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Aviation History
1956
1956 - 0608.PDF
608FLIGHT, 18 May1956 (Left) Comet 4 structure specimens—of the kind shown on the previous page— seen undergoing their water-pressure fatigue tests. (Above) A 28ft portion _ of fuselage, containing representative doors, hatches, windows, etc., about to go under water for fatigue-testing THE COMET 4 operator had to be certain that he could depend upon fillingthem, and the authors believed that there would be few world routes which would have the traffic density afforded, say, on theNew York to Miami route. With regard to structural design the philosophy was as follows.Thicker-gauge materials were used for the pressure cabin, with windows and cut-outs designed to a stress level so low thatcracks would be most unlikely to occur during the aircraft's life. Should for any reason such cracks occur, then their rate ofpropagation would be so slow that they would be seen before becoming dangerous. The aim was to demonstrate on test-specimens that cracks would not occur before a minimum of 60,000 reversals, and would not propagate seriously before another60,000. Assuming each reversal to represent a three-hour flight, the apparent life was 180,000 hours, giving a safe life of 30,000hours after application of a scatter factor of six. The Comet 4's structural test programme included firstly the underwater testingof six representative types of cut-out. In addition to Comet 4 specimens, 30 panels of the Comet 1 type also were tested.Secondly, a 28ft section of the fuselage containing representative cut-outs had up to April 1 completed 85,000 reversals—255,000"flying hours" [without factor]—without failure. A further 27ft centre-section was at present underwater, and was having winggust loads applied as well as cabin pressure. A section representa- tive of the nose had also been in the tank. The wing spars alsohad been closely investigated for fatigue, as well as the bottom wing skin (see photographs). In addition many hundreds oftests had been carried out on seam joints. The authors confidently anticipated that the test programme,culminating in fatigue-testing of a complete Comet 4 in the water- tank, would establish the structural integrity of the aircraft beyonddoubt. A Comet 2 had been in the water-tank since last autumn [described in Flight for December 30].The lecturers put forward the case for the Comet's buried engine installation. Much had been said about the advantagesof pods and it was of interest to consider the factors in favour of the wing installation: (1) The height of the intakes from the ground reduced the riskof debris being sucked into the engines. (Fine-mesh guards could reduce take-off power by 20 per cent.) (2) Control runs, and cabin and de-icing systems ducting fromthe compressors, were simpler. The mounting structure, in which the engines were hung between ribs, was reduced to its simplestpossible form. (3) Maintenance access was excellent and afforded mainten-ance engineers the protection of the wing and cowling doors in the worst weathers. (4) Unlike the pod, the spacing of ancillary equipment was lessclosely packed. (5) The close proximity of the engines allowed the greatestpossible advantage to be taken of the available fire extinguisher fluid. (6) In a wheels-up landing, whereas pods must bear the fullimpact with the ground, the wing installation provided a high degree of protection for the engines. (7) A pod striking the ground could be the cause of fuel tank Tank fatigue-testing of a Comet nose-canopy (taken from Comet I G-ALYX) to assess the life of the windscreen structure. rupture; in the case of the Comet no fuel was carried above orbehind the engines, and the serious fire hazard of the fuel being sucked into the intakes was reduced to a minimum.(8) Effective means of containing turbine blade-failure were incorporated in the Avon engine installation. In the case of discfailure, release energy could have equally severe consequences were it within a wing or a pod. The possibility was in any case remote. (9) The close grouping of the Comet's engines simplified thefuel feed system, there being no complicated system of cock operation.(10) Closeness of the engines to the aircraft centre-line virtually eliminated asymmetric thrust in the event of outboard engine
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