The impact of LO-configured forebodies on inlet approach flowfields

Low Observable (LO) shaping considerations have driven the cross-sectional shape of fighter forebodies from smooth-contoured circles and ellipses to Sharp half-breadth -. diamond and blended shapes. These new shapes induce strong vortices in. the vicinity of the fuselage half-breadth at both maneuvering and cruise flight conditions. If ingested, these vortices can reduce integrated inlet performance and can degrade inletlengine compatibility. Over the past six years, the USAF. NASA, and McDonnell Aircraft Company (MCAIR) have performed a comprehensive aerodynamic investigation to assess the influence of LO forebody shaping on inlet approach flowfields. This investigation used high-speed experimental flowfield surveys to CharacterlZe advanced fighter inlet approach flowfields. This paper summarizes the results of this testing, and assesses the impxt of these flowfields on fighter inlet integration. * Senior Engineer Technology, Senior Member AlAA ** Unit Chief Technology. Senior Hember AlAA *** Supervisory Aerospace Engineer, Member AIAA **** Research Engineer, Member AlAA Export Authority: 22 CFR 125 4(b)(13) \copyright 1991 American Institute of Aeronautics and Astronautics, inc. A I I rights reserved. IMROWCTION Future fighter aircraft require reduced observability, supersonic cruise capability. and enhanced subsonicltransonic performance to survive and defeat an increasingly capable threat. These requirements place diverse and sometimes conflicting constraints on the design and integration of air induction systems. For example. Radar Frequency ( R F ) signature reduction requires that forebodies have diamond or blended shapes with sharp half-breadths (line of maximum forebody width) as seen in the Advanced Tactical Fighter prototypes, Figure 1. I n addition, these requirements can result in highly-integrated configuratlons with inlets located in close proximity to the forebody. These sharp half-breadths induce strong vortices which can enter the inlet approach flowfield of the highly-integrated inlets, resulting in reduced inlet performance and degraded inletlengine compatibility. The USAF Wright Laboratory (WL), NASA Langley Research Center (LdRC), NASA Lewis Research Center (LeRC). and McDonnell Aircraft Company (HCAIR) nave performed a comprehensive aerodynamic investigation of the impact of LO forebody shaping on inlet approach flowfields over the past s i x years. This investigation evaluated flowfield characteristics for three generic advanced fighter inlet integration concepts, Figure 2 . These concepts were