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Aviation History
1952
1952 - 2275.PDF
FLIGHT, 22 August 1952 Design Analysis of the New Turboprop Airliner RITA I A The Proteus 2 turboprop! in the first prototype are very neatly installed, the cowling design offering maximum accessibility. The D.H. hollow- steel airscrews are 16ft in diameter. THAT the Britannia is an aircraft with the promise of outstanding merit has already been made abundantly clear in various articles published in Flight earlier this year.* These reviews, however, were concerned in the main with performance expectations, whereas now it is possible to examine the aircraft as a study in design skill. When visiting Filton a few weeks ago we were taken up to the catwalks which traverse the great roof of the Brabazon hangar, and from that eminence looked down on the first and second prototype Britannias, the former then nearing completion. Whilst gazing down at the aircraft, it was borne in on us once again that to all superficial appearances the Britannia, handsome as it is, is quite an ordinary aircraft: there is nothing about it to strike the observer as outstanding or novel; in short, there is nothing on which one can fasten as justifying the out-of-the-ordinary qualities that one believes the aircraft to possess. Why is this? In the briefest terms, the answer is simple-—and, like most seemingly simple things, has a deal of complication behind it. Aerodynamically, the aircraft is clean; it has, in fact, an overall drag-coefficient of an order according with the best modern practice for largish commercial transport types. This has been attained, moreover, in conjunction with a capacious fuselage. What is the more unusual, however, is that the carrying capacity of the body volume is usable to a much greater degree than is normally the case, this being permitted by the aircraft's astonish ingly low percentage structure-weight. Directly linked with this is the low installed weight of the power installation, and on the other side of the picture there is the high efficiency of the engines —a reflection of their low specific fuel-consumption and high specific power-output. Include with all these qualities a markedly generous fuel-tankage capacity, and one has all the essentials for an outstanding aircraft. It is as simple—and as difficult—as that The evolution of the Britannia was not, however, a smooth process; the design history goes back a long way and it is perhaps because so much time was spent in argument and conference between the company, B.O.A.C. and the Ministry, that the design had the opportunity to mature; to mature, that is, in the sense of * January 2Sth, February 29th, March 7th, May i6tk. achieving some measure of development. Let us take a glance over the background highlights. The first germ of the idea was found in the Type 3 recommenda tion of the famous 1944 Brabazon Committee, but in 1946 the essentials of the Brabazon 3 type gave way at the instigation of B.O.A.C. to the "Medium-Range, Empire" specification. During April of the following year, Bristols tendered a design-study in competition with ten other companies and learned in the July that it had been considered as the most promising of the designs submitted; this was the first concrete appearance of the Bristol 175. In October, 1948, B.O.A.C. proposed that the 175 should be fitted with Proteus turboprops instead of Centaurus piston- engines, and the following month they announced their intention to order 25 production aircraft. The actual agreement to purchase 25 production 175 s was signed in July, 1949. In November of that year the decision was taken to build a non-flying prototype and, as readers of the March 7th, 1952, issue of Flight will know, this decision was of the most profound import ance in its influence on the later progress of the aircraft. By October, 1950, scale versions of the Britannia's servo-rudder, tail-plane and elevator had been flight-proven on an adapted 170 Freighter and, following completion of the primary structure of the non-flying prototype and the full-scale wing and half-scale fuselage structural test specimens, building of the first (flying) prototype started in January, 1951. At the time of writing, five sets of detail parts for production aircraft are extant, and the fuselage structures of the first two production machines are nearly complete. So much for the historical sequence of events. As regards the design development of the aircraft, it may be said that dozens of solid shapes were tunnel-tested in order to find that which afforded the best compromise between form and structure. As already mentioned, the efficiency of the Britannia is in no small measure due to the low structure-weight, and basically this is directly linked with the use of a straight taper wing and as lengthy a parallel trunk of fuselage as the aerodynamic considerations would permit. Within these parameters, however, the actual structural form employed could have been any one of several variations on the classical semi-monocoque theme. In the event, the use of multiple close-spaced stringers with comparatively few frames or ribs, in conjunction with skin plating of the lightest practicable gauge, was decided upon largely as a result of the studies made for the Brabazon project. It is unfortunate, to say the least, that in the public mind the Brabazon should be regarded as a white elephant, and a extremely expensive white elephant at that. In point of fact, that giant aircraft still represents a considerable advance in aircraft technology—in many respects is still a good deal in advance of its time. The fundamentally important thing about the whole project is, however, that in order to build the Brabazon the Bristol Aeroplane Company were forced to make the most exhaustive investigations into structural techniques and material limitations. As a result, they obtained an enormous mass of definitive data, enabling them to select with certainty the precise optimum answer to a particular structural problem. As for structures, so for other aspects of aircraft design : we do not imply by these comments that Bristols know all there is to know about designing and building aircraft—fortunately, it is a matter which is constantly in a state of flux—but there is little room for doubt that, as a result of having built the Brabazon—as a result of having been able to build the Brabazon—the company derived a wider and, at the same time, a more exhaustively detailed knowledge of structural optima than that possessed by perhaps any other one company. The production technique adopted for the Britannia also owes more than a little to the Brabazon studies. For example, the fuselage is plated in skin panels of unusually light gauge, the material being D.T.D. 746 close-tolerance sheeting and, in order to preserve surface-finish, a dimpling process is used. The panels
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