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Aviation History
1953
1953 - 1218.PDF
374 FLIGHT, II September 1953 POD PROS and CONS . . . foreign bodies can easily arise if the engine is mounted too close to the ground. Although it is a Boeing claim that, in a recent test, one of their engines "did not pick up a candy-wrapper from a large, flat board until the board was within 18in of the engine," this might not be a fair claim, for the behaviour of paper on a flat surface can be most odd. Matter already in the air, such as stones kicked up by another aircraft, can be more serious. On marine aircraft, too, water could do a lot of damage. The Martin P6M Seamaster, fitted with four Sapphires, is eagerly awaited. This flying-boat may be the first American buried installation. The necessity of keeping the pod well clear of the ground could, in fact, rule out the adoption of this layout unless the overall air craft configuration permits it. A pod is fairly straightforward on a bomber, where the weapons bay has to be placed under the wing (always excepting the Vulcan), but the most efficient transport has a low wing which will be too close to the ground to permit the installation of pods at all. Podded engines on a transport thus not only make for a high fuselage or a mid-wing layout, but raise the wing, so making refuelling and daily servicing more difficult, unless special methods are adopted. Even in the forth coming Boeing transport, in which the wing is mounted between the two pressurized decks, the inner pods are no more than 3ft from the ground. At this height, the engines can suffer damage themselves, and their blast can cause considerable annoyance to others on the airfield and, except on good quality concrete, appear likely to cause airfield deterioration. Fuel Tankage In the matter of space available for fuel stowage, it is remarkable that proponents of both podded and buried layouts are convinced that theirs is the correct solution. The American layout can, sur prisingly enough, accommodate a considerable quantity of fuel entirely within the wing, since the latter is largely free from struc tural discontinuities and houses litde apart from control runs, piping, operating jacks, de-icing and radio equipment and similar comparatively small items, which can be housed within leading or trailing edges. With integral tankage, bounded by a wing struc ture machined from continuous slabs, efficient fuel stowage can be provided; and it is capable of remaining leak-proof even during the considerable flexing which occurs with such a wing. The B-47 Stratojet carries all its fuel in the fuselage—perhaps supplemented by two 1,780-gal under-wing tanks—but the bigger B-52 and a nu-nber of other American jet aircraft carry most of their fuel within the wing. The American argument against the British layout is that the latter fi'ls too much of the wing with engines and landing gear. But the basic configuration of our wings can provide a larger tankage than the American, all of it housed well outboard, which is genera'ly agreed to be the best place. It could also be argued that the British layout reduces the length and weight of pipe-runs, and is certainly much better from the point of view of controls and piping between the cockpit and the power plants. Both pods and buried engines can be effectively de-iced. It could be said that the buried unit is more prone to the collection of birds and other such objects, but it is doubtful if there is a real case for the fitting of intake doors, as has been suggested bv the C.A.A. and other American authorities. The standard B-47/B-36 pod is, in fact, fitted with petal-type intake shutters which stop the engine from motoring under die influence of ram air when switched off, and also reduce drag. Once a pod has been designed for one engine-type, it can be incorporated, as a standard unit, on any of a number of air frames. Changes of engine, to meet increased power require ments, are thus facilitated. But the amount of re-design required to fit, say, the Vulcan with three types of British engine is not excessive. Probably the strongest argument advanced in favour of the pod is the fact that power plants can be more readily fire-sealed from the wing. If an engine fire is experienced in the air, the theory is that the engine can be left to burn quite merrily in the know ledge that the conflagration will not spread up the supporting strut into the wing. This is probably quite true and we person ally know of a number of B-36 and B-47 engine fires which have failed to make their way into the interior of the wing. But it should not be thought, as Mr. Schairer suggested, that the British buried engine is housed in an unventilated bay bounded by thin duralumin walls. Once again proper design is the solution, and we know that all the companies now building multi-engine jet aircraft in this country know that buried engines can be made safe. The engine is. in fact, capable of complete iso'ation in a fireproof section and, in the extremely unlikely event of an engine fire, measures can immediately be taken to ensure that it cannot spread outside die sealed engine bay. Surprisingly, the worst trouble is dripping fuel which, from a variety of sources, can sometimes find its way into other portions of the airframe, there to ignite and cause trouble. Here again, forewarned is forearmed, and one or two nasty experiences in the past are probably suffi cient to ensure that loose fuel will no longer cause trouble in our buried installations. In America at least (and this is not intended to be acrimonious) the incidence of turbine failure has been considerably higher than we have ourselves experienced. So catastrophic can the results of such a failure be that it is doubtful if there is very much to choose between the two types of installation on this score. B-47 turbines or compressors have been known to come apart in the air, and on at least two of these occasions all three engines on the affected side of die aircraft have been knocked out by flying pieces of metal. But Boeings have carefully swept their pods forward, so that the line of the turbine blading lies just forward of the leading edge; loose blading can, therefore, do litde harm except to the unpressurized part of the fuselage, and any engines or under-wing tanks in the neighbourhood. In the case of a buried engine, lack of evidence makes comment difficult—although, of course, we would not have it otherwise. It does appear, however, that one can guard against minor failures, in which event the main doors of die wing engine-bay should be more capable of standing up to loose material than the thin cowling of a pod. In the case of a crash landing, a study of the probable sequence of events leads one to believe that the buried power plant is likely to be safer than one mounted in a pod. A heavy impact on a pod is at present likely to push the complete pod up through the wing, dius severing fuel and auxiliary power lines and open ing up the main wing tankage. This leaves everything ready for a serious fire. The American counter to this argument is that they are developing pods with wipe-off characteristics, so that a wheels-up landing will result in the aircraft coming to rest as a glider with the fire-provoking engines a considerable distance away. Many British engineers remain entirely unconvinced that this procedure would add significantly to the safety of the aircraft, for it has been shown that, where fuel is being spilt, fire can follow the aircraft, gunpowder-train fashion, for a considerable distance over all types of terrain, including water. In any case, it is considered doubtful if pods could be arranged to tear off cleanly, unless jettisoning cartridges were used to shear the attachments. With the buried engine, the impacts of a wheels-up landing are taken on the belly and the wing tips, which can usually be relied upon to absorb the forces thereby imposed on them without causing severe airframe-failures. It is hoped that, in such aircraft, any normal crash-landing would leave the power plants and fuel tankage completely unharmed. It may be remembered that when a Comet belly-landed at Rome nearly a year ago, at least one of the engines was lifted out of its mountings and pushed up through the upper surface of the wing. The Comet is unusual in that, in a belly landing, the engines are likely to take some of the im pact. In any case, this incident was instrumental in proving the relative safety and fire-resistance of kerosine in all but the worst cases, except for the most adverse tropical conditions. A quite unrelated point is the improbability of podded turbo- props. Unless contra-props were fitted, the pod-strut would experience a severe torque load which would be difficult to meet. There remains one factor in determining the type of layout to adopt, one which could have a profound effect where the aircraft concerned is a passenger transport. From the noise-level aspect, it is generally agreed that a cone of 120-deg included angle, with its apex at the centre of the orifice of any of the engines, should not intersect the fuselage forward of the rear bulkhead of the passenger cabin. Mr. Salmon, of Convairs, talked what sounded like good sense when he said that if this cone could be made to intersect the rear fuselage only, the passenger cabin could be kept reasonably quiet in flight with a minimum of soundproofing, provided that the rear bulkhead was properly soundproofed and contained no openings. He went on to say that if this 120-deg cone intersected the wall of the passenger cabin, no amount of soundproofing would maintain the interior noise at a reasonable level. The only turbojet transport-type aircraft in which noise measurements have been taken are the Comet, Ashton, Nene- Viking, Tay-Viscount and Avro Canada Jediner. The general consensus of opinion is that when the engines are carried well inboard against the fuselage, the jet orifices should be as far to the rear as possible. The increased length of jet-pipe observable in photographs of the Comet 3 models may have something to do with keeping noise out of the lengthened passenger cabin. This problem may, it is hoped, become less acute in such aircraft as the Vickers Type 1000 and V.C.7, in which the cooler and slower-moving outer cylinder of air from the Rolls-Royce Con way by-pass power plants serves as a most effective blanket to the hot, high-velocity central jet. Once again, of course, Avro's delta layout remains in a class of its own: the Avro Atlantic transport has no passenger accommodation behind the line of the jet-nozzles, and should therefore prove to be one of the internally quietest aircraft ever built.
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