engines have an inlet to bring free stream air into the
engine. The inlet sits upstream of the compressor and, while
the inlet does no work on the flow, there are some important
design features of the inlet. As shown in the figures
above, inlets come in a variety of shapes and sizes with the
specifics usually dictated by the speed of the aircraft.
that cannot go faster than the speed of sound (like large
airliners), a simple, straight, short inlet works quite well.
On a typical subsonic inlet, the surface of the inlet, from
outside to inside, is a continuous smooth curve with some
thickness from inside to outside. The very front (most
upstream portion) of the inlet is called the highlight,
or the inlet lip. A subsonic aircraft has an inlet with
a relatively thick lip.
An inlet for a
supersonic aircraft, on the other hand, has a relatively sharp
lip. The inlet lip is sharpened to minimize the performance
losses from shock waves that occur during supersonic flight.
For a supersonic aircraft, the inlet must slow the flow down
to subsonic speeds before the air reaches the compressor. Some
supersonic inlets, like the one at the upper right, use a
central cone to shock the flow down to subsonic speeds. Other
inlets, like the one shown at the lower left, use flat hinged
plates to generate the compression shocks, with the resulting
inlet geometry having a rectangular cross section. This kind
of inlet is seen on the F-14 and F-15 fighter aircraft. There
are other, more exotic inlet shapes used on some aircraft for
a variety of reasons.
An inlet must
operate efficiently over the entire flight envelope of the
aircraft. At very low aircraft speeds, or when just sitting on
the runway, free stream air is pulled into the engine by the
compressor. In England, inlets are called intakes,
which is a more accurate description of their function at low
aircraft speeds. At high speeds, a good inlet will allow the
aircraft to manoeuvre to high angles of attack and sideslip
without disrupting flow to the compressor. Because the inlet
is so important to overall aircraft operation, it is usually
designed and tested by the airframe company, not the engine
manufacturer. But because inlet operation is so important to
engine performance, all engine manufacturers also employ inlet