APPLICATION TO THE AXIAL FLOW COMPRESSOR:
In order for the compressor to deliver a high mass airflow for a minimum effort required to drive it, it is important that all the compressor blades are operating close to their optimum angle of attack at the designed optimum rpm of the engine.
This is achieved by setting the blades onto the rotor assembly at a large enough angle so as to make allowance for the automatic reduction in angle of attack that will occur with increase in rpm.
An axial flow compressor is designed to operate at maximum speeds in the region of 8000-10,000 rpm, depending on size. At this rpm the engine will be producing a large amount of thrust and in order to vary the thrust it is necessary to vary the compressor rpm.
When the compressor is operating at speeds below its designed rpm range, the axial velocity of the airflow through the compressor will decrease which will cause an increase in the angle of attack of the compressor blades. At low rpm, such as idling, the reduced axial velocity of the airflow may cause the angle of attack of some of the blades to increase beyond their stalling angle.
A slight amount of LP blade stalling during ‘off design' conditions is to be expected and only becomes a problem if a complete row of blades stall.
COMMON CAUSES OF COMPRESSOR STALL
Compressor stall normally occurs at low rpm and can be induced by:
a disturbance of smooth airflow due to damaged or dirty blades.
b disturbance of smooth airflow caused by damaged aircraft air intake.
c high combustion chamber pressure caused by over-fuelling during engine acceleration.
STAGGER ANGLE AND END BEND
The rotor blades are of airfoil section and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to 'twist' the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localised increase in blade camber both at the blade tip and root has been introduced. The blade extremities appear as if formed by bending over each corner, hence the term 'end-bend' Figure 4.27.
The latest engines incorporate blades that have been designed and profiled using 3-D design techniques. This produces blades, which are curved in 3 dimensions, which are more aerodynamically efficient.