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Aviation History
1951
1951 - 0210.PDF
136 FLIGHT, 1 February 1951 ASPIN 1 ... of aircraft speed is less rapid with an augmented jet than with an airscrew and consequently the throttle setting to effect a take-off can be somewhat lower. At very high aircraft speeds, and approaching sonic speed, the propulsive efficiency of the turbojet is superior, but at speeds up to about 575 m.p.h. the augmented jet shows to advantage. Furthermore, the secondary flow lends itself perfectly to the installation of auxiliary fuel-burners which, in the manner of the afterburner for the turbojet, can furnish • an increase of thrust for take-off or emergency. Between these nominal limits of 400 m.p.h. and 575 m.p.h. would appear to be the field of application for the augmented jet. It must be remembered that it possesses the incidental advantages that accrue from the elimination of the airscrew: reduced weight, absence of vibration, less noise, simpler installation, wide latitude of location and grouping, and lower and lighter aircraft undercarriage. The Turbomeca unit, Aspin I, embodies the variable circulation system developed by M. Szydlowski in which the ratio of augmentation is controllable to establish opti- mum propulsive efficiency over a range of aircraft speeds. Variable-incidence shutter vanes, located ahead of the axial fan and interconnected with the turbine speed-control, regulate the proportion absorbed by the fan of the available energy developed by the turbine. This method of control enables substantial variation to be effected rapidly without the need to overcome the inertia of the rotating assembly. Despite the fact that it is coupled to the fan through the medium of a double reduction gear the turbine can, if necessary, be suddenly unloaded. Furthermore, it resolves the major problem of starting that has handicapped other fan-augmentation projects. With the shutter vanes sufficiently closed, the torque resistance is substantially lowered and a conventional electric starter-motor is adequate for the task. Thus the shutter vanes furnish a novel method of control which, with simultaneous regulation of fuel supply, gives an effective adjustment of performance to suit differing flying conditions. In effect, it is analogous to controlling the pitch of an airscrew. Referring to the sectioned drawing, it will be seen that after passing the axial fan the air is divided into two separate flows. The primary flow is to the centrifugal compressor and thence to the annular combustion chamber. Here it receives the energy released from the fuel and is then expanded through the two-stage turbine and discharged as a propulsive jet through a diffusing efflux duct. The design of this duct will determine the static pressure behind the turbine and consequently, to some degree, the division of energy between the axial fan and the jet. It offers, there- fore, the possibility of modifying the performance character- istics of the unit to meet specific applications. Downstream of the axial fan, the secondary flow is accelerated in a duct enshrouding the turbine unit and leading to the orifice of the efflux duct to effect a mixing with the primary flow. To realize the optimum conditions of operation there is co-ordinated control of the two variables, rate of fuel supply and position of the shutter vanes. Two differing regimes, economical cruising and flight in circumstances calling for rapid manoeuvre, may be taken to indicate how this is effected. The control shaft carries actuation cams and is permitted both rotative and axial movement. In the first case, the control lever is turned through an arc to establish a speed of rotation (which is maintained constant by a centri- fugal governor) and to adjust the shutter vanes to give the" best specific rate of fuel consumption. For the second con- dition, depression of a control button brings into operation a hydro-electric servo-motor which moves the control shaft axially and thus ensures that speed of rotation is held to maximum by controlling the action of the centrifugal governor. Rotative movement of the control lever then solely influences the position of the shutter vanes, so making ? possible a rapid variation of thrusL Starting is effected at the control condition for economical cruising. As long as the speed of rotation is less than 70 per cent of the maximum, the shutter vanes remain closed, the centrifugal governor is ineffectual and the fuel supply is regulated solely by movement of the control lever. Only at rotational speeds in excess of 70 per cent of maximum does the simultaneous control of speed and shutter vane movement come into operation. The prototype Aspin I unit illustrated has the following characteristics: — Max. thrust 4401b (200kg) Specific fuel consumption 0.628 Ib/lb/hr (0.628 kg/kg/h) Weight 2641b (120 kg) ••' Max. diameter 23.6 in (0.600 m) In general design the unit follows Turbomeca practice, as exemplified in the Pimene turbojet and Artouste turboprop. It incorporates the unique "centrifugal injection" system in which the fuel, supplied along the main shaft, is atomized and projected into the annular combustion chamber through - radial holes in a thrower flange. Under the control of the French Air Ministry, the Aspin I was submitted to a 150-hr test at the take-off rating of ~ 200 kg thrust under I.CA.O. conditions. On its successful conclusion, the unit was officially examined and, being found in perfectly satisfactory condition, the test was continued for a further 850 hr at continuous maximum rating of 180 kg ~ thrust. After 674 hr the casing of the revolution counter drive was damaged as a consequence of the fracture of the starter ratchet. This component was renewed and the opportunity was taken to change the igniter plug. These were the only items replaced throughout the test during which the turbine was running at speeds from 33,500 to 36,500 r.pjn. At the end of the 1,000-hr test the performance of the unit was reported to be identical with that at the commence- ment. It was completely dismantled for official examination and found to be in excellent condition. From the experience gained in the test certain development modifications are to be made and it is confidently expected that the take-off rating will be raised from 200 kg to 220 kg. 5[~ Parusectional diagram of the " variable circulation " augmented unit. Detail showing ar- Jurbomeca ^ ^ A = Entry duet. B —Shutter vanes. C = Axial fan. D =ftm reduction-gear. E — Centrifugal compressor. F =Annular combustion chamber C — Two-stage turbine.H=Effhurduct. / =Secondary-flow duct. K = Centrifugal fuel-atomizer.
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