The variation of profile drag with Mach number up to the critical value; a comparison of recent predictions with early flight and wind-tunnel measurements and a comment on an earlier prediction

Abstract

Flight measurements made by Mair in 1942, with the wake-traverse method, are used to extend to high Reynolds numbers (2 x 107) and a thick section (18 per cent t/c) the verification that the sub-critical creep (slow increase with increasing Mach number) in profile-drag coefficient can now be predicted accurately. Good agreement is also obtained between the absolute magnitude (as distinct from creep) of the measured drag and of that predicted on the basis of boundary-layer transition having occurred near the leading edge. Similar agreement is obtained with a drag coefficient measured on a twodimensional aerofoil (R = 1.6 x 106) of an almost identical section with transition positions now known to have been at 39 per cent chord. The analysis shows that predicted profile drag, particularly at high sub-critical Mach numbers, is sensitive to the actual compressible-flow pressure distribution used in its derivation, and that the errors due to inaccuracies of the older compressibility laws can be especially important for sections of this thickness, with the particular type of velocity distribution involved. This last point is shown to be the probable reason for the failure of the analysis of Young and Winterbottom 6 to predict any significant sub-critical drag creep.