Definition
An air inlet duct, shaped like the flared end of a bell, that smoothly guides outside air into the compressor of a turbine engine. The curved, widely flared opening allows air to enter with minimum turbulence and minimum pressure loss, making it especially suitable for engines mounted on helicopters and turboprops where the aircraft itself does not provide significant ram air at the inlet.
Plain English
A flared, bell-shaped opening at the front of a turbine engine that lets air flow in smoothly and evenly before reaching the compressor.
Context Anchor
Seen in aircraft engine inlet design, engine maintenance, and ground test setups where clean airflow into the engine matters.
Derivation
Named for its resemblance to the flared end of a bell. The wide, curved opening narrows gradually toward the engine, much like a bell's mouth narrows toward its body. The shape itself does the work — guiding air in without sharp edges that would cause turbulence.
Why Pilots Care
Smooth airflow into the engine improves thrust, fuel efficiency, and prevents compressor stall or damage from turbulent intake air.
Intuition Check
Do not read “bell-mouth” as a special engine type. It describes the shape of the inlet opening: wide, smooth, and rounded like a bell.
Example Sentence 1
The helicopter's turbine engine uses a bell-mouth inlet duct because it spends most of its time hovering, with no ram air available.
Example Sentence 2
At high speed the bell-mouth inlet duct kept airflow steady so the engine produced full thrust.