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Aviation History
1954
1954 - 2107.PDF
116 FLIGHT, 23 July 1954 NAPIER ELAND . . . blading is of aluminium bronze. Napicrs have used this material from the outset and, with its high fatigue-strength and easy machinability, have never regretted their choice. The company virtually developed and tightened die relevant chemical specifica tion of D.T.D.197. The rotor blades are secured conventionally by fir-tree roots, which are all nipped together axially between stages five and nine to damp out umbrella modes of vibration; the modes are, of course, different for each stage and tend to cancel each other out. The stator blades are all held in single-stage half-rings made from light alloy. Those in die first four stator stages are fully shrouded and the remainder arc cantilever with the exception of die ninth stage, which carries die static shroud for turbine- cooling air. All are pushed in from the back of the retaining ring and peen-locked. The rings arc then bolted to the two half- casings of L.51 alloy. The compressor shaft is supported in a ball and roller bearing at the front and a roller bearing at die rear, where it is connected to me turbine by a gear-type coupling. To prevent die engine from windmilling, a parking brake is fined at die rear end of the compressor shaft. Shown in a detail sketch, this unit has an Al-Fin bonded drum with a cast-iron lining. The brake is manually operated by die starting lever, the positions of which are run, dump, positive coarse-pitch, and brake. If required, the brake can be actuated hydraulically, and all Elands have drillings to supply such power. From die compressor, the air passes through die support plate which virtually forms die basis of the whole engine. Cast in die new ZT-1 alloy, it is an impressive example of die founder's art and weighs 79 lb finished. It was, in fact, die first large casting in mis alloy, the contractor being Sterling Metals, Ltd. The plate carries diree pads upon which die engine is mounted in the air craft, and also incorporates the diffusers of the six combustion chambers, a central hole for die shafting and a form of integral Warren-girder bracing to carry die loads. The combustion chambers are Lucas tubular assemblies and their exceptional efficiency reflects the close co-operation between the two companies. Early in 1951 a chamber inlet was thoroughly tested not only as a straight diffuser but also with simulated wakes and velocity profiles. As a result Lucas had complete traverse data before they began work, and mis preliminary investigation has been more dian justified. Upstream injection is employed, the burners being of a simple swirl type with an 80-deg spray angle so diat, during the starting cycle, fuel is dirown direcdy at the igniters provided in two chambers. These igniters are high-energy K.L.G. units fed widi H.T. current from the Rotax ignition system. The materials used are aluminized S.84 for the casings and D.T.D.703 for the flame-tubes. The latter have conical noses widi annular openings dirough which the primary air swirls in to perform a complete 360-deg helical loop without appreciable pressure-loss. The fuel burns rapidly in this low-speed primary zone with a total flame-length always less than 9m, and com bustion is complete before the tertiary air enters dirough large holes in the flame-tubes. Some 0.75 per cent of die total flow is passed around the outside of the flame-rubes at die rear to cool, and relieve the pressure differential across, die turbine entry duct. Each combustion chamber can be removed in five minutes with out disturbing any other part of the engine, merely by undoing a clamping ring at each end. Similarly, the burners may be removed without draining die fuel supply lines—a most welcome achievement. The burners themselves are cooled by P2 air passed down their shrouds, and die whole system performs well on all standard turbine fuels even at very low fuel/air ratios. The Eland has three turbine discs, all splined to die same shaft and housed in a fabricated-sheet casing containing the three nozzle assemblies. The overall pressure ratio is 5.5 :1 and the design is such diat the exit swirl is low for all operating condi tions, thereby minimizing jet-pipe loss. Further, the propelling-nozzle area has been chosen to give optimum performance under altitude cruise conditions. The rotor discs, of Jessops H.40, are discussed later. They are mounted on flank-fitting splines on the turbine shaft, which is supported at die rear by a ball bearing and at die front by the gear-type drive coupling. The complete rotating assembly of the engine is thus mounted in but three bearings and is so arranged that gyro-couple loads at die centre bearing are substantially cancelled. Turbine rotor blading of Nimonic 90 material is used in stage one and Nimonic 80A in stages two and three. All blades are secured by fir-tree roots so broached as to permit the usual limited tip-rock. The nozzle blades are hollow castings in X.40 material; the first-stage blades are secured by inner and outer rings, but stages two and three have inner and outer platforms and are bolted to the external casing. The inter-stage seal plates, which carry seals at the inner diameter and cooling scrolls on each face, The parking brake is mounted on the rear of the compressor shaft. It is externally operated, the linkage being interconnected with the high-pressure shut-off cock and dump valve and the c.s.u. feathering lever. SEALING PLATE This close-up of the annulus gear and torque ring shows the construction of the torquemeter, the principle of which is described in the text. are mounted from the nozzle rings by swinging links attached to several of the stator-blade roots. Differential expansion is thus permitted while maintaining concentricity of the seals under all conditions. The drive from the turbine through the compressor to the reduction gear is shown in the large drawing. From the com pressor the drive is imparted through a quill shaft to a high-speed input pinion supported at die rear by a ball bearing and at die front by a roller bearing. This pinion drives three planet gears having integral layshafts, each mounted on two roller bearings and carrying a planet pinion meshing with the internal annulus gear. The combined planet gear and layshaft assembly is carried in a roller bearing in two halves of a carrier integral with die airscrew shaft. This carrier is, in turn, supported by a roller bearing at the front and by a ball and roller bearing in the reduc tion gear casing at the rear, the ball bearing absorbing the thrust from die airscrew. The whole gear is designed to extremely advanced techniques and, although highly stressed at the design power of 2,700 sh.p., has successfully been run for long periods at over 3,500 sh.p. The geometry of the gear is such (as the drawing shows, the drive "loops the loop" in the reduction gear) that tootii deflections largely cancel out. The writer can vouch, from visual inspection of components, to the claim that all die main-drive teeth mesh right across their faces at all torques. A trial 14:1 reduction gear was fun at full power in a "face-to-face" rig before the first Eland was built, and mechanical effi ciencies exceeding 99 per cent are normally attained. Some credit for this result is due to die fact that die gear is completely self- centring, and the quiet running and low heat-loss to the oil confirm the excellent results. The built-in torquemeter consists of an annulus gear and a torque ring, each having several paddle-shaped members carrying spring-loaded sealing vanes. The annulus gear is located inside the torque ring, the paddle-shaped members of each being inter posed and spring-loaded against the annular walls between the opposite members by the sealing vanes. The intervening spaces form the high- and low-pressure chambers of the torquemeter and
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