Where high pressure ratios on a single shaft are required it becomes necessary to introduce airflow control into the compressor design. This may take
the form of variable inlet guide vanes for the first stage plus a number of stages incorporating variable stator vanes for the succeeding stages as the shaft pressure ratio is increased (fig. 3-15). As the compressor speed is reduced from its design value these static vanes are progressively closed in order to maintain an acceptable air angle value onto the following rotor blades. Additionally interstage bleed may be provided but its use in design is now usually limited to the provision of extra margin while the engine is being accelerated, because use at steady operating conditions is inefficient and wasteful of fuel. Three types of air bleed systems are illustrated as follows: fig. 3-16 hydraulic, fig. 3-17 pneumatic and fig. 3-18 electronic.
