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Aviation History
1957
1957 - 0282.PDF
284 FLIGHT, 1 March 1957 PROPELLER DE-ICING . . . The overshoe is then mounted on a blade and the adhesivecured under pressure. The surplus rubber is trimmed off, after which each blade is put through a rigorous test programme. Aheat run is carried out, during which the power input is slowly increased to a value giving a specified surface temperature, whichis maintained for some time. During this test the heaters are closely scrutinized for hot spots and air bubbles which may betrapped between overshoe and blade, or within the overshoe. Following this, a flash test is carried out at 1.7 kV between theheater and the blade; insulation resistance must exceed 100 megohms. After testing, each blade is carefully moment-bal-anced to ensure dynamic balance on the finished propeller. The spinner is also provided with electric de-icing gear.Eighteen separate heaters of trapezoidal shape are cemented A heater under inspection, showing the zig-zag pattern of the resistance wires in Ihe glass-fibre yarn. radially to the shell of the spinner. They give a power loadingof 11 W/sq in, or a total dissipation of 2.7 kW. Current is again supplied from the slip-rings which feed the heating elements onthe blades. Spring-loaded plungers allow the shell of the spinner to be removed without disconnecting the wiring to theheaters. Economy in alternator capacity is achieved by arranging for thespinner and blade heaters to cycle alternately with those on the associated engine (Rolls-Royce Dart) air intake. Normal com-plete cycle times are between 120 and 360 sec, depending on the severity of the icing conditions encountered. In service, theinitiation of de-icing is not performed automatically. Instead, current is switched on by the pilot when ice has reached a pre-determined thickness. Cycling is purely automatic from then on, the choice of the fast or slow cycle being determined by the out-side air temperature. Although the use of this type of electric surface heater hasbeen described for de-icing a particular type of aircraft, the range of application is extremely wide. An interesting recent casehas been the simulation of high-speed flight, as far as the frictional heating of aircraft skins is concerned. Isopanels ofspecial construction, capable of short-time loads (about 30 seconds) of up to 10 kW per square foot are used. Similar heaters are also widely used on airfields and testinstallations, for frost-protection on compressor lines, storage tanks, fuelling equipment and as drum heaters to reduce theviscosity of oils and other materials before use. On the manufacturing side, electric surface heaters are rapidlyreplacing other forms of heating for the curing and part-curing of resinated plastics. Their use often eliminates the need forspecial ovens, so that considerable savings can be achieved. Further developments are likely to take place, since such surfaceheaters combine the traditional benefits of electricity in the way of efficiency, cleanliness and ease of control with the additionaladvantages of high unit loading, flexibility, compactness and ease of application. TALKING ABOUT TITANIUM (continued from page 270) Greatly extended use of titanium tube is awaiting the result of aprogramme of work now in hand on the production of tube with enhanced fatigue resistance. Many components in hydraulic systemscan already be made from alloys such as I.C.I. Titanium 314A or 318A and various firms are understood to be doing development work in thisconnection. Problems associated with the galling of titanium are hold- ing back some uses but there is much hope that various surface treatments(such as oxidation, anodizing, cyaniding, electro-plating or chemical deposition) can help in this direction. ECONOMIC APPLICATIONS IN AIR TRANSPORT By Dr. H. W. Shaw, Development Department, I.C.I. Metals Division In order to develop useful characteristics, titanium has to be preparedto a standard of purity that until recently would have been unique amongst engineering materials. It is therefore pertinent to ask whetherthe civil aviation industry can afford to incorporate the new material on a purely economic basis. We must confine ourselves to applications inconventional subsonic aircraft and to applications where the substitution of titanium complies fully with design requirements and where theprimary advantage is a weight reduction. The latter condition is stipulated to exclude the case for compressor blades.[Dr. Shaw examined the operating statistics of British European Air- ways for 1955-6 and showed that the yearly revenue from each poundcarried was slightly over £38. A similar calculation for B.O.A.C. showed a figure of around £50, and later jet aircraft on the North Atlantic mightestablish a figure between £85 to £95. After making all suitable deduc- tions one was left with yearly figures of £17-£22 for B.E.A. and fromaround £30-£80 for B.O.A.C. and these sums would have to meet the costs of incorporating titanium, together with replacement charges, andyield a suitable profit margin.] The cost of incorporating titanium in the form of a finished com-ponent is a detail on which further information would be welcomed. In the case of the first major applications, namely the substitution forstainless-steel sheet in heat- and fire-resisting zones, a little personal experience with one-off research components made in a developmentshop with traditional tools suggests an absolute upper limit of £67 per lb of weight saved at Ministry contract prices. In batch production inaircraft quantities with press tools it is felt that the cost of weight saving should not exceed £20 per lb. In the extended uses visualized in the nearer future (where stressedcomponents might be incorporated both in engines and parts of the aeroplane such as the undercarriage) fatigue and the implications it hason service life before replacement is required becomes a much more important factor. Again the utilizations of the primary material withcomplicated machining from bar or forging may be so low that the economic "break-even" point cannot be reached. This considerationsuggests that extensive developments in the field of fabrication by welding and other means may be expected in the future with the needto conserve expensive material. SHEDDING LIGHT ON WEIGHT "TIE that removeth weight doth as much advantage motion as-H he that addeth wings" wrote John Pym in the seventeenth century; an apposite quotation that was used by the Society ofBritish Aircraft Constructors to preface their third Display of Weight Control held at the Seymour Hall, London, betweenFebruary 14 and 20. The attractive layout of the exhibition was obviously the resultof considerable hard work and imagination by the Society, who used the term "weight control" to embrace various aspects ofdesign efficiency and so disseminate information on new manu- facturing techniques and processes which might be used tofurther weight-reductions. The exhibits consisted largely of components and detail parts,all contributed by manufacturers as examples of lightweight design for a specific purpose. In many cases examples of "beforeand after" weight reducing were shown—the result of a change of material, re-appreciation of the stressing problem after testing,the use of novel techniques or just better design. But regardless of source, each exhibit was cloaked in anonymity, and placardedonly with its description and the way in which its weight had been reduced. There was, consequently, a good deal of speculationamong many of the visitors as to which firms were responsible for some of the more advanced construction techniques. Materials much in evidence were resinated glass-fibre,expanded-honeycomb sandwich and titanium, fabricated examples of which were used to show that remarkable weight-savings that are possible by intelligent application. The relative merits of any choice of materials obviously requires to be weighedvery carefully; at opposing ends of the hall were examples of a machined component that was lighter—on a strength-for-strengthbasis—in forged light alloy rather than in cast magnesium- zirconium and another part where magnesium-zirconium had anadvantage over light alloy. Nor must methods of attachment (among the host of weight-saving ideas) be forgotten. "Jo-bolts"are a useful lighter alternative to hexagon bolts and nuts in structural applications; and puddle-welding—examples of whichwere also displayed—can be used to join together light-gauge metal with practically no increase in weight at all. Project analysis is an important part of the weight estimators'task, and a section of the display was devoted to a resume of the problems with which he is confronted and an analysis of themechanics of solution. Modern methods of weight control shown in this section included punch-card indexing and the Librascopebalance computer into which can be fed payload and structure- • weight information; the centre of gravity is then computed.
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