EPR v. N1 for turbofan power setting
Ed Hahn
> 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 | eh...@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.
Robert Dorsett
>
>
>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
>meaningful?
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
r...@netcom.com aero-si...@wilbur.pr.erau.edu
ftp://wilbur.pr.erau.edu/pub/av
Keith Steele
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
75126,11...@compuserve.com
KEITH...@delphi.com
Eric J. Whitney
I do not have a lot of experience in this area, but I do remember being in a
meeting a long time ago (about 8-10 years) in which there was some
discussion about the parameters used to determine power setting.
I remember that a design engineer on a military engine project said that
F/A-18 pilots used Exhaust Gas Temperature (EGT) to determine take-off power
setting. The pilot would throttle the engine to reach a specific EGT, once
reached the plane would then be catapulted off the deck. It's easy to see
that you can get the same EGT with different EPR and N1's depending on the
condition of the engine. So, when the engineer said this other designers (I
assume on other engine programs) went nuts. They said in a severly degraded
engine N1 could be so low that the plane could be catapulted into the sea! I
cannot comment on any of this because I'm just not knowledgeable in this area;
but using EGT would seem to be very iffy proposition.
I know of no commercial engines that use EGT as a measure of thrust. Perhaps
somebody with more experience in this area could shed some light on this topic.
For example, in the commercial airliner what is the relationship between EPR
or N1 and EGT?
Eric
Lakshmi N. Chilukuri
ent...@cr41.staffs.ac.uk 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
>meaningful?
>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
Carl Nordstrom
> 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.
It seems that you could use *corrected* N1 and always get a proper
indication of thrust in high bypass engines. High bypass engine fan
blades do not suffer the wear and tear of smaller engine fan blades as a
percentage of core flow, so I would wager that this would be a good
indicator. Of course, I don't know if pilots would feel comfortable with
a mathematical approach to thrust, as opposed to a direct reading of N1
(and the subsequent table lookup to arrive at thrust).
Carl Nordstrom
nord...@esa.lanl.gov
Crone
> I remember that a design engineer on a military engine project said that
> F/A-18 pilots used Exhaust Gas Temperature (EGT) to determine take-off power
> setting. The pilot would throttle the engine to reach a specific EGT, once
> reached the plane would then be catapulted off the deck. It's easy to see
> that you can get the same EGT with different EPR and N1's depending on the
> condition of the engine. So, when the engineer said this other designers (I
> assume on other engine programs) went nuts. They said in a severly degraded
> engine N1 could be so low that the plane could be catapulted into the sea! I
> cannot comment on any of this because I'm just not knowledgeable in this area;
> but using EGT would seem to be very iffy proposition.
>
> I know of no commercial engines that use EGT as a measure of thrust. Perhaps
> somebody with more experience in this area could shed some light on this topic.
EGT is particularly useful in fighter aircraft because of (among other
things) formation flight. Generally two aircraft using the same EGT have
nearly identical power output which is great for station keeping.
Fighter engines tend to be low-bypass turbofans, or turbojets (which have
only one RPM guage). With these engines EGT is a pretty good indicator
of power output although in the end I think the choice is simply a matter
of how the pilot was trained to fly. Having flown the Northrop T-38 I
can tell you that it varied from pilot to pilot as to whether he
preferred EGT or RPM for gauging his power.
dzul
: 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 .
its only me ............................. dzul
************************************************
i am who i am but i am not my company's spokesman
Ken Lewis
: Lakshmi N. Chilukuri (lchi...@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).
R. & L. Chilukuri
>: Lakshmi N. Chilukuri (lchi...@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.
K.G.A...@sussex.ac.uk says...
> 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.
I just received flight test data for a RR engine that clearly shows EPR
to be greater than 1.0 for all power settings, altitudes and freestream
Mach numbers.
Perhaps some engineers prefer to subtract 1.0 from the above definition
of EPR.