An experimental investigation of apex fence flaps on delta wings
An exploratory wind-tunnel investigation was performed to observe the flow-field effects produced by vertically deployed “apex fences” on a highly swept delta wing. The objective was to obtain an initial evaluation of the potential of apex fences as vortex devices for subsonic lift modulation as well as lateral-directional control of delta wing aircraft. A full-span, 74 deg delta and a semispan, 65 deg delta wing were used with fence pairs varying from 4.19 to 6.75 percent of the main wing area. Two fence shapes (delta & cropped delta) and four fence orientations were examined. The vortex flow field was visualized at angles of attack ranging from O to 25 deg using helium bubble and oil flow techniques; upper-surface pressures were measured along spanwise rows. The results were used to construct a preliminary description of the vortex patterns and induced pressures associated with vertical apex fence deployment. It was determined that the relatively small apex fences, when symmetrically deployed, enhance the average suction level on the wing upper surface, which may amount to a 10 percent increase in the normal force over the angle of attack range of this test. Indications are that even higher suction levels may occur between the fences, producing a nose-up pitching moment for longitudinal trimming (i.e., when trailing-edge flaps are used for lift increment). Favorable lateral-directional characteristics may be obtained by deployment of a single fence which would depend on the side force acting on the fence itself and vortex-induced effects on the downstream surfaces.
