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Aviation History
1941
1941 - 2110.PDF
156 FLIGHT SEPTEMBER IITH, 194T. JET PROPULSION OF AIRCRAFT A Whittle propulsion plant mounted directly in the tail of an aircraft. Air is first admitted to the interior of the fuselage andthen picked up by the high-speed rotary compressor. The turbine casing and rotor bearings are fluid-cooled. tion conduits may serve as structural members of a fuselageor wing. Embodied as a leading-edge wing spar they would furthermore overcome the possibility of icing athigh altitude. Both the Milo and Ljungstrom designs from Swedenare for streamlined units, presumably intended for wing installation. A feature of the first-mentionedis a scheme for withdrawing compressed air from the propulsion units to charge and warm the pressure-type cabin of high-altitude craft. In the Ljungstrom's plant special arrangements are made for rapid accelera-tion and also emergency lifting or braking efforts. Several British jet-propulsion schemes must be creditedto F. Whittle. The compact, single-jet plant illus- trated in this article utilises high-speed rotary com-ponents in order to minimise weight. Chief interest attaches to the blower-type compressor designed tosecure the high mass-flow of air through the system which is the basic requirement for propulsive effi-ciency. All the air delivered by the compressor, together with the combustion gases of the fuel, are passedthrough a single rotor turbine which drives the compressor. The earlier scheme of H. S. Harris was an interestingsingle compressor aggregate with dual combustion chambers exhausting into long diffuser tubes into which air wasadmitted to increase the jet mass. Some notes on this design will be given in a succeeding article. A British Plant To revert in more detail to the Whittle plant, it will beseen from the drawing that this is enclosed within the fuse- lage. Initially the air can be admitted either by way ofa suitable orifice facing forward or by a series of scoops suitably located to collect the boundary layer from thefuselage shell. The compressor has two air intakes, arranged one on each side of the plane of rotation of theimpeller A. This component is rotated at such speed that the air leaves the tips of the blades at super-sonic velocity.For considerations of flow it is desirable that the air speed should be reduced to a sub-sonic velocity before reachingany stationary part of the blower. Accordingly, the air is delivered first into the radial primary diffuserchamber, B, which discharges into the delivery scroll, C, of increasing cross-sectional area. At its outlet the scrollis directly connected to a helical combustion chamber, D. At first the combustion chamber is of tapering form so asto constitute a secondary difluser for the compressor, and the discharge end of the chamber is connected to the voluteturbine nozzle. This extends around the periphery of the turbine, constituting a single annular orifice, so that theblade ring of the turbine rotor is at all times open to the gas blast. At E the various parts are shown in a plane development to illustrate the gas circuit from the com-pressor chamber to the turbine. The air is heated by the addition of fuel introduced tothe combustion chamber through injector, F, surrounded by a cowl, G. The inner wall of this cowl may be coveredwith a perforated metal or a wire mesh, as indicated in the small detail drawing, to produce a boundary layer of airat a reduced velocity. The air and combustion gases give up part of their energy to the turbine rotor, H, to drivethe air compressor. After leaving the turbine, the air passes into an annular collecting chamber formed as a divergentchannel between the discharge conduit, J, and a cone mounted on the turbine casing. The conduit is continuedto the rear, and terminates in a convergent-divergent pro- pulsion nozzle in the tail of the aircraft. Where two propulsion plants of this type are installedon a single craft they may be arranged to rotate in oppo- site directions in order to reduce the gyrostatic couples toa minimum. Certain Whittle designs were reviewed in Flugsport in 1938. Some Swedish Systems Streanjlined units for wing installation, to the designs ofA. Lysholm, are sponsored by Milo Aktiebolaget, of Stock- holm. In one of these, shown in part-section, the air entersby an axial orifice, is compressed in a multi-stage blower, A, and delivered to chamber, B. This chamberhouses an annular combustion chamber, C, and the gas turbine, D, which drives the blower. The air, preheatedin its passage over the walls of the combustion chamber and the turbine stator casing, at the rear reverses the direc-tion of flow and enters the combustion chamber past fuel nozzles, E. From the combustion chamber the flow isagain reversed, and the gases expand through the turbine and pass with an accelerating velocity through an outletconduit of diminishing cross-sectional area to the discharge nozzle F. A LjungstrSms Angturbin unit embodying dual, twin-rotor blowers for compressing the air. Special arrangements are made to permit rapid speeding-up of the compressor to pro- vide sudden acceleration of the aircraft when taking-off or in case of emergency.
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