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Aviation History
1939
1939 - 0836.PDF
298 FLIGHT. MARCH 23, 1939 DRIVE FROM ENGINE GEAR PUMP PLUNGER GOVERNOR OPERATED BY TELECONTROL FROM COCKPIT PRESSURE CHL TO FRONT SIDE OF .^AIRSCREW PISTON RELIEF VALVE LOW PRESSURE HIGH PRESSURE OIL OIL RETURN TO REAR SIDE OF FROM AIRSCREW AIRSCREW A PISTON REDUCING BLADE PITCH LOW PRESSURE OIL RETURN FROM AIRSCREW The governor-pump unit of the Rotol airscrew incorporates a spring-loaded governor which controls the opening and closing of a number of ports by means of a sliding plunger. The loading of the spring is pre-determined by remote control from the cockpit. opened and oil is allowed to flow into the coarse-pitch side of the airscrew piston. As the pitch of the blades coarsens, so the load on the engine increases, and the engine r.p.m. drops until the weights in the governor are again balanced by the spring, at which point the plunger is once again back in its central position. The oil is now trapped on both sides of the airscrew piston. Adaptability It may be added that the Rotol governor-pump unit has been designed for use with engines rotating in either direc tion, requiring but the simplest alteration from one-hand rotation to another. The direction of rotation of the pump is-changed by* reversing a pair of flanged plugs. The oper ating pressure of this pump is in the neighbourhood of 300 lb., and from this it will be realised that exceptionally fine limits are necessary in the workmanship of the air screw and governor-pump unit. And now for a word on production. The Rotol factory is laid out to provide the maximum amount of natural light. High-power G.E.C. " Osira " mercury-vapour-type lamps are fitted throughout the various works departments. Modern types of machine tool plant are installed throughout the factory ; a large number of them consist of special-purpose machines, while some standard machines have been adapted for single-purpose use. The airscrew blades are received from High Duty Alloys, Ltd., in the form of blanks. The blanks are pro duced at this company's Slough works in the following manner. The magnesium ingot is removed from the mould, and before it is placed in an electric furnace and soaked for twenty-four hours at 400 degrees In the final assembly of the Rotol airscrew the blades are fitted into the hub by highly skilled specialists. It is then balanced as a complete unit prior to packing for delivery. Centigrade its casting skin is removed by a machining process. The ingot is then forged to a rough dummy shape between heated dies in a 1,000-ton hydraulic press, daring which process the ingot is returned to the furnace several times for the purpose of reheating. After cooling, the ingot is finished to dummy size. It is then reheated and forged to approximately the finished shape. Incidentally, a test sample is forged integral with every blade as an extension of the tip. In addition, one blade in every batch of twenty is cut up and thoroughly investigated. At the Rotol works the airscrew blank is centred so that an approximately equal amount of material can he removed from both sides. Three-quarters of the blade from the tip is machined on Berliner contour-milling machines. The root section of the blade is milled on a Sundstrand 3A copy-milling machine. The next operation is the machining of both trailing and leading edges of the blade. This is carried out simul taneously in a Sundstrand No. 3C Rigidmil. After gauging, certain filing and disc-grinding operations follow. During these operations the blade is attached to an adjustable balance weight. The whole assembly is pivoted at its centre of balance so that the operator can tell whether more metal should be removed, and so is able to keep within the margins permitted. Blade Balance Next, the blade root is threaded and the centre drilled out to enable balance adjustments to be made during final assembly. Finally, the blade is balanced against a master blade in a special draught-proof room set aside for this type of work. Although there are some 300 components in the hub of the Rotol constant-speed airscrew, the majority do not present any unusual production problems except for the actual hub itself. This part is forged in S.65 and is delivered with the centre pieces bored out. After com pletion of machining operations the weight is reduced to approximately 42 lb. The machining operations of this component are divided into three sections—roughing, finishing and final drilling, screwing, etc. The roughing operations are carried out on Webster and Bennett vertical bcring machines. In this machine the bore, top face, and a portion of the outside diameter of each barrel are machined. But this is not all, for the internal wall opposite each barrel is of a concave spherical form. Most of the operations concerned with the machining of the external diameters of the barrels and also the outer convex spherical form of the hub are dealt with by a line of Dean, Smith and Grace lathes. After final roughing on a Cincinnatti vertical milling machine the hubs are sub jected to a Magnaflux test for cracks, then they are de magnetised, stabilised, and finally Brinell-tested in the special works laboratories. The next group of operations is very similar to that
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