is known as stall. All pilots are familiar with this condition and
its consequences as it applies to the wing of an aircraft. The stall
that takes place on the fixed or rotating blades of a compressor is
the same as the stalling phenomenon of an aircraft wing.
Centrifugalflow compressors are usually made of titanium.
The diffuser is generally manufactured of a stainless steel alloy. A
close fit is important between the compressor and its case to obtain
maximum compressor efficiency. Correct rotor assembly balancing is
essential for safe operation because of the high rpm. Balancing the
rotor can be accomplished by removing metal from specified areas of
the compressor or by using balancing weights installed in holes in
the hub of the compressor. On some engines where the compressor and
turbine wheel are balanced as a unit, special bolts and nuts having
slight variations in weight are used.
Axialflow compressors are constructed of many different
materials, depending upon the load and temperature under which the
unit must operate. The rotor blades are generally cast of stainless
steel alloy. Some manufacturers use mdybdenum coated titanium blades
to dampen vibrations on some stages of rotor blades. The clearance
between the rotor blades and the outer case is most important. Some
companies coat the inner surface of the compressor case with a soft
material that can be worn away by the blades as they expand because
of the heat generated from compressing the air. This type of
compressor uses the "wearfit" method to form its own clearance
between the compressor case and the rotor blade tip.
Methods of attaching the blade to the disk or hub vary between
manufacturers, with the majority using some variation of the dove
tail method to hold the rotor blades to the disk. Various other
methods are used to anchor the blades in place. Some blades do not
have a tight fit in the disk, but rather are seated by centrifugal
force during engine operation. By allowing the blades to move,
vibrational stress is reduced during start and shutdown. Stator
vanes, shown in figure 1.16, can be either solid or hollow
construction, and are connected together at their tips by a shroud.
This shrouding serves two purposes. First, it provides support, and
second, it provides the necessary air seal between rotating and
stationary parts. Most manufacturers use the split compressor cases,
while some others favor a weldment, forming a continuous case. The
advantages of the split case lie in the fact that the compressor and
stator blades are readily available to inspection. The onepiece
case offers simplicity and strength because it is one piece; in most