I won't disagree with
N1 Vibes about IEPR, but there is a bit more to the picture of N1 as a thrust measurement.
In typical airline service, the fan suffers some degradation, from simple dirt and insect accumulation to outright physical airfoil damage to tip clearance erosion. All of these tend to reduce fan airflow (and thus fan thrust) at a given N1.
But the above is accompanied by similar deterioration in the core engine, and this shows up as increasing EGT over a period of time. This means the
core nozzle velocity increases, and it is producing MORE thrust at a given N1.
The net effect is the two tend to cancel each other, and the
total thrust/N1 relationship changes very little. The effect on takeoff performance will be within the normal day-to-day statistical variation.
So - is the supposed advantage of IEPR as a thrust measurement worth the added weight/expense/maintenance burden of an EPR system? The certification rules require a tachometer on each independent rotor, so removing the N1 gage on an EPR machine is not an option. Why not use it for thrust setting?
The proof is left to the student.
