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Aviation History
1957
1957 - 0009.PDF
FLIGHT, 4 January 1957 TRANSONIC TUNNELS —and Testing Techniques: All-day R.Ae.S. Discussion THE experiences of a wide selection of wind-tunnel users fromindustry and government establishments were disclosedduring an all-day discussion on Transonic Wind-tunnel Testing Techniques, organized by the Royal Aeronautical Society on December 20 in London. The basis of the general discussion on the subject consisted of five main papers, as summarized below. The chairman of the meeting was Maj. G. P. Bulman. The first paper was by Mr. H. F. Vessey, B.Sc, F.R.Ae.S., of the Royal Aircraft Establishment, Farnborough, who gave an historical and general review of the development of vented walls for transonic tunnels. The four main advantages of ventilated walls, he said, were the reduction or elimination of tunnel cor- rections at subsonic speeds (alternatively allowing larger models for the same corrections); the prevention of choking at high subsonic speeds; the establishment of uniform transonic flow (from approximately M = 0.8 to 1.4); and the reduction or elimination of Shockwave reflections. The impetus for the development of such tunnels, Mr. Vessey continued, had come from the N.A.C.A. in America, who had produced a report on the subject in 1948. The speaker went on to trace the main steps in this development since then, on both the theoretical and practical sides. The first British company to have a ventilated-wall tunnel in operation was English Electric, whose 12in tunnel was put into use in 1950. Flow through the ventilated walls could be established either by diffuser suction or (the more generally used method) by means of a separate auxiliary-suction installation. After describing the respective general principles of operation and the various possible practical arrangements, Mr. Vessey turned to consider again the four main advantages provided. Most tunnel corrections could be eliminated or calculated, and choking could be avoided in most installations; which left the establishment of uniform flow and the elimination of Shockwave reflections as the two objectives to which most effort was directed. The problem of Shockwave reflection was probably the most controversial, with a difference of opinion between those who favoured slots and those who believed in perforations as the best means of ventilation. The Shockwaves would be cancelled at the tunnel wall if the open-area ratio was correct: if this ratio were too small (in a perforated wall), weak reflected Shockwaves would appear; if too large, there would be weak reflected expansion waves. For the modification of an existing tunnel, the incorporation of slots (it was claimed) would provide the easier solution. The open-area ratio for a slotted wall would be approximately half that for a perforated wall. Examples of each type were the R.A.E. 8ft x 6ft transonic tunnel (slotted), and the A.R.A. tunnel (perforated). The speaker concluded by describing the conver- sion of the 10ft x 8ft high-speed tunnel at Farnborough to the 8ft x 6ft transonic installation. Measurements had been made with both plain slots, and slots backed by perforated plates, and calibration tests were continuing. The auxiliary suction plant would be used also to drive a small additional tunnel. Three-turbojet Tunnel The second speaker was Mr. J. A. Kirk, M.I.A.S., A.F.R.Ae.S., of the de Havilland Aircraft Company, and his subject was Design and Operational Problems of the Jet-driven Transonic Wind-tunnel. Information obtained from the U.S.A. formed the basis of the design of the transonic working-section of his com- pany's tunnel, Mr. Kirk said, and diffuser suction was employed because the source of power available (three Ghost turbojets) was sufficient to provide the required pressure-ratio without auxiliary power. The speaker went on to consider the requirements of a transonic working section and to give details of the de Havilland transonic installation. The main problems encountered had included those of temperature as the speed of the air increased. The jet engines provided a convenient source of heat for drying the air, and a proportion of the exhaust from the turbojets was used for this purpose. Modifications had been needed to obtain an even velocity distribution along the working section. Typical air temperatures were 80 deg C (subsonic), 100-120 deg C (transonic), and up to 160 deg C (at M = 1.6), and a temperature shock on the Schlieren window had been the first problem caused by these conditions. Another problem, Mr. Kirk said, was that the internal straingauge balance had to be designed with these high temperatures in mind, and difficulties also arose in the con- struction of the model itself. The Mach number distribution along the working section, which was at first affected by a series of expansion waves, had been improved by the insertion of perforated plates behind the slotted walls. Overall Mach Number variation was ±.005 throughout the transonic range. Another problem was the measurement of pitch in the airstream; variations had been found and a modified plenum chamber was to be used for further tests. Finally, the speaker reviewed the effect of shock reflection on the model and discussed the value of Schlieren pictures in inter- preting the measured results. A detailed description of the A.R.A. tunnel at Bedford (described in Flight of May 4, 1956) was next given by Mr. R. Hills, B.A., A.F.R.Ae.S., chief executive of the Aircraft Research Association, in his paper Design and Operational Problems of the Electrically Driven Transonic Wind-tunnel. This tunnel has a 9ft x 8ft working section incorporating perforated walls. A separate auxiliary suction plant had been adopted, Mr. Hills said, because the total power required would be less than that of diffuser suction with the main fan drive. Flexibility was an added advantage, enabling a small extra tunnel to be operated. The addition of a flexible supersonic nozzle had made possible the extension of the speed range up to M = 1.4. After giving results obtained from the calibration of the tunnel, Mr. Hills disclosed that additional perforations upstream of the original perforated walls had been incorporated in the A.RA tunnel, in order to attain the required increase in velocity ahead of the nose of the model. There was considerable variation in the uniformity of flow at Mach Numbers from 1.0 to 1.4, and this problem was being investigated. The size of the holes in the perforated walls was 0.5in, and the open area was 22 per cent (for Mach 1.4). Measurement of Speed The general discussion at the morning session was opened by Dr. D. W. Holder of the National Physical Laboratory, who raised the question of the measurement of tunnel speeds. He doubted the validity of using the plenum-chamber pressure to obtain the flow velocity in the tunnel, and thought the optimum position of tunnel-speed measurement varied with Mach Number. Might it not prove difficult to measure the speed with an insufficient working-section length upstream, and what length was needed downstream? Replying, Mr. Hills agreed that speed measurement was a real difficulty, but thought that the plenum-chamber pressure could give a good idea of tunnel flow. Mr. Kirk confirmed that plenum-chamber pressure was normally used in the D.H. tunnel to regulate tunnel speed. The slotted length of the working section was 7ft, and its height was 2ft, i.e. the ratio was 3.5 :1, and a length of about lft lay behind the model. Mr. Vessey agreed that the plenum-chamber pressure method of speed measurement seemed generally satisfactory. The next speaker was Dr. W. F. Hilton, of Armstrong Whit- worth, who gave details of his company's transonic tunnels (described in Flight of June 17, 1955). These comprise an 8in intermittent tunnel driven by compressed air, and a larger, elec- trically driven continuous tunnel. Because of the pressure ratio available, the speaker submitted that the operation of jet-driven transonic tunnels was marginal. After showing a graph of pres- sure ratio plotted against mass flow, Dr. Hilton explained that, on the large A.W.A. tunnel, maximum motor speed was maintained throughout and the compressor was throttled as required. Running costs (power only) for his company's electrically driven tunnel were about £30 per hour, compared with some £100 per hour for kerosine. The speaker expressed the opinion that perforated walls gave less Shockwave reflection than slotted walls. In reply, Mr. Kirk estimated the fuel cost of running the D.H. tunnel at an average of about £62 10s per hour. Dr. S. Neumark, of R.A.E., put forward the general submis- sion that, although tunnels were being used for the measurement of stability in aircraft, the tunnels themselves had a problem of instability—which, he felt, should be investigated from basic principles. Wind-tunnels, he reminded the meeting, were not a purpose in themselves: they remained only tools, and he hoped that the results obtained in these transonic tunnels could soon also be discussed. ~ i -—t [Contd. overleaf.
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