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Aviation History
1952
1952 - 0053.PDF
FLIGHT, 4 January 1952 19 BOUNDARY-LAYER CONTROL Some Basic Considerations: Methods of Research and Results Achieved By A. G. THOMSON THIS simply expressed, succinct review of boundary-layer control should be helpful to all those who have but a nodding acquaintance with the subject. Today, interest in this branch of aerodynamics is considerable, for it has long held out the tantalizing promise of reducing drag and/or increasing lift: until the gas turbine arrived, however, there was not much hope for the practical employment of this attractive system. THE conception of a thin "boundary layer" surrounding the wing of an aircraft in flight was first put forward in 1904. Even at that early stage in the history of aviation it was realized that the layer could be controlled by withdrawing it through a slot, thereby preventing its separation from the surface. It was later established that, at a certain position depending on various factors, the airflow within the boundary layer ceased to be laminar in character and changed to a state of turbulence which resulted in increased drag. This change is called "transition." Various attempts have been made to reduce the drag or increase the lift of a wing by removing the whole or part of the boundary layer before transition or separation could occur. By the outbreak of the Second World War a number of experiments had been made, but without very much success; for the most part, research was confined to sucking the boundary layer through slots cut in various positions on already existing aerofoils. By 1944 methods had been developed for designing wing sections to have any specified pressure-distribution, and this led to a revival of interest in the possibility of controlling the boundary layer. It was known at that time that neither boundary-layer transition nor boundary-layer separation (or break-away) was very liable to occur if the pressure was decreasing in the direction of the flow. A logical step forward, therefore, was for further studies to be carried out on wing sections so designed that the pressure tended to fall in the direction of the flow for the maximum possible distance, with the result that the boundary layer remained laminar for as long as possible. The methods of aerofoil design which had opened up these new possibilities for reducing drag were developed at the National Physical Laboratory by Prof. S. Goldstein. It was natural, therefore, that the Aerodynamics Division of the N.P.L. should be asked to undertake further investiga tions on the effectiveness of boundary-layer suction as a means of improving the low-drag properties of aircraft wings. The procedure adopted was to design sections with suitable pressure distributions and test them in wind tunnels. Dr. A. A. Griffith suggested that, since it was impossible for the fall in pressure to be maintained throughout the entire length of the section, attention might be accorded Fig A. GLASII aero foil section with theoretical pressure distribution at zero lift. to the feasibility of letting it continue to some chosen far-back position, and then have a sudden discontinuous rise, succeeded by another continual fall. When a section is designed on this basis the shape of the aerofoil becomes rather like that of a tadpole. Since the introduction of an abrupt pressure-rise renders the boundary layer even more liable to break away at that point, it is necessary to withdraw some or all of it by suction through a slot, or, alternatively, to blow air out parallel to the surface and so re-energize the boundary layer. Both methods have been tried, but so far most work has been done on suction, though there is still a possibility that blowing might give better results in certain practical applica tions. Greater attention has hitherto been devoted to suction, because preliminary investigations suggested that the quantity requirements were likely to be rather less than for blowing. On the other hand, the possibility that it may be easier to blow has not been overlooked. At this stage the comparative advantages of the two methods cannot be finally assessed. Expediency The investigators have tested several sections designed on the principle of the sudden discontinuous rise, the results obtained being in reasonable agreement with the theoretical predictions. The practical possibilities of this development appear to be governed largely by economic factors, the most important consideration being the amount of power required to propel an aircraft. If the wing sections are of the special suction-type, the power normally required to overcome the effects of external drag is very greatly reduced; but, on the other hand, this saving is to some extent offset by the power necessary to provide the suction. The investigators find that the added complication of suction is more likely to be justified in the case of thick wings, which offer structural advantages as well as other important gains such as increased storage capacity. The first aerofoil tested was 16 per cent thick, but, when it was realized that greater gains might be expected from thicker wings, it was decided to carry out further studies with a 30 per cent section. In common with the previous sections this was constructed with two slots located respectively on the lower and upper surface, but at this point the designers suggested that there should be only one suction slot. The next wing, known as the GLAS II (Fig. 1), was therefore designed with a single slot on the upper surface. This design has been taken up in Australia, where test flights have been carried out on a glider fitted with wings of this type. Certain difficulties were experienced, but these can be overcome, and the performance of the glider is already very satisfactory. Another method of sucking-in the boundary layer is to Fig. 2. Clauert nose-slot aerofoil with curves of theo retical pressure dis tribution at 10 deg incidence.
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