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Aviation History
1949
1949 - 1743.PDF
FLIGHi, 13 October, 1949 507 L In a room carefully controlled for tem- perature and humidity, the marriage of glass and interlayer is first made. sandwich is required, made up from two T%in, three Jin and two -,%-in panels of glass separated with six o.O2oin layers of Vinal. Flight at great altitudes complicates the problem by introducing the nuisance of mist- ing. To mitigate this the latest fighter screens are being fitted with an additional three Jin rim layers and a Jin panel on the cabin side of the screen, so providing a fin cavity in the bottom of which is fitted a desiccant pack of activated alumina. Even so, at the extremely high rates of descent of which modern fighters are capable, and despite the desiccant, it is possible for misting to occur inside the cavity, and the latest ideas in circumventing the trouble envisage the introduction of electrical wire elements, taking about 40 watts, in order to warm the air in the cavity. The modern variation of Prince Rupert's discovery is to employ air instead of water as the quenching medium. The plate of glass to be toughened is first heated to its softening point and the surface is then suddenly cooled by the blast from a number of jets of compressed air. This chills and hardens the outer surfaces of the glass while the inside is still hot and still con- tracting; by continued surface chilling, the rigid outer skin is lightly contracted and thus.put into compression. As already mentioned, glass usually fails in tension and, therefore, if a sheet of glass has a strong and uniform compressive strain in its surfaces, this must be overcome before tension can be induced. The effectiveness of this process is shown by the fact that a piece of glass so treated has its apparent ultimate tensile strength increased from about 6,000 to over 30,000 lb/sq in. When toughened glass ultimately breaks the stress is re- leased and the whole panel disintegrates into small particles incapable of inflicting serious injury. In view of the effects of high air speeds it is owing to this quality of disintegration on fracture, even though fracture is difficult to bring about, that toughened glass has not been found suitable for aircraft use. The defect can, however, be overcome by laminating a piece of plain glass on to a piece of toughened glass, a com- bination which, in fact, was widely used for bomb aimers' windows during the war. Such a combination brings together many of the most desirable properties both of laminated and toughened glass, but it also introduces the drawback that, if the toughened glass becomes fractured, the fragments are firmly held in situ by the laminated interlayer and vision through the glass may well be almost totally obscured. Strength and Fragmentation Research work is now in progress to produce a glass which, whilst retaining the essential qualities of toughened glass, breaks with a much larger fragment-size. To be known as strengthened glass, the new development is intended to be used as one of the laminee in a sandwich panel, the idea being that, on fracture, the particles of strengthened glass will be so large that the resulting cracks will offer little or no impediment to vision. Larger fragment-size is, of course, only obtainable at the expense of strength; it is, nevertheless, thought that strengthened glass will have at least two-thirds the ultimate tensile strength of fully toughened glass. For use in pressurized aircraft, if there is no objection to the employment of toughened or strengthened glass, the thickness of a panel required safely to withstand a given pressure may- be much reduced. When such heat-treated glasses are used, however, it is particularly desirable that they should not be used alone, but as one component of a laminated glass. In such circumstances, if the heat-treated glass fractures and disinte- grates, the fragments are held together by the interlayer, and the panel remains in position. This is of particular value in that cabin pressure will be retained, for it is not difficult to omploy a thickness of interlayer which will withstand any (reasonable) desired pressure. Some indication of the properties of Vinal in this connection are given by tests which have been made on glass panels de- signed for resistance to bird impact. The type and thickness of glass used has been found to have relatively little effect, the resistance to impact lying entirely in the thickness of the interlayer. Experiment has shown that a Vinal interlayer <>f |in thickness will give protection against a 14-lb bird at an impact-speed of 200 m.p.h. In oider to keep the weight of B II the screen down to a minimum, the thick interlayer is facedwith two relatively thin (about 5 in) panels of glass, prefer- ably of strengthened type. On impact, the glass shatters andthe interlayer bulges into a saC, which retains the carcase of the bird. To be effective, it is patent that the interlayer must beanchored separately, and a method (known as Triflex) has been developed in which an extra thick interlayer, if necessaryreinforced with metal, projects beyond the edge of the glass and is drilled for bolting direct to the screen or window frame.A refinement of the system embraces an additional single panel of toughened glass placed in front of the laminated panel witha £in gap between for the de-icing or de-misting circulation of hot air. To complement their work in the safety-glass field, Tripiexalso fabricate a great deal of the Perspex which is used in British aircraft. The material is, of course, an I.C.I, productand is supplied to Triplex in sheet form; the Company then manipulates it to the required shapes for cabin windows, astro-domes, "bubble" hoods, radar scanner caps, glazing panels for gun turrets, and so forth. When heated to between 90 and 100 deg C, Perspex isdeformable and, if the temperature is increased to 130-140 degC, becomes soft and rubbery. In this condition, it is readilyconformable to any required shape and, as it cools, hardens or '' sets '' in that shape. Essentially, a simple matter; but, asso often is the case, the simple matter becomes complicated. In this instance, the difficulty is that Perspex cools, and there-fore sets, rather rapidly. As a consequence, the interval be- tween taking the hot plastic sheet from the oven and clampingit on the form tool must be as short as possible. For relatively thick sheets, the total time from oven extraction to completionof the forming operation may be as long as a minute; but with very thin sheets, it is as little as ten seconds. For this reason the design of the edge-clamps must be suchthat they permit rapid action. Even so, it it necessary' to have perhaps as many as ten men on the clamping operation in orderto do it in the time, although one man alone may be sufficient for the actual forming sequence. To see the forming of a blown bubble hood intended, forexample, for a Sea Fury, is entrancing. The forming tool con- sists simply of a front wall shaped as a complement to thefront screen framing, and a smaller rear wall sloped backwards at about 45 deg. Between these walls is fitted a pair of" chines " over which the hot sheet of Perspex is draped, there- after to be clamped by a special frame which is lowered on tothe former and firmly holds the Perspex against the wall edges and along the skirts. Compressed air is then admitted to theinterior of the form tool and blows the sheet to the familiar hood shape. The former-chines serve not only to support thePerspex during the clamping operation, but also control the contours to which the sheet is blown in order to give thecorrect fore/aft curvature and, at the same time, give balloon- ing to the sides. The great advantage of the blowing tech-nique is that the surface of the Perspex is not pressed against any mould surface and, therefore, does not become marked; amatter of paramount importance when optical qualities of a. high standard are required as, of course, they are for aircraftapplications. C. B. B-W.
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