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Aviation History
1944
1944 - 2256.PDF
474 FLIGHT NOVEMBER 2ND, 1944 MODERN AIRSCREWS The airscrew shaft is hollow and bouses the cylinder stem, which is free to slide in and out. This member is a thick-walled tube, the forward end of which is fashioned as a shallow, large-diameter cylinder. Inside the cylinder stem are fitted two oil-tubes, nested co-axially, to allow the passage of oil through the bore of the inner tube and also through the cylindrical space between the inside tube and the bore of the outer rube. The inner tube screws into posi tion, whilst the other tube has a spline engagement. On the forward end of the inner tube (which projects beyond its outer fellow) is fitted the piston, the boss of which butts against the end of the outer oil tube—thus holding it in position—and the piston is locked by means of a retaining ring-nut screwed on the end of the inner tube. The piston carries on its periphery two back-to-back, cup-type, syn- thetic-rubbgr oil seals which bear against the cylinder bore. The inner oil tube conveys oil to the front side of the piston, and the outer tube carries oil to the rear of the piston, the oil emerging from the outer tube via radial holes about ^in. from the end. Oil fed from the constant- speed unit to the oil tubes determines, according to which side of the piston pressure feed is applied, the movement forward or rearward of the cylinder—the piston remaining stationary and acting as a pressure head. Movement of the cylinder is employed to rotate the blades and so change their pitch angle, the transmission of the linear motion of the cylinder to the rotary motion of the blade roots being achieved by means of articulated linkages. Each linkage comprises an eyebolt secured to the cylinder and terminating in a fork-end in which, by means of a gudgeon pin, an operating link is pivoted. This pivot connection is retained in a Tufnol sleeve, and may be likened to the "little-end" bearing of a car engine. The opposite end of the operating link terminates in a boss bored to accept a needle-roller bearing in which is fitted the oper ating pin on the blade root. It may thus be seen how oil pressure fed to one side or other of the piston causes the cylinder to move and thus rotate the blades in order to change their pitch. However, the determination of pitch change is the province of the conslant-speed governor unit, usually referred to as the c.s.u. The unit is composed of three main elements, a gear-type oil pump, a centrifugal governor and a piston valve, all of which are housed in a three-part casing. The gear-type pump is also the drive for the centrifugal governor and comprises a driving gear mounted on a long hollow shaft, one end of which is splined to transmit the drive from the engine. Meshing with the driving gear is an idler gear mounted on a short hollow shaft incorporat ing a non-return valve through which engine oil enters the unit. These gear wheels form the pump which feeds oil, via ducts in the casing, into the interior of the hollow driv ing-gear shaft, from which two outlets give access to the oil pipes connecting to the air- When the airscrew is " on speed " no oil flow takes place and the blades are held in the obtain'ng pitch. ON SPEED CONDITION ] COARSENING PITCH' HIGH PRESSURE OIL TO FRONT OF PISTON A OVERSPEED CONDITION' _^_(//\ / r* L / j£ i _____ ..;•-• "- -••----•" Fall<ng r.p.m. allow fly-weights to lower piston valve, so feeding oil to rear of piston to fine-off blade pitch. B OIL EXPELLED TO RETURN UNDERSPEED - - ) - CONDITION
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