Drawing: Cobra Drawing: Cobra


The amount of intake required by a gas turbine engine is approximately 10 times that required by a reciprocating engine. The air entrance is designed to conduct incoming air to the compressor with minimum energy loss resulting from drag or ram pressure loss, that is, the flow of air into the compressor should be free of turbulence to achieve maximum operating efficiency. Proper design contributes materially to aircraft performance by increasing the ratio of compressor discharge pressure to duct inlet pressure.

The amount of air passing through the engine depends on the —

Inlets (Figures 1, and 2 ) are classified as —

Drawing: Location of side inlet and wing inlet

Figure 1

Drawing: Bellmouth Compressor Inlet

Figure 2

There are two basic types of air entrances in use: single entrance and divided entrance. Generally, it is best to use a single entrance with an axial-flow engine to obtain maximum ram pressure through straight flow. Single entrance is used almost exclusively on wing or external installations where the unobstructed entrance lends itself readily to a single short, straight duct (Figure 3).

Drawing: Aircraft With Single-Entrance Duct

Figure 3

A divided entrance offers greater opportunity to diffuse the incoming air and enter the plenum chamber with the low velocity required to utilize efficiently a double-entry compressor (refer back to Figure 1). (The plenum chamber is a storage place for ram air, usually associated with fuselage installations.) It is also advantageous when the equipment installation or pilot location makes the use of a single or straight duct impractical. Inmost cases the divided entrance permits the use of very short ducts with a resultant small pressure drop through skin fiction.

The air inlet section of turboprop and turboshaft engines (Figure 4) also incorporates some type of particle separator or inlet screens to protect compressors from foreign object damage (FOD). Systems will vary among manufacturers. Consult the aircraft or engine technical manuals for a description of the inlet duct and its particular air inlet protective device.

Drawing: Turboprop Compressor Inlet

Figure 4

Air inlet ducts have an anti-icing system. Turbine engine air inlets use hot engine oil, hot bleed air, or a combination of both These systems prevent icing in a turbine engine air inlet. They are not designed to melt ice that has already formed on or in the inlets.

Guide vanes are included in some turbine engines These vanes direct air coming through the inlet into the compressor at the most efficient angle. The angle depends on the speed of the engine. On most engines the vanes are hollow to allow hot air or oil to flow through to prevent ice buildup.


Thanks to (ATSC) Army Training Support Center for the information and graphics.

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Updated: 12 January 2008
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