Not sure I understand or want to understand some of the replies in this thread, but I have absolute data for the A320 in front of me, and the reality is that for a given N1 RPM% value the thrust produced by the engine will change with altitude....
See here:
N1% = 78%, FL50, thrust = 29387N, CAS = 259kts
N1% = 78.2% FL370, thrust = 14130N, CAS = 260kts/M0.802
Therefore, it is my understanding that the FADEC (or whatever control system is in place) adjusts the fuel delivery to maintain the demanded RPM, with thrust changing as a function of altitude. This makes sense since at lower altitude the air is 'thicker' and hence you need more push to achieve a given forward speed, with the air density decreasing the higher you get, hence you need less push...
The point is that 78% N1 RPM is always the same irrespective of altitude (i.e. the blade passing frequency will be maintained), what varies is the fuel burn needed to maintain this RPM, as a consequence the thrust delivered by the engine.
- GY