Re: EPR v. N1 for turbofan power setting

From:         K.G.A.Lewis@sussex.ac.uk (Ken Lewis)
Organization: University of Sussex
Date:         08 Mar 95 02:59:51 
References:   1 2 3
Followups:    1
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dzul (dzulbas@merlion.singnet.com.sg) wrote:
: Lakshmi N. Chilukuri (lchiluku@ucsd.edu) wrote:

: : EPR is the ratio of the primary (hot) stream total pressure to the 
: : total pressure of the freestream. EPR should be greater than 1.0 if 
: : the engine is running. So it is tough to accept your estimate of EPR=0.3 
: : for flight idle power!

: Maybe an EPR of 0.3 for flight idle is a bit on the low side but the EPR
: reading can be less than 1. For the PW4056 (I only know about this
: particular engine from experience) the flight idle is around 0.7 .

: When the B744 is fully configured for landing ie with full flaps 
: and gears down, the EPR setting is only around 1.07 .

 The textbook I was reading last night defines Engine Pressure Ratio as
the the ratio of the pressures recorded at the inlet and in the hot
exhaust. I dont see how this can be less than one.  There must be a jet
pilot out there who can say definitively if the EPR dial starts at 1.0
or 0.0!  

EPR is a good indicator for non-high-bypass engines.   N1 may not be a
good indication if the engine is worn or has internal damage but still
runs OK(ish).