a ratio of approximately 1.25:1 per stage. The action of the  rotor increases the compression of the air at each stage and  accelerates it rearward through several stages. With this  increased velocity, energy is transferred from the compressor  to the air in the form of velocity energy. The stator blades act  as diffusers at each stage, partially converting high velocity  to pressure. Each consecutive pair of rotor and stator blades  constitutes a pressure stage. The number of rows of blades  (stages) is determined by the amount of air and total pressure  rise required. Compressor pressure ratio increases with the  number of compression stages. Most engines utilize up to  16 stages and more. Included in the ducting for double-entry compressor engines  is the plenum chamber. This chamber is necessary for a  double-entry compressor because the air must enter the  engine at almost right angles to the engine axis. Therefore,  in order to give a positive flow, the air must surround the  engine compressor at a positive pressure before entering the  compressor. Included in some installations as necessary parts  of the plenum chamber are the auxiliary air-intake doors  (blow-in doors). These blow-in doors admit air to the engine  compartment during ground operation, when air requirements  for the engine are in excess of the airflow through the inlet  ducts. The doors are held closed by spring action when the  engine is not operating. During operation, however, the doors  open automatically whenever engine compartment pressure  drops below atmospheric pressure. During takeoff and flight,  ram air pressure in the engine compartment aids the springs  in holding the doors closed. The stator has rows of vanes, which are in turn attached inside  an enclosing case. The stator vanes, which are stationary,  project radially toward the rotor axis and fit closely on  either side of each stage of the rotor blades. In some cases,  the compressor case, into which the stator vanes are fitted,  is horizontally divided into halves. Either the upper or lower  half may be removed for inspection or maintenance of rotor  and stator blades.  The diffuser is an annular chamber provided with a number  of vanes forming a series of divergent passages into the  manifold. The diffuser vanes direct the flow of air from the  impeller to the manifold at an angle designed to retain the  maximum amount of energy imparted by the impeller. They  also deliver the air to the manifold at a velocity and pressure  satisfactory for use in the combustion chambers. Refer to  Figure 1-46A  and note the arrow indicating the path of airflow  through the diffuser, then through the manifold.  The function of the stator vanes is to receive air from the  air inlet duct or from each preceding stage and increase  the pressure of the air and deliver it to the next stage at the  correct velocity and pressure. They also control the direction  of air to each rotor stage to obtain the maximum possible  compressor blade efficiency. Shown in  Figure 1-48  are the  rotor and stator elements of a typical axial-flow compressor.  The first stage rotor blades can be preceded by an inlet guide  vane assembly that can be fixed or variable.  The compressor manifold shown in  Figure 1-46A  diverts  the flow of air from the diffuser, which is an integral part of  the manifold, into the combustion chambers. The manifold  has one outlet port for each chamber so that the air is evenly  divided. A compressor outlet elbow is bolted to each of the  outlet ports. These air outlets are constructed in the form  of ducts and are known by a variety of names, such as air  outlet ducts, outlet elbows, or combustion chamber inlet  ducts. Regardless of the terminology used, these outlet ducts  perform a very important part of the diffusion process; that  is, they change the radial direction of the airflow to an axial  direction, in which the diffusion process is completed after the  turn. To help the elbows perform this function in an efficient  manner, turning vanes (cascade vanes) are sometimes fitted  inside the elbows. These vanes reduce air pressure losses by  presenting a smooth, turning surface.  [Figure 1-46B] The guide vanes direct the airflow into the first stage rotor  blades at the proper angle and impart a swirling motion to the  air entering the compressor. This preswirl, in the direction  of engine rotation, improves the aerodynamic characteristics  of the compressor by reducing drag on the first stage rotor  blades. The inlet guide vanes are curved steel vanes usually  welded to steel inner and outer shrouds.  The axial-flow compressor has two main elements: a  rotor and a stator. The rotor has blades fixed on a spindle.  These blades impel air rearward in the same manner as a  propeller because of their angle and airfoil contour. The  rotor, turning at high speed, takes in air at the compressor  inlet and impels it through a series of stages. From inlet to  exit, the air flows along an axial path and is compressed at  At the discharge end of the compressor, the stator vanes are  constructed to straighten the airflow to eliminate turbulence.  These vanes are called straightening vanes or the outlet vane  assembly. The casings of axial-flow compressors not only  support the stator vanes and provide the outer wall of the  axial path the air follows, but they also provide the means  for extracting compressor air for various purposes. The stator  vanes are usually made of steel with corrosion- and erosion- resistant qualities. Quite frequently, they are shrouded  (enclosed) by a band of suitable material to simplify the  fastening problem. The vanes are welded into the shrouds,  and the outer shroud is secured to the compressor housing  inner wall by radial retaining screws. Axial-Flow Compressor 1-41