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Aviation History
1945
1945 - 1178.PDF
•644 FLIGHT JUNE 14TH, 1945 MARSHALL CABIN BLOWER other positive-displacement types— particularly for aircraft cabin pres- surising. For example, at' ground level it is absorbing very little, power, as it is doing virtually no work, and the temperature rise across it is corre- spondingly small. Again, adiabatic- ally it is inefficient, and the moft work it is called upon to do the greater the temperature rise of the air. This is of practical value for cabin work as the air delivered is, to a cer- tain extent, preheated and thus needs only a com- parable temperature additive. In fact, where small-volume cabins are concerned, the additional heater can often be discarded. Such is by no means the condition where large volumes are involved, and it is a pointer to the price-penalty of pressurising that, for instance, in the case of such an aircraft as the new Avro Tudor 1, no less than 100-odd h.p. is expended on pressurising and heating the cabin air. Probably the most important advantage possessed by a Rootes-type blower for this particular type of application is that it is one of the only positive-displacement types in which the pumping elements do not touch, and thus the air stream delivered is completely free from oil—a vital necessity. Centrifugal and axial-type compressors are not very suitable for medium, and quite unsuitable for small aircraft installations as to -attain reasonable operative efficiency they must deal with very large volumes or run at extremely high speeds. There is undoubtedly a very large field of endeavour ROTORS NON RETURN AND SRILL VALVE CABIN DISTRIBUTION v DUCTS This sectioned perspective illustration shows the salient features of the Marshall Blower and makes clear the method of operation. awaiting exploitation in the pressure cabin sphere, and it would appear that Sir George Godfrey and Partners are in an excellent position to give a valuable contribution. A very great deal of work on pressurising has already been done, but there still remains an even greater amount yet to be done, particularly in large volume cases. This is emphasised by an American admission that it will be a full two years before they can offer a large-volume pres- surised cabin. Without being unduly optimistic, we are quietly confident that Our own aircraft manufacturers will successfully fulfil their expectations in this direction within the very near future. Certain it is that the difficulty is purely a structural one, primarily involving the sealing and stressing of the fuselage. The blowers and ancillary equip- ment have already been developed to a pitch where their performance is vastly in excess of the airframe limits. Military and civil applications are very different; the Diagrammatic layout of typicalpressurising system for a four- engined aircraft. In suchsystems the cabin atmosphere is not only pressurised but isalso fully conditioned. former is by far the more exacting, particularly in terms of operational range. A point to this is that as far back as 1940 Sir George Godfrey and Partners were producing blowers with a compression ratio of 3.7 : i, whereas in the most extreme civil case so far envisaged a compression ratio of a little over 2 : 1 suffices—a useful yardstick for measuring relative safety factors. Reliability is a great point of the Marshall Blower, as may be understood from the performance figures of the Type 15 model which are shown in the accompanying set of curves. The blower runs at 2.89 times engine speed, thus at the normal engine cruising r.p.m. of 2,100 the blower revs are 6,000/min. ; in view of this the normal overhaul period of 1,000 hours is pretty conclusive. It has been established that for long-range applications a pressurised cabin with all the an- cillary equipment entails a smaller weight penalty than does providing the passengers with oxygen from bottles—this being a purely factual calculation which does not take into consideration the very important physiolo- gical effect upon the passengers. In consideration of the latter qualities it must be appreciated that pressurisation per se is not enough. The climatic conditions in which the aircraft is to operate exercise a pro- found effect upon the air supplied to the cabin, quite apart from temperature factors. For in- stance, a machine which is, say, flying the Arctic route will be abstracting air from the atmosphere which, in comparison with tht' temperate conditions such as obtain in this country and Europe as a whole, is abnormally dry. Further drying effect incurred by the high temperature rise of the air through the blowers aggravates the condition with the result that, unless a humidifying chamber is incorporated in the system to wet the air delivered to the cabin, the poor passengers will suffer extremely sore throats. Conversely, for operation in tropic conditions, the air abstracted is so saturated with moisture that drying is imperative to preclude the whole cabin interior, and the passengers, running with rivulets of water. As previously stated, large volume cabin pressurising is disproportionately more difficult than pressurising small cabins, and, of course, the civilian passenger who has paid a not inconsiderable sum to be transported at high speed for some thousands of miles not unnaturally expects to be so transported within the lap of luxury. High-altitude operation for long ranges holds too many positive advantages to be long delayed in commercial air passenger transport, and whoever is first in the field will hold excellent cards for developing in extended markets. ROTOR COOLING INTAKE ROTOR COOLINt AIR OUTLET EXTERNAL COOLING FINS
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