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Aviation History
1953
1953 - 1217.PDF
FLIGHT, II September 1953 POD PROS and CONS 373 behaviour on the approach. Admittedly, these flans are cosrlv structures, for they have to be fitted around die j« p£s bm their efficiency is not thereby impaired, and may even be increased owing to the induced flow pulled through the flaps by the efflux from the engines themselves. American designers have made strenuous efforts to obtain high lift-coefficients, but the travel of their flaps is severely limited by the proximity of the jet effluxes. In the eight-,et B-52, for example, the flaps are dis continued in way of the jets. As a direct result of the layout of the B-47, it is imperative that both front and rear sets of wheels touch the runway together if pitching oscillations are not to develop. This in turn imposes on the pilot the necessity of keeping true air speed on the approach within something like plus or minus one knot of a value fixed for each landing weight, and read off from a chart. This necessity of landing at a precise angle of attack is by no means easy, and some what restricts the usefulness of this fine bomber. Furthermore, the B-47 is undeniably something of a "hot ship" and needs rather more deceleration than its wings and flaps can provide—particu larly since the angle of attack remains verv low throughout the landing run. On all but the best airfields, "therefore, a tail para chute is required in order to maintain brake and tyre life at an economic figure. The big-wing/buried-engine bomber, on the other hand, can settle quite gently on its bogies over a wide range of attitudes and air speeds. Even the Avro Vulcan, the prototype of which has no flaps whatever, makes but slight demands in the matter of runway length, on account of its low wing-loading and considerable air drag over a wide range of landing attitudes. The relationship of the engines, and their exhaust, to the tail is very important. The most pronounced effec* is the modifica tion to the local-flow direction, which can seriously alter the stability and trim of a multi-engined aircraft at varying throtde settings. The only solution is to move the engines up or down, or outwards towards the wing-tips. It is quite easy to see what method has been adopted in any given case; the Valiant and Victor, for example, have high-mounted horizontal tail surfaces, while the Boeing bombers have thrust lines well down beneadi the wing and further from the aircraft centre-line. The Comet is the most conventional of all multi-jets; the Vulcan, of course, is once more in a class by itself. Nacelle Drag Interference between a pod and its supporting wing-structure can be brought to a very low minimum. In fact, Mr. Schairer has claimed that "it is relatively straightforward to mount engines in such a way that interference and compressibility effects are negli gible or even favourable at moderate to high lift-coefficients." It seems a fair claim that interference drag should be negligible throughout the normal operating conditions of the aircraft. On the other hand, this ideal condition may be impossible of attain ment owing to overriding requirements in other spheres. A typical figure quoted in America for the total flat-plate drag of podded engines is a value of 18 per cent of that for the complete aircraft. Even making allowances for the larger wing and possibly inferior engine performance of a buried installation, it is likely that the drag of the latter will amount to very much less than half this value. This improvement should be even more pronounced as speed is increased. There are as yet no examples of a supersonic buried installation, and it would be instructive to examine such a layout. The pod undoubtedly scores in reducing the length of ducting through which engine air and exhaust must pass. In fact, a pod can be so designed as to permit its contained power plant to have intake and exhaust pipes of the optimum configuration. This is rarely possible in the case of a buried installation—although it may perhaps be attainable in the future if engines continue their present trend of becoming ever longer and thinner. But even such mighty ducting as is found in the Vulcan is by no means the millstone that it might appear. Proper design can reduce losses to a very low value, and the fuel consumption required for a given dirust, or the thrust available from given r.p.m., can be made virtually indistinguishable, for practical purposes, from figures recorded with a bare engine on the test-bed, assuming sea- level static conditions in each case. Ram compression can, in fact, be improved by careful design of the entry and intake ducting. That several solutions are possible is clearly shown by the variety exhibited by British designers. The Valiant was originally built with narrow, "letter-box leading-edge slits which were changed to the present configura tion when much more powerful Avons became available. I he big, plain intakes of the Olympus-Vulcan and Sapphire-Victor are carefully designed to feed both engines on each side, the ducting bifurcating at a point well inside the wing. Both these aircraft have boundary-layer-splitting shoulders at the inboard ends of the intakes. The Comet I and II, of course, have a pair of simple intakes in the leading edge of each wing which could The inner pods of the B-47 each house two General Electric J47-GE-23 or -25 turbojets. For full inspection, the complete cowling can be removed. The stalky outrigger leg retracts forward. hardly be simpler, and this layout may be maintained in later versions of the Comet. In each case, the governing factor is the maintenance of reasonably steady flow in a straight line with the minimum pressure drop and without breakaway or turbulence at the duct walls. Incidentally, we seem to recall that Prof. Messerschmitt fav oured engines mounted under the trailing edge, with direct intakes at about 50 per cent chord. This might give a lower intake- velocity for the same total head. There were, of course, all kinds of other pioneering German layouts. So much for performance, and Mr. Schairer appears to have been quite fair when he said that, on this score, there was little to choose between the podded layout and the buried engine. On the other hand, we know of several men in this country who are convinced diat the buried engine can provide better all-round performance; yet Mr. Salmon, of Convairs, appears to think that an airliner using buried engines would have to accept a penalty in cruising speed of from 50 to 75 m.p.h. in comparison with a similar design using pods. Direct comparison, of course, is impossible until directly comparable aircraft can be built using the two systems. With the podded engine, all-round accessibility can fairly easily be attained, and this is obviously the ideal state for easy main tenance. As such, the pod has an advantage over the buried engine, for one can only get at the lower half of the latter, unless the engine is removed from the airframe. But the picture is by no means as one-sided as American lecturers would have us think; and one feels that Mr. Schairer, for one, was not playing quite fair when he showed a photograph of a ground engineer's feet projecting from the wing-root intake of a Grumman F9F Panther fighter as an example of access to a buried engine. Much depends on the engine itself. We are fortunate in having a number of excellent engines specially designed for buried lay outs with the accessories disposed around the lower parts of the engine, where they are readily accessible. The engine bays in the wings of our latest large turbojet aircraft are covered by big access-doors which form part of the wing under-surface. When open, these doors reveal the entire body of the engine, and permit about 90 per cent of the daily work to be carried out forthwith. Top access is rare, although it is a feature of the Comet. Day-to day duties, therefore, can be done quite easily with either layout. Altogether, there is nothing critical about the accessory distri bution in a buried engine, and there is far more room around such a power plant than there is around a podded engine, as is reflected by the ease with which accessories and their associated pipes and cables can be distributed. There is no necessity for personnel to crawl about in confined spaces, as has been suggested. The single p8d, in particular, leaves little room for accessories, although the twin-engined unit is better in this respect. The pod probably scores where a major overhaul has to be effected, or where replacement of flame-tubes or blading is re quired. In fact, it could be argued that the frequency with which this had to be done with the earlier American axial turbojets was the principal reason for the adoption of the pod in the first place. From a performance viewpoint, it appears likely that a "rough" pod or a loose cowling is more likely to occur than is superficial damage to a buried engine installation. On the latter, the main doors are frequently load-carrying in flight, and can form part of the main airframe. Their robustness can, therefore, prove useful in preserving their original shape and smoothness under arduous service conditions which might damage an unstressed pod. Furthermore, the pod is rather vulnerable to damage by servic ing platforms or ground vehicles. Injury from stones and other
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