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Aviation History
1977
1977 - 0124.PDF
114 * TOMORROW'S POWER DOUBLE AIR FILM COOLING TRAILING EDGE GUIDE TUBE HOLES EJECTION ASSEMBLY Figure I: a current example of the cooling engineer's art is the RB.2I I high-pressure nozzle guide-vane using a combination of internal cooling, by convection and impingement, and film cooling by the ejection of small quantities of air over parts of the blade surface. This effort includes studies of the effects on aero dynamic efficiency of the emerging coolant flows, and work aimed at improving the arrangements for feeding the cooling air into the rotor system. Further ahead, research is proceeding also on transpiration cooling, in which the cooling air effuses outwards through a porous blade surface and into the main gas stream. The manufacture of this and other forms of cooled blading presents many challenges. Future progress will owe much to the continued development of blade-manufacturing methods. In the area of high-temperature materials, the success of the gas turbine has up to now been founded mainly on the extensively developed nickel alloys. Although the future now offers only limited potential for conventional alloys in terms of increased temperature capability for a given creep life, large improvements of life at a given metal temperature are in prospect. Attention is currently being paid to controlling the structure of the material, through such techniques as single-crystal and unidirectional solidification. Figure 2 shows turbine blades made at NGTE for research purposes. In addition to improved creep properties, an important gain from such structure control is substantially better thermal fatigue- resistance. Research is proceeding also on intermetallic materials and on directionally solidified euteetics incorporating strong fibres or lamellae in a relatively ductile nickel-based matrix. At present it appears that such approaches may provide an advance of about 100°C in temperature capability com pared with the best current casting alloys. Unfortunately, there is no obvious heir apparent to nickel in the reasonably near future. The materials which Figure 2: turbine blades made at the National Cas Turbine Establishment are advancing the high-temperature art by demonstrating how the structure of the material may be controlled FLIGHT International, IS January- 1977 might be expected to take us on to still higher temperatures suffer major disadvantages. Refractory metals such as niobium, tungsten and molybdenum are very prone to oxidation at high temperatures. While some progress has been achieved in the development of protective coatings, adequate durability is still some distance away. The ceramics, while offering a very high temperature capability together with chemical inertness, are notoriously brittle. Although the problems of these alternative materials may be overcome or circumvented in the longer term, the aero engine, with its need for very high integrity for safety reasons, will continue to depend on nickel-based materials and air-cooling techniques for many years to come. A wide range of materials is already used in the lower- temperature portions of aero-engines, and development here is continuing. For example, much remains to be done in developing and exploiting titanium alloys, although these materials already commonly represent more than 20 per cent of the weight of a modern turbofan engine. The material properties of other engine parts, as well as turbine blades, are being extended by control of metallurgical structure and by improvement of composition. An important example is the manufacture of rotor discs from powdered metal. Advanced methods such as this can yield production cost savings in addition to improved mechanical properties. Composite materials represent another promising area of development for low-temperature applications. Their good strength/density and stiffness characteristics make them obviously attractive. One of the problems of the modern civil turbofan is the increasing proportion of total propulsion-system weight taken up by the engine nacelle as the design bypass ratio is raised. This trend might be held at bay by using composites for parts of the nacelle, though much work needs to be done on design studies and trial constructions before production begins. Research is in progress on many aspects of detailed mechanical design. Examples are seals, bearings, oil and air systems, and so on. This work can be expected to yield important benefits for future engines. It is also important to reduce manufacturing costs wherever possible. At present, "material utilisation" is probably below 20 per cent in general—more than 80 per 1 Conventionally Cast UnkJirectionally Solidified Single Crystal -'
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