664,055. Scale-model testing of ram jets. LEDUC, R. March 10, 1949 [March 12, 1948], No. 6540/49. Class 4. Apparatus for scale-model testing an aerothermodynamic duct in a wind tunnel with simulation of the effect of combustion comprises a model 3 of the duct in combination with a coaxial member 6 mounted in that portion of the model corresponding to the combustion and exhaust zones of the actual duct, so as to reduce that part of the model to an annular passage, the member 6 extending downstream from the model without abrupt change of section for attachment at 7 to a mounting remote from the model so that the forces to which it is subjected are not transmitted to the tunnel balance. At any axial section the ratio of the area of the annular passage to the total internal area of the corresponding section of the actual duct is equal to the square root of the ratio of the densities of the heated air at the temperature obtaining at that section in the duct with combustion and of air at ambient temperature. Under such conditions the mass flow of air at ambient temperature through the model duct is the same as it would be without the member 6 in place if combustion were taking place in the model, having regard to the requirements of dynamic and thermodynamic similarity between the model and full size duct.
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