Tunnel interference at compressibility speeds using the flexible walls of the rectangular high-speed tunnel

Abstract

The results of various measurements made in the National Physical Laboratory Rectangular High-speed Tunnel using the flexible walls are compared with theory in order to throw further light on the problem of tunnel interference at very high speeds. The dependence of the wall pressures and overall aerofoil forces on the wall shape has been investigated for two-dimensional tests of various aerofoils, though most of the work relates only to the low drag section EC 1250. It is concluded that the standard methods of 'streamlining' the walls to simulate free air conditions are satisfactory up to speeds at which the shockwave from the aerofoil first reaches one wall, which in ordinary cases occurs above about M = 0.85 for a low-drag 12 per cent. t/c section, or 0.81 for a conventional 18 per cent, t/c. The 5-in. chord is about as large as should normally be used, and in this case lift can be estimated from the streamline wall pressures, a correction being made for insufficient length of tunnel. If straight walls are used, the theoretical corrections to free air seem applicable up to top speed, and in this case the lift can be obtained from the wall pressures without addition beyond the end of the tunnel.