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Aviation History
1988
1988 - 3070.PDF
layer suction, anti-icing, and anti-insect contamination with a system which purges fluid through the microscopic holes in the titanium skin. The aircraft was used to prove (or disprove) the practicality of drag reduc tion techniques for airline service by oper ating it on more than 50 flights from several hub-and-spoke airports in the United States. These trials have shown that the problems of insect debris and ice formation in the ducting of LFC systems are by no means insurmountable. Nasa claims that such a system is no more difficult to maintain than many other systems currently on aircraft in regular airline service. Smaller, but still important, drag savings can be realised by passive means—by methods of turbulence control. Many conceptual methods of drag reduc tion by turbulence control are under research. Two have been shown to produce measurable effects in many independent trials. Any such device has its own associated drag, so the reduction it provides must be greater than its own drag to realise a net improvement. The first of these methods involves the introduction of flow-aligned V-shaped grooves called riblets. Accurate results indi cate that covering surfaces with such grooves can reduce drag by as much as 8 per cent net. The reasons why riblets reduce drag are not well understood. The best explanation is that they produce a damping effect, which attenuates the levels of turbulence in the boundary layer. This, in turn, leads to a reduction in the skin friction drag which results from turbulent mixing in the boundary layer. Riblets were first tested on Stars and Stripes, the winning yacht in the America's Cup race Early research into riblets showed that riblet size was critical. Because riblets increase the surface area exposed to the flow, the most likely effect is an increase in skin friction. However, with riblets of the right depth to suit a particular flow (typically 0 • 002in for an airliner) a drag reduction can be achieved. Nasa published many of its results on riblet performance six years ago, and this led the 3M Corporation to produce a self- adhesive riblet film which it marketed to the world's aircraft manufacturers. The film was widely publicised when it was used in the 1986 America's Cup by the winning yacht, Stars and Stripes, to reduce drag on the submerged parts of the hull. The commercially available film is being widely tested by airlines to see if it lives up to the high expectations resulting from wind- tunnel tests on scale models. In Europe an 18-month trial by Airbus Industrie and Lufthansa has begun to investigate how riblets behave in everyday airline service. The trial involves the application of riblet film at 12 locations on the aircraft. The locations were chosen to ensure the film would be subjected to as wide a range of adverse conditions as possible. It would be subjected to elevated temperatures on the engine nacelles, to ultraviolet radiation on the top of the fuselage, to erosion at the lead ing edge of the fin, and to fuel/hydraulic fluid spills around servicing ports. During the trial the riblet strips will be removed from the aircraft every three months, so that engineers can see how the grooves stand up to environmental factors. Degradation and clogging of the grooves clearly will have a detrimental effect on riblet behaviour, and this needs to be fully quan tified before any airline will use riblets on its fleet of aircraft. Riblets are not only of interest for civil applications. In a trial for military applica tions at Nasa's Dryden facility, supersonic riblet tests have begun. Early results seem to indicate that the extension of riblet tech nology into the supersonic regime will meet no fundamental problems. Should riblets prove practical for airline service, we would merely be emulating a feature evolved by fish to enable them to swim faster and with less expenditure of energy. Sharks have tiny grooves on the surface of their skin which, it is believed, exist for the sole purpose of drag reduction. The second passive method of turbulence control under evaluation is the large eddy breakup device (Lebu). In its simplest form this is a thin plate which is suspended in a turbulent boundary layer to break up large vortices (eddies) which produce turbulence where the body and the flow meet. These eddies re-form downstream, but because there is some distance before this occurs drag is reduced. Lebu research initially began with experi ments on models in water-filled tow-tanks. They have now been tested in flight at many research establishments around the world. A practical Lebu design for an aircraft takes the form of a ring round the fuselage. Wind- tunnel tests have demonstrated drag reduc tions of up to 7 per cent for Lebus shaped as low-drag aerofoils. Nasa is clearly not taking Lebus lightly. Before the end of 1988 full-scale flight tests of a Boeing 737 fitted with Lebus will be conducted at the Langley research centre, so that fuel savings can be quantified. In aerodynamics, as in all things, "something for nothing" is not a reality. Nevertheless, simple concepts like those behind Lebus, riblets, and laminar-flow control appear to present large potential benefits for relatively initial small outlay. E PW4000 nacelle performance improvement riblets Very small streamline-oriented "V" grooves on surfaces reduce skin-friction drag of turbulent boundary layer. 28 FLIGHT INTERNATIONAL, 22 October 1988
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