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Aviation History
1951
1951 - 2281.PDF
620 FLIGHT, 16 November 195? HIGH-SPEED FLUTTER . . . The lecturer then referred to the positions of the curve which were covered by different testing techniques. For ground-launched-rocket tests, which were generally made at near constant density and temperature, the approach to the experimental curve was essentially along a vertical line, such as A in Fig. 2. Such rocket techniques were useful primarily for determining the lower part of the experimental curve, namely, the portion of the flutter curve that had a positive slope. By the use of freely falling bodies, it was possible to avoid the lower part of the flutter curve and to obtain data for the upper part in which the slope was negative. The reason for this was that, for a given wing, the value of N was quite high for high altitudes; and, because of the change in mass ratio, N progressively became less as the body fell to lower altitudes, while M steadily increased. The approach to the experimental curve was along a line which curved up and to the left, such as B in Fig. 2, thus making an inter- section with the flutter curve in the supersonic range of M. Air. Garrick then compared some calculations for a wing with concentrated weights. He said that further development of this subject, as well as many other parts of the flutter field, seemed to depend in large measure on the greater ease and facility in the making of the calculations (as well as on improved assumptions). Many of the aircraft companies in the United States were making good use of the I.B.M. card- programmed sequence methods. The developments now taking place in the field of automatic digital computers could not be overlooked, as these developments would undoubtedly play an increasingly important role. Dealing with flutter examples based on a single model each of wing bending and of wing torsion, and on the use of two- dimensional aerodynamic forces for compressible flow, Mr. Garrick then referred to Fig. 3. The ordinate V\VO was the speed ratio, that is, the flutter speed V, calculated by the two- dimensional theory with compressible flow coefficients divided by Vo, the speed calculated with incompressible flow theory. (The particular curves A and B were calculates for two plan forms each of angle of sweep A of 45 deg a shown. For A, the mass ratio 4ml(npc2) was 40, the ratio of the bending to the torsional frequency was 0.38, and the length/chord ratio 1/c was 3. For B, these values were respectively, 150, 0.16 and 2.5. The lecturer then mentioned some of the techniques being Fig. 4. Bending- type flutter on swept wings. A tends to B for lower air density, lower bending tor- sion frequency ratio and lower ratio of Ijc to tan X. used for flutter research in the Langley 4.5ft flutter research tunnel. This tunnel, he said, was a variable-density and a variable-medium subsonic closed tunnel by the use of Freon gas. Freon 12 was a well-behaved gas, available commer- cially, in which at standard conditions the speed of sound was less than half that in air, and which had a density more than four times that of air. The use of air and Freon 12 interchangeably, and in mixtures with variable density, permitted the use of a single model to yield "trends" research and not just "point" research. ULTRASONIC TYRE-TESTING AN instrument which embodies one of the most importantapplications of industrial ultrasonics, the non-destructive testing of materials, has recently been put into operation in the works of the Dunlop Rubber Co., Ltd., at Birmingham. The instrument, which has been developed jointly by the Dunlop Research Centre and the General Electric Co., Ltd., is designed for the production testing of aircraft and motor vehicle tyres of all The method developed depends on the fact that any internal discontinuity, e.g., imperfect bonding between rubber and fabric, will necessarily lead to .the presence of an air film, and that this rubber/air boundary will cause almost 100 per cent reflection of ultrasonic waves which reach it. With new tyres a fault of this type is possible, though rare, but with used tyres it is common and may make it inadvisable to renew the treads, notwithstanding the fact that the tyre is superficially sound. The instrument is used for tyre testing on a production basis and can detect faults having an area of $ in by | in or more. It TRANSMITTER RECEIVER RECORDING AIR FAULT. ZERO READING AND RED LIGHT TRANSMITTING CRYSTAL AIR CAVITY ENERGY REFLECTED consists of a steel cabinet which contains an ultrasonic generator and valve amplifiers, together with pneumatic handling equipnlcnt for lowering the tyres into an adjacent tank of water, where they are tested. The reason for using the water bath is that water produces a good transmission path between transmitter and receiver. If a dry contact was used it would be practically impossible to exclude, air. This would result in nearly 100 per cent reflection of the wave, which would be unable to pass freely into the rubber. Projecting from the front of the cabinet is a pneumatically- operated extensible arm, which can be given a cycle of operations to enable it to move outwards and to project beyond the tank so that a tyre can be loaded on to it. The arm then lifts the tyre, moves it inwards and lowers it onto two rollers in the tank, whereby the tyre can be rotated. The tyre is first revolved quickly for wetting purposes, then flowed for the test and, if necessary, further flowed or stripped for precise location of a flaw. The ultrasonic transmitter head houses a f in diameter quartz crystal mounted in a brass holder placed one inch from the rubber, in the well of the tyre, which is under the water. The ultrasonic beam is radiated at an angle of 120 degrees'and a single transmitter within the tyre can be used with up to six receiving crystals spaced around the outer wall. Leads are taken from each of the receiving crystals, to separate amplifiers, and the outputs of these are used for visual indication on six meters and for the operation of six alarm circuits each consisting of a relay and a red indicator lamp. SIGNAL LAMPS Diagram of test unit, showing disposition of crystals relative to the tyre GERMAN R.D.F. TRANSLATIONS of nine papers prepared by German tech-nical experts and concerning some novel arrangements to improve the accuracy of both radio direction- and position-finding are contained in Radio Direction-Finding and Navigational Aids- Some Reports on German Work Issued in 1944-45, which has been published for the D.S.I.R. by His Majesty's Stationery Office, price 3s 6d (3s 8d by post). Some of the papers contain the results of fundamental investiga tions designed to demonstrate the limitations imposed by wav- propagation and other conditions on the accuracy of direction an • position finding. The papers are published under the names c the original authors; the translations are by the Admiralty.
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