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Aviation History
1941
1941 - 1951.PDF
AUGUST 28TH, 1941. FLIGHT 117- JET PROPULSION arranged to swivel, and the cross-sectional area may be varied by means of the cone, J. On starting, there will be a negative pressure in the inlet chamber between the cabin and the enshrouding ring, and air can be admitted by the displacement of controlled lateral orifices, K. The air-admission arrangement outlined above is that employed for sub-sonic speeds. If the speed of flight is increased to a super-sonic velocity, a ring, L, is moved forward beyond the former mouth, as indicated by the dotted lines. The relative wind now enters a funnel which first converges and then widens, thereby taking account of the peculiar conditions of flow which are associated with super-sonic velocities. Streamlined Junkers propulsion unit. Fuel is injectedinto the combustion chamber between the compressor and the gas turbine. The gas-flow through the turbine andthe discharge nozzle is maintained at approximately the same sub-sonic velocity. Some German Experiments Germany, and the Junkers Company in particular, is also conducting investigations. Of the two Junkers' projects illustrated, one relates to a plant employing a multi-bank, radial, two-stroke engine as the driving unit tor the turbo-compressor. Usually in plant of this type' a portion of the air delivered by the compressor is utilised to charge the engine cylinders. The exhaust gases from the engine, and the by-passed major portion of the air, are mixed adjacent to the discharge nozzle or nozzles. Losses are occasioned by such a system in the mixing process as, although the mass leaving the nozzle is increased, the velocity is reduced. To avoid this defect the major portion of the air is delivered to a special mixing chamber, in which the exhaust gases are added to raise the pressure before transference to the discharge nozzle. Referring to the drawings, the air is forced by the com- pressor, A, into passages, B, between opposite pairs oi cylinder banks, from which a portion of the supply is taken as scavenging and charging air for the cylinders. The remainder passes round the cylinders as a cooling medium into the mixing chambers, C, arranged between the cylinder banks alternate to the passages, B. Into these chambers the exhaust gases are delivered to mix with the by-passed air, which has been pre-heated by the cylinder walls. From these chambers the mixture expands to the ooter air by way of nozzles, D. To enable the propulsive effort to be increased, combustion chambers, E, into which additional fuel can be introduced through injectors, F, are provided between th« mixing chambers and the nozzles. The air flow through the complete unit is shown diagram- matically in the plane projection of a longitudinal section of the plant. Although there may be some immediate advantage in employing a reciprocating engine, the performance and reliability of which can be calculated with accuracy from accumulated data, it would seem that a gas turbine is par- ticularly suitable for this work. The Junkers Company have proposed an attractive plant of this type. Despite the somewhat ambitious schemes of, for example, their Italian allies, this company is anxious to avoid the attainment of super-sonic speeds, not only of the aircraft, but even of the air flowing through the plant. With admirable caution they admit that such speeds introduce problems of an ex- ceedingly complex character of which there is as yet but scanty knowledge. In the main the data available are related to ballistics and the flight of projectiles. Optimum Velocity With the type of plant shown diagrammatically, com- prising a gas turbine and a turbo-compressor on a common shaft, the selection of a suitable terminal velocity at which *he gases of combustion issue from the nozzle is of great importance. If the velocity be too low the gas turbine will of necessity be of large dimensions. Should the velocity be excessive, that is, above the speed of sound, it is expected that, with our present limited knowledge of super-sonic phenomena, undue losses would be experienced and a low degree of efficiency obtained. Accordingly, for practical purposes, the optimum velocity is regarded as being as high as possible, but below the speed of sound. The selection of such a speed enables the annular exhaust aperture of the turbine to be merged into the discharge nozzle under known or ascertainable streamline conditions. Furthermore, this permits the outer casing also to follow a practical streamlined outline If the conduit from the gas turbine to the discharge orifice is reduced in the direction of flow, the gas velocity through the turbine is lowered, and this unit must be made unduly large. On the other hand, if the conduit is enlarged in cross-section up to its outlet, an excessive terminal gas velocity may be attained which, if of the order of the speed of sound, is attended by unfavourable conditions of flow. Accordingly, it is proposed to maintain the discharge orifice at about the same dimensions as that of the gas turbine to avoid such adverse effects. The same effect, promoting good flow conditions internally and making possible an efficient exterior shape, can be obtained if any modification of the discharge aperture, whether an increase or decrease, is held within a narrow limit of approximately 3 per cent. The schematic drawing shows a compressor. A, driven by a gas turbine, B. Air from the compressor is delivered to the combustion chamber, C, into which the fuel is injected. The combustion gases issuing from the turbine flow through an annular conduit, D, to the discharge orifice, E, with a velocity slightly lower than that of sound. The area of orifice, E, is calculated to be about the same as that of the exhaust aperture of the turbine, and the whole plant is arranged in a streamlined casing. A further article dealing with Je. Propulsion of Aircraft wil/'- appear in an early issue. .-.-"'••' Vapour Lock at Altitude TX>ILING of the petrol, so causing a vapour lock in the fuel~~ system, is one of the problems which high altitude flight brings in its train. Reduction of the atmospheric pressure ataJtitude allows the fuel to vaporise at much lower temperatures than at round level pressures. To combat this, it is necessary«•_ use a fuel with a low vapour pressure, increase the pressure *nnin the fuel tank, or keep the temperature of the fuel low.e first method is not promising as it would then be necessary to use an auxiliary fuel for starting and also because higt)octane fuels with low vapour pressure are not readily available. The pressure inside the tank could be increased by applying asupercharge pressure, or by "self-supercharging," which con- sists in closing the tank vent just after the fuel has startedto boil and so preventing it boiling at any greater altitude. With a fast-climbing fighter, it may be necessary to cool the-fuel cm the ground, otherwise the contents of the tank will still be near ground temperature after the fighter has risen togreat altitude.
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