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Aviation History
1954
1954 - 0962.PDF
432 FLIGHT, 9 April 1954 The Quest for Power . . . example, the designer is aiming at a low specific fuel consumption, obtained by employ ing an exceptionally high compression ratio, then individual compressor blades and cascades will be examined more extensively than usual, followed by tests on two or more complete stages, culminating in rig tests of a complete compressor. The function of the compressor is, as its name implies, compression of the intake air to the highest possible pressure preparatory to combustion. All that is needed to test the compressor is, therefore, an adequate supply of shaft power, together with a means for admitting and exhaling the pumped airflow. This may not appear difficult to provide, but if the new compressor is very big—suppose it is for a 30,000 lb thrust turbojet— there may not be any rig in the country capable of running it in anything like its design conditions. Accordingly, a new test-house has to be built at a cost of between £500,000 and £1,000,000, with plant capable of providing sufficient power. Various types of equipment have been tried, some decidedly better than others. Space and weight are not primary problems, so marine machinery can sometimes afford an economical solution. Armstrong Siddeleys at Anstey, for example, employ high-pressure steam boilers and turbines from a former Royal Navy destroyer. Bristols have chosen batteries of electric motors, while Allison, in America, have produced a novel layout, obtaining power from batteries of turbojets and turboprops. Although not particularly attractive from the viewpoints of simplicity, flexibility and running cost, this last-named installation provides 40,000 h.p. and uses components of proven aero engines. Incidentally, the set-up is an excellent example of the manner in which components of a gas turbine can be run by themselves, provided that their designed operating limits are respected. Not counting the test compressor, the Allison installation employs parts of 26 separate engines of four types. While the big test station is being built, work can proceed on dimensionally similar models of the new compressor. It is also possible to make a full-size compressor and test it in an airflow at reduced pressure, so economizing in power requirements. This can provide a very great deal of information on how near the compressor performance is to the design requirements. These test models can be made rapidly and cheaply by employing inferior materials, such as straightforward aluminium or any available plastic. Although the primary aim of the rig testing is the development of the compressor to give optimum performance, it is also essential that mechanical strength and vibration and resonance characteristics should be thoroughly worked out before the first engine is built. Straightforward mechanical testing, which embraces the entire structure of the engine, is similar in some respects to structural testing of airframes. Photo-elastic stress analysis is now an accepted method in this work. One aspect not found in aircraft construction is whirl testing of the rotating parts of the engine in a sunken test pit below ground level. Here, the whole assembly is run far beyond designed speed—possibly until destruction occurs from centrifugal loading. After a destruction test, nothing is left but a heap of splinters; but from these the trained eye can learn a great deal, particularly if strain-gauging and a high-speed or stroboscopic film record has been undertaken. It is now becoming common practice to evacuate the test chamber of air in order to cut down the power required by such testing. In the complete engine, the compressor is driven by one or more turbine stages, rotated by a flow of hot gas from the combustion chambers. To test a turbine fully, a flow of hot gas is therefore needed, although cold testing is adequate in the initial stages. In testing compressors, the power requirements can be cut down by examining one or two stages at a time. A turbine can be run only with the whole gas flow. Turbine work starts with a large number of blade rigs and cascades with which general profile investigations are undertaken. Pressure measurements may be made from small orifices on the blade surfaces, while pitot measurements can be obtained through tiny probes with pressure heads equal in size to sections of hypodermic needle. The results of such tests keep the company's mathematicians busy for weeks unless an electronic calculator is employed. For cold testing, it is possible to use a light-alloy turbine driven against a dynamo meter brake by air delivered from huge compressor sets. At the same time, blades made of appropriate high-temperature alloys are tested for resonance and fatigue, either electro-magnetically or by subjecting them to a hot-air blast applied down the blade from the tip. Vibration characteristics may also be studied by clamping the blade at the root, sprinkling it with a suitable powder and playing upon it with a violin bow. The combustion chamber is probably no longer the most difficult part of the engine to get working really well. In this country, the design of burners and "cans" is largely the prerogative of one company, and frequently this company is supplied with all the relevant requirements by the engine manufacturer and left—with guidance—to get on with the job. Various sets of combustion systems are then set up in rigs and air from industrial compressors blown through them in the correct conditions. Something can also be learnt by testing the chamber with a flow of liquid: for instance, how the assembly will accept high pressures and large flows. The most promising chamber will then be run hot with the specified fuel, and checks made to see that no hot spots exist. The delivery from the combustion chamber is then probed for uniformity across the section, both in velocity and temperature. The rig is run under all possible flight conditions, particularly those met at high altitude, where reduced air pressure affects flame stability adversely; the high altitude re-light case is probably the most difficult requirement of all. Tests of fuel and ignition systems proceed at the same time. It may be noted that, with the almost universal adoption of annular or cannular combustion chambers, it is still possible to rig-test a portion of such compressor central hub shaft and extension shaft for the same engine (S.ll nickel-chrome steel). The company also make Proteus name tubes, the material being B.A.C.E.183 heat-resistant stainless steel. The David Brown Foundries Co.. Fenistone, near Sheffield, manu facture all sizes of aero-engine steel castings. Bryan* Aeroquipment, Ltd., Willow Lane, Mitcnam Junction, Surrey, ensure that engine-test equipment is itaelf accurate by providing such test and calibration gear as a portable testing unit for jet-pipe thermo couples, rear-bearing thermometers, voltage compensators and similar temperature-measuring systems. Burnley Aircraft Products, Ltd., Fulledge Works, Burnley, Lanes, were recently the subject of an article in this journal, describing their big output of sheet-metal ware for the "hot-end" of most British engines. Their products include flame tubes, afterburners and insulating blankets. The Carbon Dioxide Co., Ltd., Great Burgh, Epsom, Surrey, have, in association with the Distillers Co., Ltd., developed the "CeDeCut" technique of using CO; as a machine-tool coolant, which was demonstrated when the Rolls-Royce East Kilbride plant was opened. Most encouraging results have been obtained, particularly with the tougher gas-turbine alloys, including Nimonic and ni-cr-mo steels. CO 2 is also widely used in America on titanium alloys. The Carborundum Co., Niagara Falls, New York, are the manu facturers of Fiberfrax, a synthetic heat-resisting fibre formed by the action of an air blast on a molten aluminium oxide/silica mix. It is widely employed as a basis for thermal insulation, and a particular application is its use in an afterburner fiameholder as a cushion between the refractory segments and their metal retaining ring. Another of the com pany's refractories is Niafrax, which is extremely resistant to combined high-temperature corrosion and ero sion. It is basically silicon carbide bonded with silicon nitride to give resistance to thermal shock far greater than has been possible with clay bonding. Niafrax is being used at temperatures up to 3,500 deg F, and is in pilot production for numer ous guided-weapon rocket motors as a nozzle and combustion liner. In nozzle form it is available in throat diameters between Jin and about lOin. Carron Parianti, Ltd., Falkirk, Stir lingshire, Scotland, have patented an advanced moulding process in which the dies are made of anodized alu minium. With such dies, "Ni-forge" stress-free castings are produced in all standard alloys, these castings having unusually good properties more akin to those of a forging. A principal product by the Carron Par ianti process is compressor blading. Cementation (Muffelite), Ltd., 39 Victoria Street, London, S.W.I, make silencing installations for all kinds of test-bed. At the Patchway facility of the Bristol Engine Division, for example, the twin Olympus test- houses have the following layout: a flow of entrained air, seven or eight times that passing through the engine, cools the flow so that it can be passed through a diffuser section. It then traverses a splitting section formed from acoustic panels and in cluding four fairly sharp bends, and finally passes out vertically to atmo sphere Much of the structure is a concrete monolith, pin-jointed for expansion, and the inner surface temperature is kept below 75 deg C by a 4in mineral-wool lining covered by reflecting aluminium. The Chemical and Insulating Co., Ltd., Darlington, hold the British Empire and European rights for "Re-frasil," described under the H. I. Thompson company. Consolidated Engineering Corpora tion, 300 North Sierra Madre Villa, Pasadena 8, California, have de veloped a vibration recorder for jet
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