65.6 Turbine Engine Inlet Systems

65.6.2 Describe the purpose of the engine inlet duct.

The engine inlet duct directs outside air to the compressor inlet.

Its purpose is to:

• provide the compressor with a smooth and continuous supply of air;

• reduce turbulence and uneven airflow before the air reaches the compressor;

• deliver the air with the minimum possible pressure loss;

• ensure the airflow is distributed evenly across the compressor inlet;

• prevent excessive distortion of the airflow, which could contribute to compressor stall or surge.

On a helicopter, the inlet duct may also incorporate screens, filters, separators, or anti-icing equipment to help prevent foreign objects, dust, water, snow, or ice from entering the engine.

An effective inlet duct allows the compressor to operate efficiently under different flight conditions and engine power settings.

65.6.4 Describe and explain the principles of operation of a Bellmouth inlet duct on a helicopter turboshaft engine.

A bellmouth inlet duct has a smooth, rounded, flared entrance that narrows progressively towards the engine compressor inlet.

As the compressor draws air into the engine, air enters around the wide bell-shaped opening and is guided smoothly into the narrower duct. The reducing cross-sectional area causes the airflow to accelerate as it approaches the compressor.