Aerofoil theory and the axial flow compressor 

The blades of the axial flow compressor are aerofoils and as such behave in a similar way to aircraft mainplanes and propeller blades. The airflow across their surfaces produces lift and the amount of lift produced by an aerofoil depends on:

a Its shape, area and smoothness of its surface.
b the speed of airflow over the aerofoil.
c the angle at which the aerofoil meets the air.

Once manufactured, their area and shape will remain the same unless they are damaged in any way. Assuming the blades are in good condition, the variables will be the speed of the airflow and the angle at which the blades meet the air (angle of attack).

SPEED OF AIRFLOW OVER BLADES

This will vary with the rpm of the compressor rotor. The faster the rotor turns, then the faster the air flows over the blades. This will result in an increase in the axial velocity of the airflow through the compressor.

ANGLE OF ATTACK

This will vary with the combination of the rotational velocity of the blades and the axial velocity of the airflow. In the normal course of events, the angle of attack (VA) becomes progressively smaller as the compressor moves from a low rpm to a high rpm.(VT)