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Aviation History
1951
1951 - 1186.PDF
22 June 1951 737 stepped-sleeve piercing the back-plate. On the forward face of the piston are bolted a pair of L-section rings, the free flanges of which serve as the superfine (starting) pitch and the flight-safety-pitch stops. The latter is effectively the normal fine-pitch stop, and it is for the removal of this stop (to permit the piston to move forward until the superfine stop is engaged) that the third oil line in the airscrew is embodied. The free flange of the flight-safety stop-rings butts against the rim shoulder of a collet in the forward mouth of the multi-stepped sleeve. Immediately forward of the collet is the front end-wall or crown of the pitch-change cylinder; this crown is of junk- head form, and is secured in the cylinder by a buttress- threaded retaining ring-nut. The junk-head itself serves as a cylinder for the lock piston, this secondary piston being enclosed by a circlip-retained cap through which are drillings venting to atmosphere the volume forward of the piston. Somewhat resembling a poppet valve, the lock piston has a cylindrical stem, around which is fitted a cup enclosing a coil spring that reacts against the rear face of the lock cylinder. The spring cup is retained on the piston stem with a sleeve nut, and at its forward end the cup is threaded to a buttress- section ring which supports the collet segments from collapsing under engagement with the flight-safety stop-ring. Third oil-line delivery to the rear of the lock piston induces forward motion of the piston, so compressing the coil spring, removing the support from the collet mouth, and thus allowing the main pitch-change piston to travel fully forward. The lock piston is returned (and the collet again supported) under the action of the coil spring immediately the delivery of pressure oil to the lock cylinder is cut off. The blade-operating eyebolts are attached direct to the pitch-change piston, and slide in guide sleeves housed in bushes flange-bolted to the hub barrels. The eyebolts terminate in fork-ends, in which are floating-pinned the shanks of operating links; the "big ends" of the links are Tufnol-bushed for engagement with the operating pins. These latter are virtually crank-pins, and each is dowelled and bolted to the lower race of the blade bearing. Front-airscrew Oil Supply Fitted around the sleeve nut which retains the coil spring cup is a bell sleeve to the flange of which is through-bolted the nested flanges of the three co-axial oil tubes that serve the front airscrew. The core line of the three carries coarse- pitch oil, the centre annulus carries fine-pitch oil, whilst the outer annulus accommodates oil for lock actuation. The (solid) nose of the innermost oil tube is threaded to the boss of the lock-cylinder crown, and is locked to it with a circlip- retained externally- and internally-splined sleeve. The periphery of the lock cylinder crown cap is splined to the junk-head crown of the pitch-change cylinder which, in turn, is locked to the buttress-threaded ring-nut by means of a dogged locking ring. The outermost oil tube carries a flange towards its rear end which is splined to the forward mouth of the hub-retaining nut. Thus the oil tube assembly acts as a lock transfer medium between the right-hand thread of the hub-retaining nut and the left-hand thread of the main cylinder crown retaining ring-nut. As already mentioned, in essentials the rear airscrew (a right-hand tractor unit) is similar to its forward left-hand tractor complement but, of course, the fact that the front airscrew shaft, together with the oil-feed assembly, has to pass through the core of the rear airscrew cylinder makes it necessary for the whole cylinder/piston assembly of the rear airscrew to be annular in character. This is made the more complicated by virtue of the fact that the diameter or the rear cylinder has had to be kept as small as is practicably possible, not only for weight reasons, but also because the delivery characteristics of the oil feed to the rear airscrew must match fairly closely those for the front airscrew, it only for reasons of standardization between pumps and control units. The rear hub driving-centre is drawn up on the usual cone seats by the hub-retaining ring-nut threaded on the nose of the (outer) No. 6 size shaft and, as in the forward airscrew, is radially flange-bolted at the front and axiaUy flange-bolted at the rear direct to the hub shell. The RELIEF V»LVE 5 Ib/cq in I BLEED TO ALLOW tj (OOpt/hr AT TO DRAIN 6Olb/sq in PRESS DIFF FEATHERING PUM AND MOTOB This schematic diagram illustrates the operation of the control units governing the supply of actuating oil to each airscrew. assembly of the blades in the barrels is precisely similar to that employed in the front airscrew but, owing to the larger spinner diameter at the rear, the transitional length of the blade shanks from a circular section at the hub barrels to the aerofoil section of the blade proper is considerably greater. Axially bolted on the front of the hub is an annular plate, to the rim of which the root flange of the cylinder, and the rear end-wall of the cylinder, are jointly through-bolted. To the front of the cylinder is axially bolted the crown, or forward end-wall, which, like its front complement, is of semi-junk-head form. In the root bore of the crown is threaded the oil-transfer sleeve, locked through a dogged ring and, at the rear, splined in the mouth of the hub- retaining ring-nut. As in the front airscrew, locking-back between hub-nut and cylinder is thus obtained. In the stepped bore of the cylinder crown is a multi- step-section steel sleeve which serves as a housing for the roller bearing supporting the inner (front airscrew) shaft. The tail of the inner shaft, and the outer shaft as a whole, are supported in the engine reduction-gear casing, and front- support loads for the inner shaft are transferred through the roller bearing and thus taken back into the outer shaft through the cylinder/hub assembly of the rear airscrew. Oil retention on the forward side of the roller bearing is effected by means of the latest type of Angus lipped-seal. This is a synthetic rubber unit of figure "2" section, bonded to an L-section metal reinforcing ring, and the inner lip of the seal is spring-retained in contact with a hardened and ground steel sleeve trapped between the rear cone of the front airscrew and the inner race of the roller bearing. Whilst on the subject of sealing, it may be mentioned that, with the exception of U-seals used on the piston, eyebolts and at one or two other locations, oil-retention is effected by the use of O-rings. The piston is an annular member, to the front face of which the flight-safety and superfine-pitch stops are bolted. The piston slides on a steel sleeve, the rear bore of which is threaded to the boss of the cylinder rear end-wall. In the bore of the piston is accommodated the lock collet, the forward mouth of which normally engages a support ring threaded on the forward end of the lock cylinder. There is considerable divergence here from the lock design of the front airscrew. The lock cylinder is a moving unit, sur- rounding a stationary (reaction-member) piston, which is trapped between the main cylinder crown and a shoulder on the outside diameter of the oil-transfer sleeve. The rear mouth of the lock cylinder is closed with an end-wall block in which are axial drillings accommodating the several coil springs for returning the lock. The springs are enclosed by retaining cups that react against the root
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