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Aviation History
1958
1958 - 0139.PDF
FLIGHT, 31 January 1958 143 Design and Development of an Outstanding British Bomber BY THE TECHNICAL EDITOR was a suggested limitation on gross weight. Strenuous efforts were being made at that time to stave off the evil day when the standard operating platform of Bomber Command would have to be lengthened and strengthened, and as runway loading index and gross weight were then considered to be directly related, it was desired that the latter parameter should be limited to a value —a remarkably low value—written into the specification. At the beginning of 1947 the Air Ministry sent copies of the specification to selected bomber firms. One of these companies was A. V. Roe at Chadderton, between Manchester and Oldham, where the project was raised under the company number 698. It was obvious from the outset that no conventional aircraft could even approach the required performance; and the \vro design team, although one of the strongest in the country, was soon acutely conscious of the very limited knowledge which was then r available on high-speed aircraft design. Virtually nothing worth- while could be culled from the R.A.E. or the U.S.A., and almost all that the company had to go upon were the reports by the field team investigating German research during the 1939-1945 war. These reports were the first to make widely known the reduc- tion in high-subsonic drag which- could be obtained by the use of swept-back wings. Avro investigated a number of possible configurations before deciding to concentrate upon a design which, although otherwise conventional, had a wing with 45 deg sweep-back at the quarter-chord line, a thickness /chord ratio of some 12 per cent at the root and a modest design lift coefficient. This seemed a good jumping-off point; but even the earliest calculations showed that any machine of this shape then capable of being constructed would have exceeded the target weight by 100 per cent, and would have failed to achieve the desired per- formance (it would have been generally comparable with the B-47). Avro's thinking next took the following lines: "a bomber carries a concentrated load and so the fuselage is needed only to carry the tailplane; since the swept wing increases longitudinal stability it may be possible to delete the tailplane and hence eliminate the fuselage also." Putting this idea into practice, the company arrived at an all-wing shape reminiscent of the Northrop aeroplanes of that time. Assessment of this showed a substantial reduction in gross weight, but one still insufficient to bring the weight down to less than 150 per cent of that officially suggested. It was clear that the wing itself (which now comprised virtually the whole airframe) was too heavy, and that the only practicable method of trimming its weight lay in a considerable reduction in aspect ratio. In effecting this the design team were increasingly attracted by the idea of filling in the gap between the wing tips and the fuselage behind the trailing edge, and thus producing the triangular delta shape. As soon as this was done the way was opened to the use of thinner material gauges and traditional manu- facturing methods, which, advantageous enough in themselves, also came hand-in-hand with a very great reduction in wing weight. Moreover, although the thickness/chord ratio had not been increased, the interior volume of the wing had risen sufficiently to provide capacity for a much greater quantity of fuel. At last the Air Ministry requirements were within Avro's grasp. An accompanying drawing shows the classically simple con- figuration of the original delta sketch. Within the wing, space was provided for an offset bomb load, two or four (upper and lower on each side) large turbojet engines and the main undercarriage, in addition to the bulk of the fuel. At the front was a rudimentary fuselage containing the crew compartment. The flying controls, which were based on those developed for the Armstrong Whit- worth A.W.52, consisted of an unusual arrangement of double elevons along the wing trailing edge; fixed fins were provided at the wing tips. In spite of its radically new shape, the design seemed on paper to be logical and correct, and the late Mr. Roy Chadwick, then Avro's technical director, decided to back the delta proposal as his company's submission to the specification. His chief designer, Mr. S. D. Davies, later said "I would like to pay tribute to the courage of Mr. Chadwick in making the decision to back the tailless delta configuration. It is one thing for a junior technician to push an unconventional idea, but for a designer with Mr Chadwick's past history of success to risk his reputation on such a venture was an act of high courage which was not perhaps sufficiently recognized." A project model of the Avro 698 a$ it was late in 1948. Similarity to the present Vulcan is close, the latter differing only in detail. This was the tender which Avro submitted in 1947, but before the Ministry of Supply accepted it the design evolved into the form shown in an accompanying photograph. The originally meagre store of knowledge of high-subsonic aerodynamics was steadily being reinforced with data from the U.S.A. and else- where. Towards the end of 1947 the Avro project staff realized that low thickness/chord ratio was just as valuable as wing sweep, and the wing of the 698 was thereupon reduced in thickness, albeit at the expense of greater structural loads. This reduction in thickness made it impossible to accommodate the powerplants (the projected Bristol B.E.10 two-spool engine) in superimposed pairs, and the upper engines were therefore relocated outboard of the lower units, each pair being served by wide slot-type intakes. The flying-control system was completely redesigned; the elcvon arrangement was replaced by inboard elevators and outboard ailerons, and the wing-tip fins gave way to a single vertical surface on the aircraft centre-line. At the same time the fuselage was allowed to grow somewhat, thus removing any suggestion of all- wing appearance. By January 1948 the Avro 698 design looked very much like the present Vulcan. In that month the Ministry of Supply awarded a prototype contract for the 698; an order had already been placed with Handley Page for the H.P.80 (the Victor). During 194S, tunnel-testing of the Avro 698 began in earnest. The tunnel programme was divided into low-speed and high- speed portions, the former being carried out jointly by Avro, the R.A.E. and the National Physical Laboratory, and the high-speed part being undertaken solely by the R.A.E., whose 1 lft tunnel was at that time the only one in the country capable of handling such work. The following is taken from a paper prepared by Avro: "As part of the high-speed model programme the R.A.E. investigated the effect of planform and section modifications on the characteristics of the aircraft at high subsonic speeds up to and above the design cruising lift coefficients. These tests indicated that substantial improvements in both the drag-rise critical Mach number and the drag rise with increase in lift coefficient could be realized by modifying the wing sections across the span. The reasoning behind these improvements was that the characteristics of a wing at high Mach numbers are closely related to the sweep- back of the maximum-suction line, and this is adversely affected by root and tip effects. At the root the peak-suction line tends to become normal to the centre-line to attain continuity, and this causes a reduction in sweep over the inner part of the wing. At the tip the high induced incidences move the peak-suction line forward, again reducing its sweep. The R.A.E. therefore evolved a wing in which the maximum thickness of the wing sections was progressively moved forward from tip to root and the sweepback of the peak-suction line maintained over a much greater proportion of the span than on a wing of similar plan- form but with constant section throughout." So advantageous were the effects of this modification that early in 1950 the 698 wing was redesigned to incorporate the R.A.E. recommendations. Although unchanged in planform, the new wing had a section which was progressively modified to bring the point of maximum thickness very near to the leading edge at the root; the flow in this area was in any case appreciably influenced by the fact that the inboard portions of the wing were occupied by the powerplants and their ducting. To some
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