EPR during take-off roll
 

Hi All

Why does EPR reduce during the take-off roll after take-off thrust being set? Does N1 do the same on engines using N1 as the reference?

Regards

1. Ram air effect increases the pressure in the intake reducing the overall pressure ratio. This is why the power set has to be checked by a certain IAS.

2. the N1 should increase slightly as the foward speed reduces the angle of attack on the fan blades, much the same as if accelerating in a fixed pitch prop aircraft. However, when I flew aircraft with GE CF6s, I never noticed it, so it must have been slight.

In the N1 case, it will depend on the vintage of the control system. If the engine is N2 governed (1970s vintage) then all kinds of thermal etc. effects will cause N1 variation - either up or down - during TO roll and initial climb.

But closed-loop N1 controls introduced in the 80s will give much nicer behavior.