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Aviation History
1955
1955 - 1835.PDF
958 FLIGHT TEST TANK Mk 2 Comet 2 in the de Havilland Tank at Hat field THE water tank is likely to be a prominent feature of theairliner-factory landscape—certainly in Britain if not in theU.S.A. also—until it has yielded sufficient information about the design of pressure cabins to render itself unnecessary. This isnot likely to be for some years, despite the vast amount of research into fatigue already carried out by Government estab-lishments and private firms. The aim, quite simply, is to evolve design principles which will yield structures that do not requireto have their fatigue-proof properties demonstrated by costly and prolonged experiment. The original "prototype" water tank was constructed by theR.A.E. in the spring of 1954, and the vital evidence it provided for the Comet 1 investigation has passed into history. As mightbe expected, the de Havilland company have now built their own tank at Hatfield, in which a Comet 2—actually the sixth pro- -.-.-•duction aircraft, modified and strengthened to R.A.F. Transport :''•', Command standards—made its first "flight" on Saturday, ..f:'November 26th. Up to the present date about 1,000 applications • of pressure, or cycles (it is logical to measure cabin fatigue life ,1in "cycles" rather than hours), have been made, equivalent to - 3,000 hr of flying if an average of three hours per flight for an vR.A.F. Comet 2 is assumed. Normal cabin pressure of 8i lb/sq in, ' with simultaneous wing gust-loads, is applied every four minutes,with respite only for detailed examination every 500 cycles. This " programme will continue 24 hours of the day until failure occurs,and even after a life beyond that likely to be achieved by R.A.F. ; Comet 2s is reached. This is certain to be not less than4,000 to 5,000 hr, an,d probably more: assuming 3 hr as an average "cycle," this would require a test to, say, 1,700 x xcycles, where x is the all-important scatter-factor. The Comet 2 in the tank incorporates modifications designedin the light of an intensive de Havilland test programme con- ducted during and since the Comet 1 investigation of 1954. Themodifications include redesigned cabin windows, now heavily reinforced, oval-shaped cut-outs, involving the completere-skinning—with heavier gauge material—of virtually the whole length of each side of the fuselage; reinforcement of all fuselageseams, joints, hatches, doors, etc.; and local reinforcing of the wing. Several thousand individual tests on components tookplace before the complete aircraft specimen was submitted to the final tribunal of the water tank. The 220,000-gal tank measures 140ft by 20ft, including aseparate 70,000-gal section to enable Comet 4 specimens to be - tested concurrently with the Comet 2. The dividing wall can beremoved for later tests of a complete Comet 4 specimen. The tank is emptied into an adjoining reservoir when the 500-cycle „'inspection is due. Construction—by bolted plates—is straight- forward, except where the wings protrude; here the walls arerecessed to facilitate inspection of wing rib 3, and are tailored to the local contour of the wing (which itself was modified to .eliminate the sharp trailing edge). The seal of dinghy fabric was made and fitted by the R.A.E. As can be imagined, the up anddown movement of this seal poses a fatigue problem of its own. Fuselage and wing bending forces are applied by hydraulicrams and lever systems as shown in the illustration. Force applied to the wings is equivalent to a lOft/sec gust, this beingthe value used by the R.A.E., based on Comet 1 flight velocity- (Concluded at foot of page 981) An aerial view of the Comet 2 in the de Havilland .-.•,;• •::. north-west side of Hattield aerodrome. tank the The main control and pump room. The double row of lights on the small panel at the top left indicates the application of up - and - down wing gust loads, and a red light below comes on when the cabin is pressurized. ::r-.-..'.:. '. . KEY 1 Nosewheel mounting ^ 2 Front fuselage ballast 3 Load carrier 4 Front fuselage lever system 5 Lateral stabilizer (stbd. only) 6 Constant-load lever system 7 Watertight wing seal 8 Rear fuselage lever system 9 Mainwheel mounting 10 Wing upload lever system 11 Wing download lever system 12 Ballast weights 13 Fore and aft stabilizers 14 Tail upload system
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