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Aviation History
1946
1946 - 2073.PDF
OCTOBER 17TH, 1946 F LI G HI 417 High-speed A irk* low A Critical Review of Qerman Research Methods, Equipment and Results By R. SMELT, M.A., A.F.R.Ae.S. A MOST interesting paper was read before the RoyalAeronautical Society on October 9th by Mr. R.Smelt, who is head of the Supersonic Division of the Royal Aircraft Establishment at Farnborough, dealing with German research on high-speed airflow. Mr. Smelt recalled that the greatest contribution to high- speed research vas made by the D.V.L. (Deutsche Ver- suchsanstalt fiir l.uftfahrtforschung) and the twin organi- zations of K.W.I. [Kaiser Wilhelm Institut) and A.V.A. (Aerodynamische Versuchsanstalt) at Gottingen. A third station, the L.F.A. (Luftfahrtforschungsanstalt Herman Goring), neai Brunswick, elaborately camouflaged, had not got really started until 1942, and had thus con- tributed less than the older institutions. The work on supersonic flow during design of V-weapons done by the Peenemunde group was directed by the Army and not by the Air Ministry, but had considerable importance in its application to high-speed aircraft of the future. The characteristics of the more important German high- speed tunnels are given in the table. The lecturer remarked that their 2.8 m tunnels gave a Reynolds Number of about 7 millions at high Mach Numbers, compared with British 1 to 2 million, but that the Germans did a ridiculously small amount ot full-scale flight research. Uncertainties in the correction for tunnel wall constraints near the speed of sound, and in the effects of wind tunnel turbulence on shock formations, made it imperative to have continual checks with the behaviour of aircraft in free flight. German subsonic wind tunnels were in general very efficient. This was achieved by making a long expansion from the end ot the working section to the first corner. That made the tunnels long and costly. Some interesting work on the effect of Reynolds Number on flow with shock around an aerofoil was done by Dr. Gothert in the 9ft tunnel of the D.V.L. He found that pressure distribution in the region of the shock wave on the, aerofoil was much changed with Reynolds Number. The pressure gradients near the shock became greater as R.N. increased and finally settled down to a very steep gradient, more or less inde- pendent of R.N , above about 5 or 6 million. German supersonic wind tunnels were almost without exception intermittently operated by evacuating a con- tainer, usually spherical, and drawing in air from the atmosphere through the working section. Evacuating plant of sufficient power to exhaust the reservoir in a few minutes was used. The most interesting innovation was Fig. 1. Working section of vertical wind tunnel Ao.a. Aram-jet model is mounted for test. that of a second throat following the working section, made variable in area by a series of screw jacks operating on flexible walls (Fig. 2). In the closed supersonic tunnels its aim was to improve the efficiency by diffusing to a Mach Number not greatly in excess of unity before the final shock wave in the tunnel. The most notable feature of German high-speed tunnel equipment was the introduction of the Mach interfero- meter as a method of examining the velocity field around the model. The principle was that diffraction fringes were obtained by the interference of two light beams, one of which passed through the tunnel airstream, while the other went around it. Changes in the density of the airstream, produced by the presence of the model, resulted in displacement of the fringes, the displacement being proportional to the density change. A typical interference pattern, together with a simul- taneous schlieren photograph, is shown in Fig. 3. If the interferometer mirrors are set exactly parallel, so that the undisturbed stream produces no interference fringes, then the disturbance due to the model in the Fig. 2. The supersonic wind tunnel at Koche^ showing adjustable throat.
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