A comparison of results in the A.R.A. transonic tunnel on a small and a large model of a slender wing

Abstract

Force, moment and pressure-plotting measurements are reported on 75 deg slender wings of 60 in. and 20 in. lengths in the Aircraft Research Association 9 ftx 8 ft perforated-wall transonic tunnel. The object of the tests was to obtain some idea of the tunnel-interference corrections for slender-wing models. The maximum blockage correction on lift-curve slope for the larger model which has a blockage ratio of 0.36 per cent reduces an apparent Mach number of 1.00 to 0.975. Earlier tests on a swept-wing model with a blockage ratio of 0.5 per cent gave a correction of only about 0.02 in M, and hence the new results confirm the expectation that transonic interference for a given blockage ratio may be larger for slender models. No blockage correction on aerodynamic-centre position appears to be needed and it is explained that this could be due to the effect of the variation of blockage along the length of the model. Reflected wave-type interferences on the larger model has the effect of changing the steep rise in C D between M = 0.99 and M = 1.01 into a relatively slow rise to a maximum near M = 1.10 at which C D is about 0.001 too high. This is due mainly to reflections of the bow shock and forward compression region. The walls would have been more effective in alleviating these reflections if the models had been placed further forward in the working section. Also, it is probably true that the bow shock was somewhat stronger with these models than with other more typical slender wings. The results suggest that in general, tests on slender-wing models placed at the most suitable position in the tunnel should give reasonably interference-free results at Mach numbers above about M = 1.1. On the basis of the evidence presented in this paper, it should be possible to devise corrections even in the range M = 1.0 to 1.1 particularly if a few pressure-plotting points are included in the models under test.