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Aviation History
1958
1958 - 0872.PDF
NORMAL GOVERNOR CENTRIFUGAL' ORIFICE CONTROL RESTRICTEDFLOW EMERGENCY HIGH RATIO LOW LOAD OUTPUT 888 FLIGHT HEATEXCHANGER A SERVICE TO INDUSTRY . . . business in the aero-engine industry, Lucas remain busy, althoughmuch of their work admittedly is in the repair of fuel system equipment built during the past few years. Beyond 18 months,Lucas say, the future appears uncertain, but they hasten to point out that their future has never been clear farther ahead than that.The Liverpool factory is an object lesson in the application of specially trained labour to the manufacture of precision com-ponents. Pistons for the fuel pumps, for example, must be round and parallel to a tolerance of 0.00003in and finished to 0.75 microinches, and the spherical-surface cam-plate (controlling piston dis- placement) lapped to be truly spherical to within five light bandswhen checked against a planar-convex glass prism; these processes are performed and checked by trained operators—very frequentlywomen. The over-riding impression to be gained at this factory (fuel system component production is also undertaken by theLucas parent factory at Shaftmoor Lane, Birmingham) is the intensity with which precision manufacture can be undertaken. It is upon this background of knowledge and production com-petence that the company are basing their hopes for the future. They are justifiably confident of their ability to retain their posi-tion as the major suppliers of fuel-system and combustion equip- ment for British gas turbines and they are determined to pursuea policy of competitive pricing as a contribution to lower engine costs. They share the concern of aero-engine manufacturers thatBritish gas turbines are as yet offered abroad at prices little less than their American counterparts. Much of the technical develop-ment that is being pursued by Lucas must remain shrouded by the veil of commercial security, but something of the trend ofdevelopment can be discerned from the discussion of the research facilities which follows. Fuel Systems. Although a gas turbine fuel system can ultimatelybe proven only in flight, considerable data analysis and the lessons of past experience have made it possible to apply known data—perhaps obtained from a ground run of an engine on a test-bed —to an analogue computer and to simulate the system perform-ance under various operating conditions. This has proved to be an inexpensive way, judged by results, of eliminating inherentlyunsound designs and of providing performance engineers with a rapid method of testing new ideas without the need for buildinghardware. A sea-level engine test house remote from the factories is equipped for recording data on six channels from an engineinstalled in a test cell and, to reduce the time involved in process- ing data, information is recorded on an automatic plotting board.Cross-wires displacing the recording pen are synchro-driven from amplified data passed from many points on the engine throughthe six channels. Providing the non-linearities of an ideal fuel flow with theparameters of r.p.m. and altitude is one of the difficult problems that the fuel-system designer has to meet. In this test laboratoryLucas have devised a method of using the automatic plotting facility more or less in reverse. A series of theoretical fuel-systemperformance curves, cut from black card, are laid over the axes of a plot of standard scale, sensed by the eye of a photocell andfed into the engine as metered fuel to a pre-determined pattern. A transient function of engine performance can then be directlyscheduled from a pre-determined fuel flow and the mechanical design of the fuel system adjusted to provide the best approxima-tion to the ideal curve. Hydraulic Drives. Swashplate-type piston pumps used forpumping fuel can readily be adapted to operate as hydraulic pumps. Lucas have achieved some success with such conversions, andhave secured an important order for an aircraft hydraulic pump based on their well-tried fuel pump in the face of strong competi-tion from established manufacturers of hydraulic equipment. In view of the association of Rotax Ltd., with Lucas (G.T.E.),it was perhaps only to be expected that the two firms should Illustrated diagrammatically is the 25 h.p. Lucas-Rotax constant-speed alternator drive. For paralleling synchronous alternators a D.C. signal is injected into the servo system. This design has twin outputs. collaborate in the design of a hydraulic constant-speed alternatordrive, particularly as there is a closely contested—if limited— market for such units on two new British airliners. The Lucas-Rotax drive is a 25 h.p. unit that maintains outputr.p.m. constant at 8,000 ± one per cent within an input speed range of 1,700 to 6,000 r.p.m. It consists essentially of two parts:a hydraulic pump and a hydraulic motor. There is no mechanical interconnection and the components can be separated if necessary;the hydraulic motor speed is held constant by varying the angle of the cam-plate in the hydraulic pump. The particular unitdeveloped by Lucas is arranged to drive both a three-phase genera- tor-alternator supplying 5kVA, 400 c/s A.C. at 208 volts and45kW D.C. at 28 volts, and, for radar purposes, a single-phase, 2,400 c/s 2kVA alternator supplying A.C. at 115 volts. The lattermachine is driven at a constant 5,143 r.p.m., through gearing. The seven-cylinder cam-plate pump is capable of delivering upto 77 gal/hr per 1,000 r.p.m. at full stroke, and is designed to run up to 6,000 r.p.m. At fun" load, continuously transmitting 25 h.p.,the pump delivery is pressurized to 2,500 lb/sq in. Return oil at 100 Ib/sq in is passed through a fuel-cooled heat exchanger.Standard rating is 25 h.p. and allowance is made for transient overloads up to 50 h.p., but a continuous rating of 33 h.p. maybe adopted on a basis of 50 per cent transient overload. Under conditions of abnormally low input drive power (such as occurwith windmilling engines) an emergency output of 8 h.p. is available, and in this condition the cam-plate of the hydraulicmotor can be altered to maintain 8,000 r.p.m. at the output. The manner in which the constant speed is achieved is illus-trated in the diagram on this page. A centrifugal governor driven by the hydraulic motor actuates a valve orifice which controls theleakage of oil from one side of the pump servo-piston chamber, so adjusting the cam-plate angle in the pump. At constant alterna-tor load a variation in input speed will cause a variation in the flow of oil to the hydraulic motor, and the resultant speed increasewill be adjusted through the governor and servo system until the flow again reaches its nominal value. At constant input speed butvarying load the flow will be maintained at the nominal value but the pressure will vary with load, since as the speed of the hydraulicmotor falls the stroke of the pump will be adjusted through the servo system to increase the system pressure to meet the highertorque requirement. Lucas claim that the response and stability of the servo is well within the limits possible with a mechanicalsystem possessing unavoidable mass and inertia. During engine starting, provision is made in the hydraulic driveto relieve the starting torque condition by means of a solenoid- operated valve which sufficiently reduces the working pressureof the drive to make the alternators inoperative. For obtaining satisfactory paralleling of synchronous alternators, provision ismade for injecting into the servo system an electrical signal pro- portional to the real power component of a given alternator load.The signal, proportional to kW load, is picked up from the A.C. control panel, rectified and fed as a D.C. component into asolenoid within the servo valve. The servo can hence be biased so as to equalize the kW loads of any two or more hydraulic drives,the "real power" D.C. signals being balanced against each other to give correct kW division under every load condition. Applied Research. A Lucas precept is that where there is alaboratory there should be a workshop; research, they say, should keep its feet on the ground. The efforts of their applied re-search section are thus directed towards improved fuel-system design and the end-product, so to speak, of the department'swork is the preparation of design standards for the drawing office. A volume of design standards is, of course, usual in most designdepartments, but it is not common practice for these to be kept really up to date on improved materials and new techniques. Lucas research has been aimed at making available technologicalknowledge with which the designer can meet extending environ- mental requirements of fuel systems for both manned aircraft andmissiles. The target conditions which were laid down were fuel at 2,000 Ib/sq in pressure and a temperature range of — 30deg Cto +250deg C and air at 250 lb/sq in pressure and up to 500deg C temperature. The work covers the investigation of the behaviourof gases and liquid passing through orifices and valves, a study of metal elasticity and investigations into the behaviour of seals. The firm estimated that it would be worth something like£100,000 a year to them to have a complete understanding of the mechanism of sealing, and the laboratory consequently undertooka programme of research into the chemistry of elastomers and the control of seal nip. To meet the conditions under which fuel-system seals must work they found it necessary to depart from the British Standard specification and, in order to avoid very fineface and diametrical tolerances to control the diameter of the seal to O.OOlin. An interesting result was obtained in the case of shaftseals by using a ring of ten times representative size; it was found
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