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Aviation History
1953
1953 - 0459.PDF
io April 1953 455 (see second illustration) which is mounted on flanged wheels running in rails set flush with the hangar floor, as is a mobile office (B). Hinged flaps (C) are provided in the deck where, although a floor is usually required, it is sometimes necessary to have a gap—for example, in the tail structure, to give unobstructed passage to the lower portion of the fins and rudders or in the front platforms to give passage to the main undercarriage, or along the inner edges of the side fuselage platforms in order to produce a close fit along the sides of the aircraft once it is in position (D). Four hydraulically operated platforms are provided (E), one at each side of both engines, to give easy access to any part of the power plant. These are so actuated that the level of any platform can be independently adjusted by the operator working upon it, by means of a hand-high control valve mounted on the moving platform. Metal safety-guards ensure that there is no danger of toes or hands being nipped by the moving platform. At the trailing edge of each wing, a light-alloy staircase (F) gives access to the upper wing surface. These staircases, when not in use, fold down below the deck level, leaving an unobstructed walkway. They carry a reel of canvas-backed piped rubber matting which, to protect the aircraft skin, can be pulled over the wing from trailing edge to leading edge, where automatic folding ladders (retracted in the illustration) can be erected between platform level and the upper wing surface. The permanent platform-levels have been carefully determined in an effort to ensure that an operator of average height can comfortably reach all parts of the aircraft, pits (G) being provided under each wing and under the tailplane, and hinging cantilevered platforms (H) being fitted at the tail. These platforms, when down, give access to the upper hinge-points of the rudder, and when up leave a clear space above the tailplane to facilitate its removal. During Check 4 operations the aircraft will always be jacked up to permit undercarriage retraction tests and inspections to be carried out, so main-wing jacks are built into the dock, as are nosewheel jacks, wing steadies and a tail trestle; the main platform levels have been arranged so that they are at the correct height around the aircraft when it is upon the jacks. The Elizabethan dock has been designed so that it is more than a series of platforms. It is a complete workshop, for on it, or built into it, are all the tools and appliances required to carry out Check 4. High-pressure grease is carried in fixed pipes under the decking, and is fed up through it in high-pressure flexible grease hose, which is wound on an automatic coiling device and which terminates in a grease-gun. To service any nipple, the fitter need only reach for the nearest gun, and pull out the hose attached before applying the gun to the nipple. When the greasing is complete the hose is retracted beneath the deck, thus avoiding a tangle of odd lines, wires and pipes on a walking platform. By limiting the lengths of the flexible hose, the designers have ensured that the gun is always to be found close to the nipple for which it is required, and cannot be taken away or mislaid. Electricity at 440 volts and 250 volts is supplied where required for lighting under the dock, in the underwing pits and at other points shaded from the hangar lights, for power, soldering irons and for inspection lamps. All the electrical equipment, including the fluorescent lighting fittings, is flameproof. Air is supplied for working hand tools and for operating suction cleaners. Water is supplied for recharging the aircraft tanks in the galley and the toilets, and the provision of a drainage system allows the flushing- out of the tanks. Dry air is provided for clearing instrument pressure static systems, and an oil drainage scheme ensures that the hangar floor will be free from spilt and dirty oil when engines are drained and filters cleaned. A hydraulic test rig is installed, so that hydraulically operated components may be tested while the aircraft is docked. The dock now being designed for the Discovery differs only in detail from the Elizabethan dock, the principle being the same. Whereas the Elizabethan is a high-wing aircraft, the Discovery is a medium-low-wing type; the Discovery fuselage is therefore higher, and it is necessary to provide a third working level along the sides of the fuselage to facilitate roof cleaning, polishing and repainting. This third level is higher than the tailplane, so it has been necessary to arrange that the whole of this high-level platform be retractable to allow the aircraft to enter and leave the dock. This has been achieved by mounting the entire high-level platform on latticed columns, the bases of which are carried on bogies whose wheels run in channel guides fixed to the low-level-side fuselage platform. Each bogie also carries a nut which engages with a leadscrew. The leadscrew is rotated by a reversible air motor through bevel gears, universal couplings and shafts. This arrangement enables the high-level platform (which cantilevers 26ft) forward over the wing to be completely under control during the retraction and extension operation. To service the fin and rudder, two elevating hydraulic platforms are provided, one at each side, working independently. As in the Elizabethan dock, all services are provided upon the platforms where they are likely to be required. [Note: A description of other maintenance docks—designed by Tiltman Langley—wqs given in "Flight" of August 15th last. One of these, for No. 3 Checks on B.E.A. Pioneers, is also in use at Renfrew.] TALKING ABOUT FATIGUE A Strenuous Discussion Under R.Ae.S. Auspices THE formidable-sounding title of the Royal Aeronautical Society's function on March 27th, A Full-day Discussion on Fatigue, in spite of its exhausting implications, did not pre clude the attendance of an extremely large and initially unweary audience. Several chief designers and many senior design men from firms and colleges were among those who rapidly over-filled University College's main chemistry theatre at 10 o'clock on that Friday morning—although the rumour heard that "Bristols have booked two whole rows of seats" could not be verified. In the chair for the two morning sessions was Major P. L. Teed, A.R.S.M., M.I.M.M., F.R.Ae.S., (Vickers-Armstrongs, Ltd.), who in his opening remarks mentioned the past disagree ment between engineers and metallurgists over the subject of fatigue in metals. To start the first discussion, Mr. H. L. Cox, M.A., F.R.Ae.S. (National Physical Laboratory), would present the engineer's point of view in an introduction on "Fatigue in Structures." Although one did not know exactly what fatigue was, much was known about it, began Mr. Cox, who went on to comment on the basic S-N relationship and scatter of points about the curve, corrosion fatigue, ordinary fatigue and crack multiplication. The lecturer then showed a number of photographs of fatigue cracks in typical test pieces, and referred to the many anomalies which made general conclusions difficult. A comparison was made between theoretical curves and experimental fatigue test data, and the effect of the size of transversely bored specimens on fatigue limit and fatigue stress concentration factor was discussed. Although in many cases theory and practice did agree, there remained doubt concerning the significance of the basic "endurance limit" concept. Mr. Cox then showed a statistical curve illustrating the scatter of endurance for test pieces equally likely to fail, and in conclusion referred again at length to stress concentration effects. The ensuing discussion was opened by the chairman, who enquired whether fatigue failures under static loads were probable in plastic materials : Mr. Cox agreed that this was possible. The distinction between tensile and shear failures, the effect of de- carburised surfaces on fatigue strengths, results of interruption of fatigue tests, and the variation of fatigue limits with frequency of excitation were among the other topics discussed. Concerning the scatter of experimental results, Mr. Cox pointed out that those mentioned had been typical of structures—for actual materials, the scatter would have been less. After a break for coffee, Mr. E. R. Gadd, F.I.M. (Bristol Aero plane Co., Ltd.), presented the viewpoint of the metallurgist— who, he hastened to emphasize, was in company with the engineer in not knowing what fatigue was nor how it was caused. However, the speaker would try to cover as many metallurgical factors affect- -ing the life of materials as was possible. Structural discontinuity was the first: and inclusions did tend to reduce fatigue strengths, although their position and size mattered more than mere quantity. The effect of grain direction on fatigue was marked, and the attain ment of satisfactory grain flow was increasingly important with higher tensile strengths. Other processes defined and considered in turn by the speaker were nitriding, cyaniding, carburising, de-carburisation, surface flame- and induction-hardening, cold working and the various forms of surface coating. Final factors of importance were temperature (an increase in which generally caused a decrease in fatigue strength) and environment. In throwing the subject open to general debate, Maj. Teed affirmed that, in fact, metallurgists did know what fatigue was—it was a special case of work-hardening. True, they did not know what work-hardening was. . . . Questions from the floor concerned undue dependence on bending tests ("Yet most failures are bend ing failures," answered Mr. Gadd), the importance of "metallurgy before mathematics" in the estimation of a component's life, the information available on steels compared with that on light alloys, and further points on grain flow and inclusion effects. An interval for lunch followed. [Continued overleaf
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