I understand that mass flow rate has to be corrected because air gets thinner as altitude increase. But what I do not understand is why the corrected mass flow rate is always plotted with corrected speed. Does that mean for higher altitude, that the RPM sensor of the engine will give different readings compared to that in the ground at the same corrected mass flow rate? In short, what is the physical meaning of corrected engine speed?
Why is corrected engine speed is used for engines, even though the RPM of the engine is the same both on ground and at cruise?
Corrected speed allows performance characteristics of the jet engine to be easily compared across operating conditions, specifically across different ambient temperatures.
Let's just say that for a given engine, when ambient temperature is 59 degrees F, 90% physical N1 gives 50,000 pounds of thrust.
When ambient temperature is 100 degrees, the air in thinner. So 90% physical N1 gives less than 50,000 pounds of thrust. When ambient temperature is 0 degress F, the air is denser, so 90% physical N1 gives more than 50,000 pounds of thrust.
On the other hand, 90% corrected speed will always be 50,000 pounds of thrust, regardless of outside temperature.
So in our example, when ambient is 100F, corrected N1 will be lower than physical N1. When ambient is 0F, corrected N1 will be higher than physical N1.