AIR INTAKE

The engine air intake serves to furnish a relatively distortion free, high energy supply of air, in the required quantity, to the face of the compressor. A uniform and steady airflow is necessary to avoid compressor stall and excessive internal engine temperatures at the turbine. The high energy enables the engine to produce an optimum amount of thrust. Normally, the air intake duct is considered to be an airframe part, but the duct itself is so important when considering engine performance that it must be included in any discussion on the complete engine.

AIR INTAKE DESIGN REQUIREMENTS

The air intake should be able to:

1. Recover as much of the total pressure of the free airstreams as possible, and deliver this pressure to the face of the engine compressor with the minimum loss.

2. Deliver the air uniformly, with as little turbulence and pressure variation as possible.

3. As far as the aircraft itself is concerned, keep aerodynamic drag to a minimum.

Air intakes can be said to fall into two groups:

1- Those that maintain optimum airflow condition up to high subsonic speed.

2- The second group has to fulfill the requirements of the first group, but in addition have to operate

efficiently at aircraft speeds exceeding Mach 1.

AIR INTAKE TYPES:

Air intakes are of either subsonic or supersonic configuration.

1. SUBSONIC INTAKES

Subsonic intakes are designed to maintain optimum airflow conditions up to high subsonic speeds. For a typical gas turbine engine, the velocity of the airflow at the compressor inlet is normally not in excess of Mach 0.5, this ensures high compressor efficiency. Under flight 'cruising' conditions, the airflow velocity must, in most circumstances, be reduced before it reaches the compressor inlet. This is achieved by designing the intake to function as a SUBSONIC diffuser; such intakes have ducts that increase in cross-sectional area from front to rear i.e. divergent ducts.

You will recall that in a divergent duct, the velocity of flow decreases, with a resulting increase in pressure and temperature above ambient.

2. SUPERSONIC INTAKES

A supersonic diffuser progressively decreases in area from front to rear, therefore supersonic intake ducts follow this general configuration, until the incoming air is reduced down to a velocity below Mach1. To further reduce the velocity to that required by a subsonic intake, the duct now increases in cross-sectional area thereby acting as a subsonic diffuser.

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