Re: EPR v. N1 for turbofan power setting

From:         ehahn@jug.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         20 Feb 95 12:05:25 
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In article <airliners.1995.183@ohare.Chicago.COM> entmlf@cr41.staffs.ac.uk (Martin at Staffs University, UK) writes:

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

Both N1 and EPR are fairly easy to measure.  N1 is measured using a
tachometer, and EPR is measured using something very equivalent to a
pitot tube stuck in the exhaust.  (A "Pt7" probe for those curious.)

>     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 MD80 series uses the JT217/JT219 series from Pratt and Whitney,
exclusively.  (The MD90 and 95, however, will not use P&Ws).

>  the Go Around limit was about 2.1, and flight-idle during
>  descent was about 0.3. 

>     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?

>     A range of 0.0 to 2.3 representing the thrust range from idle
>  to TO/GA seems small.

Given that an EPR reading of zero really means that there is a
pressure differential of 1 (i.e. both sides of the engine are at the
same pressure), multiplying the ambient pressure at sea level by 3.3
(i.e. 2.3 gauge pressure), one gets a pressure differential of 48.51
psi.

Not having the engine specs handy, I'll assume an effective engine
diameter of 2 feet (24 inches).  This works out to an engine area of
452.4 square inches.  Multiplying this by 48.51 psi, it works out to
21,945 lbs of static thrust.  (Note that assuming a 3 foot engine
diameter works out to almost 50,000 lbs!). 

This is easily within the ball park of the real answer.

ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.