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Aviation History
1954
1954 - 2110.PDF
FLIGHT, 23 July 1954 117 NAPIER ELAND . . . allow movement of the annulus gear. During normal operation, high-pressure oil is supplied to the high-pressure balancing cham bers and then leaks across the sides of the vanes to fill the low-pressure chambers. The chambers are separately interconnected by annular passages; drain-oil escapes to the engine sump through ports in the torque-ring front cover. As the torque increases on the annulus gear, the vanes move in relation to those on the torque ring, increasing the inlet port area, and thereby increasing the oil pressure in the high-pressure cham bers until the pressure is sufficient to balance the torque. Move ment of the annulus gear also increases the exhaust port area to facilitate drainage of the now-greater leakage of oil from the low-pressure chambers. In the reversed torque condition, the ports in use for normal torque are closed by the relative movement of the annulus gear and torque ring, and other ports are uncovered to reverse the functions of the high- and low-pressure chambers. By suitable connections to the pressure-balancing chambers the torquemeter can be used to give signals for operating safeguard devices in the event of component failure. Although the differential-pressure signal from the torquemeter is linear with torque, the low-pressure signal has its peak at zero torque. This feature provides a powerful signal if failure I occurs. The safeguard unit consists of an arrangement which moves to the "guarding" position when the low pressure is above a given level. The safeguard unit is rendered inoperative by a cock during starting and running up. Engine tests have consistently shown that the accuracy of the torquemeter over the normal power range is well within plus or minus one per cent of the calibrated dynamometer readings. The Eland has extensive provision for internal cooling, pressure- balancing and sealing. The front face of the first-stage turbine disc is cooled by air bled at the maximum possible pressure from two holes drilled through the inner wall of each diffuser in the support plate, the latter also embodying an integral annulus from which this air is fed to the turbine. It is noteworthy that the abrupt 90-deg turn of the air bled from the diffuser produces momentum separation of all water in the engine air; thus, any catchment of ice-cold water is not flung against the hot turbine disc but is passed into the combustion chambers. It may be appreciated that from some points of view efficient combustion chambers are a drawback. In the Eland, for example, the total pressure-loss across the entire combustion system is less than 5 lb/sq in, and this has to suffice for the extraction of cooling air and its passage through three quite lengthy pipes to the meter-mg orifices which admit to the annular chamber forward of the first turbine. From here the air flows between the front face of the disc and the front seal plate, through a scroll attached to the seal plate, and exhausts across the rotor blade roots to the trailing «dge. The object of the scroll is to lengthen the path of the cooling air up the face of the disc. Further air is tapped from the ninth compressor stage, again with almost complete water separation, and passes to the hollow haft via a static shroud. This shroud removes the free-vortex .ow which would otherwise produce a near-sonic airspeed, and hus prevents loss of available pressure. This air is then led to "he rear face of the first turbine disc, both sides of the second, "K r ?ront ^ace of lhe mir^- Bv wav of the rcar end of the ^haft, this static pressure is also communicated to the turbine oalance-piston chamber, and maintains the end-load on the rear bearing at 500 lb—the minimum residual load to prevent skid during acceleration. The rear face of die last disc and the turbine bearing are both cooled by fourth-stage air, which is filtered and piped externally. Tappings from the filter also supply air to cool the housing and' inner race of the compressor rear bearing, and to pressurize the seals at the airscrew and accessory bearings. The factor govern ing cooling of the turbine discs is that, by metering the flow to each disc-face, all three disc rims are maintained at the same temperature. By this means, the lightest possible discs are used, with the best material employed in the most efficient manner. A small supply of air—not enough to warrant a vortex breaker —is also obtained from the fifth stage, and this is taken forward, through the hollow shaft, to anti-ice the v.i.g. vanes and supply pressure to the compressor-balance chamber. In addition, cabin-pressure air can be extracted from a delivery flange on the support plate, at a rate of up to half-a-pound per second, and this is automatically compensated for by the control system. The engine-control system itself is entirely Napier in design and manufacture. The pilot's power lever actuates an inter connected airscrew governor and a fuel-metering unit which auto matically compensates for changes in forward speed and ambient pressure and temperature. A special acceleration control prevents overfuelling and surging during acceleration, and a two-position cockpit lever re-sets the fuel-metering unit when cabin air is being tapped off. Other equipment includes a variable-datum • i turbine inlet temperature control, a torque-limitation device, auto-: matic pitch-coarsening and a control for the v.i.g. vanes which is i responsive to engine speed. The aim of the Napier designers has been to achieve a control : which, while having the minimum possible lag, maintains the : appropriate turbine-inlet temperature for each selected engine speed (the two parameters vary direcdy with each other) but which, at a given r.p.m., maintains the turbine-inlet temperature : constant under all operating conditions. No details can be given l of the type of turbine-inlet temperature sensing device used, but : it can be said that the man-hours expended in the development of l the Eland control system match die total time required by the rest ; of the engine. 5 Very rapid starting and acceleration (2j sec from idling to max. j r.p.m.) is made possible by the v.i.g. vanes, "the effect of which : is shown by the small sketch on page 118. Their operation permits excess fuel to be fed during acceleration, the vane angle t being precisely related to turbine-inlet temperature. It will be , noted that the vanes are at constant incidence in all normal flight i conditions. ; The shape of the Eland is such that all the engine auxiliaries are accommodated around the compressor within the confines of : the minimum-drag cowling. The drives for the accessories are f taken from a bevel gear at each end of the central quill shaft. ; The quill shaft is looked upon as the deliberate "weak link" in I the transmission, and its failure is safeguarded by driving the I overspeed governor from the compressor side and the oil pump from the airscrew side. l The two drive-shafts lie vertically inside the twelve o'clock v and six o'clock radial spats. Each drives a train of spur gears s which transmit the drive to the auxiliaries mounted around the i reduction gear casing on what are descriptively known as "banana- 3 plates." The lower group of auxiliaries consists of the oil pres-j, sure and scavenge pumps, and torquemeter pump; the upper £ banana carries the remainder, including the fuel pump, tachometer e generator, airscrew governor, synchronizing alternator and a drive r to a remote accessory gearbox which can take out up to 250 h.p. A pure longitudinal cross-section shows the logical layout of the engine. Each of the four main assemblies—reduction gear, compressor, combustion chambers and turbine—con be removed and replaced individually. The overall cowling diameter is just three feet.
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