between bearing housing walls to cool the rear face of the second
stage PT rotor assembly. As the ambient air passes the forward face
of the No. 3 bearing seal, it helps pressurize the seal.
VARIABLE INLET GUIDE VANE SYSTEM
the inlet air to the first compressor rotor must be within the stall
free operating range of the compressor blades, and, because this
stallfree operating range varies with compressor speed (N1), it is
necessary to vary the angle of attack with changes in N1 speed. This
is done by varying the angle of the inlet guide vanes. The variable
inlet guide vanes (VIGV) are located in front of the first compressor
rotor as shown in figure 4.5.
At low N1 speeds, a high angle of attack is required, while at
higher N1 speeds, the angle of attack decreases. Refer to the blocks
in figure 4.25 for the angle of attack at high and low N1 speeds.
The VIGV's are positioned by the inlet guide vane actuator
pilot valve, located in the fuel control, which monitors N1 speed and
compressor inlet temperature (T1). While setting the desired
position of the VIGV's, the actuator relays their position back to
the fuel control through an external feedback control rod to nullify
the fuel pressure signal so that at any steadystate N1 speed between
80 and 95 percent, the inlet guide vanes will assume a constant
position. The VIGV actuator is mounted on the right side of the
compressor housing assembly, shown in figure 4.26. The actuator is
controlled by main fuel pressure from the fuel control. Two fuel
lines carry the fuel from the fuel control to the VIGV actuator.
This fuel pressure acts upon the piston inside the actuator to move
the VIGV's. The VIGV's are positioned by the inlet guide vane
actuator control rod through a synchronizing ring.
The interstage bleed system, shown in figure 4.27, consists of
a bleed band, an actuator assembly, and air hoses and connectors.
The function of the system is to improve compressor acceleration
characteristics. The system automatically unloads the compressor of
a small amount of compressed air (about one tenth) during the period
is more desirable than the slight loss in engine power due to the air