Chapter_9-Jet_Engines - Introduction Modern aircraft...

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Unformatted text preview: Introduction Modern aircraft engines have the ability to actuate massive airstreams. The total airflow ingested by the engines of a large passenger aircraft during takeoff is of the order of a ton per second. The engine airflow rate is perhaps 50 times the fuel flow rate, so that the term air- breathing engine is quite appropriate. It is with air-breathing jet and propeller engines that this chapter is concerned. Given that thrust is proportional to airflow rate and that large thrust per unit engine size must be a design objective, it follows that the jet engine designer will generally attempt to maximize the airflow rate per unit size of engine. This means maximizing the speed at which the air can enter and the fraction of the inlet area that can be devoted to airflow. Turbine engines generally are far superior to piston engines in these respects, so one never hears of a piston-type jet engine. For low flight speeds, propellers (ingesting 20 to 30 times the airflow rate of the engine) are well able to handle the massive airflow rate required for propulsion. Though piston engines may have superior efficiency at low power levels, gas turbines are favored for driving large propellers because they can generally be designed to do so with much less engine mass per unit power output. This too is related to their ability to accommodate greater airflow for a given size of engine. At low flight speeds, propeller propulsion is considerably more efficient than jet propulsion. Conventional propellers, however, become inefficient and noisy at flight speeds higher than 0.5 or 0.6 times the speed of sound. In contrast, turbojet and turbofan engines can function efficiently and quietly at flight speeds as high as 0.85 times the speed of sound. Turbojets can also operate at supersonic flight speeds. It is high-speed flight that is the chief advantage of using jet, rather than propeller, engines. In very small sizes piston engines can be made to operate more efficiently than turbine engines so, at low flight speed with very small aircraft, piston engines will have a continuing role in driving propellers. In this chapter, we will not discuss piston engines, but we will return to a discussion of propellers. Much work has been done in recent years to develop new designs of propellers that may be able to operate efficiently (and with tolerable noise) at flight speeds as high as 0.6 or 0.7 times the speed of sound. This chapter is based on air-breathing engines that operate in the gas turbine cycle. As the schematic of Fig. 9.1 indicates, they may take a number of forms depending on what is added to the bas gas generator of Fig. 9.1 (a). The output of the gas generator can be used entirely in a single propulsion nozzle (Fig. 9.1 b) or, before expansion in the core engine nozzle, to activate additional turbine stages need to drive a fan (Fig. 9.1 c) that accelerates a large 1 stream of air passing around the engine core. The flow through the outer part of the fan may be five or six times the flow through the engine core. be five or six times the flow through the engine core....
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Chapter_9-Jet_Engines - Introduction Modern aircraft...

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