Experiments with biconvex and double-wedge aerofoils in low-density, supersonic flow

Abstract

This report contains an analysis of the surface-pressuie distributions on two aerofoil sections of 10% thickness/chord ratio, at Math numbers near 2 and 4, and at Reynolds numbers between 93 and 1920, based on model chord. It is shown that with the exception of those hole positions within the upstream influence of the trailing-edge, all surface pressures over a wide range of incidence may be correlated using a viscous-interaction parameter 2, in a manner similar to that for the flat plate at zero incidence. For the front surfaces of the double-wedge aerofoil, the local inviscid flow conditions near these surfaces represent those of the equivalent free stream where these are used in formulating the appropriate value of X. For the rear wedge surfaces, and for the biconvex surfaces, correlation is achieved if it is assumed that the equivalent local flow only attains some fraction (about 0.6) of the surface expansion from the shoulder or the leading edge respectively. This reduction of the expansion angle is due to the presence of a very thick laminar boundary layer in the expansion region. A simple theoretical model is developed which provides an estimate of the effect. The pressure distributions have been integrated to give section lift and pressure-drag. The lift-curve slope is significantly higher than that predicted by inviscid theory due to the viscous-induced pressure increments. These also increase the pressure-drag so that the ratio of lift to pressure-drag is close to that measured elsewhere at high Reynolds numbers (around 6 x 10power5). The addition of skin-friction drag to the low-Reynolds-number data increases the section drag considerably and quite high incidences are needed before the lift/drag ratio exceeds unity.