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Aviation History
1945
1945 - 0332.PDF
'74 FLIGHT FEBRUARY 15™, 1945 JET PROPULSION as a sparking plug. Once ignited the combustion is con- tinuous, so the igniter can be cut out. Upon being heated to such a high degree the gases rapidly expand and are forced to enter the venturi at a very high velocity. Now, the venturi has the same effect on these gases as does the camber on the upper surface of an aerofoil; that is, it speeds up or accelerates thajflovv of air and burnt gases, and causes an area of low pressure. This area can be utilised to draw in an extra quantity of air to supplement the existing mass, as is seen below. This is thrust augmentation. It was mentioned above that extra efficiency would be gained if the boundary layer over the wing could be con- trolled. Here, then, is our chance. If tubes extend from the combustion chamber to orifices in the wing surface, part of the boundary layer will be drawn in and the main flow over the wing will be sucked closer and prevented from eddying and hence causing loss of efficiency,. Getting back to our venturi, the gases are tearing down it at a very high velocity. They encounter the turbine which drives the compressor and impellor. And now we have reached the jet orifice ; this is the only exit for the gases, which force their way out, their momentum creating the reactionary thrust forwards in the aircraft. To increase manoeuvrability the orifice could be a tube so mounted as to move up and down with the elevator and from side to side with the rudder. Pre-heating the Air All the time this is going on, there is more air being staked in from the surrounding atmosphere, and for more efficient operation it is essential that it be preheated so a ' portion of the high temperature gases in the combustion chamber can be conducted back and mixed with the cold incoming air to raise the intake temperature. This is called heat exchanging, and can raise the efficiency of a jet system by as much as 20 per cent. Now let us take a look at the closed-circuit system and its opportunities. Here no material is wasted at all; in other words the cycle is completely closed. Since this is so, an inert gas ot comparative rarity can be used as the working medium, preferably of the monatomic type, which is simply a gas in which the molecule, the smallest particle that retains all the characteristics of the gas, con- tains only one atom. We use such a gas because a high ratio of specific heat at constant volume to specific heat at constant pressure can be obtained, hence a slower com- pression value will be obtained. This is the gas turbine, a-nd the working of such a system is as follows: —The medium (e.g. Argon or Krypton) is compressed in an axial-flow compressor, passed through pipelines to furnace C, there expanded and then passed to the inlet side of the turbine B, which is of the axial-flow type. The gas is then passed back through a regenerator or heat-exchanger D and thence to the compressor for recirculation. The turbine in this case drives the airscrew and compressor, but a separate engine /can be used to drive the compressor if it is thought that efficiency can be so gained. A t T c B — —^ 1_ In this closed-circuit system A is the axial-flow compressor,C the furnace, B the turbine, and D the regenerator or heat- exchanger. Such a system of course gives perhaps only a few advan- tages over the reciprocating engines of to-day in regard to power/weight ratio and fuel consumption, and can hardly be called jet propulsion, but the last and final system under discussion is the combined turbine airscrew/jet combina- tion, where the gases are compressed and adiabatically expanded, passed through a turbine which is connected by shafting to the airscrew and compressor, then gathered again and expelled as a jet through a venturi in the normal way. In both the; latter systems we are using an airscrew, and hence we introduce the old headache of tip speeds, since the rotational speeds of a turbine are considerably higher than those of a reciprocating engine, large gearing systems will have to be used, and a reduction in diameter can possibly be effected by the use of four- or five-blade contra-rotation airscrews. Of course, the great advantage gained is the fact that take-off conditions will be greatly improved compared with those obtained by pure jet, and rate of climb will be very good. Whether pure jet or the turbine systems will supersede is a matter for thought, but it might be suggested that for large transports, etc., of the future, the airscrews/jet com- bination will be favoured, for it is also possible to "make the combination applicable to retardation as well as to provide the propulsive thrust, and perhaps the pure jet propulsion will be put to use in small high-speed aircraft. To finish up, let us look at the advantages the jet system gives us over the modern reciprocating engine. A summary is as follows: — Power is applied direct and without transmission losses." No airscrew used, hence any amount of power available can be used and no torque reaction effects are produced. Thrust is completely axial. Fuel of less critical formula can be used. Airframes of better aerodynamic shape. -Low U/'C. gives better ground stability and good view forward for the pilot. The boundary layer can be more easily controlled. Heating problems are solved by diverting part of the heated gases. Better power/weight ratio. Vibrationless running. Structure of system much simpler than modern recipro- cating engines. BALLOON COMMAND'S LAST PARADE AFTER five and a half years of active service R.A.F. BalloonCommand officially came to an end last week when, as brii'flv reported in the February 8th issue of Flight, the Secre- tary of State for Air (Sir Archibald Sinclair) attended a fare- well parade, l>ut told the massed ranks of men and women of the Command that their balloons would still continue to "fly" under oilier Commands of the R.A.F. Saving that although farewells are sad occasions, this one was marked by feelings of ihaiiklulness tor devoted service m;infully rendered, l»-c;ius«- " tor more than five years Balloon l\>inn:and has fought the evii machinations of the Nazis and helped to frustrate their knavish tricks." The Command came to life m November, 19^8, under Air Vice-Marshal O. T. Boyd, and was the logical development of the London Balloon Barrage, which actually came into being with Air Comilre. J. G. Hearson as A.O.C. the previous year, and was largely manned by the Auxiliary Air Force. Recruit- ing was put into effect early in 1938, and Londoners first saw their balloons in the air during the Munich crisis in September of that year. When war broke out, Balloon Command had some 600 balloons in the air, but by the end of the blitz they liad increased to 2,400. During the life of the Command, balloons not only protected London and many British industrial centres, but they helped to guard convoys, went into action on L>-Day over the Normandy beaches and over the beaches of Italy and Sicily, and elsewhere. Sir Archibald particularly referred to their splendid work against the flying bomb and to the part played by the W.A.A.F. in Balloon Command. In 1943 over 1,000 balloon sites were taken over by the W.A.A.F., thus releasing many squadrons for overseas.
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