Experimental Investigation of a Wing with Controlled Midspan Flow Separation

Abstract

A wing with a bounded region of separated flow at high angles of attack was tested in the McDonnell Aircraft Company low-speed wind tunnel. The model configuration was chosen to provide a test case for the develop- ment of an analytical method for the calculation of the flow about stalled wings. Extensive data were obtained on wing surface pressure, the boundaries of the separated flow bubble above and aft of the wing, and flow visualization. Flowfield velocity measurements were integrated to determine the displacement thicknesses of the viscous flow, which were subsequently used to determine an equivalent inviscid shape. The equivalency was verified by comparing the calculated potential flow pressures of this shape with the measured pressures. AVORABLE aerodynamic characteristics in the stalled flight regime are becoming increasingly important to modern fighter aircraft as flight envelopes are expanded to exploit the high-angle-of-attack combat environment. This requires a capability to design departure resistance into fighter aircraft in the early stages of development. Recent evidence for unswept wings suggests that controlled midspan separation, in concert with attached inboard and outboard flow, may significantly improve departure characteristics by softening the abrupt stall associated with other separation patterns.1-2 It is not obvious at this time that this type of controlled stall progression is the best design approach for increased departure resistance for fighter aircraft. However, it is desirable to determine analytically the stalling charac- teristics of fighter wings in order to supplement develop- mental wind-tunnel testing. To do this, it is necessary to predict the conditions under which flow separation will occur, regardless of the type of flow separation. In addition, it is necessary to predict the loads on the wings and control sur- faces when there are regions of separated flow present. Analytical techniques are being developed at McDonnell Aircraft Company to model regions of flow separation and to predict the aerodynamic forces under these conditions.3 To support this effort, it was necessary to provide experimental data for the description of the flow within the separation bubble and additional wing flowfield data. On the basis of a preliminary analysis, the required data were determined to be: 1) extensive surface pressure measurements to provide estimates of spanwise pressure gradients, 2) measurements of the boundaries of the separated flow bubble above and aft of the stalled wing, 3) an estimate of the shed vorticity, and 4) flow visualization data to identify regions of flow into and out of the bubble. A review of the pertinent experimental data indicated that no data were available that satisfied all of these requirements. Therefore, an experimental program was initiated, under MCAIR Independent Research and Development funding, to supply the necessary measurements.4 This paper discusses the results of this ex- perimental effort.