The calculation of lift slopes, allowing for boundary layer, with applications to the RAE 101 and 104 aerofoils

Abstract

The need to know two-dimensional low-speed characteristics accurately in designing swept-back wings for transonic aircraft has led to a revival of interest in calculations of the loss of lift below the ideal-flow value caused by the boundary layer. The existing methods for performing this calculation have now been reduced to a straightforward routine. This is sufficiently sensitive for coefficients to be evaluated at rather small incidences, e.g., α = 2 deg, for which the boundary-layer part of the calculation is tractable; from these results the lift at higher incidences may be estimated. The method has been applied to RAE 101 and 104 sections of 6, 10 and 15 per cent thickness at R = 10power6, 10power7 and 10power8 with a wide range of transition positions. Good agreement exists between prediction and measurement for the 10 per cent thick 101 section tested by Brebner and Bagley. The predictions have also been compared with those of the Royal Aeronautical Society data sheet Wings 01.01.05, but the measure of agreement in this case lies just outside the assessed accuracy of the data sheet, Although the discrepancies are not fully accounted for, they may be due in part to over-simplification of the effects of transition and incidence on the data sheet, and the present method should, it is thought, yield accurate results when the correct transition positions are used. An Appendix shows that circulation in the wake, which is treated as zero in Refs. 1 and 2, leads to reduction of lift by a factor estimated as 1 - 0.214 √CD (CD = drag coefficient); this has not been taken into account in these calculations, since CD is of the order of 0.005.