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Aviation History
1949
1949 - 1665.PDF
FLIGHT, zq September 1949 REAR-AtRSCREWREDUCTION DRIVE STARBOARD-ENGINE 4 20 TRANSMISSION SHAFT Drive between the trans- mission shafts of a pair of Centaurus engines and their respective airscrews. OUTER AIRSCREW SHAFT INNER AIRSCREW SHAFT FRONT-AIRSCREW REDUCTION DRIVE PORT-ENGINE TRANSMISSION SHAFT AUXILIARIES-DRIVE CARDANSHAFT THE STORY OF THE BRABAZON Means are provided for measuring the engine torque by electrically determining the degree of torsion in the primary drive shaft tube. For this purpose, two toothed inductor rings encircle the universal joint; one of these is carried directly by the forward end of the drive shaft tube and the other is supported on a spider member, deriving its location from the engine end of the drive shaft. Associated with the inductor rings is a double "generator," in which electrical impulses are induced. Phase differences between the impulses of the two portions of the generator provide an indication of true engine torque. Two complete and independent reduction gears are incorporated in the magnesium-alloy casing, with means of driving the aircraft accessories and a simple system of lubrication and cooling, both of which function whether either or both the engines are running. The reduction ratio is 0.400. The right-hand power input shaft carries at its forward extremity a straight-tooth bevel pinion, the teeth of which mesh with the ground teeth of an internal bevel gear. This gear is serrated to the large-diameter tubular shaft, carried on ball and roller bearings in the magnesium casing*, which drives the rear of the coaxial airscrews. The shaft is made up of three sections, flange-bolted together. Nesting within it, and running in a series of ball and roller bearings, is the inner airscrew shaft, carrying at its forward extremity the forward airscrew and at its rear extremity the main driving bevel wheel. In mesh with the teeth of this wheel are the teeth of the left-hand input bevel pinion. Notwithstanding the high efficiency of the gear trains, considerable heat is generated at the actual tooth-contact point, so to maintain a satisfactory temperature level, oil jets impinge upon the tooth flanks as they come out of mesh. Oil draining away is cooled and recirculated. Accessory Drives There are two subsidiary gear trains in the gear box. The first of these picks up the drive from the outer air- screw shaft and conveys this through idler gears and a shaft to a free-wheel unit and thence to the aircraft acces- sory drive shaft; the second train picks up a drive from the inner airscrew shaft and similarly conveys this to the same accessory drive shaft. This shaft carries a small gear which drives the main oil-pressure and scavenge pumps. If either engine is closed down, the section of the free-wheel unit associated with the drive from that engine commences to slip, or "free-wheel," the other engine carrying on with the drive. With both engines running, notwithstanding that their speeds are nominally synchronized, there will inevitably be minor speed fluctuations between the two drives to the free-wheel unit. In order to avoid any pos- sibility of resulting shock-loading, the drive ratios have been carefully selected so that under synchronized condi- tions the entire accessory load is carried by the right-hand engine. Meanwhile, that portion of the free-wheel unit associated with the left-hand engine "free-wheels" at a very low speed difference. The ancillaries serving the coaxial airscrews are located on the rear cover of the dual reduction gear unit. These consist of the airscrew controller units and the synchroniz- ing alternators. Drives for these units are tapped off the gear trains which drive the free-wheel units. Lubrication of the working parts is by a supply of cool oil pumped to vital points by a gear type pressure pump; this pump also supplies the oil necessary for hydraulic air- screw operation, a filter being interposed in the circuit to exclude any small particles of matter from the constant speed control mechanism. Feathering During single-engine operation, the non-operative air- screw is, of course, feathered. If the front airscrew only is feathered, the aerodynamic forces acting on the blades do not produce any airscrew turning moment. In the case of a feathered rear airscrew, however, the blade angle for minimum drag is such that a reverse airscrew-rotating moment is produced. Since this is undesirable, means are provided inside the dual reduction gear to prevent it. Carried on the forward face of the internal bevel gear is a large ratchet wheel. Three pawl units equally spaced round this wheel, but attached to the front cover of the gear box, are brought into action automatically by the airscrew feathering oil supply. The pawls '' click '' over the ratchet wheel teeth while the latter is rotating in a forward direction with the rear airscrew. As the feathering process is completed, the airscrew slows down and comes to a standstill before the reverse torque builds up. At this point the teeth of the ratchet wheel are brought into abutment with the pawls, thus preventing reverse rotation. In normal operation an arrangement of springs completely retracts each pawl. As a means of precluding excessive torques in the reverse direction, e.g., in the event of a backfire, the ratchet wheel mounting incorporates a high- torque slipping clutch. Three entirely independent oil systems supply respec- tively, the left hand and right hand engines and the dual reduction gear and each consists of an oil cooler and a service oil tank. As already intimated, air to the oil coolers is ducted from the forward portion of each engine cell; the ducts run straight to the twin oil coolers which discharge their air through variable-area shutters into the main wing space. The reduction gear cooler is jointly fed by small ducts from both the main cooler ducts. This also discharges into the wing space. Air outlets from the wing space are provided through the rear spar and upper wing skin, near the trailing edge. Oil consumption is made good by maintaining a constant level in the service
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