Re: EPR v. N1 for turbofan power setting

From: (Lakshmi N. Chilukuri)
Organization: Univ of California at San Diego
Date:         28 Feb 95 22:57:35 
References:   1
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In article <airliners.1995.183@ohare.Chicago.COM>, says...

>Which is the better parameter to use, ie who has got it
>right?   Maybe EPR is harder to gauge than N1 but is more 

>b) On aircraft that use EPR for thrust setting, why is
>the range from 0.0 to about 2.3?  
>if flight-idle is 0.3, then I'd have guessed that full
>power EPR would have been about 50 on that scale? ....

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!

Percent N1 is a measure of the fan rpm, and hence is directly related to 
the total pressure rise across the fan. Thus, percent N1 is a measure of 
the work being done by the fan. Since the bypass stream dominates the 
thrust generated by modern turbofans, I would prefer to use percent N1 to 
regulate power setting? Any comments? 

EPR can also be used to regulate power setting. However, the power 
expended by the fan can be estimated from EPR only if the thermodynamic 
cycle efficiency is well understood.

Krish Chilukuri
Aero Group Leader
Rohr Industries