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Aviation History
1958
1958 - 0404.PDF
420 FLIGHT,28 March 1958 COMET RESURGENT "Flight" photograph A DECADE OF D.H. JET TRANSPORT DESIGN THAT the Comet is "the most structurally tested aeroplanebuilt to date" is a claim unlikely to be challenged. Thede Havilland Comet 1, when it went into service in 1952, possessed a structure which could itself fairly claim to have under-gone static tests at least as comprehensive as any to that date. Subsequent events, which exposed so publicly the wide gaps inman's knowledge of metal-fatigue, have now passed into die per- spective of history. But the completely redesigned Comet structurewhich has since emerged probably represents—as it should—the contemporary art of structural design at its best. The rig-testing programme behind the Comet 4 has been inprogress without interruption since the start of the Comet 1 inves- tigation in the early summer of 1954. The programme has beenin two parts: (1) basic research into materials and design-methods to establish a new anti-fatigue design formula, (2) applied researchto test actual detail Comet 4 components designed according to these new principles. All this work, with one important exception, is now complete—at least so far as the Comet 4 is concerned, though obviously general basic research into fatigue goes on. The exception is thefinal proof-of-the-pudding test of a complete Comet 4 wing and fuselage. This assembly is now being immersed in the D.H. watertank at Hatfield where, during the next six months, it will be subjected to the now familiar programme of tank tests (Flight,December 30, 1955). A photograph appears on page 424. It may be asked why, after such a comprehensive programme ofdemonstrating the safe life of the detail ingredients of the Comet 4 structure, an expensive verification test such as this is necessary.Scientifically, the results of satisfactory component-testing—which embraced seams, joints, materials, windows, canopies, centre sec-tions, spars, undercarriage, etc.—might be proof enough. The fact that the complete test is nevertheless being made is a testimonyto the thoroughness of the whole approach to Comet testing, as exhibited by DJL, the Air Registration Board and—not leastimportant, since they have borne the greater burden of the cost —die Ministry of Supply. The aim throughout die structural design and test-programmehas been a minimum safe life for die Comet 4 higher than its useful operational life. If one assumes for "useful operational life" afigure of not less dian 30,000 hr die magnitude of die test-pro- gramme can be appreciated. To provide for die scatter charac-teristics of fatigue, each specimen component has been tested to a figure five to six times in excess of this life (factors of 6 for diepressure cabin and 5 for die wing have in fart been applied). An unexpected consideration in the early days of die Comet 4design was the more severe fatigue-case presented by die (at diat stage unforeseen) Comet 4B. This shorter-range member of diefamily—developed only during 1957 for B.E.A.—will obviously spend much more of its "useful operational life" riding gusts, pres-surizing its cabin, landing, and so on. But D.H. are confident that the basic test-programme has been comprehensive enough to The first production Comet 4, structurally quite different from previous Comets, undergoes a resonance test at Hatfield. Flight tests will begin next month, and deliveries to B.O.A.C. will start in September. ensure that die 4B will be as good for 30,000 hr as the Comet 4.A point worm noting particularly is that die fatigue-proofing process has been applied not just to the structure—i.e., to die partsof die aeroplane subject to flight loads. It has, in die words of a senior D.H. designer, been extended to "everydiing in the aero-plane that gets a towsing"; in order words, to a hundred-and-one components in die power systems and ancillary equipment. On die controversial structural question of "safe life" versus"fail-safe," die D.H. principle seems to be that there is no clear- cut distinction between eidier philosophy. If it is possible to designcomponents to fail-safe, tiien this should obviously be done; but whether a component is fail-safe or not, it must certainly bedesigned for, and demonstrated to possess, a safe life. General Comet 4 structural details wordi highlighting are exten-sive modification of the rear fuselage to withstand jet-blast (die inner pipes of the 4, like diose of die 2, are of course canted out-wards to minimize die effect of blast experienced on die Comet Is); redesign of die tailplane and elevators with thicker skins anddoubled-up ribs to deal with die fatigue effects of jet noise; the use of 24ST (naturally aged aluminium alloy to American specifi-cation) on die lower tension surface of die wings; exclusion of zinc-alloy skins from die fuselage; and die use of steel for dieimportant lug-forgings as employed for die attachment of wings, tailplane, and so on. Structurally, dierefore, die Comet 4 is a completely new design,to die extent that diose familiar with die Comet 1 can examine almost any detail of die new Comet and find that it is different.All that remains die same, allowing for die different proportions, is die external shape. Re-engineering of die Systems. As was the case widi die struc-ture, each system in die Comet 4 was completely reassessed and in certain instances (e.g., electrics, cabin-air-system control, and fly-ing controls) modified to embody features suggested by operational experience and modern ideas. A good new idea, for example, isdie complete separation of all components associated widi, respec- tively, die electrical and hydraulic systems. In accordance widi modern practice, the electrical power
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