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Aviation History
1947
1947 - 1782.PDF
436 FLIGHT OCTOBER I6TH Pressurisafion be controllable within close limits, and to maintain a low temperature with higher ambient temperature, necessitated refrigeration Humidity, when flying at altitude, should be maintained at a minimum of 20 or 30 per cent, which neces- sitated the carrying of water. Methods for increasing humidity were eithei to boil water and to inject steam into the stream of incoming air; to Employ water jets; or to incorporate water in the air blower. As a guide it was considered that two hours' flight would require 1 gal of water, and five hours about 2 gals of water. Safety precautions were necessary to prevent damage to the cabin by excessive pressure, crushing of the cabin during rapid descent, and also sudden loss of pressure. Apart from structural Strength, the present equipment was designed to yrovide generous safety valves, inwards relief valves and a complete duplication of the air supply system. Mr. Widgery then outlined the basic layout of a pressurised system and mentioned several of the refinements which had been necessary. It had been found necessary to fit a silencer between the blower and the cabin on the Welkin, which was the first aircraft to be designed in this country, ab initio, for pressure. It was also found necessary to introduce a measure of air cooling by allowing jets of air to impinge on the silencer casing. Briefly, the system was to install a blower, fitted with a filter to exclude dirt and dust, from which air was passed through inwards relief and spill valves into the cabin. Initial difficulties had been experienced in sealing the cabin. The lecturer considered the Meteor to be of special interest since the air supply was obtained by bleeding the compressor instead of by employing a separate blower. For heating the cabin the lightest and most efficient heaters available were of the petrol-burning variety. There was, however, an alternative in the employment of heat exchangers placed in the exhaust system. Having already dealt with humidification, Mr. Widgery referred briefly to dehumidifica- tion. In the absence of refrigeration, a chemical drier such as activated alumina could be used to absorb the excess water vapour. Mr Widgery observed that there was extensive literature on heating and ventilation of buildings, but the aircraft cabin presented a difficult problem, largely because of the small volume pei person. Extensive investigations had been carried out by Westlands on distribution of air, and for this a cylin- drical test chamber had been built. The cylinder represented a short section of fuselage, and they appreciated that there was the added difficulty of bringing about the distribution from one end of a long cabin to the other, and he considered it imperative that prototypes should be constructed so that distribution could be corrected during flight trials by the use of butterfly valves within the ducts or at the outlets. Since a re-circulating system distributed tobacco smoke and other odours there was need for a good and effective air filter and purification system. Air Supply and Blowers.—Ii was customary, the lecturer explained, to employ special blowers for pressurising the cabin, the only alternative being to tap from the turbo compressor where suitable povei parts were employed. Blowers at present in use in this country were of the Roots type, whereas in the U.S.A. the centrifugal type had been exclusively used. Owing to greater operating altitudes, cabin pressure blowers will be required to work at higher compressor ratios, and in future single stage will no longer suffice, and multi-stage centrifugal types will be necessary. The necessity for refrigeration had long been the subject of controversy, but there was now almost general agreement that it was necessary on a high proportion of air routes. In the opinion of Mr. Widgery, modern fuselage construc- tion and shapes lent themselves to pressurisation, but addi- tional weight resulted from necessary sealing and local stiffen- ing. Attempts had been made at assessing the additional weight, and it seemed fair to assume that the structural weight of the fuselage should not be increased by more than 5 per cent on account of pressurisation. Experience of complete installation was still limited, but the actual extra weight carried on the Tudor 1 amounted to approximately 770 lb. It must be remembered, however, that an economy of weight was effected by the elimination of oxygen gear, and it had been calculated that there was an economy of nearly 500 lb due to the use of pressure instead of oxygen on the Tudor I. Future Development.—In summing up, Mr. Widgery con- sidered that the experience and lessons learnt in operations of the smaller civil types of pressurised aircraft were already being •applied to military types and the requirements of both were rapidly converging. It was well known, he said, that in the near future civil transports would be flying regularly at heights up to 50,000ft, and at such a height a sudden loss pf pressure would be fatal. In fact, in the new projected aircraft a cabi failure would be just as disastrous as a main structural failure Mr. Widgery pointed out probable lines of development and suggested that provision of fresh air meant considerable usag« of horse-power for pumping air into the fuselage, none of which at present is recovered, and development might possibly result in a reduction of the supply of fresh air in conjunction with an effective air purification plant, or, alter- natively, equipment might be developed to recover the power expended^on initially compressing the air. He considered the problem of de-humidification to be a serious one, but if it were possible to reduce the amount of fresh air required, the amount of water required would automatically be reduced. Alterna- tively, water-recovery plant could extract water from the air discharged from the cabin or from the engine exhaust He felt also that further development would lead to a design in which control of cabin atmosphere was effected by condition- ing the equipment carried within the aircraft, and blowers would be used only to make up the necessary amount of fresh air. There was reason to believe, he said, that pressurisation would be demanded by operators for purely economical reasons, as the ability to make rapid climbs and descents would result in a saving of flight time that would more than compensate the cost involved in carrying and servicing the equipment. v The Discussion : Mr. N. E. Rowe, Controller of Research and Long-term Development in B.E.A., opened the discussion by drawing attention to the large attendance, which indicated the interest in and importance of the subject. He felt that since there was only 4 lb/sq in difference between the amount of pressure required to obtain ground-level conditions, and conditions of 8,000ft, designers should go the whole hog and aim at ground-level conditions for all altitudes. Mr. Rowe considered that the danger due to explosive decompression would have to be eliminated and that there must be no risk of such a happening. He could see a long period of development ahead but considered that the greater economy and comfort which would result were essential to air operations. Mr. Saunders, of the Gloster Aircraft Company, spoke of the possibility of providing emergency systems other than those already suggested by the lecturer, and wondered what ground- testing equipment was being developed, as flight testing of pressurisation was a lengthy process. He considered that temperature was a serious problem. Mr. Scott-Hall, Principal Director of Technical Development (Air) at the Ministry of Supply, had no criticism of the system but asked if there were data on the humidity and temperature effects in different climates. He challenged the lecturer that military and civil requirements were converging, since there was always the danger of military action on the one hand, and we must not forget, he said, that civil aviation could not afford the risk of explosive decompression. Mr. Widgery replied that there were considerable data on humidity and temperature, effects at the meteorological offices. He said that military machines provided a safety margin by j the use of oxygen, but as future operations would take both 1 civil and military aircraft to even greater heights, they had a similar problem through the complete lack of oxygen. He also considered that the collapsing of a cabin by enemy action had an application to civil uses. Mr. R. Graham made a point of the boredom of long- distance travel and considered that pressurisation was essential to comfort. He was very impressed with the flight he had made under pressure conditions in a Gold Plate Constellation. Mr. Petei Masefield said that pressure was one of the three great problems, and at the present stage we were getting / familiar with pressure up to 25,000ft, but we must be pre- pared to fly up to 40,000ft and even higher, and the time would come when we would be flying at 250,000ft, and then we would probably have to carry air around in the aircraft as there would be none to suck in. He considered that on the success of pressurisation depended the success or failure of our future aircraft. Humidity he considered to be a very great problem, and gave instances of flying when the tempera- ture was high but the humidity was low, so causing the passen- gers discomfort from cold. The opposite effect could be felt when the temperature was low but humidity was high. Re' ferring to the permissible rate of leak, he revealed that flush riveting on the Brabazon had not been adopted as the cabin would not hold the pressure, and mushroom-head rivets had been substituted. Mr. Willis, of the design office of Avro's. paid tribute to the pressure equipment on the Tudors, but h« said there had been the problem of fitting after construction He thought that in future the pipes should be laid first.
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