bell-mouth inlet duct

A form of convergent inlet air duct used to direct air into the inlet of a gas turbine engine. The area of the duct gets smaller as the air flows into the engine. The duct is very efficient, especially where ram pressures are low, as in helicopter engines where it is used widely. Bell-mouth inlets are also installed on engines being calibrated in a ground test stand to lead the outside static air into the inlet guide vanes of the compressor. Most inlets of this type are detachable.