For an N1-rated engine, the question of airfoil deterioration and its effect on thrust is often asked. The response I've heard from GE (CFMI) goes like this:
Gradual airfoil deterioration (e.g. sand ingestion)
does cause a small loss of thrust (maybe 1 or 2%). However, during this period, there is also deterioration in the core compressor and turbine, resulting in higher EGT. The effect here is to
increase thrust at a given N1.
So the NET result is less fan thrust, and more core thrust, the two approximately cancelling each other.
