Re: EPR v. N1 for turbofan power setting

From:         Keith Steele <>
Organization: Delphi ( email, 800-695-4005 voice)
Date:         20 Feb 95 12:05:26 
References:   1
Followups:    1
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EPR stands for engine pressure ratio.  It is simply the exhaust pressure
divided by the intake pressure.  For practical purposes the intake pressure
equals ambient pressure.  What produces thrust is the difference between
intake and exhaust pressure.  For example lets say you are at an airport with
an elevation of about 1000 ft.  Ambient pressure is 14 pounds pounds per
square inch.  The engine on one of the B-727 variants that I fly is a
JT-8D with 14,000 lbs takeoff thrust.  The diameter of the exhaust is 3 ft
for a total of 1000 square inches.  Takeoff EPR is 2.0.  Therefore the
   The air pressure at the front
of the engine is 14psi X 1000 sq in or 14,000 lbs of pressure.  The air
pressure at the rear of the engine is 28psi X 1000 sq in or 28,000 pounds.
the total pressure is 28,000 pounds at the back of the engine opposed by
14,000 lbs of pressure at the front of the engine giving a total difference
of 14,000 lbs of forward thrust.  As long as you know what ambient pressure
and the diameter of the engine is you can always use the EPR indicature to
figure exactly how much thrust or push each engine is delivering.  At 18,000
ft the air pressure is only 7 psi, therefore using the math you can see that
2.0 EPR would only give you 7000 lbs of thrust per engine.
Also I believe you misrread the low end of the EPR indicator.  1.0 EPR is no
thrust because the pressure at the front and the back of the engine would be
the same therefore there would be no thrust being produced.
EPR is an excellent thrust indicator because regardless of engine condition
it tells you exactly how much thrust or push the engine is delivering.  The
problem is that with high bypass engines it doesn't work very well.  The
reason is rather complex but if you want to know send me Email.
In the aircraft I fly N1 is used as a backup thrust indicator should the EPR
indicator fail.
N1 is an indirect measure of thrust.  The engineers determine that if N1 is
rotating at a certain speed than so much thrust must be produced.  The probvlem
The problem with N1 is that it does not take into account wear an tear on the
fan blades.  As they get worn they may not produced the thrust they did when
they were new.  On the newer high bypass engines they are the best choice
because EPR is not accurate on high bypass engines.
Keith Steele
Captain/Flight Instructor B-727