635,609. Wind tunnels. ENGLISH ELECTRIC CO., Ltd., ELLIS, D. L., FOWLER, R. G., HARRISON, H. C., and CREASEY, R. F. Nov. 4, 1947, Nos. 29374/47 and 9312/48. [Class 4] A wind tunnel plant is operated by at least one gas turbine jet engine connected in the duct between the working section and a discharge. diffuser so as to act both as a suction pump and ejector pump. The air entering the working section may be uniformly heated by engine exhaust gases or a core of cold air may be formed within a surrounding flow of heated air to enable measurements at high Mach number to be recorded. In the embodiment shown a gas turbine jet engine 1 is mounted in a nacelle 2 which in turn is surrounded in spaced relationship by a bulge 3 in the tunnel wall. A diffuser section 4 supplies a portion of the jet efflux to a collector 13 ducted to an intake 14 feeding a chamber 11 which, by this arrangement, is supplied with a mixture of cold and heated air controlled by the valves 15, 16. The heated air from the chamber 11 enters the working section 7 through a flare 8 into the mouth of which is introduced a secondary flare 19 to admit a core of cold air into the centre of the surrounding heated air to enable measurements to be recorded at a high Mach number. The temperature of the core may be controlled by the valves 20. In alternative arrangements described in the first Provisonal Specification a flow of air of uniform temperature may be circulated by omitting the secondary flare 19, or the heat exchange between the jet efflux and the intake air may be effected by a surface heat exchanger or that portion of the tunnel between the sections 8 and 4 may be operated as an open circuit. In the further embodiment described in the Complete Specification the tunnel may be branched on the downstream side of the working section to enable two or more jet engines to be operated.