From Water Tunnel to Poststall Flight Simulation: The F/A-18 Investigation

Abstract

Introduction T HE prediction of poststall maneuvers and departure characteristics of high-performance aircraft continues to be challenging,1;2 due, in particular, to the highly three-dimensional nature of the motions and aerodynamic responses. A case in point is the F/A-18 aircraft (Fig. 1), which has experienced departures from controlled  ight, typically involving large sideslip excursions with attendant high body-axes rate variations leading to high angles of attack ® (Ref. 4). The lateral–directional departure susceptibility of the F/A-18 provided part of the motivation within the NASA High-Alpha Technology Program for the improvement of high-alpha predictionsbased on ground testing. The gyrations and responsesexperiencedon departureare intrinsicallynonplanar, that is, involvingarbitraryrotationsnot alignedwith theprincipalaxesof the aircraft.3 When the  ight mechanical deŽ ciencies of an F/A-18 simulation were identiŽ ed, a Canadian programwas undertaken to enhance simulator Ž delity.6 The areas in which the airframe model could be improved included the poststall aerodynamicdatabase, the multivariabletable lookuparchitecture,and the aerodynamicmodel. The present paper focuses on the improvements to the database. The shortcomings of the poststall aerodynamic database were attributed largely to facility interference and scaling effects not accounted for and gaps in the parameter space covered. The original rotary database for F/A-18 was generated in the NASA Langley Research Center Spin Tunnel.11 The F/A-18 poststall maneuvering aerodynamics is dominated by forebody/leading edge extension (LEX) vortex interactions at moderately high angles of attack (30 55 deg). In the light of the sensitivity of the vortex interactions to sideslip angle (Ref. 12), it was deemed necessary to generate a more detailed rotary database to deŽ ne the associated relationshipsover a large sideslip range. When the advantages of the water-tunnel approachwere considered, it seemed appropriate to use this ground-testmethodology to generate supplementary data. The orbital platform rotary balance system (OPLEC) at the Institute for Aerospace Research (IAR) water tunnel, a diagnostic tool for studying support interference7;8 and unsteady wall interference, was subsequently used in highresolution tests as well as diagnostic experiments15i18 on the baseline F/A-18. A thorough understanding of scale effects on vortex