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Aviation History
1951
1951 - 1143.PDF
712 FLIGHT ENGINEERING IN AVIATION... to be used because it was undesirable to introduce combustionproducts into the wing, owing to the danger of severe corrosion on condensation. Electric de-icing could be both efficient andlight, because current could be used cyclically as, indeed, it was in the case of airscrew blade de-icing. The services embraced in the function of cabin atmospherecontrol cost about i h.p. per person for pressurization, and up to 0.5 h.p. for refrigeration. With a gas turbine powerinstallation, it was attractive to tap the compressor, as the necessary pound of air per person per minute amounted to lessthan 1 per cent of the gas turbine mass-flow, and the effect on compressor/turbine matching was small. However, there weredisadvantages : (i) no trace of oil must leak from the compressor bearings into the airflow; (ii) the range of engine speed waslimited by the need to maintain adequate pressure at the tapping; (iii) the use of methanol for de-icing or power boosting wasprecluded; and (iv) if combustion gases were used for de-icing, they had to be sealed off from the compressor airflow. For thesereasons, it had been considered advisable in some cases to use separate cabin superchargers. Lighting and galley services were included in the cabin servicesgroup. Lighting required up to 1.4 kW on aircraft of 60,000 lb a.u.w., and about 6 kW on aircraft of Brabazon size. Galleyloads varied between 7 and 12 kW in this size range. Under the heading of accessory power supply, the lecturerstated that the turbine engine presented new potentialities, not only in the availability of compressed air, but also in the existenceof a high-speed drive for an electric generator. These possibilities seemed attractive, but it had to be remembered that compressedair was not free, and if the compressor was robbed of either air or power, the cost might be disproportionate if turbine/compressormatching was appreciably affected. An approximate general rule was that the abstraction of 1 per cent of compressor powerfor air delivery cost ij per cent in power output, and T.\ per cent in specific consumption, but if the compressor or turbinecharacteristics were sharply critical (as with high compression ratios) the percentage loss was appreciably more. In this con-nection, however, power could be taken from a free turbine either in the form of hot gas or shaft power, without affectingthe compressor matching. The accessory power unit offered undoubted advantages, but entailed penalties of duplication andweight. Interest had recently been re-awakened, however, by the introduction of low-powered gas turbines which, althoughtheir specific fuel consumption was inevitably high, nevertheless deserved consideration for short-period applications by virtue oftheir low weight. Dealing with future prospects, Mr. Owner stated that theturboprop engine had two exclusive design potentialities : (a) higher combustion temperatures, and (b) high-speed-airscrewdevelopment. With regard to (a) the output increased rapidly, and the efficiency steadily, up to any temperature which couldbe contemplated from present understanding of materials and design. Much work was in hand on both aspects, and the use ofblade cooling combined with external coatings would lead to marked advances. As to (b) it had been theoretically indicated,and factually confirmed that, although an airscrew specially designed for supersonic rotational speed was less efficient than aconventional airscrew at flight speeds appreciably below that of sound, it could, nevertheless, retain a fairly high efficiency—about 70 per cent—well into the range of supersonic flight speeds. Mr. Owner suggested that the possible field of utilization of thesupersonic airscrew was limited to speeds between Mach numbers of 0.8-1.2 where, however, it might be able to demonstrate asignificant improvement in range. It was in this speed region (M 0.8-1.2) that the ducted fan or by-pass engine would findtheir sphere of utilization, if any. Development of the turbojet was, the lecturer thought, straight-forward. Higher temperatures would improve specific thrust at the cost of increased fuel consumption, both at sea-level and ataltitude. The only avenue open for improvement in economy without sacrificing specific output lay in the use of higher com-pression ratios, for which higher component efficiencies were a prerequisite. Unfortunately, however, both these factors werebeginning to reach the region of diminishing returns in respect of specific consumption. At supersonic speeds, high compressionratios were of less advantage, but, fortunately, this was offset by the concurrent improvement in propulsive efficiency. The very high thrusts required at transonic and supersonicspeeds would limit range and duration for a long time to come, and since for short duration specific output assumed increasedimportance compared with specific consumption, combustion temperatures might well be increased for these duties by theuse of blade cooling. This, observed the lecturer, was a method of increasing thrust which was far less extravagant than re-heat;which did not require the complication of a variable-jet nozzle; and which need not interfere with the attainment of good cruisingconsumption at lower temperatures. Aircraft Structures The two aeronautical papers presented on Monday, June 1 ith,at the Institution of Mechanical Engineers, were The Development and Design of Aircraft Structures, by Professor A. G. Pugsley,O.B.E., D.Sc, M.I.C.E., and Planning and Construction of Airfields, by Mr. J. A. Dawson, C.B.E., B.Sc, M.I.C.E. PROFESSOR PUGSLEY, having given an historical review followedby a brief survey of current design, turned his attention to research and the future, and dealt first with the problems offatigue. One of the difficulties was that too little was known of the loading cycles that occurred in practice, and, in the absenceof full evidence, the current British tendency was to concentrate on tests representing a fluctuation of roughly ±10 per cent aboutthe loads corresponding to level flight, and to aim at the produc- tion of materials and structures which would withstand suchfluctuations for at least 2 x io6 cycles. A major problem of construction was the stabilization of thethin metal skin of wing or fuselage for reasons both of strength and aerodynamic smoothness. Interest had been focused onmetal sandwich construction, and experiments had been made on a wide range of possible fillers, including expanded formvarand paper honeycombs impregnated with resin. It was now known that sandwich structures of this type could be madehaving a strength of the order required for wing covering, and a weight of the same order as that of stringer-reinforced plating. Initially tried out in miscellaneous minor applications, reinforcedplastics, particularly those using resin reinforced by asbestos fibres, had since been developed to give strengths and weights ofthe order required for general use in aircraft construction. Again, the method offered chances of improved surface smoothness and,in addition, had obvious potentialities for large-scale production. Another interesting material was magnesium, and the productionof thick magnesium plating of high purity (and therefore with less liability to corrosion) had now become practicable, andexperiments were in hand on the possible use of this for the main structures of aircraft. Such thick plating—of the order ofhalf an inch—should be capable of replacing the reinforced sheet of wings or fuselages, and was found to have strength/weightcharacteristics which might lead to increased efficiency. Advanced design of aircraft, such, for example, as delta wingand swept wing types, introduced new structural problems. The old concept of wings built around two spar? had largely todisappear in delta wings, where "ribs" and "spars" became of almost equal status. Experiments on the natural modes ofvibration of such wings had shown that the aero-elastic problems of the future would relate more to those of a delta-shaped "jelly"or of a ship's propeller blade, rather than to the bending and torsional modes of a conventional wing. Airfield Layout MR. DAWSON began his paper on the planning and constructionof airfields with a reference to the development of grass airfields, and then went on to give a brief description of the growth ofhard-runway airfields during the war. In the last 40 years, the area of airfields had grown from about 100 acres or less to morethan 1,500 acres in the case of the largest Service stations, and 3,000 acres in the case of London Airport. The cost of prepara-tion of one of the early grass airfields probably did not exceed £5,000, whereas the cost of the largest Service airfields was inthe neighbourhood of £2,000,000, and the estimated cost for the present scheme for London Airport stood at about £7,000,000. The lecturer then went on briefly to describe the developmentof airfield lighting, and stated that the installation at London Airport was the most advanced arrangement in the U.K. Mr. Dawson thereafter sketched in the chief factors governingthe selection of an airfield site, and then dealt with the respective advantages and disadvantages of the parallel-runway andtangential-runway layouts. On the score of design and con- struction of airfield pavements, the lecturer emphasized thenecessity for having preliminary expert examination of the soil structure. For heavy-duty runways, there had been a tendencyto select rigid concrete construction, but this was not necessarily the most suitable form of pavement. The choice of pavementshould be governed by the bearing capacity of the sub-grade, within the limitations imposed by the depth of sub-base requiredover the natural sub-grade. The thickness factor in a cubic content of concrete, which, on a large airfield, might amountto not less than half a million cubic yards, might involve a very large sum of money. For this reason, Mr. Dawson thought that,whilst the Westergaard formulae have been of great value, it was possible that the data now being collected by the Air Ministrymight prove to be a more reliable basis for economical design. On the subject of expansion and contraction joints, it wasstated that, although it was normal practice in Britain to provide
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