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Aviation History
1937
1937 - 2143.PDF
JULY 29, 1937 3 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 132c In the high gear the planet cage was held stationary by means of a hydraulically operated clutch. For the low gear, the cage was released, the drive reaction being taken by a centre gear co-axial with the rotor shaft, this gear being adapted to run freely when high gear was in engagement. Mechanically this unit was very successful but the gearing arrangement, although having the advantage of simplicity, limited the range of variation between high and low gears. Actually the low gear ratio proved to be too low in practice and the next design, although incorporating the same principle, introduced a third set of gearing which allowed greater latitude in the selection of ratios. In this design great attention was given to saving weight, and the unit with its necessary additional control gear was produced at the same weight as the single-speed blowers then in use. This unit also proved successful. Owing to the absence at the time of a suitable C.P. airscrew, the full advantage to be gained by the use of a two-speed supercharger could not be realised, and as a result further development was delayed. The unit now in use on the Armstrong Siddeley Tiger VIII engine has successfully passed its type test and is now in production. It embodies the same principle of a triple gear train having a simple back gear drive for high speed, and epicyclic gear for the low gear. It has been completely redesigned to incorporate the benefit of the experience recently gained as the result of prolonged experiments on the supercharger research bench. Mechanically the chief point of difference from the previous unit is the elimination of the free wheel and the substitution of a simplified hydraulic control to both the high and the low gear while the experimental work referred to has enabled the design of the rotor and diffuser to be much improved. A feature of all the Armstrong Siddeley two-speed supercharger units is the ease of control of the change- speed mechanism. The operating clutch is engaged by oil pressure from the main engine supply, control being effected by a simple " push-pull " type of plunger valve which directs the oil as required. This control is light in operation and has proved satisfactory, automatically providing sufficient delay action to prevent a too sudden change in gear ratio. Back cover and accessories, including two-stage blower, of the Siddeley Tiger VIII engine, which is the standard power plant of the Armstrong-Whitworth-Whitley heavy bomber.
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