BIBLIOGRAFÍA

Highly swept wings produce strong vortices over their upper-wing surfaces at higher-AoA flight conditions, such as those associated with takeoff, landing, or aggressive maneuvering. These vortices can result in greater lift for takeoff and maneuvering, better control of the aerodynamic center of lift, and improved airflow over a wide range of angle of attack and Mach number. Unfortunately, the vortex flows created by flow separation from the wing leading edge result in a large drag increase for a highly swept wing. The leading-edge vortex flap is specially designed to control the airflow over highly swept wing leading edges.

Properly designed vortex flaps can enable the aircraft designer to reorient part of the vortex force vector in the forward direction instead of directly normal to the wing-chord plane without compromising other aerodynamic characteristics such as stability and control.³⁴

Although not adopted on the final F-16XL configuration, the potential application of full-span leading-edge vortex flaps on the highly swept wing inboard segments of supercruise fighters like SCAMP was a major area of investigation during NASA Langley research efforts. If additional reductions in transonic drag were attainable by the proper formation of leading-edge vortices to improve low-speed and transonic maneuvering capabilities, increased lift-to-drag ratios were possible. Edward C. Polhamus led the Langley vortex research

program with his research group concentrated in the Transonic Aerodynamics Division, led by Percy J. Bobbitt. This team had previously made many sig-nificant contributions in the area of vortex lift during earlier NASA-industry research programs as well as in direct support of important military aircraft programs. These included the Air Force’s F-16 and the Navy F/A-18. The F-16 and the F/A-18 had evolved from the Air Force’s LWF program of the 1970s, a program that highlighted the importance of tailored vortex lift to improve fighter maneuverability and also indirectly led to the F-16XL. John E. Lamar and James F. Campbell, along with other members of the NASA Langley vortex lift research team, would be heavily involved in the cooperative SCAMP effort with General Dynamics as well as later research efforts involving NASA’s use of the F-16XL to better understand and predict the aerodynamics of cranked-arrow wings at higher angles of attack.³⁵ Follow-on NASA F-16XL efforts ori-ented toward vortical airflow research are discussed in detail in a later chapter.

During wind tunnel tests in the Langley 7- by 10-foot High-Speed Tunnel and associated analysis efforts, the NASA-GD team focused on achieving a 4-g transonic turning maneuver using a cranked-arrow wing planform with a highly swept leading edge similar to that used on the SCAMP configuration.

Langley’s John Lamar, in collaboration with GD engineers, conducted wind tunnel and computational analyses in an attempt to optimize SCAMP wing camber and shape to best meet the conflicting demands of supersonic efficiency and acceptable transonic maneuverability. Langley’s earlier experience with vortex flow research suggested that emerging vortex control concepts could be effectively applied to the SCAMP configuration. One of these concepts was the vortex flap. This concept involves using specially designed leading-edge flaps to modify undesirable leading-edge flow separation behavior. Vortex flaps could enable highly swept wings to recover lost leading-edge thrust without compromising other aerodynamic characteristics, such as stability and con-trol. With the vortex flap concept, the vortex force vector can be reoriented in the forward direction instead of directly normal to the wing-chord plane.

In exploratory testing, certain combinations of deflected full-span leading-edge and trailing-leading-edge flaps on a zero-camber wing produced almost the same drag improvements at transonic speeds as the highly refined cambered wing implemented on SCAMP.

NASA wind tunnel test results with vortex flaps showed great promise.

Well-designed leading-edge vortex flaps provided nearly the same supersonic efficiency as a highly tailored wing designed purely for supersonic flight during NASA Langley wind tunnel testing. Lift-to-drag ratio during subsonic cruise using the vortex flap concept was nearly as good as that of the standard F-16 and better than that of the supersonic wing design. L/D ratio during transonic maneuvering flight was midway between that of the standard F-16 and the

wing tailored for supersonic flight. Issues associated with vortex flap design, fabrication, and testing generated many in-house and NASA contractor stud-ies. These focused on refining and validating vortex flap design methodologies as well as investigating innovative applications of vortex control with deflected flaps. NASA vortex flap research would lead to the publication of a large number of professional papers and technical reports that were widely dissemi-nated throughout the aerospace industry and the aircraft design community.³⁶ Neal T. Frink led a NASA Langley team assessing the effects of varying wing leading-edge sweep angles and the geometric characteristics of various leading-edge vortex flap approaches. The overall effectiveness of vortex flaps was evaluated in an extensive series of wind tunnel tests that resulted in per-formance information for highly swept delta wings with constant-chord lead-ing-edge vortex flaps. This formed the basis for development of an analytical approach to the vortex flap design process. As a result, forces and moments on highly swept wings equipped with vortex flaps, as well as detailed pres-sures for different vortex flap configurations, could be predicted. Frink’s work resulted in the development of a leading-edge vortex flap design procedure in 1982.³⁷ Separately, Dhanvada M. Rao determined that reducing inboard flap length actually improved the efficiency of leading-edge vortex flaps on very highly swept wings. Tailoring the shape of the flap in the spanwise direc-tion also improved vortex flap efficiency and favorable vortex formadirec-tion over wing leading edges. Rao and a separate independent team led by W. Elliott Schoonover, Jr., of NASA Langley and W.E. Ohlson of Boeing determined that increasing the geometrical area of the vortex flap delayed the movement of the vortex inboard with some reduction in overall drag. The use of differentially deflected vortex flaps to improve aircraft roll control was also evaluated with some promise.³⁸

During 1981, NASA Langley researchers Long P. Yip and Daniel G. Murri investigated the effects of vortex flaps on the low-speed stability and control characteristics of generic arrow-wing configurations in the 12-foot Low-Speed Tunnel at Langley. Test results showed improvements in both lateral stability and lift-to-drag ratio; however, an unacceptable nose-up pitching moment was produced by the flaps. Geometric modifications to the vortex flap configura-tion, including adjusting spanwise flap length and leading-edge geometry, were evaluated in the wind tunnel. A vortex flap concept that incorporated a deflected tab on its leading edge was shown to moderate the nose-up pitching moment. The tabbed leading-edge vortex flap was installed on a 0.18-scale cap-tive free-flight wind tunnel model of a later SCAMP configuration, which had been transformed into the F-16XL configuration. Captive free-flight tests with this model were conducted in the Langley 30- by 60-Foot (Full-Scale) Tunnel in 1982. The tabbed vortex flap was also evaluated on a 1/25-scale model of the

F-16XL in the NASA Langley spin tunnel. The vortex flap did not produce adverse effects on aircraft spin recovery characteristics.

NASA vortex flap research with the F-16XL would eventually continue well into the 1990s, especially driven by the High-Speed Research program with its focus on technical risk reduction for the High-Speed Civil Transport. This program is described in “Chapter Test” results reported by General Dynamics’

Dennis B. Finley and Langley researcher W. Elliot Schoonover in 1986; the results indicated that F-16XL maneuvering performance could be enhanced with vortex flaps with no adverse effects on configuration aerodynamics.³⁹ Later, during the NASA flight research effort with the F-16XL, there was a plan to install and test leading-edge vortex flaps to support risk reduction activities associated with the High-Speed Civil Transport. However, that effort was ter-minated before the vortex flap modification to F-16XL-1 was accomplished, despite the fact that the necessary tooling had already been delivered to Dryden.