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Aviation History
1955
1955 - 0084.PDF
84 FLIGHT Hydraulic Drive for Alternators A Well-proved Variable-speed Transmission IT is doubtful if there are any aircraft electrical engineerswho remain to be convinced that the type of electricsupply best suited to meet the power requirements of advanced aircraft is the three-phase system. Altogether, a very strong case can be made for generating current as 400- cycle, three-phase A.C. at 200/115 V; this can carry most of the load by itself, voltages being simply transformed off; where D.C. is specifically needed, a dry-plate rectifier can be used. There is only one major difficulty in generating the power asA.C.: where more than one alternator is used—as would obviously be the case in a multi-engined aircraft—load-sharingis virtually impossible unless all alternators are precisely tied together in both phase and rotational speed. It is impracticableto keep all the engines of, say, a modern large transport or bomber all turning at precisely the same speed. In any case,electric power is also needed on the ground, when it is very undesirable to open up the main engines above a fast idling speed. These requirements can be met by a number of types ofalternator-drive system, all of which have originated in the United States. That with which this description is concerned was de-veloped by the Sundstrand Aviation division of the Sundstrand Machine Tool Company, of Rockford, Illinois. The firm haslong been famous for hydraulic control equipment of machine tools, and this work was of great assistance in developing a com-pact and efficient hydraulic transmission which, within wide limits of input r.p.m., is capable of maintaining the output drive toan alternator at a constant rotational speed. The basis of the system is that the input shaft drives a variable-displacement hydraulic pump which supplies oil to a hydraulic motor mounted on the output shaft. Pump and motor aremounted in the same housing, as a differential system, so that the pump/motor assembly handles only the power represented bythe difference between the input and output speeds. Both pump and motor are of the multi-plunger axial type. Theplungers of the pump are moved over a stroke which varies infinitely between two maxima (passing through zero) accordingto the angle of the driving swash-plate. In contrast, the swash- plate driven by the plungers of the hydraulic motor has a fixedangle. Between the two sets of plungers are positioned a pair of valve plates—one mounted on a fixed axis and the other,separated by a port plate, carried on an eccentric axis rotating with die motor—which direct the oil flowing between the pumpand motor. The input shaft drives the main pump/motor cylinder block,as well as the make-up and scavenging pumps. The make-up pump is required to prime the plunger assemblies and ensurethat they arc maintained full of high-pressure oil. As the main plunger assemblies rotate, the pump swash-plate takes up anangle governed by the relationship between the under-drive and over-drive control pressures. According to the output from theplunger pump, the plungers of the motor are fed with high- pressure oil, and they therefore impart a clockwise torque to thefixed-angle output swash-plate. The output swash-plate can rotate independently of the mainplunger assembly, and the governing system is such that, until the output speed reaches 6,000 r.p.m. (or whatever speed isrequired by the characteristics of the alternator), the pump swash- plate remains in the full-stroke position, in which the hydraulicsupply tends to speed up the output compared with the input r.p.m. There is, of course, a minimum input speed below whichthe required output r.p.m. cannot be obtained. When the required output speed has been established, nothingfurther happens until the input speed is increased, when this occurs, the over/under-speed governor pressure reduces the pumpswash-plate angle so that the pump delivery is reduced. This has the effect of reducing the differential speed between thecylinder block assembly and the motor shaft, so maintaining the correct output r.p.m. When the input speed is equal to the re-quired output speed, the pump delivers no oil, and the unit behaves as a direct drive. Further increase in input r.p.m. reverses This photograph shows a pair of Sundstrand-made drives on test with 40 kVA alternators at the Napier factory at Acton. These units have been subjected to the test schedule shown in the table opposite. the pump swash-plate angle, so that the original motor behavesas a pump while the pump meters back the oil in a reverse direction. The whole system is self-contained, and fits approximately intoa 15 in cube. The weight of the unit varies according to the application, but 70 to 80 lb is a fair average. The arrangement was steadily developed during the years im-mediately following World War 2, and it early attracted U.S.A.F. interest. The obvious attractions of a fully paralleled A.C.system were so great that, after protracted testing, the company were given a production contract. By far the most important application in the early days wasto the great Convair B-36, in which Sundstrand units have now completed hundreds of thousands of hours. In this aircraft, sixalternators are fitted, one being driven off the forward-facing "rear" end of each of the pusher piston-engines. The hydraulicdrives are accessible from inside the wing; in fact, if one engine at a time is shut down to reduce the airflow through the wing,they can be reached in flight. There is now a strong core of Strategic Air Command flightengineers who have become so used to having a load-sharing A.C. system mat they take it for granted. In recent years, webelieve, the Sundstrand units have given virtually no trouble and their overhaul life is high. All the worst snags were eliminatedin the early days; and the B-36, which can cruise at 45,000ft for over 30 hours, is certainly an arduous test-bed. Sundstrand have now developed a whole series of hydraulicdrives tailored to fit individual types of engine and aircraft. They can be mounted either on an engine—wherever a drive pad canbest be placed—or on the airframe, and types exist which are not dependent on engine oil but have a self-contained hydraulicsystem. Several new types of aircraft, notably the Douglas B-66A (two Allison J71s), are now coming into service with the drive. It was announced last year that the English Electric companyhad concluded an agreement under the terms of which the British company will manufacture and market an anglicized version ofthe Sundstrand unit. During the past year, the company's Brad- ford factory has been handling the new device, while the actualtest-bed running (to obtain an independent check on the perform- ance of the American-made equipment) has been carried out byD. Napier and Son, at Acton (Napier are members of the English Electric Group). The unit wim which English Electric are immediately con-cerned is that for a 40 kVA alternator, which is maintained at A typical 40kVA alternator (on the left) and hydraulic drive.
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