, the use of fly-by-wire flight controls also made XL feasible where the pure SCAMP configurations with conventional controls would not have been. The lateral-directional control from the outboard leading-edge flaps and the rudder-aileron interconnect from the basic F-16 flight control logic also helped make the aircraft controllable.²²

By mid-1980, SCAMP had converged on the aerodynamic layout that would soon materialize in the actual F-16XL design. This progression in aerodynamic and geometric configuration depicts the design evolution that occurred during the cooperative test program. The Greek symbol η denotes the geometrical loca-tion of the wing crank measured outboard from the fuselage centerline of the aircraft. For example, η=0.63 indicates that the wing crank is located 63 percent of the distance from the aircraft centerline to the wingtip. During the F-16XL design evolution, the location of the wing crank moved from its initial 63-percent position to one that was 70 percent of the distance to the wingtip. Although the inner wing’s leading-edge sweep angle remained little changed (going from 71 degrees to 70 degrees), the sweep angle of the outboard wing sections were decreased from 57 degrees to 50 degrees to provide better lateral control. As discussed earlier, to improve pitch stability, the trailing-edge sweep angle on the inner section of the wing was revised from the earlier 8.6-degree aft sweep to a forward (negative) sweep angle of –8 degrees. This, along with the S-shaped curvature implemented on the forward portion of the wing, gave the F-16XL its final distinctive wing planform.

General Dynamics had spent $15.9 million of corporate funding on the SCAMP effort by the time that the jointly developed SCAMP–F-16XL con-figuration was selected in late 1980. Up until that time, somewhat over 1,397

During the course of the cooperative program, the early SCAMP concept evolved to the cranked-arrow-wing configuration eventually adapted for the F-16XL. (Lockheed Martin)

hours of wind tunnel testing had been accomplished. This included 611 hours of low-speed testing, 419 hours at transonic speeds, and 367 hours at super-sonic conditions. On December 1, 1980, GD corporate management approved initiation of the next phase of the F-16XL effort. This would focus on detail design and construction of two flight-test prototypes. During this phase, General Dynamics would expend $41.7 million of corporate Independent Research and Development (IRAD) funding with an additional $7.8 million provided by their vendors and suppliers. Additional wind tunnel tests, focused on detailed aero-dynamic refinements and technical issues important to the flight-test program, would continue well into 1982. GD continued to use NASA Langley facilities on an as-required contract basis and also used facilities located at the NASA Ames Research Laboratory at Moffett Field near San Francisco, CA. Other wind tunnel testing related to the F-16XL was accomplished in cooperation with Calspan Corporation (previously Cornell Aeronautical Laboratories) and at GD’s Convair Division in San Diego, CA.²³

Follow-On Flutter, Spin, Drop Model,