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Aviation History
1949
1949 - 0077.PDF
JANUARY 13TH, 1949 FLIGHT 55 been caused by nationalization, nor that there was anyparticular shortage of that commodity in the Corpora- tions which could not be attributed to the difficult stateof affairs. It had, he thought, probably provided a very iavourable degree of stability which would enable genuineprogress to be made in the future. That could perhaps, he thought, have been achieved by making long-term agreementswith private operators, but because of the large sums of money involved, and the uncertainty of the future, such agreementswould have been difficult. Nationalization had provided adequate Government supportto develop a full system of routes on a permanent basis, and it was unfortunate that so much expenditure had beenabsorbed in overcoming inadequate equipment instead of being devoted to building up trade in the future. Mr. Branckerthought that because of Government ownership the Corpora- tions had been subject to rather more criticism than wouldhave been the case in normal circumstances. One effect which he thought should be recorded was that, if main air transport services were operated privately, fre- quencies and schedules would be arranged to give the bestcommercial results. In view, however, of the special status of the'Corporations, there was a tendency on the part of otherGovernment organizations to require their own interests to be given special consideration. It was quite erroneous to thiukthat the nationalized corporations had not a commercial out- look. It was not, of course, permissible to adopt other thanstraightforward tactics, and they were, also, not as free as the private commercial operator to cut rates or adopt similardevices. If, however, stability was required, that situation was, perhaps, not particularly disadvantageous. He empha-sized that the operators should be free to handle such matters themselves. Mr. Brancker suggested that the key to the situation was,perhaps, less the actual ownership of the operating organiza- tions than the way in which they were controlled, andif they could be permitted to function flexibly and inde- pendently, as ordinary operators, there was little doubt thatthey would be successful, since many of the ills attributed to nationalization were, in fact, due to other causes. ,•• • MODERN ELECTRICAL SYSTEMS Use of High-Voltage Supplies on Large Civil Types : A New Battery Design WITH the increasing tendency towards large size in civilaircraft, and the correspondingly heavy demands ontheir electrical equipment, considerable attention is now being given to the advantages offered by high-voltage elec-trical systems. (The term "high-voltage" is used here in a general sense, though it now has a specific meaning, as willbe explained.) It is well known, even among those not closely concernedwith electrical engineering, that use of a high voltage can effect a considerable weight-saving in cables, because (to putit briefly, if rather loosely) high voltage is able to overcome the resistance offered by a small core; and the weight savedby use of a small core is by no means counterbalanced by the increased weight of any extra insulation which may be neces-sary. Other important advantages, -however, are conferred by the use of a high-voltage A.C. supply (rectified for D.C. appli-cations) as compared with the conventional 24-volt D.C. Chief among them, perhaps, is the fact that the use of an alternatorand rectifier makes full power available over a wide range of engine speed, an important consideration in passenger aircraftwith their heavy, and often simultaneous, current demands for such services as cooking, non-hydraulic actuation and radio.The use of alternating current also makes it a simple matter to transform the supply to higher or lower voltages for specificpurposes, using in many cases, transformers with no rotating parts. It is, perhaps, indicative of the widespread adoption of thesesystems that voltages up to 250 are already officially known as "low voltage," while the old 24 has become "extra-lowvoltage." The term " high-voltage " is now reserved for systems above 250 volts. f A.C. systems in the up-to-25ov category, now underdevelopment, include a 400-cycIe 3-phase 208V supply. This, while effecting even greater economies in cable-weight, will alsoshow a further saving in the weight of motors, which are be- coming steadily more numerous in aircraft of large size. Useof the squirrel-cage type of A.C. motor in turn confers an addi- tional advantage, for in this motor there is no commutator orbrushes, and thus maintenance is simplified, radio interference is re-duced, and the present problems of excessive brush-wear at high altitudesare eliminated. Adoption of a 400-cycle high-voltage supply, of course, postulates • the availability of accessory equip-ment designed to work on it, and already a number of such componentsare under development. Two of the first British civil aircraftto incorporate medium-high-voltage systems are the Handley PageHermes IV and Bristol Brabazon. Also, although no details have yetbeen made available, it is a safe guess that a similar system will be used inthe Saunders-Roe SR45. In the case of the Hermes poweris supplied by two three-phase Rotax The 24-vo/l Exide B 25 alternators, one on each of the inboard Bristol Hercules 763engines. The 120V output is rectified by metal rectifiers of the selenium type, while a separate 115V A.C. system for operatingthe radar and gyro services is provided by two motor-generator sets operating from the D.C. 120V supply. The generating system " balances" battery discharge byproviding an additional charging supply at 28.5V for the 24V batteries catering for the extra-low-voltage services. The 10SVExide battery consists of four 24V type 12ZCS11 units and one 12V type unit of similar type, having a capacity of 15AI1for a total weight of 150 lb. The use of four 24V and one 12V unit, connected in series, gives a total of 54 2-volt cells, whichis all that is required within the nominal 120V system. One 24V unit is tapped to give a supply at that nominal voltagefor radio and instrument services. Physically, this arrange- ment of 24 and 12V units gives a "flexible" battery installa-tion having overall dimensions of only 35111 x 15m x 6fin high. A somewhat similar installation will be used in the Hermes V(four Bristol Theseus airscrew-turbines), but 108V Exide 12- ZC11 batteries of rather larger capacity, giving 25AI1 for atotal weight of 200 lb, are being employed.. On the Bristol Brabazon, a 208V 3-phase 400-cycie systemis being utilized, the current being generated by six 3okVA alternators, giving a total D.C. capacity of 36k\V obtainedfrom six 6k\V rectifiers. Only a small percentage of the total electrical installationcalls for extra-low-voltage D.C. operation, for which purpose nominal 24-volt batteries will be used. In order to provide asufficient battery output and storage reserve, accumulators having a total ampere-hour capacity of 240 a.h. at 24V weredecided on, provided by two separate banks each consisting of four 12V 60 a.h. Exide units type 6FZ17-3 coupled in series/parallel, the necessary charging current being drawn from the main 208-volt A.C. supply converted through selenium rectifiersand voltage-controlled at approximately 28V. The 24V " ZCS" type of Exide battery as fitted to theHermes is an entirely new design of somewhat striking external appearance, for the familiar black ebonite case has given placeto a cream-coloured polythene mould- ing. This material, in spite of itscomparative lightness, is stated to have a high degree of toughness andshock-resistance, to be unaffected by oil or petrol and, of course, sulphuricacid, and to have exceptionally low susceptibility towards softness orbrittleness at very high and very low operating temperatures respectively. Each cell is fitted with " micro-porous " P.V.C. separators, and it is claimed that the battery can bestored indefinitely in a new and un- filled state without ill-effects. Fittingsinclude a hinged lifting handle and a foolproof topping-up device which^ automatically limits the electrolyte level, while the terminal connections,which are completely polarized, are of tottery, type ZCS. a new quick-release type.
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