Stators. --The stator vanes project radially
toward the rotor axis and fit closely on either side
of each stage of the rotor. The function of the
stators is twofold: (1) they receive air from the
air inlet duct or from each preceding stage of the
rotor and deliver the air to the next stage or to
combustors at a workable velocity and pressure;
(2) they control the direction of air to each rotor
stage to obtain the maximum compressor blade
efficiency. The stator vanes are made of alloys
with corrosion- and erosion-resistant qualities.
Frequently, the vanes are shrouded by a band of
suitable material to simplify the fastening
problem. The outer shrouds are secured to the
inner wall of the compressor case by radial
Some manufacturers machine a slot in the
outer shrouds and run a long, thin key the length
by retaining screws to prevent the stators from
turning within the case. This method is used when
a one-piece compressor case is slid over the
Figure 1-14.--Blade with squealer tip.
compressor and stator assembly.
Each pair of vanes in a stator acts as a
Another method of maintaining minimum
diffuser. They use the divergent principle: the
clearance is to metal-spray the case and stators.
outlet of the vane area is larger than the inlet. In
Thin squealer tips on the blades and vanes (fig.
this diverging area, the high-velocity, LP air from
1-14) contact the sprayed material. The abrasive
the preceding rotor stage is converted to a low-
action of the blade tip cuts into the sprayed
material, thus obtaining minimum clearance.
velocity, HP airflow and directed at the proper
angle to the next rotor stage. The next rotor stage
will restore the air velocity that was lost because
The primary causes of rubbing are an
of the pressure rise. The next stator will give a
excessively loose blade or a malfunction of a
further pressure rise. This process continues for
compressor support bearing. This causes the
each stage in the compressor.
compressor rotor to drop.
A pressure rise of about 1.2 times the
Large compressors have loose-fitting blades
preceding stage is about as much as a single stage
on the first several stages. These move during
can handle. Higher pressure rises result in higher
through critical speed ranges. Once up to speed,
diffusion rates with excessive turning angles. This
causes excessive air instability and low efficiency.
centrifugal force locks the blades in place and little
or no movement occurs. Movement of the blades
Preceding the first stage compressor blades is
also occurs during rundown. On a clean engine
a row of vanes known as inlet guide vanes (IGVs).
some of the blades may have as much as 1/4-inch
radial movement, which can cause a tinkling
The function of the IGVs varies somewhat,
depending on the size of the engine and the air-
sound during rundown.
inlet construction. On smaller engines the air
inlet is not totally in line with the first
stage of the rotor. The IGVs straighten the
the first stage. They have a wing tip on the blade
airflow and direct it to the first-stage rotor.
faces called a midspan platform (fig. 1-15). The
On large engines the IGVs are variable and
platform gives some radial support to the blades
move with the variable stators. The variable
IGVs on large engines direct the airflow at
needed because of the length and amount of
the proper angle to reduce drag on the first-stage
movement of the blades.