Critical Mach numbers for thin untapered swept wings at zero incidence

Abstract

In this paper, which is a continuation of two earlier ones (R. & M.'s 2713 & 2717), the subsonic flow past untapered swept wings, at zero incidence, is further investigated using linear theory. Methods for calculating 'lower' and 'upper' critical Mach numbers are given, the solution of the main problem being preceded by a short analysis of critical Mach numbers for the simpler cases of infinite wings (straight, sheared and yawed). The determination of critical Mach numbers depends on the knowledge of velocity distribution over the wing surface, the problem dealt with in the previous reports mostly for the case of the simple biconvex parabolic profile. These earlier results have been extended here to cover a wide class of profiles. Hence it has been possible to determine critical Mach numbers for wings with four different profiles, showing the effect of thickness ratio and of angle of sweep-back (or sweep-forward) in each case. The method applies strictly to wings of large aspect ratio, but no significant corrections are necessary except for very low aspect ratios. The results and examples, illustrated by a number of tables and graphs, provide a basis for more general discussion. Several conclusions concerning the practical use of swept-wing design are presented.