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Aviation History
1946
1946 - 0360.PDF
FLIGHT COMPRESSOR CASING AIR INTAKE revolutions of the crankshaft. Stroke No. 3 is the only power stroke, the expansion of the ignited fuel /air mixture, previously compressed to a ratio of about 7 to 1, forcing the piston downwards. In an engine operating at a crankshaft speed of 3,000 r.p.m., each stroke is completed in 1 / 100th second and a complete cycle of operations in 1 / 25th second. As the power stroke occurs only once in every two revolutions of the crankshaft the working energy is applied to the shaft intermittently, the frequency depending, of course, upon the number of cylinders. It will be understood that the greater the number of cylinders the nearer is the approach to con- tinuous application of energy. Above this set of diagrams is a sectional diagram of a gas turbine with a multi-stage a^xial compressor and a single-stage turbine. The positions of the two diagrams have been arranged in order to show their corresponding func- tions in relation. In com- paring the action of the two types of engines it is im- portant to appreciate that the turbine produces power continuously, that is, there are no intermittent explo- sions as in a two- or four- stroke reciprocating engine How it Works A turbine /jet engine con- sists of three essential com- ponents, (1) a rotary air compressor at the front, (2) a series of combustion chambers into which kero- sene is sprayed and burnt continuously, and (3) a tur- bine revolving with the com- pressor on a common shaft. There are no pistons, valves or ignition apparatus save for two igniter plugs to attain a start. Initially an electric starter motor spins the main shaft at about one - eighth speed. Atmospheric air is induced at the front by the action of the ' rotating compressor, forced by the same com- pressor into the combustion chambers at a ratio of about 4 to 1 and into this compressed air, aviation ker*. sene is pumped at a pressure of about 750 lb. sq. in. and burnt continuously, somewhat in the manner of a plumber's blowlamp. Only a small quantity of air is used for combustion and the tremendous heat generated expands the main volume of air surrounding each of the flame tubes. As the chambers have an open end, in contrast to the closed cylinder of a piston engine, there is no rise in pressure, but by reason of the rapid expan- sion, the heated air and combustion products at increased velocity force their way through the only exit from the chambers, namely, between the blades on the periphery of the turbine wheel. The rapid expansion of the heated mixture through the airfoil shaped blades (about 50 of them) causes the wheel to rotate rapidly—from 8,000 to 16,000 r.p.m., for example, at full speed. In this manner mechanical energy is created at the shaft, and the compressor fixed to the front end of the shaft is made to do its work in compressing more air into the combustion chambers and the process goes on so long as fuel is introduced. All the power delivered by the TURBINES fc turbine is expended in driving the air compressor and the necessary auxiliary equipment. To drive the com- pressor approximately 100 h.p. is required for each lb. of air delivered per second. A pound of air per second gives about 50 lb. thrust, therefore the turbine of a unit developing 2,000 lb. thrust must deliver about 4,000 h.p. From the rear of the turbine the heated air, still expanding, emerges from the tail nozzle as a high- AIR INTERCONNECTING PIPE BURNER CENTRE BEARING HOUSING BEARING COOLING FAN velocity jet, say, 1,800 ft. per second, or 1,227 rn.p.h. The temperature at the outlet is down to about 300 deg. C, compared with 690 deg. C. maximum as it enters the jet pipe. Actually, it is not strictly correct to term the efflux "propulsive gases." The propulsive effort is the reaction of the force expended in ejecting the gases. A familiar example of jet reaction is the garden rotary water sprinkler. Because there are no fluctuating pressures as in a piston engine, the combustion chambers can be of a light welded sheet-steel construction. Rotary action and continuous operation impart a smoothness of operation to a turbine never before achieved in aircraft power units. They virtually have no out of balance forces. The relatively small bulk, smooth exterior and light weight give the turbine an immense advantage over a piston engine. It requires no cooling radiators and because there are no metallic
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