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Aviation History
1959
1959 - 0160.PDF
64 Ground-running of the Rolls-Royce Tyne engines of the first Vanguard, G-AOYW, started early in December last year. These slim two-spool turboprops drive 14ft 6in de Havilland propellers, the latter haying an activity factor of 160 in later machines in order to absorb over 5,000 h.p. The intakes beneath the outboard nacelle serve a de-icing heat exchanger Vanguard . . . showed that the wing/body intersection could be made almost ideal for the chosen cruising-speed/Mach relationship, and no fillets were found necessary. Likewise the over-wing jet-pipe layout— dictated by mechanical and practical considerations—is extremely clean and requires no fillets. As might be expected, Mach considerations require empennage t/c ratios rather lower than those chosen for the wing; the values adopted are 14 per cent at the root and 12 per cent at the tip and, as in the Viscount, the tailplane has 15 deg dihedral to give mini- mum variation of longitudinal stability with engine power. Another noteworthy feature is the selection of conical propeller spinners, affording maximum intake efficiency at high flight speeds. Notwithstanding Vickers' advanced model-making techniques, over 75,000 man-hours were expended in the manufacture of the 22 models used during Vanguard development. Tunnel time is currently 1,500 hr, and the tempo is likely to increase as actual flight-test results require further examination in tunnels. Separate models have been employed for aerial-polarization and ditching trials. Structural Testing: It is still too early for there to be any conformity on this subject between Britain and the U.S.A., or even between manufacturers, but the Vanguard programme naturally incorporates procedures influenced by the ten years of structure-testing of the Viscount. The Vanguard will involve less testing to destruction, but greater emphasis will be placed on proof-load tests. Major investigations number three: proof tests on airframe under gust, manoeuvre and other design cases, with tanks pressurized and control circuits and flaps in operation (speci- mens subsequently to undergo ultimate-load tests); proof and ultimate tests on a forward-fuselage specimen; and fatigue pressure testing in the stratosphere chamber at Weybridge on another forward-fuselage specimen. In addition a great number of detail structure-test investigations are in hand.- Structure The Wing. A main double torsion box is built up from integrally machined combination skin/stringer panels and three shear webs. To this box are added the readily detach- able leading-edge sections, the aileron fairings and attachments and the flap shrouds; the flap guide-rails also cantilever from its rear face. The torsion box'is manufactured in five sections: a short section spans the fuselage, and each mainplane is in two sections, with manufacturing joints at the root and just outboard of the outer nacelle. Altogether there are 54 machined skin panels, the largest starting as a billet 340in X 28in X 2in, weighing almost 2,000 lb. Upper-surface panels are to D.T.D. 5050 (high- strength alloy) and under-skins of American 24ST-4. The latter are attached by taper-head (1:13) bolts, which project about 0.03in unloaded and have the correct interference for maximum fatigue life when pulled flush. Four mainplane sections form integral fuel tanks; no fuel is carried in the fifth portion in the fuselage. In the top surface of the inboard tank are four large access panels for entry to the two torsion boxes, and in consequence the tank ribs incorporate elliptical crawl-ways. Access to the rather shallow outer-wing tanks is by removable panels in the front and rear shear webs. The four engine-mounting longerons in the inboard nacelle are attached to the torsion box at two integrally machined ribs, which also carry the fittings for the undercarriage. A great deal has been said of "fail safe" or multiple-load-path structures; and much of it has come from salesmen rather than from engineers—or from engineers who, in order to safeguard their livelihood, had to become salesmen. The fallacy of the multiple-load-path argument when used by the wrong people is exposed unwittingly by a major American manufacturer, who has placed two highly stressed machined forgings side-by-side and boasts that either can take the load if the other fails. A careful check would seem to indicate that the integrity of the design is such that a failure would not be exposed, other than at a major tear-down, whilst the effects of such a failure as a stress-raiser in the remaining half of the "multi" load-path gives serious food for thought. Why, then, is the Vanguard using such a wing structure, bearing in mind Vickers' experience and satisfaction with mono-spar con- struction in the latest form of the art? Basically it is because of the fuel-capacity demands of a turbine aircraft of relatively high wing-loading. Bag tanks on both the Viscount and Valiant have been developed to a highly satisfactory state, and in accordance with the overall design philosophy of the Vanguard should have been retained; but it was quite impossible to obtain adequate internal capacity with them. Accordingly integral tanks were chosen, and the Viscount mono-spar, nil-stringer construction— quite unsuited to this—was abandoned. As soon as a stringer type of wing with integral tanks was indicated, the long-term advantages of the machined-skin/ stringer construction were seen, not only for the Vanguard but for future aircraft. The logical development of this thinking was the twin torsion-box, triple shear-web design (cross-section sketch, p. 74). A feature of the Vanguard wing is that not only is there the fail- safe concept for bending loads on both top and bottom surfaces, but in addition there are two torsion boxes; thus, even major dam- age to a shear web could still leave a temporarily workable torsion structure. On page 76 is seen the root-joint configuration which results from the differing philosophies of the Viscount and Vanguard wing structures. Another feature is that an unpressurized wing stub has been incorporated, so that the pressure cabin is isolated from the fuel-tank end rib.
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