Re: EPR v. N1 for turbofan power setting

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         20 Feb 95 12:05:25 
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In article <airliners.1995.183@ohare.Chicago.COM> (Martin at Staffs University, UK) writes:
>A two-part question, and probably a pretty stupid one too:-
>a)  RR Engines usually use Engine Pressure Ratio as
>the parameter used to set engine power/thrust for
>take-off, climb, noise abatement, cruise, etc. Other
>manufacturers use N1.
>Which is the better parameter to use, ie who has got it
>right?   Maybe EPR is harder to gauge than N1 but is more 

EPR is engine pressure ratio, the ratio of air pressure leaving the
back to the air entering the engine.  There are two types of EPR, a 
"simple" EPR, with two probes, one at the inlet and the other at the
fan outlet; another is a sort of "composite" EPR, with two outlet probes:
one for the fan, the other for the turbine exhaust (a small percentage
of total thrust goes through the turbine, and counts).

Why EPR?  Well, on high-bypass turbofans, it just makes more sense.  You're
using a power setting, not a performance setting.  A tach indication can
be unreliable, and can result in different readings among different engines
in identical conditions.  It is also more sensitive to engine differences, so
a peak N1 indicator has to be monitored throughout the regime (some imple-
mentations which use N1 also have an N1 computer showing the numerical
maximum in the gauge; this is one reason why Boeing 747s with CF6 engines
tended to have electromechanical tape displays).

Some engines which use EPR include the JT8D, the RB.211, and JT9.  The 
GE CF6 uses N1.

Another consideration is the type of nacelle the engine is positioned in. 
In this case, the airplane design also becomes a factor; I'm sure one can
find exceptions for all of these cases.

A big, high-bypass fan (e.g., CF6) might benefit more from a tach indicator.

>b) On aircraft that use EPR for thrust setting, why is
>the range from 0.0 to about 2.3?  On a recent MD81
>flight, thrust was set using EPR (I don't know what
>engines, but not RR?) and the Go Around limit was about
>2.1, and flight-idle during descent was about 0.3.

The range is usually 1.0 to 3.0.  At 1.0, the ratio is 1:1; this will
happen on the ground with the airplane standing still.  Most gauges may go 
peg at 0.7 (more ram air than exhaust), but I've never seen one go to 0.0.

>I can't calculate the actual pressure ratio, but IF
>it represents the pressure rise along the engine, and
>if flight-idle is 0.3, then I'd have guessed that full
>power EPR would have been about 50 on that scale?

When you apply full power, you're also creating "suction" at the engine
inlet.  The probles are within the inlet, not on the outside.

Robert Dorsett                         Moderator, sci.aeronautics.simulation