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Aviation History
1952
1952 - 1784.PDF
778 FLIGHT, 27 June 1952 HIGH-SPEED AERODYNAMICS —at the N.P.L.: Progress Reviewed in Annual Report AMONG the best values for money offered by Her Majesty's Stationery Office is the annual report of the L work of the National Physical Laboratory. The report for the year 1951 is now available, a 70-page publica tion priced at 3s (by post 3s 2d), and it gives condensed sur veys of the work of the several divisions which together make up the establishment. The Aerodynamics Division has been mainly engaged during the year on basic research for the Ministry of Supply, particularly on new aerodynamic problems connected with the design and performance of high-speed aircraft. Investigations have also been made for the Admiralty, the Ministry of Transport, research associations and private firms. Progress has been made on the scheme to provide the new wind runnels needed for future research. The plans made for the new High Speed Laboratory on the West Aero Site are now being implemented. Work on a building to house three 500-h.p. com pressors has started. The first high-speed wind tunnel (36m by 14m) is being made and a contract for the second tunnel (25m by 2oin) has been placed. A tentative layout for a new low- turbulence pressurized wind tunnel (8ft by 6£ft) has been pre pared. As in past years, close liaison has been maintained with the Royal Aircraft Establishment and with the aircraft industry for exchange of views on current aerodynamic research and its bear ing on aircraft problems. Experiments have continued on the maintenance of laminar flow on an aerofoil by withdrawing boundary-layer air through porous material. The new thin nose-slot aerofoil designed for high maximum lift has now been tested in the 4-ft tunnel. With moderately large suction quantities the maximum lift coefficients obtained were not greatly different from those expected when the section was designed, but with the low suction quantities which it was hoped would be sufficient there was no improvement on previous designs. The improvement of stalling characteristics by discharging air over trailing edge flaps has also been kept under review. Consider ably increased maximum lift coefficients can be obtained by this means for both straight and swept-back wings, but power require ments for swept-back wings are likely to be rather large and the increased pitching moments may prove troublesome, particularly for tailless aircraft. A systematic investigation of the aerodynamic characteristics of three swept-back wings of different aspect ratios has been started in wind tunnels. The object of the work is to provide information for modifying lifting-surface potential flow theory to allow for viscosity. Measurements in the boundary layers and wakes will be included. The basic aerofoil section of the model wings has been tested in two-dimensional flow to provide essential data to link with three-dimensional test results. A cambered version of the aerofoil has also been tested in two-dimensional flow to give values for the camber derivatives involved in wing-loading theory. Further tests of a symmetrical aerofoil on the Whirling Arm have also provided a measure of these camber derivatives. A critical comparison of routine methods for calculating the lift slope and aerodynamic centre of symmetrical two-dimensional aerofoils has been developed and described in a recent report. Calculations made for families of plan forms show the effects of compressibility and aspect ratio on the loading of swept wings; related wind-tunnel tests are being made to provide a link between theory and experi ment. Calculations of compressible flow for aerofoils of various thickness-chord ratio are planned; allowance for wake effects and comparisons with wind-tunnel results are to be made. Much work has been done on wind-tunnel interference both for two- and three-dimensional models; tables are now available for computing, in particular, wind-tunnel interference for swept-back tapered aerofoils in closed rectangular tunnels. The effect of tunnel blockage at high subsonic speeds is being investigated by two- dimensional theory. Attention has been given to the use of high speed computing machines for the solution of aerodynamic problems; for example, the ACE (Automatic Computing Engine) Pilot Model has been used to solve the large number of linear simultaneous equations which arise in wing-loading calculations. Tables based on the approximate numerical method of aerofoil design are being prepared which will permit the calculation of the velocity distribution over the surface of a given two-dimensional aerofoil when its co-ordinates are specified at convenient chordwise stations, instead of at equal intervals of the angular co-ordinate in the conformal transformation. A technique for use in the exact method of design is being developed for obtaining a specified leading-edge radius of curvature. A catalogue has been prepared giving details of all aerofoil sections which have been designed at the N.P.L. and which have been the subject of theoretical investi gations, aircraft design-studies or wind-tunnel tests. Tests in the compressed air tunnel have confirmed that the maximum-lift characteristics on bluff-nosed aerofoils like the H.S.A. V aerofoil are very dependent on small changes in nose shape. The programme of research on high-speed flow consists mainly of investigations designed to give information on fundamental problems and, where possible, comparisons with theory. Several such investigations have been made during the year. Considerable effort has also been devoted to the design and development of new techniques and apparatus. Detailed studies of the flow round thin two-dimensional aero foils at high subsonic speeds have been continued in the 2oin by 8in wind tunnel, and work on aerofoils fitted with control flaps is to begin soon. The models are so constructed that transition from laminar to turbulent flow in the boundary layer can be fixed near the leading edge by allowing a minute quantity of air to flow into the boundary layer through a span-wise row of small holes. This technique enables tests to be made with either laminar or turbulent flow over most of the surface, and thus permits the scale effects associated with the low Reynolds numbers of the tests to be studied and minimised. The maximum angle of incidence used in tests in the 2oin by 8in tunnels is usually sufficiently large for detailed information to be obtained on the flow changes which occur in the region of maximum lift coefficient. Further information on scale effect at high subsonic speeds is to be obtained by tests on an aerofoil in the i8in by 14m tunnel over a wide range of Reynolds number. Tunnel Development An examination of tunnel-wall interference on a two-dimen sional aerofoil at Mach numbers close to unity has been completed in the 9m by 3in tunnel, and the information obtained is to be used to extend the Mach number ranges of the 2oin by 8in and 9^in by 9|in tunnels. The 9m by 3m tunnel has also been used at high subsonic speeds for photography of the flow round a two- dimensional aerofoil performing pitching oscillations similar to those in the damping measurements made in the 9|in by gjin tunnel. The photographs show that the major difference between the steady and oscillatory flow is that of phase; the phase differ ences can be deduced from the direct measurements of damping and are found to be in reasonable agreement with those observed. A small direct-discharge tunnel has been completed during the year. It operates at Mach numbers between 1J and 4 with an intake pressure which can be varied between about £ atmosphere and 20 atmospheres, to enable tests to be made over a wide range of Reynolds number. The tunnel is being used for an investigation of the interaction of an o*blique shock wave of adjustable strength with the boundary layer on a flat plate at Reynolds numbers up to about 15 million. Measurements of the skin friction on the plate and of the boundary layer profile are to be attempted. Further improvements have been made in the technique of schlieren photography, and, with the collaboration of the Electricity Division, in the design of short-duration light sources. Exposures down to about 10-7 second have been obtained and have proved to be particularly useful for examining the micro-structure of wakes. The calibration of the i8in by 14m pressurized wind tunnel at subsonic speeds has been completed; the calibration at supersonic speeds is in progress. The ancillary equipment now installed includes manometers, a schlieren apparatus and a Mach-number gauge. A three-component balance using wire-resistance strain gauges is being constructed. Modifications and repairs to the nin by nin supersonic tunnel have continued throughout the year; a new working section and cooler have been constructed, and the control circuits have been re-designed. The re-blading of the axial-flow compressor is to begin shortly and it is hoped that the tunnel will come into service within the next six months. When the i8in by 14m and r 1 in by 11 in tunnels are available for experi mental work, the ranges of both Reynolds number and Mach number that can be covered will be increased appreciably; it will then be possible to do more work on bodies and aerofoils of finite aspect ratio than formerly.
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