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Aviation History
1957
1957 - 1681.PDF
15 November 1957 769 The Hobson constant-speed alternator drive, developed in conjunction with Integral, the associate company. This drive forms the basis of the engine starting system shown below, right. spring. This force would have to be held by the pilot while theseizure persisted. With one addition, the complete circuit is now proof againstany one major failure. If one valve seizes, the surface-section can be moved in one direction (by both jacks operating in unison) butnot in the other (jacks in opposition) and the servo stiffness—with both valves open—is lost. This is easily remedied by inserting anon-return valve in the supply line and providing a by-pass orifice to act as a damper sufficient to suppress flutter; the surface-sectioncan then trail to a position of zero hinge moment. Feel simulation. It is now widely appreciated that artificialfeel is required with powered flying-control systems, but it is interesting to relate that, although Hobson (from an EnglishElectric suggestion) could provide some time ago any feel required, q-feel, corrected if necessary for Mach number effects to provideroughly constant stick force/g, has been almost universally adopted where springs would not serve. There is now, however, aresurgence of interest in special (i.e., unlike "natural" q-feel) systems, and a feel simulator is under development which pro-vides an approximation to the "V" law—stick rate proportional to e.a.s. This is accomplished by a modification to the well-knownhydraulic feel simulator and makes use of the fact that a diaphragm changes its area progressively with deflection. Apart from offeringa choice of system, the component manufacturer has little say in the characteristics to be adopted, since this is a prime function ofthe control system of the aircraft. But where, for instance, it is suggested that the rudder control with advantage be made toincrease foot load very rapidly with speed, a system might be developed which follows (as a basis) a V3 law; and this is in factbeing done by an inversion of the mechanical processes adopted to achieve a V law. The q-feel simulator or its like provides an indication (in pitch)of the loads on the elevator and tailplane, but if the e.g. range is large, it provides no indication of the normal g built up or of theloads on the wings. On the other hand, a bobweight system pro- vides a completely accurate indication of the normal g and thewing load, but does nothing to protect the tailplane during the period in which g is building up, and its inherent lagging qualitiestend towards instability. Angular accelerometers can provide one solution; and another, proposed by Hobson, is to use a damperpiston supplied with fluid at a pressure varying as q in conjunction with the bobweight. In this way, initially the stick force isaffected both by the rate of application of surface angle and by the airspeed, while in the steady-state condition the stick force isentirely due to the bobweight. Integrated starting systems. The advent of full A.C. electricalsystems—a little slow to materialize on British aircraft, but an assured feature of new civil types—has resulted in a needfor constant-speed drives to couple the engine and constant- frequency alternator. Several such drives (the Sundstrand/English Electric is typical) are already on the market, but a new design by Hobsons, in association with B.T-H., is so arrangedthat the alternator may be used as a motor for engine starting. At first sight the torque characteristics of an A.C. machine areunsatisfactory (see curves in col. 2 on the opposite page) in com- parison with a D.C. starter or combined D.C. starter/generator,but advantage can be taken of the variable gear ratio of the constant-speed drive to run the alternator at its optimum con-ditions in the engine starting cycle, when it will function very efficiently. The starting time will, however, generally limit itsijse to bomber and transport aircraft, although the power curve during the starting cycle is completely flat and 45 h.p. should beavailable from an alternator of 40 kVA size. The implications of A.C. starting in this manner are interesting.Self-contained starting of multi-engined aircraft could be achieved « one engine had a conventional starter, and the A.C. power itprovided would be used to start the remaining engines. Similar systems have been suggested using both air-bleed and hydraulicpower, but the A.C. system can be shown to have an overall weight advantage. There would be a small increase in weight of thealternator and drive combination. Conventional engine starters would not, of course, be requiredother than for one engine. Alternatively, if an A.P.U. had been installed for other purposes, then an alternator on the A.P.U.would provide the electrical power for engine starting. As a further alternative, an electrical ground trolley might provide thepower for normal starts, the self-contained starter being used only in emergency when no ground facilities are available. The requirements for A.C. starting in this way are thatthe electrical machine is accelerated into, and run in synchronism with, the supply, and the engine is accelerated by changing theratio in the constant-speed drive. It is obviously necessary that the constant-speed drive should have an overall gear ratio ofinfinity (the shaft connected to the engine being stationary, while the shaft connected to the electrical machine runs at any speed). A typical starting sequence would be as follows: (1) Masterengine running at ground idling r.p.m. and delivering A.C. power at the normal voltage and frequency to die aircraft electricalbusbar; (2) starter motor (alternator) of the next, engine to be started connected to line and accelerated to synchronous speed;(3) ratio control on the C.S.D. progressively altered to start engine —during this time the ratio is automatically governed by starter-motor current; (4) engine self-sustaining—control of the ratio transferred from starter motor current to alternator frequency; (5)repeat for remaining engines. Another aspect of A.C. starting in this manner is thatduring the starting cycle it is essential that a mechanical reduc- tion gear be interposed between the electrical machine and theengine, since although the C.S.D. could select a suitable torque multiplication ratio this would produce excessive internalhydraulic pressures. If an epicyclic reduction gear is arranged to include two free wheels, these can be used to ensure automaticselection of the correct ratio according to whether the engine or the electric motor is driving, and die reduction in torque trans-mitted by the hydraulics is accompanied by a proportionate reduc- tion in fluid swept volume, size and weight. An accrued advantageof the interposed reduction-gear arrangement is that for a given size of unit the normal throughput of power is doubled and aC.S.D. nominally rated at 20 kVA can then transmit 40 kVA. Before passing on to a consideration of further systems, it isworth commenting on the obviously undesirable differences in American and European practice on hydraulic fluids used insimilar systems. Skydrol 500 has now been standardized in new American aircraft, but British designs continue to use mineral-based fluid to D.T.D. 585 specification. The main contention appears to be that Skydrol—which is water-based—isinsufficiently non-inflammable to be considered a satisfactory substitute for the admittedly more inflammable D.T.D. 585;and, until the Ministry of Supply decree that a change should be made, no change is likely. Skydrol has a deleterious effect onsome paints and needs special seals; but seals present no difficulties, and a wide range of sizes is already available. Apossible disadvantage—although this does not apply to the same extent to civil aircraft—is that die maximum working temperatureof Skydrol must be limited to 85 deg C. Fuel systems. While this account has avoided any extensivereview of Hobson's current activities, the progress of development has no clearly defined end or beginning and, in looking ahead atpossible fuel-system developments, brief notice should be taken Diagram of an engine starting system for a four-engined aircraft, operating through A.C. alternators and constant-speed drives. The auxiliary power unit or master-engine starter gives a self-contained system when external supplies are not available. CONTROLLER MASTER ENGINE CONSTANT SPEED UNITS ALTERNATORS GROUND CONNECTIONS A.PU ALTERNATOR A/C SERVICES
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