New Issues: The F/A-I8E/F Program

The U. S. Navy funded the F/A-18E/F Super Hornet program in 1992 to design its next-generation fighter as a replacement for the canceled A-12 aircraft and the earlier legacy F/A-18 versions. Although some­what similar in configuration to existing F/A-18C aircraft, the new design was a larger aircraft with critical differences in wing design and other features that impact high-angle-of-attack behavior. Two of the first configuration design issues centered on the shape of the wing leading-edge extension and the ability to obtain crisp nose-down control for recovery at extreme angles of attack. Representatives of Langley’s high-angle-of-attack specialty areas were participants in a 15-member NASA-industry-DOD team who conducted wind tunnel studies and anal­yses that provided the basis for the final design of the F/A-18E/F LEX.[1324]

Aerodynamic stability and control characteristics for the Super Hornet for high-angle-of-attack conditions were conducted in the Full – Scale Tunnel to develop a database for piloted simulator evaluations using the Langley and Boeing simulators. Once again, the Spin Tunnel was used for identifying spin modes, spin recovery characteristics, an acceptable emergency spin recovery parachute, and measurement of rotational aerodynamic characteristics using the rotary-balance tech­nique. Langley used an extremely large (over 1,000 pounds) drop model for departure susceptibility and poststall testing at the NASA Wallops Flight Facility to provide risk reduction for the subsequent full-scale flight-test program.[1325]

One of NASA’s more critical contributions to the Super Hornet pro­gram began in March 1996, when a preproduction F/A-18E experienced an unacceptable uncommanded abrupt roll-off that randomly occurred at high angles of attack (below maximum lift) at transonic speeds and involved rapid bank angle changes of up to 60 degrees in the heart of the maneuvering envelope. Engineering analyses indicated that the wing drop was caused by a sudden asymmetric loss of lift on the wing, but the fundamental cause of the problem was not well understood. Following the formation of a DOD Blue Ribbon Panel, a research pro­gram was recommended to be undertaken to develop design methods to avoid such problems on future fighter aircraft. This recommenda­
tion was accepted, and a joint NASA and Navy Abrupt Wing Stall (AWS) program was initiated to conduct the research.[1326]

Meanwhile, extensive efforts by industry and the Navy were under­way to resolve the wing-drop problem through wind tunnel tests and "cut and try” airframe modifications during flight tests. Over 25 potential wing modifications were assessed, and computational fluid dynamics studies were undertaken without a feasible fix identified. Subsequently, the automatically programmed wing leading-edge flaps were examined as a solution. Typical of current advanced fighters, the F/A-18E/F uses flaps with deflection programs scheduled as functions of angle of attack and Mach number. A revised deflection schedule was adopted in 1997 as a major improvement, but the aircraft still exhibited less serious wing drops at many test conditions. As the Navy test and evaluation staff con­tinued to explore further solutions to wing drop, exploratory flight tests with the outer-wing fold fairing removed indicated that the wing drop had been eliminated. However, unacceptable performance and buffet characteristics resulted from removing the fairing.

Langley personnel suggested that passive porosity be examined as a more acceptable treatment of the wing fold area based on NASA’s exten­sive fundamental research. Subsequently evaluated by the Navy flight – test team, the porous fold doors became a feature of the production F/A-18E/F and permitted continued production of the aircraft.

With the F/A-18E/F wing-drop problem resolved, NASA and the Naval Air Systems Command began their efforts in the AWS research program that used a coordinated approach involving static and dynamic tests at Langley in several wind tunnels, piloted simulator studies, and compu­tational fluid dynamics studies conducted by the Navy and NASA. The scope of research focused on the causes and resolution of the unexpected wing drop that had been experienced for the preproduction F/A-18E/F and the wealth of aerodynamic wind tunnel and flight data that had been collected, but the program was intentionally designed to include assessments of other aircraft for validation of conclusions. The stud­ies included the F/A-18C and the F-16 (both of which do not exhibit wing drop) and the AV-8B and the preproduction version of the F/A-18E (which do exhibit wing drop at the extremes of the flight envelope).

After 3 years of intense research on the complex topic of transonic shock-induced asymmetric stall at high angles of attack, the AWS program produced an unprecedented amount of design information, engineering tools, and recommendations regarding developmental approaches to avoid wing drop for future fighters. Particularly signifi­cant output from the program included the development and validation of a single-degree-of-freedom free-to-roll wind tunnel testing technique for detection of wing-drop tendencies, an assessment of advanced CFD codes for prediction of steady and unsteady shock-induced separation at high angles of attack for transonic flight, and a definition of simulator model requirements for assessment and prediction of wing drop. NASA and Lockheed Martin have already applied the free-to-roll concept in the development of the wing geometry for the F-35 fighter.[1327]