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Aviation History
1949
1949 - 1999.PDF
tUQHl, x% December CAN BE R R A Design Analysis of Britain's First Jet Bomber WHEN photographs of the English Electric Canberra were initially released for publication in May, the low- aspect-ratio wing evoked a good deal of interest and speculation. It also came as a comforting relief to many thinking people who had long been somewhat uneasy about the general Gadarene trend toward higher and higher wing loadings. A low aspect ratio is not by any means synonymous with a low wing- loading, but the all-up weight of the Canberra could scarcely be great enough to give, with a wing of that area, a loading anything like so high as that which has now become almost common practice. And so it has since proved. , The origin of the aircraft as a specific design is interesting in that it shows a preoccupation with efficiency, modified only by a sensible, down-to-earth appreciation of practic- ability. As mentioned in our December ist issue, the germ of the idea from which the Canberra finally grew first occurred to Mr. Petter in 1944, when he was technical director of Westland Aircraft, Ltd: This, however, was something in the nature of mental " doodling " such as all designers in- dulge in, and it was not until a specification tor a high- performance jet bomber was issued to the English Electric Co., Ltd. (of whose aircraft division Mr. Petter had newly become chief engineer), that the " doodle " had a chance of being developed into something serious. The specification outlined speed, range, load, cruising alti- tude and single-engine performance minima, take-off and landing distances and parameters of strength requirements, and stipulated the employment of two Rolls-Royce Avon axial-flow turbojets. Within this framework. Mi. Petter and his team were free to design any aircraft which would fulfil the requirements and embody the ancillary items of the specification, such as a pressure cabin, ejector seats, radio and navigational equipment, the required variety of bomb loads and so forth. The next step was to plot out a large range of alternative design studies, the chief variables of which were wing area and plan form. • In making these studies, there was a constant preoccupa- tion toward keeping something in hand for development, particularly in the matter of increasing range, and as a result the bias was in the direction of low loadings. Exam- ination and analysis of the several factors—for example, the plotting of wing loading and aspect ratio against per- formance and wing weight for a fixed thrust—clearly indicated an optimum among the (by modern standards) low wing-loadings. This, in turn, permitted a cruising condition at a low CL which, further, allowed the use of a low aspect-ratio. Structurally, wing thickness was virtu- ally determined by the necessity to stow the main under- carriage wheels, and the choice of aspect ratio was influenced by this required wheel-stowage depth and the aerodynamic factors which set an upper limit to thickness / chord ratio. The decision to use an aspect ratio of 4.3 was eventually taken, the span loading being of a conventional order, i.e., approximating to the value used for a modern fighter with an aspect ratio of about 6. Incidental advantages accruing from a design of this kind were the attainment of a high degree of manoeuvrability, particularly at extreme altitudes, coupled with outstanding low-speed-handling characteristics, the latter being safe- guarded by the decision to eschew a high taper ratio. In the event, it has been established that the Canberra virtually sets a new standard among contemporary military types for low-speed handling, approach and landing characteristics. The stall, in particular, is gentle, straightforward and utterly viceless, and the speed range of the aircraft is believed to be wider than that of any other known fixed-wing type: it is, in point of fact, something over 7:1. A symmetrical aerofoil section is employed for the main- planes as a means of avoiding violent pitch-changes when the critical Mach number is exceeded. The actual section used (a modification of a German contour) was designed to give a reasonably high critical Mach number without sacri- fice of good control characteristics. It is essentially a matter of compromise between the opposing desiderata of having the maximum thickness sufficiently far back to give good MCBIT yet sufficiently far forward to give good control. Help is given in regard to the latter quality by designing the section with a narrow trailing-edge angle. In this connec- tion, the control surfaces are flat-faced "wedges" and if any aerodynamic penalty has to be paid for lack of chorda! curvature, it is so small as to be negligible; additionally, it is of small moment in relation to the advantages that such surfaces permit in the achievement of a high standard of inspection during manufacture. They are, furthermore, very much easier to produce.
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