Wind Tunnels for Aeronautical Research

Abstract

THE Aeronautical Research Committee's “Reports and Memoranda No. 1569” (H.M. Stationery Office. 1s. net), recently issued, gives a description of the new open jet wind tunnel at the National Physical Laboratory, and also describes the preliminary model experiments carried out in order to ensure the most efficient aerodynamic performance from the actual tunnel. The results are a striking vindication of the exponents of the use of the principles of dynamical similarity in comparing the behaviour of objects of similar form but varying sizes. These principles offer a convenient, and often the only possible, way of investigating questions in aircraft design and aerodynamic problems generally. Two model tunnels were made, the second based upon experience with the first—and also the compressed air tunnel—in matters of the shape of the ducts, shape and positions of guide vanes at the corners, design of air screws, etc. The power factor of the models was subject to a large scale effect. At the jet speed mainly used during the experimental work, namely, 50 ft./sec., the power factor was 1-8. The variation with Reynolds's number indicated that a full-scale power factor of about 2-6 might be expected. The full-scale tunnel now completed has exactly equalled expectations. The distribution of velocity in the jet is as good as was anticipated, and the power factor has the predicted value of 2-6. The elliptical nozzle of the tunnel has a horizontal major axis measuring 9 ft. 1J in., and a minor axis of 7 ft. 0 in and an input of 375 B.Ht.p. at the airscrew yields an airspeed of about 210 ft./sec. in the jet. The final model is being used for further small-scale research.