| Effect of the leading-edge extension (LEX) fence on the vortex structure over the F/A-18 (1997) | |||||||||||||
Abstract | |||||||||||||
| The high performance of the F/ A-18 fighter in air combat manoeuvres is due in large part to its aerodynamic shape, and in particular, to the effect of the sharp, highly- sweptback leading edge extension (LEX) that extends forward from each wing root. At angles of attack typical in air combat manoeuvres, each LEX generates a large vortex above the aircraft. The lifting forces on the F / A-18 due to the LEX vortices give the aircraft its manoeuvring capability. The manoeuvrability of the F/ A-18 does not come without penalty, however. The centreline core of the LEX vortex can undergo the phenomenon of vortex bursting or vortex breakdown. This occurs when smooth, steady airflow along the vortex core suddenly breaks down and becomes disturbed and unsteady. The core also expands considerably in diameter. Under air combat manoeuvring conditions the unsteady flow downstream of the vortex burst position impacts on the fins and tailplane of the aircraft, causing high dynamic loads on these surfaces. Severe structural vibration results, with consequent detrimental effects on the fatigue life of the aircraft structure. To alleviate the vibration problem the aircraft manufacturer developed the 'LEX fence'. This is a flat trapezoidal plate standing perpendicular to the LEX upper surface and aligned parallel to the aircraft's longitudinal axis. The interference between the fence and the flow beneath the LEX vortex causes a reduction in the dynamic loads applied to the tail, with a consequent reduction in tail vibration and improvement in fatigue life. The LEX fence was developed using a cut and try process of wind tunnel and flight testing. The aerodynamic mechanism by which the fence achieved its favourable effect was not known. The purpose of the work described in this report was to develop an understanding of the details of the aerodynamic interaction between the fence and the LEX vortex. This understanding will add to DSTO's existing body of knowledge of the aerodynamic characteristics of the F / A-18, will assist in dealing with any future problems that may arise involving the fence, and will assist the International Follow On Test Program (IFOSTP) of fatigue tests on the F/ A-18 rear fuselage. It will also assist in the assessment of possible alternatives to the LEX fence should this ever become necessary. The approach to the problem was to use flow visualisation techniques in a water tunnel and in a wind tunnel to examine the detailed behaviour of the flow in the vicinity of the LEX fence. The vortex flow patterns over the F / A-18 are complex and flow visualisation provides a powerful tool to assist in the understanding of such patterns. In addition, measurements were made of the pressures acting on the aircraft fins and of the accelerations of the fin tips. The results show that the LEX fence achieves its effect by triggering the formation of a sequence of additional vortices that result in the favourable modification of the vortex breakdown flow. The flow around the fence itself leads to the formation of a vortex originating from a point on the LEX upper surface just outboard of the fence. This vortex in turn modifies the flow around the nearby LEX leading edge and causes the formation of yet another vortex, in this case originating from a point on the LEX leading edge outboard of the fence. This last vortex trails downstream over the LEX and as it does so it intertwines and interacts with the main LEX vortex. It is this interaction that modifies the LEX vortex breakdown process and reduces the dynamic loads on the aircraft fins. The outcome of this study has been a more detailed knowledge of the flow around the F/ A-18 under air combat manoeuvring conditions. In particular, the conditions leading to severe fin vibration, and the way in which these conditions are modified by the LEX fence, are better understood. This understanding will assist in the development of fin loading programs for structural testing, and may be of assistance in the future, should any problems arise with the fence itself. In addition, a detailed knowledge of the fence effect will be useful should it become necessary to seek alternatives to the fence for reducing fin vibration.. The effects of the Leading Edge Extension (LEX) fence on the vortex structure over the F/A-18 at moderate to high angles of attack were studied using a 1/9 scale wind tunnel model and 1/48 scale water tunnel model. Measurements at the tip of one of the vertical fins of the wind tunnel model confirmed that the fence reduced fin vibration caused by vortex breakdown. Flow visualisation in the water tunnel and in the wind tunnel showed that the fence caused the formation of a second LEX vortex, of the same sense as the main LEX vortex, but originating on the LEX leading edge outboard of the fence. Visualisation of the surface around the fence showed in detail how the fence caused the second LEX vortex. The effects on vortex breakdown and fin vibration of the interaction between the main and second LEX vortices are discussed.. DST | |||||||||||||
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