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Aviation History
1945
1945 - 1160.PDF
63 2 FLIGHT JUNE 14TH, 1945 GERMAN JET AIRCRAFT planes were shot down by R.A.F. pilots over the Western front. The Junkers Jumo unit was approximately 2ft. gin. in diameter, 11ft. 9m. long and weighed about 1,600 lb. It had an eight-stage axial flow compressor and a single stage turbine operating at about 8,800 r.p.m. As re- gards performance, it has been credited with develop- ing a thrust of 1,980 lb. and consuming from 300 to 330 gallons of fuel per hour. An opposed twin-cylinder two- stroke engine of 70x35 mm. bore and stroke was used for starting. Just why a small two-stroke reciprocating en- gine was chosen for starting is not clear. On the ground, such an engine would no doubt function perfectly. At high altitudes, the possibility of a pure turbine jet cutting out cannot be ruled out entirely and one would assume that a non-supercharged engine would not be easy to get going to give a new start to the turbine. An electric starter seems altogether preferable. The Jumo replaced the Ileinkel unit originally intended for the Me 262. Like- Diagram of a jet propulsion unit employing an eight-stage axial flow compressor in conjunction with a single-stage turbine. Junkers and B.M.W. designs follow this layout. wise the He 280—now obsolete—was originally intended to have two Heinkel jet units but, owing to the slowness of their development, Jumo units were fitted in this case also. Heinkel-Hirth The more modern Heinkel-Hirth on erigine was of slightly greater diameter but shorter length—actually about 3ft. diameter and 8ft. 6in. long—and scaled roughly 1,050 1b Features of the design were a compressor having a first radial stage with axial outlet to the three axial stages and a two-stage turbine, running at 9,000 r.p.m. The shaft ran on ball bearings at the compressor end, roller bearings being used at the turbine end. Continental manufacturers from the outset employed multi-staging, particularly for compressors. Captured units were regarded as unduly heavy and not outstandingly efficient Their "overhaul " life is reported to be quite short—less than 25 hours—which contrasts with the 250 hours' overhaul life of a modern reciprocating type aircraft engine. Whereas turbines by comparison are more simple to service and maintain, the Germans do not appear to have taken advantage of this basic fact. Instead they appear to have aimed more at quick removal and replacement of units rather than rapid overhaul. This is in line with German policy in connection with orthodox engines, which for some years past have been designed to be installed as self-contained units, readily detachable and interchangeable. A 9 to speed capabilities the Me 262 was reported to have The Arado 234, a high-wing bomber-reconnaissance type with two Jumo turbine jet units. been timed over a distance of 12 kms. at a velocity of 745 k.p.h. (463 m.p.h.) and the ability to ascend to an altitude of 12 km. (39,000ft.) in 4.5 minutes. It was not operational at that height. The highest indicated speed was reported as 880 k.p^h. (546 m.p.h.) "at altitude." A reconnaissance type also with Jumo units carried 3,000 litres (660 gallons) of fuel weighing 5,610 lb. Much of the speed of the Me 262 must be attributed to the improved aerodynamic characteristics of the airframe structure and the absence of tail vibration. Thfe Me 262 had an extremely thin wing to minimise compressibility factors. It is clear that the importance of wing and fuselage design going forward hand in hand with speed attainments with modern turbine uriits has been well appre- ciated by the enemy. The design of the engines is interesting indeed, and if the weight is high, their life short, and performance limited these disadvantages may largely be ascribed to inferior materials and, particularly in the case of the Jumo engines, to the urgent need for getting into production. It must be remembered that Germany was seriously hampered by constant Allied bombing, particularly during the closing stages of the war. As a result production and research were curtailed and the enemy's metallurgical difficulties were great. Thus the lack of high quality heat-resisting steels such as are imperative for efficiency with turbine- compressor units prevented complete development. Not only Junkers and Hirth were busily producing jet i AIR FUEL COOL-AIR COMBUSTION INTAKE INJECTION VENT CHAMBER JET EXHAUST GENERATOR STAfiTEffETi IMPELLER ,(COMPRESSOR) DRIVE TURBINESHAFT The simplicity of the basic Whittle jet propulsion unit is well exemplified in this drawing from an American source. The double entry centrifugal compressor is driven by a single-stage turbine.
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