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A variable cycle engine (VCE) is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic. The next generation of Supersonic transport (SST) may require some form of VCE. SST engines require a high Specific Thrust (net thrust/airflow) at supersonic cruise to keep the cross-sectional area of the powerplant to a minimum, so as to reduce aircraft drag. Unfortunately, this implies a high jet velocity not only at supersonic cruise, but at take-off, which makes the aircraft noisy. A high specific thrust engine has a high jet velocity by definition, as the following approximate equation for net thrust implies:〔(【引用サイトリンク】url=http://exploration.grc.nasa.gov/education/rocket/thrsteq.html )〕 where: intake mass flow rate fully expanded jet velocity (in the exhaust plume) aircraft flight velocity Rearranging the above equation, specific thrust is given by: So for zero flight velocity, specific thrust is directly proportional to jet velocity. The Olympus 593 engines in Concorde had a high specific thrust in supersonic cruise and at dry take-off power. This alone would have made the engines noisy, but the problem was compounded by the need for a modest amount of afterburning (reheat) at take-off (and Transonic Acceleration). An SST VCE would have to increase the engine airflow substantially at take-off, to reduce the jet velocity at a given thrust (i.e. a lower specific thrust) ==Examples== One SST VCE concept is the Tandem Fan engine. The engine has two fans, both mounted on the low-pressure shaft, with a significant axial gap between the units. In normal flight, the engine is in the series mode, with the flow leaving the front fan passing directly into the second fan, the engine behaving much like a normal turbofan. However, for take-off, climb-out, final-descent and approach, the front fan is allowed to discharge directly through an auxiliary nozzle on the underside of the powerplant nacelle. Auxiliary intakes are opened on each side of the powerplant, allowing air to enter the rear fan and progress through the rest of the engine. Operating the fans in this parallel mode substantially increases the total airflow of the engine at a thrust, resulting in a lower jet velocity and a quieter engine. Back in the 1970s, Boeing modified a Pratt & Whitney JT8D to a Tandem Fan configuration and successfully demonstrated the switch from series to parallel operation (and vice versa) with the engine running, albeit at part power. In the Mid Tandem Fan concept, a high specific flow single stage fan is located between the high pressure (HP) and low pressure (LP) compressors of a turbojet core. Only bypass air is allowed to pass through the fan, the LP compressor exit flow passing through special passages within the fan disc, directly underneath the fan rotor blades. Some of the bypass air enters the engine via an auxiliary intake. During take-off and approach the engine behaves much like a normal civil turbofan, with an acceptable jet noise level (i.e., low specific thrust). However, for supersonic cruise, the fan variable inlet guide vanes and auxiliary intake close-off to minimize bypass flow and increase specific thrust. In this mode the engine acts more like a 'leaky' turbojet (e.g. the F404). In the Mixed-Flow Turbofan with Ejector concept, a low bypass ratio engine is mounted in front of a long tube, called an ejector. This silencer device is deployed during take-off and approach. Turbofan exhaust gases induce additional air into the ejector via an auxiliary air intake, thereby reducing the specific thrust/mean jet velocity of the final exhaust. The mixed-flow design does not have the advantages of the mid-tandem fan design in terms of low-speed efficiency, but is considerably simpler. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Variable cycle engine」の詳細全文を読む スポンサード リンク
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