On the effect of a strake-like junction fillet on the lift and drag of a wing

Abstract

SummaryAn experimental study of the effects on the low-speed aerodynamic characteristics of a strake-like fillet is described, modelled on one used on an Airbus A320 variant fitted at the leading edge of a swept wing-plate junction. The wing, swept back at 20°, was of NACA 0015 section and chord 500 mm, both normal to its leading edge. A turbulent boundary layer had developed on the plate well ahead of the junction. The tests were conducted at a unit Reynolds number of 1.56 x 106 m-1.Surface pressure distributions were measured on the plate in the neighbourhood of the leading edge junction and also on the aerofoil and fillet at wing incidences of 0°, 3°, 6°, 9° and 12°. These were supplemented by surface oil-flow studies.The mean velocity and turbulence intensity fields around the leading edge were examined for incidences of 0° and 9°, using both a single tube yaw meter developed for the purpose and an X-wire anemometer. The X-wire anemometer was also used downstream of the trailing edge of the swept wing; five of the Reynolds stresses and the mean velocity field were measured.The sectional lift coefficients on the aerofoil were found to diminish as the junction was approached, slightly more so with the fillet than without it. The sectional drag coefficients due to pressure increased as the junction was approached, the fillet moderating this increase to only a small extent.However, the addition of the drooped fillet modified the flow considerably. The horseshoe-like vortex was less well defined than without it. At zero incidence, the peak in the turbulence intensity levels was virtually eliminated on what became effectively the compression side of the wing due to the local camber introduced by the asymmetric fillet. The turbulence levels were also reduced by the addition of the fillet at an incidence of 9°. However, the turbulent activity was spread through a larger proportion of the viscous region. The secondary flows and the turbulence activity in the wake are associated with unrecoverable kinetic energy and will be manifest as drag on the surfaces forming the junction.It is concluded that a carefully designed fillet, optimised for the cruise incidence of an aircraft, can reduce the peak turbulence levels in the junction. It remains unclear whether the total drag associated with the junction flow can be reduced significantly.However because of its effects on the turbulence, there may be other benefits, for example on the efficiency of downstream elements, such as fuselage-mounted engine intakes or the following stages of an axial flow machine. Junction fillets might also be used to control the scouring of river beds around bridge piers.